Maybe this is like blindsight but for the experience of pain? It appears humans with lesions to their primary visual cortex and who report being unable to see can still make good guesses about objects they look at. The lesions still impair performance on some tasks. I think there’s a good case to be made that those people totally lack visual experience, as they report (not just being unable to access hidden visual qualia). The same could be happening with decorticate rodents, as rodents with lesions of the primary visual cortex also exhibit some signs of blindsight. So, normal rats could experience pain, while decorticate rats don’t pain, even if decorticate rats continue to react in many (but probably not all) of the same ways.
There’s some discussion of some (mostly older) evidence related to blindsight across animals in Tehovnik et al., 2021:
In rodents (e.g., hamsters and gerbils) when the visual cortex including the putative object and motion encoding areas (see Figs. 5 and 7) is lesioned, the animals can still orient to visual targets (Mlinar and Goodale 1984; Schneider 1969), but they lose the capacity to perform feature vision such as discriminating between horizontal versus vertical black and white stripes or between speckled patterns versus diagonal stripes (Schneider 1969). Moreover, orientation discrimination is abolished in such animals (i.e., in mice, Schnabel et al. 2018) and the tracking of component motion as assessed using plaid stimuli is compromised (i.e., in mice, Palagina et al. 2017). As already mentioned, animals (i.e., gerbils) with lesions of the visual cortex fail to anticipate the trajectory of moving stimuli and to perform motion parallax (Ellard et al. 1986; Ingle 1981; Ingle et al.1979). Animals (i.e., hamsters and gerbils) that receive only collicular lesions can still discriminate between patterned stimuli and demonstrate motion parallax (Schneider 1969; Ellard et al. 1986), but they fail to orient to punctate targets (at 98% contrast) beyond 40 degrees of eccentricity (Mlinar and Goodale 1984) and they fail to respond to looming visual stimuli (> 20 degrees in size) throughout their ‘panoramic’ visual field [Schneider 1969; also see Shang et al. 2018 for pulvinar participation in response to looming stimuli], a function that moreover depends on an intact retrosplenial cortex (Ellard and Chapman 1991). Note that the superior colliculus of rodents (i.e., mice) contains neurons that respond to expanding flow fields presented from overhead (Dräger and Hubel 1975; also see Li et al. 2020). When both the visual cortex and the superior colliculus are lesioned, gerbils are no longer able to orient to visual stimuli anywhere in the visual field including to high contrast targets of 98% (Mlinar and Goodale 1984). Hence,rodents with lesions of the visual cortex and colliculus are rendered totally blind, failing to exhibit blindsight.
The lesioned gerbils performed as well as the normal animals for aperture locations anywhere within 90 degrees with respect to the left and right side of the head in the horizontal visual field. This result concurs with the results based on frogs and toads whose pretectal nuclei have been found to mediate aperture detection (Ingle 1973, 1980). If the gerbils (i.e., those with collicular lesions or those with no lesions) were given lesions of V1, they failed to orient to the low-contrast aperture, but if the aperture was of high contrast (a black aperture on a white background) the animals could respond to the aperture, which could be considered an additional type of blindsight (Ingle 1980). Indeed, a human subject with bilateral V1 damage and with no visual awareness was able to walk around and avoid large, salient obstacles as placed within a hallway (De Gelder et al. 2008).
Here, we try to bridge the divide. After reviewing key consciousness concepts, we identify “red herring” measures that should not be used to infer sentience because also present in non-sentient organisms, notably those lacking nervous systems, like plants and protozoa (P); spines disconnected from brains (S); decerebrate mammals and birds (D); and humans in unaware states (U).
When fed, decerebrate chicks “followed the grain with striking pecking precision when it was moved in front of them by a tweezer” (81). Likewise, “blindsighted” humans, unable to see because of damage to the visual cortex, are still able to avoid walking or reaching into obstacles, as well as to visually track or grasp stimuli that they report that they cannot see [(32), p. 33, 90; although as outlined in the next section, the position of such obstacles cannot be remembered]. Further unconditioned reactions to harmful stimuli in decerebrate mammals are notable because of their seemingly affective nature. Decerebrate animals can react to noxious stimuli “by flight or attack” (82). Decerebrate rats, “respond to noxious stimuli with a flexion withdrawal response, vocalization, turning to the site of the injury, licking or biting the site of the injury, complex escape response and attack responses”, although removing the noxious stimulus causes immediate return to passivity or grooming as if nothing had occurred [(71); see also (83) for similar reports]. They show startle responses to sudden sounds [e.g., (84)]. And decerebrate chicks “emitted contentment calls [sic] when warm and distress calls [sic] when cold” (81).
Such unlearned responses to stimuli can also be modulated in S.P.U.D. subjects, including by emotionally-relevant cues: they are not fixed and stereotyped. For example, faced with a startling stimulus, “jump” reflexes, and increases in heart rate and skin conductance (reflecting the sympathetic activation of sweat glands) are typically greater in fearful than relaxed humans, including subjects exposed to distressing images. Yet such images can still have this modulatory impact on the startle reflex even when presented in a way that precludes their conscious perception [e.g., (85–87)]. Thirsty humans also drink more (and rate the drink as more positive) if exposed to happy faces than angry faces, even when these are subliminal (88). Likewise, decerebrate rats show a greater ingestive response to sucrose if food deprived rather than sated (89). Furthermore, tail withdrawal reflexes in decerebrate and spinally transected rats are reduced by morphine (83, 90). And even in plants like the sensitive mimosa, whose leaves close when touched, responses to aversive stimuli like lit matches are dampened when the leaves are sprayed with lidocaine (91). These responses by S.P.U.D subjects thus show that modulation of avoidance or ingestive behaviors by affectively-relevant manipulations does not require sentience.
That strikes me as plausible, but if so, then rats are much more competent than humans in their ‘blindsight’ like abilities. My impression is that in humans, blindsight is very subtle. A human cannot use blindsight to walk into the kitchen and get a glass of water. Rats seem like they can rely on their midbrain to do this sort of thing. If rats are able to engage in complex behavior without consciousness, that should make us wonder if consciousness ever plays a role in their complex behavior. If it doesn’t, then why should we think they are conscious?
You might think that we have evidence from comparative neuroanatomy. Humans have some abilities in both places, and something in the cortex adds consciousness. Maybe the same is true for rats. But if so, that would push the questions down the phylogeny to creatures who can achieve as much or more than rats with their midbrains and don’t have any complex cortex.
Maybe this is like blindsight but for the experience of pain? It appears humans with lesions to their primary visual cortex and who report being unable to see can still make good guesses about objects they look at. The lesions still impair performance on some tasks. I think there’s a good case to be made that those people totally lack visual experience, as they report (not just being unable to access hidden visual qualia). The same could be happening with decorticate rodents, as rodents with lesions of the primary visual cortex also exhibit some signs of blindsight. So, normal rats could experience pain, while decorticate rats don’t pain, even if decorticate rats continue to react in many (but probably not all) of the same ways.
There’s some discussion of some (mostly older) evidence related to blindsight across animals in Tehovnik et al., 2021:
Another more recent relevant paper: Mason and Lavery, 2022:
That strikes me as plausible, but if so, then rats are much more competent than humans in their ‘blindsight’ like abilities. My impression is that in humans, blindsight is very subtle. A human cannot use blindsight to walk into the kitchen and get a glass of water. Rats seem like they can rely on their midbrain to do this sort of thing. If rats are able to engage in complex behavior without consciousness, that should make us wonder if consciousness ever plays a role in their complex behavior. If it doesn’t, then why should we think they are conscious?
You might think that we have evidence from comparative neuroanatomy. Humans have some abilities in both places, and something in the cortex adds consciousness. Maybe the same is true for rats. But if so, that would push the questions down the phylogeny to creatures who can achieve as much or more than rats with their midbrains and don’t have any complex cortex.