The Subjective Experience of Time: Welfare Implications

Ex­ec­u­tive Summary

The sub­jec­tive ex­pe­rience of time refers to how slow or fast time ap­pears to pass for an in­di­vi­d­ual. An­i­mals with faster rates of sub­jec­tive ex­pe­rience un­dergo more sub­jec­tive mo­ments per ob­jec­tive unit of time than an­i­mals with slower rates of sub­jec­tive ex­pe­rience. Roughly speak­ing, an­i­mals with faster rates of sub­jec­tive ex­pe­rience per­ceive the world as if it were slowed down com­pared to the per­cep­tions of an­i­mals with slower rates of sub­jec­tive ex­pe­rience. Based on hu­man re­ports of al­ter­a­tions in the sub­jec­tive ex­pe­rience of time, as well as gen­eral differ­ences in be­hav­ior, neu­rol­ogy, and tem­po­ral re­s­olu­tion across an­i­mals, I es­ti­mate there is a ~70% chance that there ex­ist morally rele­vant differ­ences in the sub­jec­tive ex­pe­rience of time across species.

Differ­ences in the sub­jec­tive ex­pe­rience of time would af­fect the per­ceived du­ra­tion of ex­pe­riences and thus would af­fect the qual­ity of ex­pe­riences. An an­i­mal’s sub­jec­tive rate of ex­pe­rience de­ter­mines the num­ber of sub­jec­tive mo­ments of pain (or plea­sure) a painful (or plea­surable) event of a given ob­jec­tive du­ra­tion gen­er­ates. Such differ­ences would be rele­vant to most plau­si­ble the­o­ries of welfare be­cause most plau­si­ble the­o­ries of welfare hold that the sub­jec­tive na­ture of ex­pe­rience mat­ters morally.

An an­i­mal’s max­i­mum rate of sub­jec­tive ex­pe­rience helps de­ter­mine its ca­pac­ity for welfare. An­i­mals with faster rates of sub­jec­tive ex­pe­rience will, all else equal, have a higher ca­pac­ity for welfare than an­i­mals with slower rates of sub­jec­tive ex­pe­rience. Un­like many other de­ter­mi­nants of ca­pac­ity for welfare, the sub­jec­tive ex­pe­rience of time is also di­rectly rele­vant to an an­i­mal’s re­al­ized welfare. The qual­ity of ex­pe­rience is the product of its valence, phe­nom­e­nal in­ten­sity, and sub­jec­tive rate of ex­pe­rience. For many an­i­mals, the phe­nom­e­nal in­ten­sity of ex­pe­riences varies con­sid­er­ably through­out an in­di­vi­d­ual’s life­time. Sub­jec­tive rates of ex­pe­rience may be malle­able, but for most an­i­mals they ap­pear to vary much less fre­quently than phe­nom­e­nal in­ten­sity. For this rea­son, even those skep­ti­cal of the prac­ti­cal im­port of ca­pac­ity for welfare will want to in­cor­po­rate rates of sub­jec­tive ex­pe­rience into their welfare mea­sures.

Differ­ences in neu­rol­ogy, re­ac­tion times, and tem­po­ral in­te­gra­tion win­dows provide means to roughly mea­sure the sub­jec­tive ex­pe­rience of time across species. Based on 13 rele­vant met­rics I have iden­ti­fied, I es­ti­mate that char­ac­ter­is­tic differ­ences in the sub­jec­tive ex­pe­rience of time span no more than two or­ders of mag­ni­tude, with hu­mans fal­ling ap­prox­i­mately mid­way on the spec­trum. Ar­rang­ing an­i­mals on this spec­trum is likely to pro­duce a rad­i­cally differ­ent or­di­nal rank­ing than ar­rang­ing an­i­mals ac­cord­ing to neu­ron count, en­cephal­iza­tion quo­tient, brain-mass-to-body-mass ra­tio, or other met­rics re­lated to brain size. In­cor­po­rat­ing con­sid­er­a­tions about the sub­jec­tive ex­pe­rience of time into our in­ter­species com­par­i­sons (cur­rently dom­i­nated by brain size con­sid­er­a­tions) would likely change the way we pri­ori­tize an­i­mals.

Although much un­cer­tainty re­mains, it ap­pears many an­i­mals have rates of sub­jec­tive ex­pe­rience faster than that of hu­mans. For ex­am­ple, con­ver­gent ev­i­dence from their neu­rol­ogy, be­hav­ior, and the tem­po­ral re­s­olu­tion of their senses in­di­cates song­birds and hon­ey­bees ex­pe­rience 2-10 times as many sub­jec­tive mo­ments per ob­jec­tive unit of time as hu­mans. Thus, all else equal, the painful ex­pe­riences of an­i­mals like song­birds and honey bees likely gen­er­ate more suffer­ing per ob­jec­tive unit of time than com­pa­rable an­i­mals with slower rates of sub­jec­tive ex­pe­rience. In­cor­po­rat­ing this sort of in­for­ma­tion into our pri­ori­ti­za­tion de­ci­sions may al­low us to im­prove the effi­ciency with which we dis­tribute re­sources across differ­ent groups of an­i­mals.

Mo­ral Weight Series

  1. Com­par­i­sons of Ca­pac­ity for Welfare and Mo­ral Sta­tus Across Species

  2. How to Mea­sure Ca­pac­ity for Welfare and Mo­ral Status

  3. The Sub­jec­tive Ex­pe­rience of Time: Welfare Implications

  4. Does Crit­i­cal Flicker-Fu­sion Fre­quency Track the Sub­jec­tive Ex­pe­rience of Time?

In­tro­duc­tion and Context

This post is the third in Re­think Pri­ori­ties’ se­ries ex­plor­ing com­par­a­tive moral value across species. The pri­mary goal of this se­ries is to im­prove the way re­sources are al­lo­cated within the effec­tive an­i­mal ad­vo­cacy move­ment. A sec­ondary goal is to im­prove the al­lo­ca­tion of re­sources be­tween hu­man-fo­cused cause ar­eas and non­hu­man-an­i­mal-fo­cused cause ar­eas. In the first post I lay the con­cep­tual frame­work for the rest of the se­ries, out­lin­ing differ­ent the­o­ries of welfare and moral sta­tus and the re­la­tion­ship be­tween the two. In the sec­ond post, I pre­sent and eval­u­ate two method­olog­i­cal schema for mea­sur­ing and com­par­ing ca­pac­ity for welfare and moral sta­tus. In this, the third en­try in the se­ries, I ex­plain what the sub­jec­tive ex­pe­rience of time is, why it mat­ters, and why it’s plau­si­ble that there are morally sig­nifi­cant differ­ences in the sub­jec­tive ex­pe­rience of time across species. In the next en­try in the se­ries, I ex­plore crit­i­cal flicker-fu­sion fre­quency as a po­ten­tial proxy for the sub­jec­tive ex­pe­rience of time. In the fol­low­ing batch of posts, I in­ves­ti­gate vari­a­tion in the char­ac­ter­is­tic range of in­ten­sity of valenced ex­pe­rience across species.

The Met­ric Problem

In the sec­ond post in this se­ries, I dis­cussed ways to mea­sure moral sta­tus and ca­pac­ity for welfare across differ­ent types of an­i­mals. To make com­par­a­tive judg­ments con­cern­ing moral sta­tus or ca­pac­ity for welfare, we must find em­piri­cally mea­surable prox­ies for the char­ac­ter­is­tics that mat­ter morally. And it must be pos­si­ble to com­pare these prox­ies across a wide ar­ray of species.

The an­i­mal king­dom en­com­passes a vast and di­verse range of an­i­mals that plau­si­bly have moral stand­ing.[1] Hu­mans di­rectly ex­ploit many of these an­i­mals: pigs, cows, goats, sheep, rab­bits, rats, chick­ens, turkeys, ducks, geese, frogs, tur­tles, her­ring, an­chovies, carp, tilapia, cod, cat­fish, eels, oc­to­puses, squid, shrimp, bees, silk­worms, snails, and many oth­ers.[2] An even greater va­ri­ety of an­i­mals plau­si­bly suffer in the wild (in­de­pen­dent of hu­man ac­tion), and the welfare of these an­i­mals mat­ters too. To fully judge in­ter­ven­tions and poli­cies, we need to be able to eval­u­ate welfare gains and losses across many species.[3] To pro­duce such com­par­i­sons, we need to know how many an­i­mals are af­fected, how their con­di­tions are changed by the pro­posed in­ter­ven­tion or policy, and how those con­di­tion changes trans­late into welfare changes. To un­der­stand how con­di­tion changes trans­late into welfare changes, it’s helpful to know about differ­ences in ca­pac­ity for welfare, which is how well or poorly an an­i­mal’s life can go. If an an­i­mal’s welfare can only vac­illate be­tween mild end­points, then big differ­ences in con­di­tions won’t trans­late into big differ­ences in welfare. If an an­i­mal’s welfare can vac­illate be­tween ex­treme end­points, big differ­ences in con­di­tions could trans­late into big differ­ences in welfare.[4] And if an­i­mals differ in moral sta­tus, then we will also want to weight welfare changes by moral sta­tus so as to com­pare in­ter­ven­tions and poli­cies by their effects on sta­tus-ad­justed welfare.

Such com­par­i­sons re­quire some way to as­sess moral sta­tus and ca­pac­ity for welfare. A met­ric to as­sess moral sta­tus or ca­pac­ity for welfare would ideally meet a va­ri­ety of desider­ata. The met­ric must be valid—that is, it must ac­tu­ally track one or more morally salient char­ac­ter­is­tics that con­tribute to ca­pac­ity for welfare or moral sta­tus. The met­ric must be ac­cu­rate—that is, it must be sen­si­tive to differ­ences in ca­pac­ity for welfare or moral sta­tus. The met­ric ought to be ap­pli­ca­ble across species—that is, it ought to be ac­cu­rate and valid with re­spect to phy­lo­ge­net­i­cally dis­tant an­i­mals oc­cu­py­ing differ­ent ecolog­i­cal niches. Fi­nally, the met­ric must be prac­ti­cally mea­surable—that is, it must be fea­si­ble (now or in the near fu­ture) to col­lect ac­tual data for a wide va­ri­ety of species.[5]

Below I pre­sent the sub­jec­tive ex­pe­rience of time as an ex­am­ple of a morally rele­vant fea­ture that can po­ten­tially be op­er­a­tional­ized in a way that ap­pears to satisfy those four desider­ata. Of course, the var­i­ous ways to op­er­a­tional­ize the sub­ject ex­pe­rience of time are not perfect. And nat­u­rally, the met­rics I pro­pose do not give any­thing like a full pic­ture of ca­pac­ity for welfare or moral sta­tus. At best, they de­liver a glimpse at one fac­tor that plau­si­bly con­tributes to moral sta­tus and/​or ca­pac­ity for welfare. In the ideal case, we would want to iden­tify and com­pare mul­ti­ple such met­rics to see where recom­men­da­tions con­verge and to test how sen­si­tive our de­ci­sion-mak­ing is to choice of met­ric. A full anal­y­sis of ca­pac­ity for welfare or moral sta­tus would iden­tify dozens of morally rele­vant fea­tures, each with mul­ti­ple op­er­a­tional­iza­tions.[6]

Nonethe­less, the sub­jec­tive ex­pe­rience of time is use­ful to in­ves­ti­gate for sev­eral rea­sons. First, the sub­jec­tive ex­pe­rience of time is di­rectly rele­vant to re­al­ized welfare as well as ca­pac­ity for welfare, so even those who are skep­ti­cal of the prac­ti­cal im­port of ca­pac­ity for welfare will want to ac­count for differ­ences in the sub­jec­tive ex­pe­rience of time. Se­cond, the sub­jec­tive ex­pe­rience of time is rel­a­tively ne­glected as a po­ten­tial met­ric of com­par­a­tive moral value. The most com­monly used met­ric for com­par­i­sons of moral value across species is neu­ron count (or some var­i­ant thereof).[7] As we’ll see be­low, many of the po­ten­tial prox­ies for the sub­jec­tive ex­pe­rience of time gen­er­ate a rad­i­cally differ­ent or­di­nal rank­ing of an­i­mals com­pared to the rank­ing gen­er­ated us­ing neu­ron counts. We don’t know which gen­eral fea­ture is more im­por­tant, but prox­ies for the sub­jec­tive ex­pe­rience of time are not ob­vi­ously much worse than neu­ron count at ac­cu­rately track­ing salient moral char­ac­ter­is­tics,[8] so the fact that us­ing these met­rics gen­er­ates a rad­i­cally differ­ent rank­ing is sig­nifi­cant for two rea­sons. First, it gives us ad­di­tional data points to con­sider. If we in­cor­po­rate var­i­ous time ex­pe­rience met­rics into our rea­son­ing (per­haps weight­ing them equally with var­i­ous brain size met­rics), we would change the way we rank an­i­mals. Se­cond, un­like the rank­ing gen­er­ated by neu­ron count, the rank­ings gen­er­ated by time ex­pe­rience met­rics don’t ac­cord with our pre-the­o­retic in­tu­itions about com­par­a­tive moral value. This is a helpful re­minder about how lit­tle we know about com­par­a­tive moral value (and pos­si­bly about the dan­ger of speciesist prej­u­dices creep­ing into our rank­ings). We should try to in­cor­po­rate this un­cer­tainty into our de­ci­sion-mak­ing pro­cess.

The Sub­jec­tive Ex­pe­rience of Time

In sim­ple terms, the sub­jec­tive ex­pe­rience of time is how fast time ap­pears to pass from the view­point of a par­tic­u­lar in­di­vi­d­ual.[9] Its con­trast is ob­jec­tive time (some­times called phys­i­cal time in the liter­a­ture), which, hold­ing the refer­ence frame fixed, always passes at the same rate and never differs among in­di­vi­d­u­als.[10] The sub­jec­tive ex­pe­rience of time refers to the felt du­ra­tion of events. Two ob­servers can agree on the ob­jec­tive length of an event but dis­agree on its felt du­ra­tion. Other things equal, the faster one’s rate of sub­jec­tive ex­pe­rience, the more thoughts and sen­sa­tions one can ex­pe­rience in a given unit of ob­jec­tive time. To con­cep­tu­al­ize differ­ences in the sub­jec­tive ex­pe­rience of time, one can imag­ine a movie ei­ther slowed down or sped up. The faster one’s rate of sub­jec­tive ex­pe­rience, the more the world will re­sem­ble a movie played in slow-mo­tion; the slower one’s rate of sub­jec­tive ex­pe­rience, the more the world will re­sem­ble a movie in fast-for­ward.[11]

It’s pos­si­ble that the sub­jec­tive ex­pe­rience of time in­vari­ably tracks ob­jec­tive time and hence that time ap­pears to pass at the same rate for all in­di­vi­d­u­als. In that case, sub­jec­tive ex­pe­rience of time would not be a use­ful con­cept. And be­cause the sub­jec­tive ex­pe­rience of time is a fea­ture of in­ter­nal ex­pe­rience, we can­not mea­sure it di­rectly and thus can­not be cer­tain whether or not it varies among an­i­mals (es­pe­cially non­hu­man an­i­mals who can­not offer self-re­ports). In this way, the sub­jec­tive ex­pe­rience of time is no differ­ent than any other as­pect of phe­nom­e­nal ex­pe­rience, such as the per­cep­tion of pain. How­ever, the fact that we can­not mea­sure in­ter­nal ex­pe­riences di­rectly does not en­tail that we can­not some­times jus­tifi­ably in­fer their ex­is­tence. We can use in­fer­ence to the best ex­pla­na­tion to jus­tifi­ably con­clude that some non­hu­man an­i­mals feel pain.[12] Similarly, we can in prin­ci­ple use in­fer­ence to the best ex­pla­na­tion to jus­tifi­ably con­clude that some an­i­mals differ with re­spect to their sub­jec­tive ex­pe­rience of time. As we’ll see be­low, there is in­deed mod­est ev­i­dence that sug­gests that the sub­jec­tive ex­pe­rience of time char­ac­ter­is­ti­cally differs among species.[13]

It’s im­por­tant to dis­t­in­guish veridi­cal differ­ences in the sub­jec­tive ex­pe­rience of time from illu­sory differ­ences in the sub­jec­tive ex­pe­rience of time. We’re fa­mil­iar with illu­sory dis­tor­tions of the pas­sage of time from ev­ery­day ex­am­ples. Some days fly by; oth­ers crawl. Wait­ing and bore­dom seem to slow time down; en­joy­able ac­tivity ap­pears to speed time up. This is un­doubt­edly a real phe­nomenon, but we need not ap­peal to differ­ences in the sub­jec­tive ex­pe­rience of time in or­der to ex­plain the psy­chol­ogy of these per­ceived dis­tor­tions.[14] In fact, ap­peal­ing to differ­ences in the sub­jec­tive ex­pe­rience of time ap­pears to be a rather poor ex­pla­na­tion.[15] After all, it’s not as if on the days that time crawls that peo­ple ap­pear to be mov­ing in slow mo­tion or that the tempo of one’s fa­vorite song is slowed.[16] (How­ever, there are cer­tain ‘fight-or-flight’ events that do ap­pear to slow time down—this class of events is dis­cussed in more de­tail be­low.)

The key differ­ence be­tween veridi­cal differ­ences in the sub­jec­tive ex­pe­rience of time and illu­sory differ­ences in the sub­jec­tive ex­pe­rience of time is that veridi­cal differ­ences af­fect the moral value of events and illu­sory differ­ences do not. Veridi­cal differ­ences are always ex­pe­ri­en­tial: the num­ber of sub­jec­tive mo­ments one ex­pe­riences per ob­jec­tive unit of time has changed. Illu­sory differ­ences are of­ten (though not ex­clu­sively) memo­rial: one ret­ro­spec­tively re­mem­bers an event as longer or shorter than it ac­tu­ally was. To judge whether some phe­nomenon con­sti­tutes a veridi­cal or illu­sory differ­ence in the sub­jec­tive ex­pe­rience of time, it’s helpful to ask whether the phe­nomenon is morally rele­vant to the event. All else equal, in­creas­ing the sub­jec­tive du­ra­tion of a plea­surable event in­creases the amount of ex­pe­rienced plea­sure. In­creas­ing the ret­ro­spec­tively rec­ol­lected du­ra­tion of an event does not in­crease the amount of ex­pe­rienced plea­sure.[17] (See Ap­pendix 1 for more dis­cus­sion of the dis­tinc­tion be­tween tem­po­ral ex­pe­rience and tem­po­ral judg­ment.)

The sub­jec­tive ex­pe­rience of time is sub­jec­tive in the sense that it is rel­a­tive to some per­ceiver. How­ever, the sub­jec­tivity of the phe­nomenon ought not im­pugn its gen­uine­ness. If it’s true that rates of sub­jec­tive ex­pe­rience vary, then it is an ob­jec­tive fact that an­i­mals with faster rates of sub­jec­tive ex­pe­rience un­dergo sub­jec­tively longer ex­pe­riences per unit of ob­jec­tive time. For two oth­er­wise similar in­di­vi­d­u­als who differ in their sub­jec­tive ex­pe­rience of time, the in­di­vi­d­ual with the faster rate of sub­jec­tive ex­pe­rience can (in prin­ci­ple) think more thoughts and feel more sen­sa­tions than the in­di­vi­d­ual with the slower rate of sub­jec­tive ex­pe­rience. The per­cep­tion of pain is sub­jec­tive in the same way. Two sub­jects might be ex­posed to a nega­tive stim­u­lus (an elec­tric shock, say) of the same in­ten­sity but differ with re­spect to the felt bad­ness of the sub­se­quent pain. Such a differ­ence is sub­jec­tive (in that the painful­ness of the shock is rel­a­tive to the sub­ject be­ing shocked) but no less gen­uine in virtue of the sub­jec­tivity. What mat­ters morally is not the ob­jec­tive in­ten­sity of the stim­u­lus, but the sub­jec­tive bad­ness of the ex­pe­rience.

As we’ll see be­low, there are con­di­tions un­der which it is plau­si­ble (though not cer­tain) that an in­di­vi­d­ual’s sub­jec­tive ex­pe­rience of time changes. Hu­mans who have in­gested cer­tain mind-al­ter­ing drugs some­times re­port dis­tor­tions in their ex­pe­rience of time. Hu­mans who have ex­pe­rienced in­tensely sur­pris­ing, fright­en­ing, and life-threat­en­ing events of­ten re­port that time seemed to slow down un­til the dan­ger passed. Th­ese sorts of events may also in­duce changes in the sub­jec­tive ex­pe­rience of time in non­hu­man an­i­mals. How­ever, for the pur­poses of com­par­ing species, it’s use­ful to fo­cus on an an­i­mal’s char­ac­ter­is­tic ex­pe­rience of time, by which I mean the species-typ­i­cal rest­ing ex­pe­rience of time.

Why the Sub­jec­tive Ex­pe­rience of Time Matters

In the first post in this se­ries, I out­lined three broad fam­i­lies of the­o­ries of welfare: (1) he­do­nis­tic the­o­ries, ac­cord­ing to which welfare is the bal­ance of ex­pe­rienced plea­sure and pain, (2) de­sire-fulfill­ment the­o­ries, ac­cord­ing to which welfare is the de­gree to which one’s de­sires are satis­fied, and (3) ob­jec­tive list the­o­ries, ac­cord­ing to which welfare is the ex­tent to which one at­tains non-in­stru­men­tal goods like hap­piness, virtue, wis­dom, friend­ship, knowl­edge, and love. The sub­jec­tive ex­pe­rience of time is a fun­da­men­tal as­pect of ex­pe­rience, and ac­cord­ing to each of these fam­i­lies of the­o­ries, the qual­ity of ex­pe­rience is morally sig­nifi­cant.

An an­i­mal’s sub­jec­tive ex­pe­rience of time refers to the phe­nom­e­nal du­ra­tion of its ex­pe­riences. The phe­nom­e­nal qual­ity of an ex­pe­rience is the product of its valenced in­ten­sity (ei­ther pos­i­tive or nega­tive) and its du­ra­tion. We know that the valenced in­ten­sity of a stim­u­lus is sub­ject-rel­a­tive. There is not a unique an­swer to the ques­tion ‘How painful (plea­surable) is this stim­u­lus?’ The an­swer de­pends on the per­cep­tion of the sub­ject.[18] It’s pos­si­ble that the phe­nom­e­nal du­ra­tion of a stim­u­lus is sub­ject-rel­a­tive in a similar way. If an­i­mals differ with re­spect to the sub­jec­tive ex­pe­rience of time, then there is no unique an­swer to the ques­tion ‘How long did this stim­u­lus ap­pear to last?’

We could in­sist on calcu­lat­ing the qual­ity of an ex­pe­rience by mul­ti­ply­ing its sub­ject-rel­a­tive in­ten­sity by its sub­ject-in­var­i­ant du­ra­tion (as mea­sured by ob­jec­tive time). But if it is the per­ceived in­ten­sity of a stim­u­lus that mat­ters morally, rather than any ob­jec­tive fea­ture of the stim­u­lus, it’s un­clear why we shouldn’t ap­ply the same rea­son­ing to du­ra­tion. It seems in­tu­itively clear that a stim­u­lus that feels more painful on one oc­ca­sion is worse than a stim­u­lus that feels less painful on an­other oc­ca­sion, even if the ob­jec­tive mag­ni­tude of the stim­uli is the same in both cases. If that’s right, then it seems in­tu­itively equally plau­si­ble that a painful event that feels longer in some ro­bust sense[19] is worse than a similar event that feels shorter, even if the ob­jec­tive du­ra­tion of the two events is the same.

If two an­i­mals ex­pe­rience time differ­ently, then a pain of the same phe­nom­e­nal in­ten­sity ex­pe­rienced for the same ob­jec­tive du­ra­tion would not in gen­eral gen­er­ate the same amount of suffer­ing. The an­i­mal with the faster sub­jec­tive ex­pe­riences would suffer more be­cause it would feel the pain longer. Hence, we should not naively equate the phe­nom­e­nal ex­ten­sion of pain with its du­ra­tion ex­pressed in ob­jec­tive time. The true qual­ity of an ex­pe­rience is the product of its phe­nom­e­nal in­ten­sity and phe­nom­e­nal ex­ten­sion.

The claim here is that the qual­ity of an ex­pe­rience is the product of its felt valenced in­ten­sity and its felt du­ra­tion. Felt du­ra­tion is dis­tinct from es­ti­mated du­ra­tion. Es­ti­mated du­ra­tion, like es­ti­mated in­ten­sity, is ret­ro­spec­tive. As we’ll see be­low, there are many con­di­tions un­der which our es­ti­mated du­ra­tions are sys­tem­at­i­cally skewed in ways that don’t re­flect our felt du­ra­tion. Similarly, there are con­di­tions un­der which our ret­ro­spec­tive es­ti­mates of painful­ness and plea­surable­ness are sys­tem­at­i­cally mis­taken in ways that don’t re­flect our felt ex­pe­rience.[20] But what mat­ters morally is not ret­ro­spec­tive es­ti­mates of pain, but ex­pe­rienced pain.[21] When some­one with short-term mem­ory loss stubs her toe, the bad­ness of the ex­pe­rience isn’t erased be­cause no mem­ory of the ex­pe­rience forms. Our ret­ro­spec­tive per­cep­tions of our ex­pe­riences are im­por­tant, but the moral value of an ex­pe­rience doesn’t change when our mem­o­ries of the ex­pe­rience change. (See Ap­pendix 1 for more dis­cus­sion of the dis­tinc­tion be­tween tem­po­ral ex­pe­rience and tem­po­ral judg­ment.)

Ac­cord­ing to he­do­nism, ex­pe­riences are all that ul­ti­mately mat­ter morally. Although other the­o­ries of welfare deny that ex­pe­riences are all that mat­ter, no plau­si­ble the­ory of welfare de­nies that ex­pe­riences do mat­ter in some re­spect. Ex­pe­riences are rele­vant to de­sire-fulfill­ment ac­counts of welfare. We gen­er­ally de­sire plea­surable ex­pe­riences and de­sire to avoid painful ex­pe­riences. (And painful ex­pe­riences of­ten get in the way of our non-ex­pe­ri­en­tial de­sires.[22]) Ob­jec­tive list the­o­ries main­tain that there are a va­ri­ety of in­trin­sic goods, but all plau­si­ble ob­jec­tive list the­o­ries in­clude goods ei­ther con­sti­tuted by our ex­pe­riences (such as plea­sure) or for which our ex­pe­riences are di­rectly rele­vant (such as life satis­fac­tion or hap­piness). So he­do­nis­tic the­o­ries, de­sire-fulfill­ment the­o­ries, and ob­jec­tive list the­o­ries all agree that ex­pe­riences con­tribute to welfare. Since differ­ences in the sub­jec­tive ex­pe­rience of time al­ter the value of ex­pe­riences, such differ­ences con­tribute to ca­pac­ity for welfare. All else equal, an­i­mals with faster rates of sub­jec­tive ex­pe­rience will have a greater ca­pac­ity for welfare than an­i­mals with slower rates of sub­jec­tive ex­pe­rience.

The sub­jec­tive ex­pe­rience of time is also di­rectly rele­vant to re­al­ized welfare. Some fac­tors in­fluence ca­pac­ity for welfare by ex­tend­ing one’s in­ten­sity range of pos­si­ble ex­pe­riences, with­out nec­es­sar­ily in­fluenc­ing the in­ten­sity of one’s ac­tual ex­pe­riences. (A hu­man, with our vast in­tel­lec­tual and emo­tional so­phis­ti­ca­tion, may be ca­pa­ble of tow­er­ing heights of hap­piness and steep valleys of de­spair. Nonethe­less, a given hu­man might only vac­illate be­tween very mild hap­piness and suffer­ing.) The sub­jec­tive ex­pe­rience of time is not like that. Rather than merely ex­tend­ing the in­ten­sity range of pos­si­ble ex­pe­riences, differ­ences in the sub­jec­tive ex­pe­rience of time act as mul­ti­pli­ers (ei­ther greater or less than one) on all ac­tual ex­pe­riences. Even if we were un­cer­tain about most of the fac­tors that con­tribute to ca­pac­ity for welfare (or skep­ti­cal of the im­por­tance of ca­pac­ity for welfare con­sid­er­a­tions), we could still use prox­ies for the sub­jec­tive ex­pe­rience time to help us com­pare in­ter­ven­tions that tar­get differ­ent species. For a given con­di­tion that causes suffer­ing, an­i­mals with a faster rate of sub­jec­tive ex­pe­rience will, all else equal, suffer more than an­i­mals with a slower rate of sub­jec­tive ex­pe­rience.

It’s less cer­tain that the sub­jec­tive ex­pe­rience of time is rele­vant to moral sta­tus. On some views of moral sta­tus, ca­pac­ity for welfare plays a role in de­ter­min­ing moral sta­tus. Since the sub­jec­tive ex­pe­rience of time con­tributes to ca­pac­ity for welfare, on views ac­cord­ing to which ca­pac­ity for welfare and moral sta­tus are tightly linked, the sub­jec­tive ex­pe­rience of time will be rele­vant to moral sta­tus by virtue of its con­nec­tion to ca­pac­ity for welfare. How­ever, for views ac­cord­ing to which ca­pac­ity for welfare is in­de­pen­dent of moral sta­tus, the rele­vance of the sub­jec­tive ex­pe­rience of time to moral sta­tus is less clear. It doesn’t ap­pear as if the sub­jec­tive ex­pe­rience of time is re­lated in any in­ter­est­ing way to self-aware­ness, au­ton­omy, emo­tional com­plex­ity, nor­ma­tive thought, so­cia­bil­ity or many of the other char­ac­ter­is­tics that plau­si­bly de­ter­mine moral sta­tus.

The Gen­eral Case for Differ­ences in the Sub­jec­tive Ex­pe­rience of Time

There’s no point search­ing for a met­ric that tracks char­ac­ter­is­tic differ­ences in the sub­jec­tive ex­pe­rience of time across species un­less there’s rea­son to think there are char­ac­ter­is­tic differ­ences in the sub­jec­tive ex­pe­rience of time across species. Cer­tainly, there’s no guaran­tee that such differ­ences ex­ist. How­ever, for the rea­sons out­lined be­low, I think it’s plau­si­ble that such differ­ences do ex­ist. I cur­rently es­ti­mate there is a ~70% chance that there ex­ist char­ac­ter­is­tic and sig­nifi­cant differ­ences in the sub­jec­tive ex­pe­rience of time across differ­ent types of an­i­mals. (I op­er­a­tional­ize ‘char­ac­ter­is­tic and sig­nifi­cant differ­ences in the sub­jec­tive ex­pe­rience of time’ as the claim that for at least half their daily wak­ing lives, some an­i­mals main­tain sub­jec­tive rates of ex­pe­rience at least twice as fast as some other an­i­mals.) Given an­other hun­dred hours of re­search, I don’t think my cre­dence would change by more than ten per­centage points in ei­ther di­rec­tion. How­ever, new em­piri­cal ev­i­dence, most of which is ob­tain­able with cur­rent tech­nol­ogy and which could be fea­si­bly gen­er­ated within a decade (given the right fi­nan­cial in­cen­tives), could plau­si­bly push my cre­dence be­low 50% or above 90%.

Mor­pholog­i­cal, Be­hav­ioral, and Neu­rolog­i­cal Differences

First, we know that an­i­mals differ with re­spect to their per­cep­tual ex­pe­rience. Con­sider the many sen­sory modal­ities that some an­i­mals have that oth­ers lack: “Some fish emit elec­tric discharges and then use elec­tric sense or­gans to de­tect dis­tor­tions of the elec­tric field pro­duced by nearby ob­jects. Some bee­tles and snakes use heat-sen­si­tive sense or­gans to de­tect sources of heat. Sev­eral an­i­mals nav­i­gate by the stars or by po­larized sun­light. It seems that an­i­mals in sev­eral phyla nav­i­gate by de­tect­ing the Earth’s mag­netic field” (Howard 2012). More­over, an­i­mals of­ten ex­ploit the same gen­eral modal­ities in differ­ent ways. Some an­i­mals rely pri­mar­ily on sco­topic rather than pho­topic vi­sion;[23] bats and some marine mam­mals use au­di­tion to nav­i­gate (echo-lo­ca­tion); dogs uti­lize ol­fac­tion much differ­ently than hu­mans. Since the per­cep­tual ex­pe­rience of an­i­mals varies so widely, it may ap­pear prima fa­cie sur­pris­ing if tem­po­ral ex­pe­rience were uniform across an­i­mals.

Even be­fore con­sid­er­ing any ex­per­i­men­tal ev­i­dence, given what we know about differ­ences in be­hav­ior, mor­phol­ogy, and neu­rol­ogy across species, it would be hugely sur­pris­ing if there weren’t char­ac­ter­is­tic differ­ences in, say, vi­sion, au­di­tion, or ol­fac­tion. If time per­cep­tion is at all like vi­sual, au­di­tory, or ol­fac­tory per­cep­tion, we should also ex­pect char­ac­ter­is­tic tem­po­ral differ­ences across species. How­ever, I think there’s good rea­son to be­lieve that time per­cep­tion is not at all like vi­sual, au­di­tory, or ol­fac­tory per­cep­tion. In­deed, time per­cep­tion prob­a­bly isn’t even per­cep­tion, strictly speak­ing. The term time per­cep­tion is a bit of a mis­nomer. The mechanisms that allegedly in­fluence the sub­jec­tive ex­pe­rience of time are not meant to con­sti­tute some novel sen­sory modal­ity. Time it­self is not the sort of thing that is amenable to di­rect per­cep­tion. Rather, there is a tem­po­ral di­men­sion to ev­ery sort of per­ceiv­ing. What we ac­tu­ally ob­serve are changes or events in time.[24] So if gen­eral mor­pholog­i­cal, be­hav­ioral, and neu­rolog­i­cal differ­ences across species in­crease one’s cre­dence in char­ac­ter­is­tic differ­ences in tem­po­ral ex­pe­rience across species, they shouldn’t in­crease one’s cre­dence by much.

Hu­man Re­ports of Changes in the Sub­jec­tive Ex­pe­rience of Time

Se­cond, un­der cer­tain con­di­tions, hu­mans of­ten re­port differ­ences in their own sub­jec­tive ex­pe­rience of time. For in­stance, peo­ple fre­quently claim that dur­ing fright­en­ing events, such as au­to­mo­bile ac­ci­dents or com­bat in war­fare, time seems to slow down.[25][26] Hu­mans also some­times re­port that mind-al­ter­ing drugs mod­ify one’s sub­jec­tive ex­pe­rience of time. Re­gard­ing these re­ports, there are two pos­si­bil­ities. One is that such re­ports are liter­ally true: the sub­jec­tive ex­pe­rience of time re­ally does change in some ro­bust, gen­uine way. The other pos­si­bil­ity is that such re­ports are illu­sory: fright­en­ing events and psychedelic drugs heighten the senses, pro­duc­ing much more vivid, fine-grained mem­o­ries which upon rec­ol­lec­tion give us the mis­taken im­pres­sion that we ex­pe­rienced the world at a differ­ent pace.[27] I’ll dis­cuss the two pos­si­bil­ities in turn.

Some au­thors urge us to take re­ports of differ­ences in the sub­jec­tive ex­pe­rience of time liter­ally. Valt­teri Arstila writes, “When peo­ple are sud­denly faced with situ­a­tions that they per­ceive as threat­en­ing, later they of­ten re­port hav­ing had ex­pe­riences with un­usual phe­nomenol­ogy. I ar­gue here that the phe­nomenol­ogy should be taken as given and re­ported in its en­tirety. This is be­cause oth­er­wise some of the most rele­vant phe­nom­ena in these situ­a­tions—such as how peo­ple can act in un­usu­ally fast and pur­pose­ful ways to save their lives—would be left un­ad­dressed” (Arstila 2012: 8). The ex­act mechanism(s) that allegedly pro­duces this phe­nomenon is un­clear. The gen­eral idea is that the ex­treme stress of the situ­a­tion al­ters one’s brain such that “(1) our senses record data at higher den­sity; (2) our brains sam­ple more of these data; [or] (3) our brains pro­cess these sam­pled data faster” (Buck­ley 2014: 1). Ac­cord­ing to Ian Phillips, the world ap­pears to slow down dur­ing trau­matic ex­pe­riences be­cause we have many more thoughts per unit of time than we nor­mally do. “Ac­cord­ing to this hy­poth­e­sis, what sub­jects are re­port­ing in terms of ‘time slow­ing down’ are ex­pe­riences in which an un­usu­ally large amount of non-per­cep­tual men­tal ac­tivity oc­curs within a cer­tain ob­jec­tive pe­riod: much more ac­tivity than would nor­mally oc­cur dur­ing such a pe­riod” (Phillips 2013: 233).[28] (See Ap­pendix 2 for more dis­cus­sion of the­o­ret­i­cal mod­els of tem­po­ral rep­re­sen­ta­tion.)

If differ­ences in the sub­jec­tive ex­pe­rience of time are pos­si­ble across mo­ments, there is prob­a­bly an evolu­tion­ary ac­count of the phe­nomenon that ex­plains why changes in the per­ceived pas­sage of time are benefi­cial. (For in­stance, in life-or-death situ­a­tions, in­di­vi­d­u­als with faster rates of sub­jec­tive ex­pe­rience have more sub­jec­tive time to re­act than in­di­vi­d­u­als with slower rates of sub­jec­tive ex­pe­rience; it’s plau­si­ble that in­di­vi­d­u­als with faster rates of sub­jec­tive ex­pe­rience tend to sur­vive such situ­a­tions more of­ten than in­di­vi­d­u­als with slower rates of sub­jec­tive ex­pe­rience.) If hu­mans can ex­pe­rience differ­ences in the sub­jec­tive ex­pe­rience of time, it ap­pears quite likely that non­hu­man an­i­mals would be able to ex­pe­rience such differ­ences as well. It’s hard to come up with a plau­si­ble rea­son that the phe­nomenon would be re­stricted to hu­mans. And if in­di­vi­d­ual an­i­mals ex­pe­rience time differ­ently de­pend­ing on the situ­a­tion, it seems plau­si­ble that there would be char­ac­ter­is­tic differ­ences in the sub­jec­tive ex­pe­rience of time across species. Phys­iol­ogy differs fairly rad­i­cally among an­i­mals, and differ­ent an­i­mals in­habit differ­ent ecolog­i­cal niches, which may op­ti­mize for differ­ent rates of sub­jec­tive ex­pe­rience. Differ­ences in the speed of preda­tors and/​or prey could ex­ert differ­en­tial se­lec­tion pres­sures lead­ing to differ­ences in the sub­jec­tive ex­pe­rience of time.

On the other hand, some au­thors cau­tion against in­ter­pret­ing anec­do­tal re­ports of time slow­ing down liter­ally. Chess Stet­son, Matthew Fiesta, and David Ea­gle­man ar­gue that “at this stage there is no ev­i­dence to sup­port the hy­poth­e­sis that sub­jec­tive time as a whole runs in slow mo­tion dur­ing fright­en­ing events. Rather, we spec­u­late that the in­volve­ment of the amyg­dala in emo­tional mem­ory may lead to di­lated du­ra­tion judg­ments ret­ro­spec­tively, due to a richer, and per­haps sec­ondary en­cod­ing of the mem­o­ries. Upon later read­out, such highly salient events may be er­ro­neously in­ter­preted to have spanned a greater pe­riod of time” (Stet­son, Fiesta, & Ea­gle­man 2007: 3). The ba­sic idea is that dur­ing stress­ful events “our mem­o­ries store data at higher den­sity” (Buck­ley 2014: 1) and upon rec­ol­lec­tion this differ­ence in den­sity in­duces a memo­rial illu­sion of time slow­ing down. (See Figure 1.) Dan Zakay re­ports ex­per­i­men­tal sup­port for this po­si­tion: “Ret­ro­spec­tive du­ra­tion judg­ment is a func­tion of the amount of re­trieved con­tex­tual changes. The more con­tex­tual changes are re­trieved, the longer the du­ra­tion is judged to be. As a re­sult, when in­for­ma­tion pro­cess­ing dur­ing an in­ter­val is com­plex (i.e., re­mem­ber­ing a com­plex ge­o­met­ri­cal figure), the in­ter­val is judged to be longer in ret­ro­spect than a re­spec­tive in­ter­val in which in­for­ma­tion pro­cess­ing was sim­ple (i.e., re­mem­ber­ing a sim­ple ge­o­met­ri­cal figure like a cir­cle). This is be­cause com­plex in­for­ma­tion pro­cess­ing is caus­ing more con­tex­tual changes to be en­coded than sim­ple in­for­ma­tion pro­cess­ing” (Zakay 2014: 1).

Figure 1: In­creases in the data den­sity of stored mem­o­ries in­duce the illu­sion of time slow­down (source: Han­cock & Weaver 2005: 201)

How­ever, even if the per­ceived slow­down is illu­sory (merely the product of more vivid mem­ory, say), the phe­nomenon still demon­strates the con­cep­tual pos­si­bil­ity of differ­ences in the sub­jec­tive ex­pe­rience of time.[29] Once the con­cep­tual pos­si­bil­ity of the phe­nomenon is made salient, we can per­haps more eas­ily look for ev­i­dence of the phe­nomenon among non­hu­man an­i­mals.

Differ­ences in Re­ac­tion Times

The third, oft-cited, rea­son to think an­i­mals differ in their char­ac­ter­is­tic ex­pe­rience of time is vari­a­tion in re­ac­tion times.[30] Some an­i­mals, such as hum­ming­birds and house­flies, seem to re­act to the world much more quickly than oth­ers. If some of those re­ac­tions are un­der con­scious con­trol, then it’s plau­si­ble that the faster re­ac­tions de­mand a faster rate of sub­jec­tive ex­pe­rience.[31]

In gen­eral, re­ac­tion time is nega­tively cor­re­lated with body size.[32] One study found that the small­est ter­res­trial mam­mals re­act about 17 times faster than the largest ter­res­trial mam­mals (More & Donelan 2018). Star­tle re­flexes can be in­cred­ibly fast in small in­sects, as low as 5 ms in some flies (Sourakov 2011: 367). That’s much faster than the 30-50 ms it takes a typ­i­cal hu­man to blink when a puff of air is blown on her eye (Sourakov 2009: 653). Some squid can ex­e­cute a jet-pro­pel­led es­cape re­ac­tion in as lit­tle as 50-75 ms (Otis & Gilly 1990: 2912). The star-nosed mole has a tac­tile re­flex time of just 120 ms (Cata­nia & Rem­ple 2004: 519), 20% to 40% faster than hu­mans.

How­ever, it’s plau­si­ble that many of those im­pres­sively fast re­ac­tions op­er­ate be­low the level of con­scious per­cep­tion. As an ex­am­ple, think about how quickly a hu­man can pull back her hand from a burn­ing stove. Since the re­ac­tion is me­di­ated by the spinal cord, it hap­pens be­fore the con­scious sen­sa­tion of pain is pro­duced. In gen­eral, fast knee-jerk-type re­flexes, such as star­tle and stretch re­flexes, don’t provide ev­i­dence for fast sub­jec­tive ex­pe­rience. And flight con­trol re­flexes op­er­ate much be­low con­scious ex­pe­rience, so hum­ming­bird and house­fly flight sta­bi­liza­tion, while im­pres­sive, doesn’t tell us much about the sub­jec­tive ex­pe­rience of time. More­over, al­though small an­i­mals have much faster ab­solute re­ac­tion times than large an­i­mals, this ad­van­tage is mostly offset by differ­ences in move­ment rate. Heather More and Maxwell Donelan note that “de­spite a 1-mil­lion-fold range in mass, a 100-fold change in leg length and a more than 15-fold differ­ence in ab­solute de­lay, rel­a­tive de­lays in the largest ter­res­trial mam­mals are only dou­ble the du­ra­tion of those in the small­est” (More & Donelan 2018: 4).

All told, I think differ­ences in re­ac­tion time provide ev­i­dence for differ­ences in the sub­jec­tive ex­pe­rience of time, but the strength of the ev­i­dence is rather mod­est.

Differ­ences in Tem­po­ral Resolution

Differ­ences in tem­po­ral re­s­olu­tion across species offer a fourth rea­son to think an­i­mals differ with re­spect to their sub­jec­tive ex­pe­rience of time. Tem­po­ral re­s­olu­tion is a mea­sure of the rate at which a per­cep­tual sys­tem sam­ples in­for­ma­tion from its en­vi­ron­ment. In an­i­mals it is best stud­ied in the vi­sual and au­di­tory do­mains, though in prin­ci­ple the tem­po­ral re­s­olu­tion of other sense modal­ities could be mea­sured. One com­mon mea­sure of vi­sual tem­po­ral re­s­olu­tion is crit­i­cal flicker-fu­sion fre­quency (CFF). CFF is “the thresh­old at which an an­i­mal ceases to per­ceive a flick­er­ing light source as a se­ries of flashes, but rather as a con­tin­u­ous stream of light” (Inger et al. 2014: 2). This thresh­old char­ac­ter­is­ti­cally varies across species (and, to a milder ex­tent, across in­di­vi­d­u­als of the same species). CFF is mea­sured as a fre­quency and ex­pressed in hertz (Hz). Higher val­ues rep­re­sent bet­ter tem­po­ral re­s­olu­tion.

If CFF does track the sub­jec­tive ex­pe­rience of time, it only does so for a sub­set of an­i­mals. It’s im­por­tant to re­mem­ber that CFF is a vi­sual mea­sure. As such, it is in­evitably bi­ased against crea­tures that in­habit low-light en­vi­ron­ments, like noc­tur­nal or ben­thic an­i­mals[33] (Inger et al. 2014: 2).[34] It is also bi­ased against crea­tures that pri­mar­ily rely on other senses like au­di­tion (e.g., bats), ol­fac­tion (e.g., guinea pigs), and whisk­ing (e.g., rats). The lower CFF thresh­olds for these an­i­mals prob­a­bly do not give us ev­i­dence that their rates of sub­jec­tive ex­pe­rience are cor­re­spond­ingly lower. What­ever evolu­tion­ary ad­van­tage a faster rate of sub­jec­tive ex­pe­rience con­fers, it seems like that ad­van­tage would gen­er­ally ac­crue across most sen­sory modal­ities, rather than be­ing re­stricted to vi­sion. (For in­stance, all else equal, echo-lo­cat­ing bats with bet­ter tem­po­ral re­s­olu­tion would be bet­ter preda­tors than con­speci­fics with worse tem­po­ral re­s­olu­tion.) The ex­is­tence of fast-mov­ing, highly ma­neu­ver­able noc­tur­nal and ben­thic an­i­mals give us prima fa­cie rea­son to think that differ­ences in the sub­jec­tive ex­pe­rience of time won’t be wholly cap­tured by vi­sual mea­sures such as CFF.

CFF val­ues vary con­sid­er­ably across the an­i­mal king­dom, though most stud­ied an­i­mals are within an or­der of mag­ni­tude of each other. As a refer­ence point, hu­mans are gen­er­ally reck­oned to have a CFF of around 60 Hz. About two-thirds of an­i­mals that have been stud­ied have CFF thresh­olds lower than hu­mans.[35] For in­stance, the Euro­pean eel (An­guilla an­guilla) has a CFF of 14 Hz (Healey et al. 2013), the Sergestid shrimp (Ser­gia filic­tum) has a CFF of 24 Hz (Frank 2000), the com­mon cut­tlefish (Sepia offici­nalis) has a CFF of 42 Hz (Nel­son 2003), and the Amer­i­can crayfish (Cam­barus spp) has a CFF of 53 Hz (Inger et al. 2014). About one-third of stud­ied an­i­mals have CFF val­ues higher than hu­mans. For in­stance, the com­mon oc­to­pus (Oc­to­pus vul­garis) has a CFF of 72 Hz (Ha­masaki 1968), the do­mes­tic chicken (Gal­lus gal­lus do­mes­ti­cus) has a CFF of 87 Hz (Healey et al. 2013), and the Tsetse fly (Glossina mor­si­tans) has a CFF of 145 Hz (Inger et al. 2014).

The idea that crit­i­cal flicker-fu­sion fre­quency is cor­re­lated with char­ac­ter­is­tic differ­ences in the sub­jec­tive ex­pe­rience of time across species ap­pears to be taken se­ri­ously in the sci­en­tific com­mu­nity. For in­stance, Bar­tosz Jura of the Nalecz In­sti­tute of Bio­cy­ber­net­ics and Biomed­i­cal Eng­ineer­ing, has ex­plored the con­nec­tion be­tween CFF and sub­jec­tive time ex­pe­rience. He writes, “As the val­ues of CFF dis­play a specific species-varied pat­tern that can be at­tributed to evolu­tion­ary-ecolog­i­cal pro­cesses, sug­gest­ing that the per­sis­tence of vi­sual per­cepts is pre­cisely tuned in par­tic­u­lar species, it seems rea­son­able to as­sume that the per­sis­tence of other types of con­tents of con­scious ex­pe­rience will also be shaped evolu­tion­ar­ily, in ac­cord with the vi­sual sys­tem’s CFF, so that they last as long as it is suit­able for in­di­vi­d­u­als from species liv­ing in given cir­cum­stances, not too long and not too short, likely be­ing pro­por­tional to in­ter­vals sep­a­rat­ing be­hav­iorally rele­vant events in which a cor­re­spond­ing type of in­for­ma­tion is pro­cessed” (Jura 2020: 3). Jura con­cludes, “Sub­jec­tive time flows with vari­able rate, as we know it from in­tro­spec­tion, and it seems that it flows with yet differ­ent rates for in­di­vi­d­u­als from differ­ent species” (Jura 2020: 6).

How­ever, there are rea­sons to doubt that tem­po­ral re­s­olu­tion tracks differ­ences in the sub­jec­tive ex­pe­rience of time. In­creases in tem­po­ral re­s­olu­tion (as mea­sured by in­creases in CFF) may en­able faster re­flexes with­out any ac­com­pa­ny­ing in­crease in the rate of sub­jec­tive ex­pe­rience. Much of the sen­sory in­for­ma­tion that an­i­mals (in­clud­ing hu­mans) ab­sorb is pro­cessed un­con­sciously. Differ­ences in the speed of un­con­scious re­flexes don’t re­veal any­thing about sub­jec­tive ex­pe­rience. We know that spinal cord re­flexes op­er­ate more quickly than con­scious ex­pe­rience. Many im­pres­sively quick re­flexes in the an­i­mal king­dom are also prob­a­bly re­flex-driven. The flight sta­bi­liza­tion mechanisms that al­low hum­ming­birds to flit from flower to flower in the blink of an eye prob­a­bly op­er­ate be­low con­scious aware­ness. In flies, track­ing and pur­suit ini­ti­a­tion mechanisms are trig­gered by fairly sim­ple rules and ap­pear to be hard­wired. More­over, there is some limited ex­per­i­men­tal ev­i­dence that CFF val­ues do not in­crease dur­ing fright­en­ing events, even when du­ra­tion es­ti­mates in­crease (Stet­son, Fiesta, & Ea­gle­man 2007).

Case Study: Birdsong

A fi­nal in­de­pen­dent line of ev­i­dence for differ­ences in the sub­jec­tive ex­pe­rience of time is bird­song. Although the ar­gu­ment de­serves more scrutiny than it is af­forded here, the in­tri­cate sys­tem of call and re­sponse that has evolved among some song­birds sug­gests that those birds are char­ac­ter­is­ti­cally ex­pe­rienc­ing time at a differ­ent rate than that of hu­mans. By many mea­sures, song­birds dis­play ex­traor­di­nary tem­po­ral pro­cess­ing (Lohr, Dool­ing, & Bar­tone 2006: 247).[36] For in­stance, “Ze­bra Finch con­tact calls have very short fun­da­men­tal pe­ri­ods of about 1.5–2.0 ms, shorter than most es­ti­mates of tem­po­ral re­s­olu­tion in the hu­man au­di­tory sys­tem” (Dool­ing & Lohr 2006: 15).[37] Un­like flight con­trol re­flexes, it seems like bird­song is prob­a­bly con­sciously pro­duced and con­sciously ex­pe­rienced. If that’s right, then it’s plau­si­ble ze­bra finches re­quire a faster rate of sub­jec­tive ex­pe­rience than hu­mans (and other ter­res­trial mam­mals) in or­der to reg­ister and re­spond to the calls of con­speci­fics.[38] Or­nithol­o­gists ap­pear to have ad­vanced just such a hy­poth­e­sis: “the re­sults we have re­viewed sug­gests that the avian au­di­tory sys­tem may effec­tively be ‘stretch­ing’ time when per­ceiv­ing com­plex sounds such as species-typ­i­cal vo­cal­iza­tions. Spec­tro­graphic anal­y­sis would miss such de­tail—per­haps in a sense confirm­ing the clas­sic re­frain of avian ethol­o­gists that much of the com­plex­ity in bird song can­not be ap­pre­ci­ated by hu­man hear­ing un­less the tape is slowed down” (Dool­ing & Lohr 2006: 20, em­pha­sis added).[39]

This claim is sug­ges­tive, but it’s un­clear ex­actly what it proves. As an anal­ogy, con­sider a na­tive speaker of some lan­guage com­mu­ni­cat­ing with a per­son who is profi­cient but not adept in the lan­guage. If the na­tive speaker speaks slowly, the non-na­tive speaker can un­der­stand the speech. How­ever, if the na­tive speaker speaks quickly, the non-na­tive speaker quickly loses track of in­di­vi­d­ual words and no longer un­der­stands the speech. A fel­low na­tive speaker, by con­trast, is eas­ily able to fol­low the rapid speech. Does that mean that the fel­low na­tive speaker per­ceives time as slowed down com­pared to the non-na­tive speaker? No.[40] The na­tive speaker is just bet­ter able to pro­cess the rapid spo­ken in­for­ma­tion than the non-na­tive speaker. Similarly, song­birds may just be bet­ter able to pro­cess rapid bird­song than hu­mans.[41] Of course, the anal­ogy is not perfect. As the non-na­tive speaker con­tinues to study the for­eign lan­guage, she will get bet­ter at un­der­stand­ing rapid speech. The differ­ence be­tween hu­man un­der­stand­ing of bird­song and avian un­der­stand­ing of bird­song ap­pears more per­ma­nent, per­haps fixed by phys­iolog­i­cal differ­ences be­tween birds and hu­mans. As a differ­ent anal­ogy, no mat­ter how hard a hu­man prac­tices or stud­ies, she will never be able to nat­u­rally see in­frared light like a vam­pire bat does. It is as yet un­clear which anal­ogy (if ei­ther) is more apt.

Po­ten­tial Prox­ies for the Sub­jec­tive Ex­pe­rience of Time

Below I dis­cuss thir­teen po­ten­tial prox­ies for the sub­jec­tive ex­pe­rience of time. (For ease of ex­po­si­tion, I will here­after drop the ‘po­ten­tial’ in ‘po­ten­tial prox­ies.’) I have di­vided the prox­ies into three cat­e­gories. Neu­rolog­i­cal mea­sures track the speed with which an an­i­mal’s cen­tral ner­vous sys­tem sends and re­ceives sig­nals. Be­hav­ioral mea­sures track the speed with which an an­i­mal re­sponds to ex­ter­nal stim­uli. Tem­po­ral re­s­olu­tion mea­sures track the rate at which an an­i­mal’s per­cep­tual sys­tem sam­ples in­for­ma­tion about its en­vi­ron­ment. This tri­par­tite clas­sifi­ca­tion is a use­ful or­ga­niz­ing tool, but it is nei­ther ex­haus­tive nor con­cep­tu­ally deep.[42] Nor is it the case that each met­ric in each cat­e­gory is equally in­for­ma­tive. My aim in this piece is not to offer a com­pre­hen­sive cat­a­logue of po­ten­tial prox­ies but rather a rep­re­sen­ta­tive sam­pling of po­ten­tial mea­sures.

Each of the prox­ies I dis­cuss is em­piri­cally mea­surable with cur­rent sci­en­tific tools. How­ever, there are large differ­ences in the ex­tent to which the prox­ies have been in­ves­ti­gated across species. Crit­i­cal flicker-fu­sion fre­quency (CFF), a mea­sure of tem­po­ral re­s­olu­tion, is the most di­versely stud­ied proxy. It has been stud­ied in at least 70 species across 30+ or­ders and 3 phyla. Many other prox­ies have only been well-stud­ied in mam­mals. With the right fund­ing, such stud­ies could prob­a­bly be ex­panded to non-mam­malian ver­te­brates and to some in­ver­te­brates.

Fi­nally, it’s im­por­tant to re­mem­ber that all met­rics that mea­sure com­pli­cated phe­nom­ena are sub­ject to some amount of ran­dom noise and most mea­sures are bi­ased un­der some con­di­tions. The goal is not to find a perfect proxy for the sub­jec­tive ex­pe­rience of time. In all like­li­hood, none ex­ist. I would count it a suc­cess if we found a mea­sure that roughly tracks the sub­jec­tive ex­pe­rience of time for some an­i­mals un­der some con­di­tions. As such, no sin­gle mea­sure should be weighted too heav­ily. How­ever, when mul­ti­ple in­de­pen­dent prox­ies all point in the same di­rec­tion, we can be rea­son­ably con­fi­dent that they prob­a­bly in­di­cate some fun­da­men­tal differ­ence in the sub­jec­tive ex­pe­rience of time.

Neu­rolog­i­cal Measures

In com­put­ers, clock speed is the rate at which a micro­pro­ces­sor ex­e­cutes op­er­a­tions. Clock speed is an im­por­tant de­ter­mi­nant of a com­puter’s pro­cess­ing power.[43] All other things equal, the faster a com­puter’s clock speed, the more op­er­a­tions it can ex­e­cute in a given unit of time. If biolog­i­cal brains are analo­gous in im­por­tant re­spects with com­put­ers,[44] then the speed at which an an­i­mal’s brain can send and re­ceive sig­nals may de­ter­mine, in whole or in part, the brain’s ‘clock speed.’ Differ­ences in neu­rolog­i­cal clock speed may gen­er­ate differ­ences in the sub­jec­tive ex­pe­rience of time, though there is no guaran­tee that this would oc­cur.[45] (See Ap­pendix 2 for more dis­cus­sion of the­o­ret­i­cal mod­els of tem­po­ral rep­re­sen­ta­tion.)

The speed at which an an­i­mal’s cen­tral ner­vous sys­tem can send and re­ceive sig­nals de­pends on four main fac­tors: (1) in­terneu­ronal dis­tance, (2) transsy­nap­tic trans­mis­sion time, (3) axon di­ame­ter, and (4) axon myeli­na­tion (Roth & Dicke 2017: 142). Although I dis­cuss these four fac­tors sep­a­rately, it might be con­cep­tu­ally more ap­pro­pri­ate to think that they col­lec­tively con­sti­tute a sin­gle po­ten­tial proxy.

In­terneu­ronal dis­tance is a mea­sure of the space be­tween neu­rons, some­times re­ported equiv­a­lently as neu­ron pack­ing den­sity. All else equal, neu­rons that are closer to­gether can com­mu­ni­cate faster than neu­rons that are farther apart. Two brains might con­tain the same ab­solute num­ber of neu­rons, but if the neu­rons are packed into a smaller vol­ume in one brain, then that brain will have a sig­nifi­cant ad­van­tage be­cause “a short in­terneu­ronal dis­tance, the corol­lary of the ex­tremely high pack­ing den­si­ties of their neu­rons, likely re­sults in a high speed of in­for­ma­tion pro­cess­ing” (Olkow­icz et al 2016: 7259).[46] Due to both neuro-ar­chi­tec­tural differ­ences and differ­ences in the size of neu­rons, neu­ron den­si­ties vary across taxa. For in­stance, grey par­rots (Psit­ta­cus er­itha­cus) have roughly the same num­ber of neu­rons as owl mon­keys (Ao­tus trivir­ga­tus) de­spite brains only half as large.[47] (See Figure 2.) Among ver­te­brates, par­rots and song­birds tend to have the small­est in­terneu­ronal dis­tances, and elephants, dolphins, and whales tend to have the largest in­terneu­ronal dis­tances.

Figure 2: Birds tend to have smaller in­terneu­ronal dis­tances be­cause their neu­rons are smaller and more densely packed (source: Pavel Němec, via “Are you smarter than a macaw?”)

Transsy­nap­tic trans­mis­sion time refers to the speed with which a synapse can pass a chem­i­cal or elec­tri­cal sig­nal to an­other neu­ron or tar­get cell. The faster the sig­nal can tra­verse the phys­i­cal gap be­tween cells, the faster in­for­ma­tion can be trans­mit­ted to differ­ent parts of the brain or body. Im­por­tantly, transsy­nap­tic trans­mis­sion time is not uniform across the cen­tral ner­vous sys­tem. Elec­tri­cal synapses have a much quicker transsy­nap­tic trans­mis­sion time than chem­i­cal synapses, and in many an­i­mals, the brain re­gions that gov­ern star­tle and es­cape re­flexes (which need to be very rapid) are char­ac­ter­ized by faster transsy­nap­tic trans­mis­sion. In gen­eral, transsy­nap­tic trans­mis­sion times don’t differ much among ter­res­trial mam­mals (More & Donelan 2018: 3), but it is un­clear how ter­res­trial mam­mals com­pare to other groups of an­i­mals.

Axon di­ame­ter and axon myeli­na­tion jointly de­ter­mine ax­onal con­duc­tion ve­loc­ity, the speed at which sig­nals can be sent along nerve fibers. Ax­onal con­duc­tion ve­loc­ity in­creases with axon di­ame­ter: thicker nerves trans­mit sig­nals faster.[48] In most an­i­mals, axon di­ame­ter isn’t uniform. Some ax­ons are much thicker than oth­ers. In hu­mans, the au­di­tory and vestibu­lar ax­ons are the thick­est, giv­ing these ax­ons im­proved tem­po­ral pre­ci­sion and higher in­for­ma­tion rates (Perge et al 2012: 636).

Myelin is an in­su­la­tion sheath found around some nerve fibers. Myeli­nated ax­ons trans­mit sig­nals faster than un­myeli­nated ax­ons. The size of the myelin sheath is not uniform across species. Among mam­mals, the myelin sheath is thick­est in pri­mates and thinnest in elephants and cetaceans (Roth & Dicke 2017: 142). Myeli­na­tion is only found in ver­te­brates, though not all ax­ons in a ver­te­brates’ ner­vous sys­tem are myeli­nated. In­ver­te­brates, es­pe­cially cephalopods, have mostly adopted the evolu­tion­ary strat­egy of in­creas­ing axon di­ame­ter to in­crease ax­onal con­duc­tion ve­loc­ity (Salzer & Zalc 2016: R972).

Be­cause an­i­mals con­tain a wide va­ri­ety of ax­ons of differ­ent sizes and de­grees of myeli­na­tion, there is no sin­gle ve­loc­ity at which sig­nals are trans­mit­ted through the ner­vous sys­tem. In hu­mans, ax­onal con­duc­tion ve­loc­ity ranges from ~0.5 m/​s to ~120 m/​s (Purves 2000).[49] For this rea­son, it would be hard to com­pare ax­onal con­duc­tion ve­loc­ity across species. In suffi­ciently neu­rolog­i­cally similar an­i­mals, one could com­pare the ax­onal con­duc­tion ve­loc­i­ties of ho­molo­gous nerve fibers. It’s not clear how far this ap­proach would take you nor how valuable the com­par­i­son would be.

Another met­ric that might be in­ves­ti­gated is neu­ronal firing rates. How­ever, this is prob­a­bly not a good proxy for the sub­jec­tive ex­pe­rience of time.[50] Differ­ent parts of the brain fire at differ­ent rates and with differ­ent reg­u­lar­ity. Among mam­mals, ho­molo­gous brain re­gions ap­pear to ex­hibit similar firing regimes de­spite differ­ences in brain size (Mochizuki et al. 2016).

Be­hav­ioral Measures

If an­i­mals do differ sig­nifi­cantly in their sub­jec­tive ex­pe­rience of time, the best causal ex­pla­na­tion of these differ­ences will prob­a­bly ap­peal to differ­ences in the speed at which cer­tain neu­rolog­i­cal sig­nals prop­a­gate and are pro­cessed. How­ever, the fore­go­ing doesn’t en­tail that neu­rolog­i­cal mea­sures are our best proxy for differ­ences in the sub­jec­tive ex­pe­rience of time. The neu­rolog­i­cal mechanisms that give rise to differ­ences in the sub­jec­tive ex­pe­rience of time might be too hard to mea­sure and com­pare across species, es­pe­cially when the neu­ral ar­chi­tec­ture of the com­pared an­i­mals differs con­sid­er­ably.

Our best cur­rent ev­i­dence for differ­ences in the sub­jec­tive ex­pe­rience of time may be be­hav­ioral. Many an­i­mal be­hav­iors are more eas­ily ex­plained if we posit differ­ences in the sub­jec­tive ex­pe­rience of time. If our best ex­pla­na­tions ap­peal to differ­ences in the sub­jec­tive ex­pe­rience of time, then, in the ab­sence of defeaters, we are li­censed to pre­fer the sup­po­si­tion that such differ­ences ex­ist.

Some of these be­hav­iors have already been dis­cussed above. Song­birds act on in­for­ma­tion in bird­song that hu­mans can only dis­cern if the bird­song is slowed down. This sug­gests that the song­birds ex­pe­rience the world at a faster rate than hu­mans. Some small an­i­mals ap­pear to re­spond to stim­uli amaz­ingly fast. This video shows a fly leav­ing a flower as a bee ap­proaches, then im­me­di­ately re­turn­ing to the flower.[51] The elapsed time is less than a sec­ond. The de­ci­sion to re­turn to the flower is not ob­vi­ously purely re­flex­ive. Such a quick de­ci­sion is per­haps more ex­pli­ca­ble if we posit that the fly’s rate of sub­jec­tive ex­pe­rience is faster than our own.[52] Rats can dis­crim­i­nate and be­gin re­spond­ing to a novel odor in as lit­tle as 140 ms (Wes­son et al. 2008). Mice, moths, and honey bees also ex­hibit ex­tremely fast odor de­tec­tion, with both re­spond­ing to odors in as lit­tle as 200 ms (Abra­ham et al. 2004; Cardé & Willis 2008; Wright, Car­l­ton, & Smith 2009).[53]

Un­for­tu­nately, quan­tify­ing such be­hav­iors in a way that en­ables them to be com­pared across species is not easy. Re­ac­tion times have been in­ves­ti­gated in a wide va­ri­ety of an­i­mals. (See Figure 3.) Th­ese stud­ies gen­er­ally con­firm that smaller an­i­mals re­act faster than larger an­i­mals. How­ever, the sort of re­ac­tions that are stud­ied in the liter­a­ture are al­most always the sort of re­ac­tions that we would ex­pect to be ini­ti­ated un­con­sciously. While per­haps sug­ges­tive, these stud­ies provide lit­tle di­rect ev­i­dence for differ­ences in the sub­jec­tive ex­pe­rience of time.

Figure 3: Com­pre­hen­sive re­view of the con­trib­u­tors to re­ac­tion time in ter­res­trial mam­mals (source: More & Donelan 2018: 3)

Ideally what we would like are stud­ies that com­pare differ­ences in con­scious de­ci­sion-mak­ing speed. One po­ten­tial way to iden­tify con­scious de­ci­sions is to train non­hu­man an­i­mals on an at­ten­tional blink paradigm. At­ten­tional blink is the tiny gap in con­scious at­ten­tion that re­sults from shift­ing one’s fo­cus be­tween stim­uli. At­ten­tional blink can be mea­sured ex­per­i­men­tally by mea­sur­ing the min­i­mum du­ra­tion at which two tar­gets can be cor­rectly iden­ti­fied. (See Figure 4.) In hu­mans, if the tar­gets are pre­sented in rapid suc­ces­sion, roughly 100 ms apart, they are both con­sciously pro­cessed and are likely to be cor­rectly iden­ti­fied. If the tar­gets are sep­a­rated by a du­ra­tion of more than ~700 ms, they are also likely to be cor­rectly iden­ti­fied. How­ever, tar­gets pre­sented roughly 300 ms apart, the sec­ond tar­get is much harder to iden­tify.[54] This rep­re­sents the length of a typ­i­cal hu­man’s at­ten­tional blink.[55] Re­cent re­search sug­gests that “in­di­vi­d­u­als with bet­ter AB [at­ten­tional blink] task perfor­mance use a shorter time win­dow to in­te­grate in­for­ma­tion, and there­fore have higher preser­va­tion of tem­po­ral in­for­ma­tion” (Willems et al. 2016: 1). Mea­sur­ing the at­ten­tional blink of non­hu­man an­i­mals may help us un­der­stand how those an­i­mals con­sciously ex­pe­rience time.

Figure 4: By al­ter­ing the lag be­tween tar­get 1 and tar­get 2, re­searchers can mea­sure a sub­ject’s at­ten­tional blink (source: Danck­ert & All­man 2005: 239)

Of course, the line be­tween con­scious de­ci­sions and un­con­scious de­ci­sions is not always clear. Hard-wired fixed ac­tion pat­terns can be rel­a­tively com­plex. But even set­ting aside the difficulty of dis­t­in­guish­ing con­scious de­ci­sion-mak­ing from un­con­scious re­flex or in­stinct, nu­mer­ous hur­dles re­main. In gen­eral, the speed of a de­ci­sion can be in­creased by sac­ri­fic­ing ac­cu­racy, and the ac­cu­racy of a de­ci­sion can be im­proved by sac­ri­fic­ing speed. Differ­ent types of de­ci­sions will call for differ­ent ways to bal­ance speed and ac­cu­racy (Chit­tka, Sko­rup­ski, & Raine 2009). Thus, to com­pare de­ci­sion-mak­ing speed across species, one would have to com­pare similar de­ci­sion types. As the phy­lo­ge­netic, ecolog­i­cal, and mor­pholog­i­cal differ­ences be­tween the com­pared species grows, our abil­ity to iden­tify rele­vantly similar de­ci­sion types will in­creas­ingly be im­paired.[56]

Tem­po­ral Re­s­olu­tion Measures

Even when they are awake and alert, an­i­mals, in­clud­ing hu­mans, do not mon­i­tor their sur­round­ings con­tin­u­ously. In­stead, our per­cep­tual sys­tems check for new in­for­ma­tion at (mostly) fixed in­ter­vals. Th­ese in­ter­vals are so short that we gen­er­ally don’t per­ceive the gap be­tween them, thus giv­ing rise to the illu­sion of con­tin­u­ous per­cep­tion. How­ever, it’s pos­si­ble to ex­per­i­men­tally mea­sure the du­ra­tion of these gaps. The char­ac­ter­is­tic length of the gaps varies by species.

Tem­po­ral re­s­olu­tion is the rate at which a per­cep­tual sys­tem sam­ples in­for­ma­tion about its en­vi­ron­ment. Tem­po­ral re­s­olu­tion is analo­gous to the re­fresh rate of a mon­i­tor. The higher a per­cep­tual sys­tem’s re­fresh rate, the faster it ab­sorbs new in­for­ma­tion, re­sult­ing in a more fine-grained tem­po­ral per­cep­tion of the world. Greater tem­po­ral re­s­olu­tion means higher tem­po­ral pre­ci­sion, gen­er­ally en­abling an an­i­mal to bet­ter track fast-mov­ing ob­jects in its vicinity or re­spond to rapidly un­fold­ing events more quickly. (See Figure 5.)

Figure 5: Differ­ence be­tween the ac­tual move­ment of a ground squir­rel [CFF = 120 Hz] (box a), the move­ment as per­ceived by a con­spe­cific (box b), and the move­ment as per­ceived by a short-eared owl [CFF = 70 Hz] (box c) (source: Healy et al. 2013: 686)

Un­der some con­di­tions, differ­ences in tem­po­ral re­s­olu­tion might be cor­re­lated with differ­ences in the sub­jec­tive ex­pe­rience of time. In­creas­ing tem­po­ral re­s­olu­tion has en­er­getic costs. In gen­eral, nat­u­ral se­lec­tion should only fa­vor an­i­mals with bet­ter tem­po­ral re­s­olu­tion if (1) there are fit­ness-rele­vant fast-mov­ing ob­jects or quickly-un­fold­ing events in its en­vi­ron­ment[57] and (2) the an­i­mal can take fit­ness-im­prov­ing ac­tions as a re­sult of the greater tem­po­ral re­s­olu­tion. When the fit­ness-im­prov­ing ac­tions are likely to re­quire con­scious de­ci­sion-mak­ing, we can ex­pect that in­creases in tem­po­ral re­s­olu­tion will gen­er­ally be ac­com­panied by in­creases in the rate of sub­jec­tive ex­pe­rience.

In prin­ci­ple, tem­po­ral re­s­olu­tion can be mea­sured for any sen­sory modal­ity. In prac­tice, tem­po­ral re­s­olu­tion is widely stud­ied for vi­sual sys­tems, slightly less ex­ten­sively stud­ied for au­di­tory sys­tems, and only very rarely stud­ied in other senses.

Crit­i­cal flicker-fu­sion fre­quency (CFF) is a well-stud­ied mea­sure of vi­sual tem­po­ral re­s­olu­tion. I have com­piled a spread­sheet com­par­ing CFF val­ues across 70 species and 33 or­ders of an­i­mals. (See Figure 6.) Green-shaded cells rep­re­sent an­i­mals with higher CFF val­ues than hu­mans; red-shaded cells rep­re­sent an­i­mals with CFF val­ues lower than hu­mans. (Note that the or­der av­er­ages in the sec­ond tab can’t be in­ter­preted liter­ally be­cause we don’t have a rep­re­sen­ta­tive sam­pling of the species from the or­ders.) The spread­sheet com­bines the val­ues from sev­eral re­view ar­ti­cles as well as in­di­vi­d­ual re­sults from a hand­ful of stud­ies on species of par­tic­u­lar in­ter­est. The spread­sheet is not in­tended to be com­pre­hen­sive. My re­view of the liter­a­ture was some­what cur­sory, and hence I do not claim that this is an ex­haus­tive list of the an­i­mals that have been stud­ied. Nonethe­less, I think the spread­sheet does offer a some­what rep­re­sen­ta­tive sur­vey of the breadth of an­i­mals stud­ied and the range of CFF val­ues recorded.

Figure 6: Screen­shot of CFF spread­sheet (note that the screen­shot does not cap­ture the full table)

The spread­sheet offers some some­what sur­pris­ing find­ings. An­i­mals as pu­ta­tively dis­similar as her­mit crabs (53 Hz), hu­mans (60 Hz), mi­gra­tory lo­custs (65 Hz), and gold­fish (67 Hz) have roughly similar CFF thresh­olds. On the other hand, an­i­mals as pu­ta­tively similar as trout (27 Hz) and salmon (72 Hz), geckos (20 Hz) and igua­nas (80 Hz), and guinea pigs (50 Hz) and ground squir­rels (120 Hz), have dras­ti­cally differ­ent CFF thresh­olds. Rep­tiles, in­sects, fish, and mam­mals all ex­hibit tremen­dous di­ver­sity in CFF val­ues.

I dis­cuss the case for and against CFF track­ing the sub­jec­tive ex­pe­rience of time in much more de­tail in the next post in this se­ries. As of June 2020, I es­ti­mate there is ap­prox­i­mately a 40% chance that CFF roughly tracks the sub­jec­tive ex­pe­rience of time for an­i­mals that char­ac­ter­is­ti­cally in­habit bright en­vi­ron­ments, rely pre­dom­i­nantly on vi­sion to learn about the world, and ex­hibit high be­hav­ioral plas­tic­ity.

For an­i­mals that rely on senses other than vi­sion, CFF is not a good proxy for the sub­jec­tive ex­pe­rience of time. For­tu­nately, we can mea­sure the tem­po­ral re­s­olu­tion of other types of per­cep­tual sys­tems. When ap­plied to au­di­tion, “[t]em­po­ral re­s­olu­tion refers to the pre­ci­sion with which the au­di­tory sys­tem can ex­tract time-vary­ing in­for­ma­tion from acous­tic stim­uli” (Gall, Henry, & Lu­cas 2012: 61). To mea­sure the tem­po­ral re­s­olu­tion of au­di­tory per­cep­tion, sci­en­tists have used au­di­tory-evoked po­ten­tials and au­di­tory flut­ter-fu­sion thresh­olds. The tem­po­ral re­s­olu­tion of au­di­tory sys­tems has been ex­ten­sively stud­ied in birds and mam­mals and oc­ca­sion­ally in fish,[58] but much less so in other taxa. To fa­cil­i­tate use­ful com­par­i­sons across species, it would be helpful to have more and more di­verse data on au­di­tory tem­po­ral re­s­olu­tion, es­pe­cially from an­i­mals that hu­mans ex­ploit in large num­bers.

Odon­to­cete marine mam­mals (i.e., dolphins, por­poises, and toothed whales) ap­pear to have the high­est au­di­tory tem­po­ral re­s­olu­tion of all an­i­mals thus far stud­ied. Th­ese an­i­mals use echolo­ca­tion to hunt prey. The abil­ity to quickly pro­cess fast-trav­el­ing un­der­wa­ter sound al­lows them to track prey bet­ter and thus con­fers a fit­ness ad­van­tage. The white-beaked dolphin (Lagenorhynchus albirostris) has an au­di­tory tem­po­ral re­s­olu­tion ap­prox­i­mately ten times greater than hu­mans (Mooney et al. 2009: 375).[59] Among ter­res­trial ver­te­brates, song­birds (sub­or­der Passeri, e.g. finches and spar­rows) ap­pear to have the best au­di­tory tem­po­ral re­s­olu­tion. Th­ese an­i­mals use com­plex bird­song to com­mu­ni­cate iden­tity, lo­ca­tion, and sex­ual in­ten­tions. Stud­ied species in this taxon gen­er­ally have au­di­tory tem­po­ral re­s­olu­tion 3-5 times finer than hu­mans (Dool­ing & Lohr 2006: 15), which ranks them bet­ter than most ter­res­trial mam­mals but worse than odon­to­cete marine mam­mals (Henry et al. 2011: 358).

Tem­po­ral re­s­olu­tion can be stud­ied in yet other sen­sory modal­ities. The tem­po­ral re­s­olu­tion of ol­fac­tion has been stud­ied in a hand­ful of species, pri­mar­ily arthro­pods.[60] In one study (Se­hdev et al. 2019), re­searchers in­ves­ti­gated the ol­fac­tory tem­po­ral re­s­olu­tion of fruit flies. To do so, they iden­ti­fied two be­hav­iorally rele­vant odors, one at­trac­tive and one aver­sive. The flies were paired with the two odors in a wind tun­nel, and the re­searchers varied the timing of the re­lease of the odors. When the two odors ar­rived si­mul­ta­neously, the flies did not move to­ward the com­bined odor. How­ever, if the two odors ar­rived asyn­chronously, the flies did move to­ward the source of the odor. (The ba­sic is that if the two odors ar­rive si­mul­ta­neously, the flies can’t pair the source of the odor with a be­hav­iorally rele­vant ob­ject. How­ever, if the two odors ar­rive at differ­ent times, the flies can dis­t­in­guish the two odors and thus in­fer that there is an at­trac­tive ob­ject nearby.[61]) The re­searchers demon­strated that the flies could dis­t­in­guish asyn­chronous gaps in the ar­rival of the odors as short as 33 ms.[62]

The ol­fac­tory tem­po­ral re­s­olu­tion of bees is even more im­pres­sive than flies. Szyszka et al. 2009 “found that for hon­ey­bees a 6-ms tem­po­ral differ­ence in stim­u­lus co­her­ence is suffi­cient for odor-ob­ject seg­re­ga­tion, show­ing that the tem­po­ral re­s­olu­tion of the ol­fac­tory sys­tem is much faster than pre­vi­ously thought” (1). They con­clude “that hon­ey­bees can de­tect tem­po­ral in­co­her­ence be­tween odor­ant stim­uli in the mil­lisec­ond range and use this in­for­ma­tion to ex­tract odor­ants’ iden­tity. This seems a re­mark­able perfor­mance con­sid­er­ing that the sense of smell is re­garded to be a rel­a­tively slow sense as com­pared to the au­di­tory or vi­sual senses” (Szyszka et al. 2009: 3).

In ad­di­tion to in­sects, the ol­fac­tory tem­po­ral re­s­olu­tion of crus­taceans has also oc­ca­sion­ally been in­ves­ti­gated. Harzsch & Krieger 2018 re­port that the “tem­po­ral re­s­olu­tion of odor sam­pling” in crus­taceans is “rel­a­tively slow” (31). Rei­den­bach & Koehl 2011 re­port that the ol­fac­tory tem­po­ral re­s­olu­tion of the Amer­i­can lob­ster (Ho­marus amer­i­canus) is 3-5 times slower than the ol­fac­tory tem­po­ral re­s­olu­tion of C. cautella and P. gossyp­iella moths (3140). It thus ap­pears that crus­taceans have rel­a­tively poor vi­sual and ol­fac­tory tem­po­ral re­s­olu­tion, while fly­ing in­sects have com­par­a­tively much bet­ter vi­sual and ol­fac­tory tem­po­ral re­s­olu­tion. More stud­ies in this area could help us de­ter­mine how well com­par­a­tive tem­po­ral re­s­olu­tion cor­re­lates across sen­sory modal­ities.

Tem­po­ral In­te­gra­tion Win­dows and Cor­ti­cal Oscillations

A tem­po­ral in­te­gra­tion win­dow is the min­i­mum du­ra­tion that ap­pears to last more than a sin­gle in­stant. Stim­uli that ar­rive dur­ing the same tem­po­ral in­te­gra­tion win­dow ap­pear to be si­mul­ta­neous. Tem­po­ral in­te­gra­tion win­dows are the in­verse of tem­po­ral re­s­olu­tion mea­sures: the greater an an­i­mal’s tem­po­ral re­s­olu­tion, the shorter its tem­po­ral in­te­gra­tion win­dow will be. An­i­mals with shorter tem­po­ral in­te­gra­tion win­dows may have faster rates of sub­jec­tive ex­pe­rience.

Like crit­i­cal flicker-fu­sion fre­quency, tem­po­ral in­te­gra­tion win­dows can vary con­sid­er­ably even across similar an­i­mals. In hu­mans, the min­i­mum tem­po­ral in­te­gra­tion win­dow is around 40 ms, though some paradigms elicit tem­po­ral in­te­gra­tion win­dows as long as ~200 ms (Melcher et al. 2014 and Wutz et al. 2016).[63] Rh­e­sus mon­keys have a min­i­mum tem­po­ral in­te­gra­tion win­dow of around 25 ms (Os­borne & Lis­berger 2009: 2023).[64] (Com­pare hu­man CFF at 60 Hz to rhe­sus mon­key CFF at 95 Hz.)

Tem­po­ral in­te­gra­tion win­dows may be gov­erned by cor­ti­cal os­cilla­tions.[65] (See Figure 7.) Some re­searchers hy­poth­e­size that “con­scious per­cep­tion oc­curs in dis­crete tem­po­ral win­dows, clocked by the fre­quency of alpha os­cilla­tions [cita­tions omit­ted]. Un­der this hy­poth­e­sis, vari­a­tion in the fre­quency of oc­cip­i­tal alpha os­cilla­tions should pre­dict vari­a­tion in the tem­po­ral re­s­olu­tion of vi­sual per­cep­tion. Speci­fi­cally, when two stim­uli fall within the same alpha cy­cle, they may be per­ceived as a sin­gle stim­u­lus, re­sult­ing in per­cep­tion with lower tem­po­ral re­s­olu­tion when alpha fre­quency is lower” (Samaha & Pos­tle 2015: 2985). Re­cent ex­per­i­men­tal re­sults “sup­port the no­tion that the oc­cip­i­tal alpha rhythm may dic­tate the re­s­olu­tion at which vi­sual in­for­ma­tion can be con­sciously sam­pled” (Samaha & Pos­tle 2015: 2988). Mea­sur­ing these kinds of os­cilla­tory brain ac­tivity in non­hu­man an­i­mals may im­prove our un­der­stand­ing of the way other an­i­mals ex­pe­rience time.

Figure 7: When two stim­uli fall within the same alpha cy­cle, they are more likely to be per­ceived as one (source: Samaha & Pos­tle 2015)

How Con­sid­er­ing the Sub­jec­tive Ex­pe­rience of Time Could In­fluence Re­source Allocation

To the best of my knowl­edge, con­sid­er­a­tions re­gard­ing char­ac­ter­is­tic differ­ences in the sub­jec­tive ex­pe­rience of time across species cur­rently ex­ert lit­tle to no in­fluence over the way re­sources are al­lo­cated within the effec­tive an­i­mal ad­vo­cacy move­ment.[66] In large part this lack of at­ten­tion is well jus­tified. The is­sue is com­plex, and with­out sig­nifi­cant care and metic­u­lous re­search, it’s difficult to be con­fi­dent about one’s views on the sub­ject. (Even with sig­nifi­cant care and metic­u­lous re­search, con­fi­dence can be hard to come by!) How­ever, I think the mo­ment is right to start in­cor­po­rat­ing such con­sid­er­a­tions into our al­loca­tive de­ci­sion-mak­ing.

Reflect­ing on the sub­jec­tive ex­pe­rience of time raises four main points per­ti­nent to the al­lo­ca­tion of re­sources. The first point con­cerns un­cer­tainty. Presently, the most com­monly used met­rics to com­pare the moral value of an­i­mals across species ap­pear to be neu­ron count, en­cephal­iza­tion quo­tient, brain mass:body mass ra­tio, and re­lated mea­sures.[67] As best as I can tell with­out gath­er­ing more data and con­duct­ing de­tailed statis­ti­cal anal­y­sis, the po­ten­tial prox­ies for the sub­jec­tive ex­pe­rience of time cor­re­late weakly or not at all with these mea­sures. Thus, the prox­ies for the sub­jec­tive ex­pe­rience of time sug­gest a much differ­ent or­di­nal rank­ing of an­i­mals than brain mass, neu­ron count, or en­cephal­iza­tion quo­tient. (And no­tably, un­like the brain-re­lated mea­sures, hu­mans don’t come out on top of the rank­ing gen­er­ated by time ex­pe­rience mea­sures.) The gen­eral les­son is that not all morally rele­vant met­rics point in the same di­rec­tion. Brain-re­lated mea­sures and time-re­lated mea­sures both pur­port to ex­press (differ­ent) morally rele­vant char­ac­ter­is­tics. Be­cause there don’t seem to be de­ci­sive rea­sons to weight brain-re­lated mea­sures much heav­ier than time-re­lated mea­sures, it would be pru­dent to give each group of met­rics at least mod­est con­sid­er­a­tion. The fact that two groups of prox­ies de­liver such di­ver­gent re­sults use­fully high­lights our deep un­cer­tainty with re­spect to the com­par­a­tive moral value of differ­ent types of an­i­mals. It’s not always clear what to do in the face of deep un­cer­tainty, but in this case I think it’s plau­si­ble that we ought to re­act by adopt­ing a slightly more uniform dis­tri­bu­tion of re­sources across an­i­mals.[68]

The sec­ond point con­cerns di­ver­sity. To date, effec­tive an­i­mal ad­vo­cates have mostly fo­cused on two or three tax­o­nomic fam­i­lies.[69] But there are more than 50 tax­o­nomic fam­i­lies of an­i­mals that hu­mans ex­ploit in large num­bers (to say noth­ing of the di­ver­sity of wild an­i­mals hu­mans don’t di­rectly ex­ploit).[70] Un­sur­pris­ingly, these groups of an­i­mals differ from each other in many ways. De­spite these differ­ences, there is a ten­dency to lump many groups of an­i­mals to­gether. Peo­ple talk about in­sects (at least eight tax­o­nomic fam­i­lies ex­ploited by hu­mans), crus­taceans (at least ten tax­o­nomic fam­i­lies ex­ploited by hu­mans), and fish (at least ten tax­o­nomic fam­i­lies ex­ploited by hu­mans) as if they rep­re­sent ho­mo­ge­neous cat­e­gories. Mea­sures of the sub­jec­tive ex­pe­rience of time helpfully re­mind us that this is not the case.[71] Fish and in­sects in par­tic­u­lar ex­hibit a huge range of tem­po­ral re­s­olu­tions. (For in­stance, sword­fish have a CFF of 22 Hz; tuna have a CFF of 80 Hz. Cock­roaches have a CFF of 42 Hz; honey bees have a CFF of 200 Hz.) The les­son is that fish and in­sects (and to a lesser de­gree, crus­taceans) should not be treated as if they con­sti­tute ho­mo­ge­neous groups with the same moral sta­tus or ca­pac­ity for welfare.

The third point con­cerns the fu­ture. Con­sid­er­a­tions per­tain­ing to the sub­jec­tive ex­pe­rience of time have up un­til re­cently been largely ig­nored (again, with good rea­son) within the effec­tive an­i­mal ad­vo­cacy move­ment. Although there is a fairly large sci­en­tific liter­a­ture on tem­po­ral per­cep­tion, most (but not all) au­thors don’t ad­dress how their re­search re­lates to the sub­jec­tive ex­pe­rience of time. Dis­cus­sions of the moral im­por­tance of the sub­jec­tive ex­pe­rience of time are even rarer. All this is to say that the field is fairly ne­glected rel­a­tive to its po­ten­tial im­por­tance. Fu­ture work in this area could yield con­sid­er­able early re­turns on in­vest­ment. With the right fo­cus, a coal­i­tion of psy­chol­o­gists, neu­ro­scien­tists, philoso­phers, and an­i­mal ad­vo­cates could po­ten­tially un­cover a wealth of new in­sights. Such re­search might help us bet­ter pri­ori­tize among an­i­mals, es­pe­cially within di­verse taxa like fish, in­sects, and crus­taceans.

The fourth and fi­nal point con­cerns the pre­sent. Although there is much we don’t know about char­ac­ter­is­tic differ­ences in the sub­jec­tive ex­pe­rience of time (in­clud­ing whether or not such differ­ences ac­tu­ally ex­ist!), there are at least a few ac­tion-rele­vant items about which we can be rel­a­tively con­fi­dent. Although it’s un­clear ex­actly how most an­i­mals com­pare to each other in terms of their sub­jec­tive ex­pe­rience of time, the rough bounds of the dis­tri­bu­tion are visi­ble. Judg­ing by the po­ten­tial prox­ies I’ve iden­ti­fied, rates of sub­jec­tive ex­pe­rience prob­a­bly don’t differ by more than two or­ders of mag­ni­tude across the an­i­mal king­dom (as­sum­ing such differ­ences ex­ist at all). The char­ac­ter­is­tic hu­man ex­pe­rience of time ap­pears to be roughly in the mid­dle of this spec­trum, so non­hu­man an­i­mals are un­likely to ex­pe­rience time more than ten times faster or ten times slower than hu­mans.

We can even say some­thing about spe­cific an­i­mals. Con­ver­gent ev­i­dence sug­gests that song­birds and hon­ey­bees are likely to have fast rates of sub­jec­tive ex­pe­rience (and thus merit more moral con­cern than com­pa­rable an­i­mals with slower rates of sub­jec­tive ex­pe­rience). Song­birds have fan­tas­tic tem­po­ral re­s­olu­tion across both au­di­tory and vi­sual do­mains and their brains pack neu­rons to­gether very densely, al­low­ing for rapid com­mu­ni­ca­tion. Honey bees also have ex­cel­lent tem­po­ral re­s­olu­tion across mul­ti­ple do­mains (vi­sion and ol­fac­tion) and pack al­most a mil­lion neu­rons in a brain less than a cu­bic mil­lime­ter in vol­ume (Men­zel & Giurfa 2001). Song­bird and honey bee be­hav­ior ap­pears to be more eas­ily ex­plained if we posit faster rates of sub­jec­tive ex­pe­rience. Taken to­gether, at the mar­gin and all else equal, we should look for op­por­tu­ni­ties to im­prove the welfare of these an­i­mals.


In­ves­ti­gat­ing the sub­jec­tive ex­pe­rience of time is nec­es­sar­ily a spec­u­la­tive en­ter­prise. There is no way to know with cer­tainty how other in­di­vi­d­u­als ex­pe­rience time. With fel­low hu­mans, we can usu­ally rely on ver­bal re­ports.[72] With non­hu­man an­i­mals, the best we can do is make in­fer­ences on the ba­sis of differ­ences in be­hav­ior, neu­rol­ogy, and sen­sory per­cep­tion. Ab­sent some rad­i­cal new tech­nol­ogy, the pri­vate men­tal lives of non­hu­man an­i­mals will re­main just that—pri­vate.

Although cer­tainty is out of our reach, jus­tified be­liefs are not. By think­ing care­fully about this is­sue, en­gag­ing deeply with both the sci­en­tific liter­a­ture on tem­po­ral per­cep­tion and the philo­soph­i­cal liter­a­ture on tem­po­ral ex­pe­rience, we can be­gin to piece to­gether con­ver­gent lines of ev­i­dence sug­gest­ing which an­i­mals (if any) ex­pe­rience time faster or slower than hu­mans. In­cor­po­rat­ing in­sights from this topic into our moral wor­ld­view could lead us to fun­da­men­tally re­think the value we place on differ­ent sorts of an­i­mals. Of course, much un­cer­tainty re­mains.

In many ways, study­ing differ­ences in the sub­jec­tive ex­pe­rience of time across species is but a proof of con­cept for the larger pro­ject of study­ing differ­ences in ca­pac­ity for welfare more gen­er­ally. Some nat­u­ral les­sons have emerged from this trial run. First, for most morally salient char­ac­ter­is­tics, it will prob­a­bly be nec­es­sary to op­er­a­tional­ize the char­ac­ter­is­tic in more than one way. Se­cond, for most op­er­a­tional­iza­tions of morally salient char­ac­ter­is­tics, there will be holes in the em­piri­cal liter­a­ture; we should re­flect on the most cost-effec­tive way to fill those holes. Third, even with near-com­plete em­piri­cal knowl­edge, our best the­o­ret­i­cal anal­y­sis of the con­nec­tion be­tween the ob­jec­tive data and sub­jec­tive welfare will of­ten re­main ten­ta­tive; we ought to re­sign our­selves to and ac­cept this in­evitable un­cer­tainty.

Fi­nally, this: there is still so much to dis­cover. I spent three months study­ing the sub­jec­tive ex­pe­rience of time, and al­most ev­ery day I learned some­thing new. There has never been a com­pre­hen­sive in­ves­ti­ga­tion into the morally rele­vant differ­ences among differ­ent sorts of an­i­mals. There is a wealth of knowl­edge wait­ing to be un­cov­ered. The or­chard is ripe with low-hang­ing fruit.

Ap­pendix 1: Tem­po­ral Ex­pe­rience vs. Tem­po­ral Judgments

We ought to clearly dis­t­in­guish the ex­pe­rience of time from the judg­ments we make about time. Un­der some con­di­tions, differ­ences in tem­po­ral ex­pe­rience will lead to differ­ences in tem­po­ral judg­ments. Im­por­tantly though, differ­ences in tem­po­ral judg­ments need not be the product of differ­ences in tem­po­ral ex­pe­rience. Just as there are a num­ber of tech­niques to re­li­ably in­duce op­ti­cal illu­sions, there are similarly a num­ber of tech­niques to re­li­ably in­duce tem­po­ral illu­sions.[73] For ex­am­ple, “in a re­peated pre­sen­ta­tion of au­di­tory or vi­sual stim­uli, an un­ex­pected ob­ject of equiv­a­lent du­ra­tion ap­pears to last longer” (Par­iya­dath & Ea­gle­man 2007: 1). This is known as the odd­ball illu­sion. (See Figure 8.) De­scribing the ex­act phe­nomenol­ogy of such ex­pe­riences is a bit tricky, but it seems as if the odd­ball illu­sion (and its many cous­ins), does not af­fect tem­po­ral ex­pe­rience.[74] “The odd­ball and de­but illu­sions in­volve dis­tor­tions in du­ra­tion judg­ments but do not af­fect per­ceived au­di­tory pitch or high vi­sual flicker fre­quen­cies” (Par­iya­dath & Ea­gle­man 2007: 4). In other words, the odd­ball illu­sion in­duces a lo­cal dis­tor­tion in du­ra­tion es­ti­mates, but it does not have the sort of global effects we would ex­pect if it in­duced a gen­eral dis­tor­tion in the sub­jec­tive ex­pe­rience of time.

Figure 8: The Od­dball Illu­sion (source: Par­iya­dath & Ea­gle­man 2007: 2)

In gen­eral, “time judg­ments show a range of task, con­text, and or­der effects” (Matthews & Meck 2016: 869). Th­ese effects can dis­tort judg­ments of du­ra­tion, or­der, and even si­mul­tane­ity. “For ex­am­ple, per­ceived du­ra­tions can be warped by sac­cades or by an odd­ball in a se­quence. Tem­po­ral or­der judg­ments of ac­tions and sen­sa­tions can be illu­so­rily re­versed by ex­po­sure to de­layed mo­tor con­se­quences, and si­mul­tane­ity judg­ments can be ma­nipu­lated by re­peated ex­po­sure to non-si­mul­ta­neous stim­uli” (Stet­son, Fiesta, & Ea­gle­man 2007: 1 [cita­tions omit­ted]). Es­ti­ma­tion tasks (e.g., ‘how long was that image visi­ble?’) and pro­duc­tion tasks (e.g., ‘press but­ton when image has been on the screen for x sec­onds’) of­ten pro­duce di­ver­gent re­sults (Matthews & Meck 2014: 430-431).[75] Au­di­tory stim­uli are re­li­ably judged longer than vi­sual or vibro­tac­tile stim­uli of the same du­ra­tion (Matthews & Meck 2016: 871). Non­hu­man an­i­mals also ex­hibit some of these time dis­tor­tions un­der cer­tain con­di­tions. (See Zen­tall & Singer 2008 for ex­am­ples in pi­geons.) Other non­tem­po­ral vari­ables that af­fect tem­po­ral judg­ments in hu­mans in­clude sex differ­ences, de­vel­op­men­tal changes, age, gen­eral in­tel­li­gence, body tem­per­a­ture, emo­tional state, clini­cal con­di­tions (e.g., schizophre­nia, ADHD, PTSD, de­pres­sion), at­ten­tion, mem­ory, prior ex­pe­rience with the stim­u­lus, stim­u­lus in­ten­sity, and the per­cep­tual prop­er­ties of the sen­sory in­put (Matthews & Meck 2016: 865).[76]

The les­son here is that not all in­fluences that af­fect the judg­ments we make about time re­flect gen­uine differ­ences in the ex­pe­rience of time. For in­stance, there are good rea­sons an­i­mals (in­clud­ing hu­mans) might be hard­wired to judge that some events last longer than oth­ers. “The feel­ing that a stim­u­lus lasted for a long time might be a use­ful sub­jec­tive in­dex of its im­por­tance, one which can com­bine with other per­cep­tual, af­fec­tive, and mo­ti­va­tional cues to de­ter­mine the op­ti­mum course of ac­tion” (Matthews & Meck 2016: 892). It’s easy to see how evolu­tion would fa­vor an­i­mals that de­voted more at­ten­tion to novel stim­uli. It has been sug­gested that “es­ti­mates of du­ra­tion cor­re­spond to the ex­pen­di­ture of neu­ral en­ergy used to pro­cess a stim­u­lus” (Jura 2019: 2). So a novel stim­u­lus (such as the de­viant pic­ture in the odd­ball illu­sion) grabs our at­ten­tion, caus­ing us to com­mit a greater share of neu­ral en­ergy to pro­cess the stim­u­lus, thereby pro­duc­ing the illu­sion of in­creased du­ra­tion, which makes the stim­u­lus more salient to our con­scious de­ci­sion-mak­ing, thereby im­prov­ing the odds that we re­act to the stim­u­lus ap­pro­pri­ately. This is a plau­si­ble story, but note that noth­ing in the story sug­gests that the sub­jec­tive ex­pe­rience of time changes dur­ing the novel stim­u­lus.

Ap­pendix 2: The­o­ret­i­cal Models of Tem­po­ral Representation

De­spite decades of in­ves­ti­ga­tion, there is no con­sen­sus in the liter­a­ture about how the hu­man brain rep­re­sents and pro­cesses tem­po­ral in­for­ma­tion.[77] Mak­ing progress on this ques­tion could shed light on whether and how hu­mans ex­pe­rience differ­ences in the sub­jec­tive ex­pe­rience of time, which would in­form our es­ti­mate of the like­li­hood that other an­i­mals ex­pe­rience time at differ­ent sub­jec­tive rates. Learn­ing more about tem­po­ral rep­re­sen­ta­tion and pro­cess­ing in hu­man brains might also al­low us to in­fer some­thing di­rectly about the tem­po­ral ex­pe­rience of a non­hu­man an­i­mal by ex­am­in­ing its brain. Think­ing more gen­er­ally about the var­i­ous ways tem­po­ral in­for­ma­tion can be pro­cessed and rep­re­sented might al­low us to spec­u­late on the way such in­for­ma­tion is pro­cessed and rep­re­sented in brains sig­nifi­cantly differ­ent from hu­man brains.

Be­fore dis­cussing spe­cific the­o­ries, it’s worth not­ing that there is no a pri­ori guaran­tee that there is a sin­gle way tem­po­ral in­for­ma­tion is rep­re­sented and pro­cessed in the brain. Hu­man ac­tivity de­pends on tem­po­ral in­for­ma­tion on many differ­ent scales, from mil­lisec­onds (for au­to­matic re­flexes) to decades (for long-term plan­ning) and many scales in be­tween. It’s plau­si­ble that the brain rep­re­sents and pro­cesses in­for­ma­tion on these differ­ent scales in differ­ent ways.[78] Some au­thors even sug­gest that the best ex­pla­na­tion for cer­tain ex­per­i­men­tal data is that the brain uti­lizes differ­ent timing mechanisms for differ­ent sen­sory modal­ities.[79]

There are two main ap­proaches to mod­el­ing tem­po­ral rep­re­sen­ta­tion in the brain: cen­tral­ized timing mechanisms and dis­tributed, in­trin­sic timing mechanisms (Mul­ler & No­bre 2014).[80] The most pop­u­lar cen­tral­ized timing mechanism model is the pace­maker-ac­cu­mu­la­tor model.[81] Ac­cord­ing to the pace­maker-ac­cu­mu­la­tor model, a neu­rolog­i­cal pace­maker pulses at reg­u­lar in­ter­vals, and when an ex­ter­nal sig­nal trig­gers a neu­rolog­i­cal switch, these pulses are counted by a neu­rolog­i­cal ac­cu­mu­la­tor. Du­ra­tion es­ti­mates are based on the num­ber of pulses stored in the ac­cu­mu­la­tor. The ac­cu­racy of du­ra­tion es­ti­ma­tion im­proves through child­hood as one learns to bet­ter cor­re­late the num­ber of pulses in the ac­cu­mu­la­tor with ex­ter­nal mea­sures of time’s pas­sage. (See Figure 9.) Though not a re­quire­ment of the model, one could imag­ine that pace­mak­ers run at differ­ent speeds pro­duce differ­ent rates of sub­jec­tive ex­pe­rience.

Figure 9: The Pace­maker-Ac­cu­mu­la­tor Model of Tem­po­ral Rep­re­sen­ta­tion (source: Taat­gen, van Rijn, & An­der­son 2007: 578)

When the field was young, psy­chol­o­gists as­sumed there was some phys­i­cal mechanism ded­i­cated to timing lo­cated in a dis­crete re­gion of the brain. How­ever, as years passed and in­creas­ingly so­phis­ti­cated neu­rolog­i­cal in­ves­ti­ga­tions failed to re­veal any dis­tinct brain area as­so­ci­ated ex­clu­sively (or even pri­mar­ily) with tem­po­ral judg­ments, psy­chol­o­gists be­gan to en­ter­tain the idea that tem­po­ral in­for­ma­tion is pro­cessed in a dis­tributed man­ner.[82] The main al­ter­na­tive to cen­tral­ized timing mechanism mod­els is dis­tributed, in­trin­sic timing mechanism mod­els.[83]

The ba­sic idea be­hind most dis­tributed, in­trin­sic mod­els is that tem­po­ral rep­re­sen­ta­tion is a product of the effi­ciency of neu­ral cod­ing; stim­uli which evoke larger neu­ral re­sponses are per­ceived as longer. In other words: “tem­po­ral rep­re­sen­ta­tions may be ab­stracted from time‐de­pen­dent changes in lo­cal cor­ti­cal net­works that are ac­ti­vated by spe­cific stim­uli. That is, in the same way that one can in­fer how long ago a rock was thrown into a pond by the pat­tern of rip­ples on the sur­face, so the brain may ex­tract tem­po­ral in­for­ma­tion from the pat­tern of neu­ral ac­tivity trig­gered by the pre­sen­ta­tion of a stim­u­lus” (Matthews & Meck 2014: 430). Again, though not a re­quire­ment of the model, one could imag­ine that differ­ent char­ac­ter­is­tic pat­terns of neu­ral ac­tivity pro­duce differ­ent rates of sub­jec­tive ex­pe­rience.


This es­say is a pro­ject of Re­think Pri­ori­ties. It was writ­ten by Ja­son Schukraft. Thanks to Kim Cud­ding­ton, Mar­cus A. Davis, Neil Dul­laghan, Rem­melt Ellen, Derek Foster, David Moss, Abra­ham Rowe, and Gavin Tay­lor for helpful feed­back on ear­lier drafts. Thanks to Ja­son Samaha and Bar­tosz Jura for helpful con­ver­sa­tions. If you like our work, please con­sider sub­scribing to our newslet­ter. You can see all our work to date here.

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  1. See Re­think Pri­ori­ties’ In­ver­te­brate Sen­tience Table (and re­lated doc­u­ments) for an overview of the ev­i­dence of sen­tience in a va­ri­ety of an­i­mals. As I dis­cuss in the first post in this se­ries, sen­tience is gen­er­ally thought to be suffi­cient for moral stand­ing. ↩︎

  2. Through habitat de­struc­tion, en­vi­ron­men­tal degra­da­tion, and an­thro­pogenic cli­mate change, hu­mans in­di­rectly af­fect al­most all an­i­mals on the planet. ↩︎

  3. Pre­sum­ably not all an­i­mals are welfare sub­jects. It’s con­tentious where to draw the line be­tween an­i­mals that are welfare sub­jects and an­i­mals that are not welfare sub­jects. Even if one adopted a con­ser­va­tive frame­work ac­cord­ing to which only mam­mals are welfare sub­jects, the in­ter­species com­par­i­son prob­lem would re­main (though it would be more tractable). ↩︎

  4. In ei­ther case, the end­points need not be sym­met­ric be­tween pos­i­tive welfare and nega­tive welfare. ↩︎

  5. This list is adapted from Brown­ing 2020. Her fo­cus is on mea­sur­ing re­al­ized welfare, but I think the desider­ata ap­ply equally well to mea­sur­ing ca­pac­ity for welfare and moral sta­tus. ↩︎

  6. The met­rics would also be weighted by plau­si­bil­ity and im­por­tance and then com­bined. ↩︎

  7. See, for ex­am­ple, Bu­dolf­son & Spears 2019. ↩︎

  8. In some ways, the time ex­pe­rience met­rics are much bet­ter. It’s clearer what these met­rics pur­port to track and clearer why what they pur­port to track is morally rele­vant. (See the sec­tion “Why the Sub­jec­tive Ex­pe­rience of Time Mat­ters.”) In con­trast, it’s not clear ex­actly what neu­ron counts are sup­posed to track. If they are meant to track some­thing like in­ten­sity of ex­pe­rience, then it’s not clear whether the cor­re­la­tion is pos­i­tive or nega­tive. If they are meant to track some­thing like cog­ni­tive so­phis­ti­ca­tion, then it’s not clear that cog­ni­tive so­phis­ti­ca­tion is more im­por­tant than the sub­jec­tive ex­pe­rience of time. ↩︎

  9. Some philoso­phers dis­pute the claim that time ap­pears to pass. Here I aim to em­ploy the phrase only in its ev­ery­day, the­o­ret­i­cally un­prob­le­matic sense. To be clear, the pas­sage of time is not the sort of thing that is di­rectly per­ceived; rather there is a tem­po­ral as­pect to the de­tec­tion of change in ev­ery sen­sory modal­ity. Time ap­pears to slow down when one is sub­ject to more ex­pe­riences per ob­jec­tive unit of time; it ap­pears to speed up when one is sub­ject to fewer ex­pe­riences per ob­jec­tive unit of time. ↩︎

  10. Rel­a­tivis­tic physics makes the no­tion ob­jec­tive time some­what prob­le­matic. Those difficul­ties, how­ever, are or­thog­o­nal to the is­sue at hand. Ac­cel­er­at­ing a crea­ture to a sig­nifi­cant frac­tion of the speed of light shifts its refer­ence frame with re­spect to non-ac­cel­er­ated crea­tures such that the crea­tures may not agree on which events are si­mul­ta­neous. But it doesn’t al­ter the crea­ture’s sub­jec­tive ex­pe­rience of time. ↩︎

  11. How­ever, it’s im­por­tant to re­mem­ber that this is only a metaphor. Vi­sual per­cep­tion is im­por­tantly dis­analo­gous to me­chan­i­cal record­ing de­vices. As Hol­combe 2009 notes, “Video cam­eras have a sin­gle tem­po­ral limit set by the frame rate. The hu­man vi­sual sys­tem has mul­ti­ple tem­po­ral limits set by its var­i­ous con­stituent mechanisms” (216). ↩︎

  12. We can at least use in­fer­ence to the best ex­pla­na­tion to in­fer that other hu­mans feel pain. Re­ject­ing this in­fer­ence pushes one dan­ger­ously close to solip­sism. ↩︎

  13. There is also some ev­i­dence that the sub­jec­tive ex­pe­rience of time varies slightly among in­di­vi­d­u­als within a species, though this vari­a­tion may be so slight that its moral im­por­tance is neg­ligible. Ad­di­tion­ally, there is ev­i­dence that cer­tain events can trig­ger tem­po­rary changes in the sub­jec­tive ex­pe­rience of time. The moral sig­nifi­cance of these events (which ap­pear to be com­mon in some species and rare in oth­ers) is as yet un­known. ↩︎

  14. In one study (Watt 1991), bored sub­jects re­ported that the time passed slowly when en­gaged in a te­dious num­ber-cir­cling ac­tivity, but they were no bet­ter or worse at es­ti­mat­ing the ac­tual du­ra­tion of the ac­tivity than non-bored sub­jects. A later study (Danck­ert & All­man 2005) found that bore­dom-prone in­di­vi­d­u­als were more likely to over­es­ti­mate the pas­sage of time. How­ever in this study there was no cor­re­la­tion be­tween be­ing more or less bore­dom-prone (as scored by the Bore­dom Prone­ness Scale) and ac­tu­ally re­port­ing that the ac­tivity was more or less bor­ing. (Ba­si­cally ev­ery­body thought it was su­per bor­ing.) The au­thors are care­ful to note that “the differ­ences we ob­served in tem­po­ral per­cep­tion were due to ro­bust differ­ences in trait sus­cep­ti­bil­ity to bore­dom” and that “it would ap­pear that state feel­ings of bore­dom did not in­fluence our re­sults” (243). ↩︎

  15. Ian Phillips ap­peals to an anal­ogy with illu­sory spa­tial per­cep­tion: “we can agree that when viewed at an an­gle a cir­cu­lar coin looks el­lip­ti­cal with­out think­ing that we mis­per­ceive the coin as el­lip­ti­cal” (Phillips 2013: 240). We need not posit an el­lip­ti­cal ob­ject to ex­plain why the coin looks el­lip­ti­cal. Similarly, al­though a day of bore­dom seems to drag on for longer than a day of ex­cite­ment, we need not posit ac­tual differ­ences in the rate of sub­jec­tive ex­pe­rience to ex­plain the tem­po­ral illu­sion. ↩︎

  16. See Sten Nadolny’s novel The Dis­cov­ery of Slow­ness for the fic­tional story of a per­son with a slowed rate of sub­jec­tive ex­pe­rience. As a re­sult, the world ap­pears very fast to him and his re­ac­tions ap­pear very slow to oth­ers. ↩︎

  17. Of course, re­mem­ber­ing a plea­surable event as longer than it ac­tu­ally was might in­crease one’s plea­sure of ac­tively re­call­ing the event. But the in­crease in plea­sure comes from one’s pre­sent rec­ol­lec­tion, not the past event. ↩︎

  18. The sub­ject’s per­cep­tions may vary in pre­dictable ways, but the ex­is­tence of such psy­cho-phys­i­cal laws does not change the fact that it is the sub­ject’s in­ter­nal feel­ings that ground the valence and in­ten­sity of the ex­pe­rience, not any ex­ter­nal prop­erty of the stim­u­lus. ↩︎

  19. The rele­vant sense of ‘longer’ is that the crea­ture ex­pe­rienc­ing the ‘longer’ event un­der­goes more sub­jec­tive mo­ments of pain than the crea­ture ex­pe­rienc­ing the ‘shorter’ event. ↩︎

  20. See Kah­ne­man 2011: 378-381 for a dis­cus­sion of the peak-end rule of pain and du­ra­tion ne­glect in mem­ory, which nicely illus­trate that “[c]on­fus­ing ex­pe­rience with the mem­ory of it is a com­pel­ling cog­ni­tive illu­sion” (381). ↩︎

  21. There are, of course, many rea­sons to be at­ten­tive to sys­tem­atic vari­a­tions in peo­ple’s ret­ro­spec­tive es­ti­mates of pain. Sup­pose there is some use­ful med­i­cal op­er­a­tion that is benefi­cial but also re­puted to be painful. If the goal is to re­duce how painful pa­tients re­port the pro­ce­dure to be (and thus hope­fully in­crease the num­ber of pa­tients who get the op­er­a­tion), it would be more helpful to re­duce the peak-in­ten­sity of pain dur­ing the op­er­a­tion than to re­duce the du­ra­tion of the pro­ce­dure. See foot­note 20. ↩︎

  22. Non-ex­pe­ri­en­tial de­sires are de­sires for ob­jec­tive changes in re­al­ity in­de­pen­dent of our ex­pe­rience of those changes. Plea­surable ex­pe­riences (e.g., the plea­sure of eat­ing ice cream) can also de­rail the pur­suit of our non-ex­pe­ri­en­tial goals. ↩︎

  23. Pho­topic vi­sion is gen­er­ally em­ployed in well-lit con­di­tions, while sco­topic vi­sion is gen­er­ally em­ployed in low-light con­di­tions. In many an­i­mals, pho­topic vi­sion en­ables color per­cep­tion while sco­topic vi­sion does not. In hu­mans, pho­topic vi­sion is me­di­ated by cone cells in the eyes, while sco­topic vi­sion is me­di­ated by rod cells in the eyes. ↩︎

  24. For more on the philo­soph­i­cal difficul­ties re­lated to the ‘per­cep­tion’ of time, see this en­try in the Stan­ford En­cy­clo­pe­dia of Philos­o­phy. ↩︎

  25. “Car crash vic­tims, pi­lots forced to eject from their planes, rock climbers suffer­ing se­ri­ous falls, and other sur­vivors of life-threat­en­ing dan­ger, re­li­ably re­port that the trau­matic events which they ex­pe­rienced ap­peared to last much longer than events of the same ob­jec­tive length in nor­mal con­di­tions” (Phillips 2013: 228). See foot­note 4 in Phillips 2013 for cita­tions to sup­port this claim. ↩︎

  26. It’s im­por­tant not to over­state the phe­nomenon. Han­cock & Weaver 2005 re­port on an un­pub­lished sur­vey of fighter pi­lots who had ejected from their air­craft, a paradig­mat­i­cally in­tense ex­pe­rience (Fair 1984). Of the 28 sur­veyed pi­lots, only 15 re­ported an ap­par­ent slow­ing of the pas­sage of time. ↩︎

  27. “Since it is eth­i­cally un­jus­tifi­able to cre­ate an ex­per­i­men­tal set­ting that is dan­ger­ous, it re­mains un­clear whether peo­ple in such situ­a­tions re­ally ex­pe­rience slow mo­tion, or whether—af­ter the event, and on the ba­sis of the emo­tional charge of what they have gone through—they sim­ply be­lieve that they felt ev­ery­thing hap­pen­ing more slowly” (Wittman 2016: 35). ↩︎

  28. Although Phillips con­cedes that “our ex­pe­riences of du­ra­tion in trauma are not strictly speak­ing illu­sions” (236), he ap­pears to be in­tend­ing to offer a defla­tion­ary ac­count of the phe­nomenon, though ad­mit­tedly the dis­tinc­tion be­tween a reify­ing ex­pla­na­tion and a defla­tion­ary ex­pla­na­tion is blurry in this case. ↩︎

  29. The con­cep­tual pos­si­bil­ity of differ­ences in the sub­jec­tive ex­pe­rience of time has been dra­ma­tized in a num­ber of for­mats. For ex­am­ple, see Sten Nadolny’s novel The Dis­cov­ery of Slow­ness for a char­ac­ter with slowed sub­jec­tive ex­pe­rience. See the Black Mir­ror epi­sode “White Christ­mas” for char­ac­ters with dra­mat­i­cally ac­cel­er­ated sub­jec­tive ex­pe­rience. ↩︎

  30. For ex­am­ple, see Luke Muehlhauser’s com­ments here and Brian To­masik’s com­ments here. ↩︎

  31. For in­stance, per­haps it’s hard to swat a fly be­cause the fly per­ceives your hand as if it were mov­ing in slow-mo­tion. ↩︎

  32. Smaller mus­cles are faster to move, which con­tributes to faster re­ac­tion times. ↩︎

  33. Ben­thic an­i­mals are an­i­mals that oc­cupy the low­est level of a body of wa­ter. In deep wa­ters, the ben­thic zone re­ceives lit­tle sun­light. ↩︎

  34. “It has been known for some time that an­i­mals adapted for low light en­vi­ron­ments tend to have lower CFFs than an­i­mals found in more in­tense light en­vi­ron­ments” (Inger et al. 2014: 2). ↩︎

  35. Note that this fact does not en­tail that two-thirds of all species have CFF val­ues lower than hu­mans. The an­i­mals that have been stud­ied thus far are not rep­re­sen­ta­tive of wider taxa. ↩︎

  36. “Stud­ies of the periph­eral and cen­tral au­di­tory sys­tems of birds have long sug­gested that birds should have ex­tremely fine tem­po­ral pro­cess­ing abil­ities” (Lohr, Dool­ing, & Bar­tone 2006: 247). ↩︎

  37. That’s about an or­der of mag­ni­tude faster than hu­mans (Leshow­itz 1971). ↩︎

  38. “In these ex­per­i­ments we have shown that birds can dis­crim­i­nate sub­tle tem­po­ral changes within the con­text of differ­ences typ­i­cally found in their nat­u­ral vo­cal com­mu­ni­ca­tion sig­nals. Our re­sults sup­port more re­cent stud­ies of periph­eral au­di­tory sen­si­tivity in birds that have be­gun to demon­strate their en­hanced tem­po­ral acu­ity, be­yond the abil­ities re­ported for hu­mans and many other mam­mals” (Lohr, Dool­ing, & Bar­tone 2006: 249). ↩︎

  39. One in­trigu­ing as­pect of the song­bird ex­am­ple is that it ap­pears as if differ­ences in the sub­jec­tive ex­pe­rience of time emerged as a product of sex­ual se­lec­tion (mat­ing calls are a key de­ter­mi­nant of re­pro­duc­tive suc­cess) rather than the usual sur­vival-of-the-fittest se­lec­tion. It would be in­ter­est­ing to com­pare the re­ac­tion time and de­ci­sion-mak­ing speed of a song­bird against a com­pa­rably sized non-song­bird that in­hab­its the same ecolog­i­cal niche. Thanks to Gavin Tay­lor for notic­ing this point. ↩︎

  40. After all, it’s not as if the na­tive speaker per­ceives the other na­tive speaker’s lips mov­ing more slowly than the non-na­tive speaker does. ↩︎

  41. An al­ter­nate defla­tion­ary ex­pla­na­tion is that the birds are stor­ing the au­dio sig­nals at a fine tem­po­ral re­s­olu­tion in sim­ple ‘cache reg­isters,’ which are then pro­cessed more slowly. This idea could be tested ex­per­i­men­tally by in­ves­ti­gat­ing the min­i­mum de­lay be­tween a bird hear­ing bird­song and the bird re­spond­ing to it. Thanks to Rem­melt Ellen for the sug­ges­tion. ↩︎

  42. For in­stance, be­hav­ioral pro­ce­dures are some­times used to as­sess tem­po­ral re­s­olu­tion, blur­ring the dis­tinc­tion be­tween the two cat­e­gories. And differ­ences in neu­rolog­i­cal fea­tures might be ul­ti­mately re­spon­si­ble for differ­ences in be­hav­ioral fea­tures and tem­po­ral re­s­olu­tion, mak­ing neu­rolog­i­cal fea­tures con­cep­tu­ally more fun­da­men­tal. ↩︎

  43. There are, how­ever, many other de­ter­mi­nants of pro­cess­ing power. In the early days of com­put­ers, clock speed was a de­cent proxy for over­all perfor­mance. For mod­ern com­put­ers, how­ever, clock speed is an in­creas­ingly poor met­ric by which to judge pro­cess­ing power. ↩︎

  44. There are, how­ever, many rea­sons to be skep­ti­cal of the brain-com­puter metaphor. ↩︎

  45. A faster clock speed might make un­con­scious pro­cess­ing more effi­cient with­out af­fect­ing the speed at which in­for­ma­tion is rep­re­sented con­sciously. ↩︎

  46. This is one rea­son (among many) that com­par­ing an­i­mals in terms of neu­ron count can be mis­lead­ing. ↩︎

  47. See Olkow­icz et al 2016 for the grey par­rot es­ti­mate and Her­cu­lano-Houzel et al. 2007 for the owl mon­key es­ti­mate. See McCaslin 2019 for gen­eral dis­cus­sion. ↩︎

  48. Dou­bling ax­onal di­ame­ter in­creases sig­nal ve­loc­ity by about 40% (Fox 2011: 40). ↩︎

  49. In­ter­est­ingly, the ax­ons of new­born hu­mans are typ­i­cally only about half as fast as adult ax­ons. Chil­dren nor­mally reach the adult value by 3 or 4 years old (She­lat 2019). ↩︎

  50. Similarly, differ­ences in brain metabolism across species prob­a­bly tell us rel­a­tively lit­tle about differ­ences in the sub­jec­tive ex­pe­rience of time. More com­plex brains prob­a­bly re­quire more metabolic en­ergy. But there are many ways in which a brain can be more or less com­plex that ap­pear un­likely to in­fluence the sub­jec­tive ex­pe­rience of time. ↩︎

  51. Video credit: Emily Baird ↩︎

  52. It’s pos­si­ble that as the bee ap­proaches the flower, the fly for­mu­lates a plan to leave the flower and then im­me­di­ately re­turn. In that sce­nario, the de­ci­sion to re­turn is not made mid-flight, and so the speed of the de­ci­sion need not be as quick. ↩︎

  53. Com­pare to crabs who “be­gin rapid lo­co­mo­tion only 500–600 ms af­ter their an­ten­nules in­ter­cept an odor pulse” (Rei­den­bach & Koehl 2011: 3139). ↩︎

  54. “Only a tiny part of all available vi­sual in­put can be per­ceived con­sciously. The pro­cess of se­lec­tion hap­pens through al­lo­ca­tion of at­ten­tion to rele­vant in­for­ma­tion that is pre­sent in our sur­round­ings. Although this sys­tem of se­lec­tive at­ten­tion works rel­a­tively well in most situ­a­tions, if two to-be-iden­ti­fied tar­gets (T1 and T2) are pre­sented in rapid tem­po­ral suc­ces­sion (200–500 ms), iden­ti­fi­ca­tion of T2 nev­er­the­less fre­quently fails. This cog­ni­tive limi­ta­tion is called the at­ten­tional blink (AB;[1]), a phe­nomenon that has al­lowed re­searchers to study the mechanism of tem­po­ral se­lec­tive at­ten­tion on the bor­der of suc­cess and failure” (Willems et al. 2016: 2). ↩︎

  55. See this Schol­ar­pe­dia ar­ti­cle for more de­tail on at­ten­tional blink. ↩︎

  56. Another prob­lem is that de­ci­sion-mak­ing speed varies by in­di­vi­d­ual within a species. This is true even of in­sects like honey bees (Mul­ler & Chit­tka 2008). ↩︎

  57. The most ob­vi­ous ex­am­ples are prey species with fast-mov­ing preda­tors and preda­tor species with fast-mov­ing prey. ↩︎

  58. See, e.g., Wysocki & Ladich 2002. ↩︎

  59. “The es­ti­mated max­i­mal tem­po­ral re­s­olu­tion of white-beaked dolphins and other odon­to­cetes was ap­prox­i­mately twice that of pin­nipeds and man­a­tees, and more than ten-times faster than hu­mans and ger­bils. The ex­cep­tion­ally high tem­po­ral re­s­olu­tion abil­ities of odon­to­cetes are likely due pri­mar­ily to echolo­ca­tion ca­pa­bil­ities that re­quire rapid pro­cess­ing of acous­tic cues” (Mooney et al. 2009: 375). ↩︎

  60. In what fol­lows, I dis­cuss the ol­fac­tory tem­po­ral re­s­olu­tion of honey bees and fruit flies. How­ever, it’s worth not­ing that in­sects use two types of ol­fac­tory cod­ing, a com­bi­na­to­rial scheme and a la­bel­led scheme. Odors that have la­bel­led lines can be re­sponded to much faster than those that re­quire dis­crim­i­na­tion with a com­bi­na­to­rial code (Haverkamp, Hans­son, & Knaden 2018). Thanks to Gavin Tay­lor for bring­ing this point to my at­ten­tion. ↩︎

  61. It is un­clear whether the odors are pro­cessed con­sciously or un­con­sciously and whether the fly’s move­ment to­ward the odor source is the product of a sim­ple fixed ac­tion pat­tern or a more cen­tral­ized de­ci­sion. ↩︎

  62. “We found that flies can de­tect a short differ­ence in the ar­rival of two odor­ants (on­set asyn­chrony of 33 ms)” (Se­hdev et al. 2019: 115). ↩︎

  63. In­ter­est­ingly, the length of one’s tem­po­ral in­te­gra­tion win­dow seems to de­pend in part on one’s ex­pec­ta­tions: “The tem­po­ral win­dow in which these vi­sual events are in­te­grated was fur­ther­more found to be adapt­able. By vary­ing stim­u­lus pre­sen­ta­tion rate in a clas­sic alphanu­meric AB [at­ten­tional blink] task, it was shown that the ex­pec­ta­tion of a slow pre­sen­ta­tion rate in­duced more tem­po­ral in­te­gra­tion (mea­sured in­di­rectly with or­der re­ver­sal fre­quency), which was thought to re­flect a longer in­te­gra­tion win­dow. In con­trast, the ex­pec­ta­tion of a fast pre­sen­ta­tion rate in­duced less tem­po­ral in­te­gra­tion, which was thought to re­flect a shorter in­te­gra­tion win­dow. The ob­served changes in be­hav­ior were thus in­ter­preted as ev­i­dence for adap­tive con­trol of in­te­gra­tion” (Willems et al. 2016: 3). ↩︎

  64. See Itoh et al. 2019 for more on com­par­i­sons of tem­po­ral in­te­gra­tion win­dows be­tween mon­keys and hu­mans ↩︎

  65. See Ar­nal & Giraud 2012 and Samaha et al. 2015 for helpful overview. ↩︎

  66. It’s pos­si­ble that Brian To­masik’s 2016 blog post “Do Smaller An­i­mals Have Faster Sub­jec­tive Ex­pe­rience?” ex­erted some in­di­rect in­fluence. Luke Muehlhauser spec­u­lated about differ­ences in the “clock speed of con­scious­ness” across species in this 2018 LessWrong post, but it’s un­clear how much (if at all) those spec­u­la­tions have in­fluenced his sub­se­quent de­ci­sions. ↩︎

  67. In academia, see, for in­stance, Bu­dolf­son and Spears 2019. Within effec­tive al­tru­ism, see Do­minik Peters’ eth­i­, Evan Sand­hoefner’s Farm An­i­mal Suffer­ing Com­par­i­sons, and this 2013 blog post from Carl Shul­man. ↩︎

  68. I’ve pre­vi­ously es­ti­mated that ver­te­brates cur­rently re­ceive more than 99% of EAA fund­ing, and most of that money goes to ter­res­trial (rather than aquatic) an­i­mals. This de­spite the fact that fish are ex­ploited in greater num­bers than birds and mam­mals and de­spite the fact that in­ver­te­brates make up more than 99.9% of all an­i­mals on the planet. ↩︎

  69. I have in mind Bovi­dae (e.g., cows), Suidae (e.g., pigs), and Phasi­anidae (e.g., chick­ens), though var­i­ous fam­i­lies of fish are be­gin­ning to re­ceive more at­ten­tion. ↩︎

  70. See this spread­sheet for my best ac­count­ing of the tax­o­nomic di­ver­sity of an­i­mals ex­ploited by hu­mans. ↩︎

  71. Th­ese an­i­mals also differ with re­spect to brain-re­lated mea­sures, so we shouldn’t ac­tu­ally need to re­flect on time-re­lated mea­sures to see this point. ↩︎

  72. Although even these are not de­ci­sive—there’s ro­bust de­bate about whether re­ports of differ­ences in the sub­jec­tive ex­pe­rience of time allegedly trig­gered by life-threat­en­ing events should be taken liter­ally ↩︎

  73. It’s per­haps helpful to con­sider an anal­ogy be­tween spa­tial per­cep­tion and spa­tial es­ti­ma­tion. When I move from look­ing at unadulter­ated straight lines to the Müller-Lyer illu­sion, noth­ing in the qual­i­ta­tive na­ture of my spa­tial per­cep­tion has changed. The ad­di­tion of the pointed ar­row shafts to the straight lines re­li­ably in­duces a dis­po­si­tion to judge that the lines are of un­equal length. (If I know about the illu­sion I may not ac­tu­ally judge that the lines are un­equal, but I still feel the pull of the dis­po­si­tion.) How­ever, it’s not clear that the ad­di­tion of the pointed ar­row shafts makes the per­cep­tion of the lines non­veridi­cal. After all, the Müller-Lyer illu­sion is ex­actly what two straight lines with differ­ently pointed ar­row shafts look like. In con­trast, if I view two unadulter­ated straight lines then in­gest psychedelic drugs and look at the lines again, I might judge that they are differ­ent lengths be­cause my spa­tial per­cep­tion has changed due to the drugs. ↩︎

  74. Other tem­po­ral illu­sions in­clude the Kappa effect, flash-lag de­lay, and chronos­ta­sis. ↩︎

  75. Es­ti­ma­tion tasks and pro­duc­tion tasks ought to be in­versely cor­re­lated. Sup­pose that one’s tem­po­ral judg­ments are skewed by a fac­tor of 2. Then, for an image that has been on the screen for 5 sec­onds, one should give an es­ti­mated du­ra­tion of 10 sec­onds. When asked to press a but­ton when the image has been on screen for 5 sec­onds, one should press the but­ton at 2.5 sec­onds. Ex­per­i­men­tal re­sults in­di­cate that es­ti­ma­tion tasks and pro­duc­tion tasks of­ten don’t obey this cor­re­la­tion. ↩︎

  76. I leave open the pos­si­bil­ity that some of these fac­tors in­fluence the sub­jec­tive ex­pe­rience of time. How­ever, I find it im­plau­si­ble that all the fac­tors af­fect the sub­jec­tive ex­pe­rience of time. So long as some fac­tors af­fect tem­po­ral judg­ments with­out af­fect­ing tem­po­ral ex­pe­rience, we can draw a use­ful dis­tinc­tion be­tween the two. ↩︎

  77. “The mechanisms by which the brain tracks and com­pares time from sec­ond to sec­ond is not un­der­stood; in­deed, there is de­bate in the liter­a­ture as to whether ded­i­cated time-keep­ing mechanisms ex­ist or whether timing-re­lated be­hav­iors emerge from the in­trin­sic prop­er­ties of other cir­cuits” (Faber 2017: 2806). ↩︎

  78. “Im­por­tantly, timing of in­ter­vals in the sub-sec­ond range is thought to in­volve differ­ent brain re­gions than timing of longer ones” (Jura 2019: 2). ↩︎

  79. “The re­sults of our study im­ply that two differ­ent pro­cess­ing mechanisms may un­der­lie tem­po­ral-or­der judge­ments, de­pend­ing on the stim­uli used. A cen­tral-timing mechanism in­de­pen­dent of sen­sory modal­ity can be as­sumed for the tem­po­ral-or­der judge­ment with click, colour and po­si­tion stim­uli. In con­trast, the tem­po­ral-or­der of two differ­ent tones is pro­cessed by in­de­pen­dent fea­ture-specific mechanisms” (Fink et al. 2006: 351) ↩︎

  80. Ian Phillips has pro­posed a re­la­tional model of tem­po­ral rep­re­sen­ta­tion based on level of men­tal ac­tivity ac­cord­ing to which “vari­a­tion in du­ra­tion judg­ments… may be the re­sult of vari­a­tion in con­cur­rent men­tal ac­tivity rel­a­tive to which we mea­sure the du­ra­tions of en­vi­ron­men­tal events” (Phillips 2013: 244). Ac­cord­ing to Phillips, “a purely rel­a­tive ac­count of per­ceived du­ra­tion could make sense of du­ra­tion dis­tor­tions by ap­peal to the idea that du­ra­tion is in part per­ceived rel­a­tive to con­cur­rent non-per­cep­tual men­tal ac­tivity” (Phillips 2013: 246). Though seem­ingly promis­ing, this type of ac­count does not ap­pear to have caught on in the liter­a­ture. ↩︎

  81. Other pop­u­lar mod­els in­clude the pro­cess-de­cay model and the os­cilla­tor-co­in­ci­dence de­tec­tion model. ↩︎

  82. “the re­cent view is that there is no sin­gle, cen­tral­ized clock in the brain, ded­i­cated to perform­ing any op­er­a­tions re­lated to timing, re­gard­less of the na­ture of a task. In­stead, imag­ing data show that dis­tributed brain ar­eas are in­volved in time-keep­ing, which sug­gests that spe­cial­ized ar­eas have their own built-in mechanisms, that may vary among sys­tems” (Jura 2019: 1). “At least over a short time frame, our tem­po­ral per­cep­tion of events is far from veridi­cal, and mul­ti­ple timelines are ca­pa­ble of dy­namic re­cal­ibra­tion. This is in­com­pat­i­ble with the no­tion of a uni­tary cen­tral­ized and ded­i­cated clock, from which all timing is performed” (Mul­ler & No­bre 2014: 69). ↩︎

  83. “there is no time‐sense or­gan or sin­gle path­way car­ry­ing tem­po­ral in­for­ma­tion from the periph­ery to the brain. Rather, all sen­sory chan­nels sup­port time per­cep­tion, and it is un­clear how far these rep­re­sen­ta­tions are me­di­ated by com­mon struc­tures and mechanisms. More­over, timing oc­curs over mas­sively vary­ing scales, from microsec­onds to years, and at in­ter­me­di­ate du­ra­tions mul­ti­ple mechanisms likely op­er­ate in par­allel, com­pli­cat­ing the search for sim­ple in­for­ma­tion pro­cess­ing mod­els and neu­ral sub­strates” (Matthews & Meck 2014: 429). ↩︎