Short version, very little in that paper has stood the test of time and the particular passage you quote has many problems. I’d encourage you to reconsider including it!
Hi Rob. A few more thoughts. I grant you that the evidence for sentience in PWS is thin and I certainly don’t want to suggest otherwise. Still, I’m not sold on some of the bases for skepticism that you’ve flagged.
First, you suggest that the miniaturized hemiellipsoid bodies in PWS provide evidence against integrative processing of the sort relevant to pain. That inference is plausible in some contexts, but not all. Small structures can be highly functional, and in small animals “miniaturization” is often just what efficiency looks like rather than what loss looks like. After all, selective pressures have done that in many other species and current breeding practices are, effectively, intense selective pressures: they might push toward compact integrative circuitry rather than cause the loss of integration. In addition, morphology is a weak proxy for function. For instance, people used to think that the mushroom bodies in insects were solely for processing olfactory information, largely because the olfactory afferents were obvious and the visual inputs weren’t. We now know that visual information also reaches the mushroom bodies and that these regions integrate multiple sensory modalities and support cross-modal learning. Even in very well-studied taxa, we’ve repeatedly underestimated what regions are doing. (Moreover, insofar as we know what the hemiellipsoid bodies in crustaceans are for, the comparative neuroanatomy suggests that variation in their size and complexity may track the amount of olfactory input, which has nothing to do with sentience. See the end of this paper for details.) Finally, even if the hemiellipsoid bodies are small relative to our expectations, other integrative regions—like the medulla terminalis and structures plausibly homologous to the insect central complex (central body + protocerebral bridge)—aren’t obviously reduced in the same way. So, we should be wary of placing much weight on this neuroanatomical point.
Second, I worry that the post puts too much weight on that one negative behavioral result—namely, the failure to observe directed grooming/rubbing in PWS under extreme pH stimuli. First, null results in animal behavior are notoriously common. Second, there are some additional reasons for caution in this particular case. For example, the authors used extremely high concentrations of NaOH and HCl for two species because lower concentrations produced no response in preliminary trials; they also explicitly note that such pH levels may be ecologically irrelevant, which raises the possibility that the stimulus wasn’t being processed in the expected way at all. On top of that, the behavioral coding differed from the studies they were attempting to replicate: for example, they didn’t count antennae contact with the tank wall as grooming, whereas earlier work did. More broadly, directed grooming is only one possible self-protective response. In many taxa, pain manifests as withdrawal, guarding, reduced feeding, or altered exploration. Third, animals are sensitive to many kinds of damage. Even if we don’t see responses to pH, it would be shocking if there weren’t any sensitivity to mechanical injury, for instance. Taken together, this makes it hard to treat the null grooming result as a strong strike against pain-like processing in PWS.
Third, you’re skeptical that nociception plus pharmacological modulation should shift our priors much. But granting that anesthetics can suppress movement in a fairly general way, pharmacological modulation is still one of the classic tools used to distinguish mere reflexive responsiveness from centrally mediated aversive processing in animals. So I think there’s some risk of people reading the post as saying “nociceptors, therefore basically nothing,” when a more accurate characterization is that we have at least some of the kinds of markers that pain researchers take seriously in other contexts.
Finally, while you’re right to caution us with respect to extrapolating across taxa, that’s abandoning one of the only tools available in our current, data-starved situation: phylogenetic inference. Decapods probably share a lot of conserved neural and behavioral machinery, and many pain-relevant capacities—learning, motivational tradeoffs, and injury-related protective behavior—appear across the group. Given that, the default evolutionary expectation isn’t obviously that pain-like processing appears in crabs and lobsters but is absent in shrimp unless we have strong reasons to think a major functional transition or loss event occurred. And I’m not sure we do. So, yes, taxonomic differences matter, and suborder distinctions may well track meaningful neurobiological differences, but they don’t by themselves defeat inference from related taxa.
Again, none of this is to claim that the evidence for pain in PWS is already strong or that skepticism is unreasonable. Just trying to push back a little on how skeptical we should be.
Hi Bob. Thanks for the clarifying comment, and linking to Rob’s reply. I had linked to your and Meghan’s paper in the middle of the crosspost (in the paragraph with for “Bob Fischer’s”), but it is great to have additional context.
I know the OP may not read this comment. I made it on his Substack post and I’m sharing it here in case it’s of interest to others on the Forum.
Thanks for your post, Rob. Meghan Barrett and I have a detailed reply to Eisemann et al. 1984 in the Quarterly Review of Biology. You can see it here:
journals.uchicago.edu/d…
Short version, very little in that paper has stood the test of time and the particular passage you quote has many problems. I’d encourage you to reconsider including it!
Hi Rob. A few more thoughts. I grant you that the evidence for sentience in PWS is thin and I certainly don’t want to suggest otherwise. Still, I’m not sold on some of the bases for skepticism that you’ve flagged.
First, you suggest that the miniaturized hemiellipsoid bodies in PWS provide evidence against integrative processing of the sort relevant to pain. That inference is plausible in some contexts, but not all. Small structures can be highly functional, and in small animals “miniaturization” is often just what efficiency looks like rather than what loss looks like. After all, selective pressures have done that in many other species and current breeding practices are, effectively, intense selective pressures: they might push toward compact integrative circuitry rather than cause the loss of integration. In addition, morphology is a weak proxy for function. For instance, people used to think that the mushroom bodies in insects were solely for processing olfactory information, largely because the olfactory afferents were obvious and the visual inputs weren’t. We now know that visual information also reaches the mushroom bodies and that these regions integrate multiple sensory modalities and support cross-modal learning. Even in very well-studied taxa, we’ve repeatedly underestimated what regions are doing. (Moreover, insofar as we know what the hemiellipsoid bodies in crustaceans are for, the comparative neuroanatomy suggests that variation in their size and complexity may track the amount of olfactory input, which has nothing to do with sentience. See the end of this paper for details.) Finally, even if the hemiellipsoid bodies are small relative to our expectations, other integrative regions—like the medulla terminalis and structures plausibly homologous to the insect central complex (central body + protocerebral bridge)—aren’t obviously reduced in the same way. So, we should be wary of placing much weight on this neuroanatomical point.
Second, I worry that the post puts too much weight on that one negative behavioral result—namely, the failure to observe directed grooming/rubbing in PWS under extreme pH stimuli. First, null results in animal behavior are notoriously common. Second, there are some additional reasons for caution in this particular case. For example, the authors used extremely high concentrations of NaOH and HCl for two species because lower concentrations produced no response in preliminary trials; they also explicitly note that such pH levels may be ecologically irrelevant, which raises the possibility that the stimulus wasn’t being processed in the expected way at all. On top of that, the behavioral coding differed from the studies they were attempting to replicate: for example, they didn’t count antennae contact with the tank wall as grooming, whereas earlier work did. More broadly, directed grooming is only one possible self-protective response. In many taxa, pain manifests as withdrawal, guarding, reduced feeding, or altered exploration. Third, animals are sensitive to many kinds of damage. Even if we don’t see responses to pH, it would be shocking if there weren’t any sensitivity to mechanical injury, for instance. Taken together, this makes it hard to treat the null grooming result as a strong strike against pain-like processing in PWS.
Third, you’re skeptical that nociception plus pharmacological modulation should shift our priors much. But granting that anesthetics can suppress movement in a fairly general way, pharmacological modulation is still one of the classic tools used to distinguish mere reflexive responsiveness from centrally mediated aversive processing in animals. So I think there’s some risk of people reading the post as saying “nociceptors, therefore basically nothing,” when a more accurate characterization is that we have at least some of the kinds of markers that pain researchers take seriously in other contexts.
Finally, while you’re right to caution us with respect to extrapolating across taxa, that’s abandoning one of the only tools available in our current, data-starved situation: phylogenetic inference. Decapods probably share a lot of conserved neural and behavioral machinery, and many pain-relevant capacities—learning, motivational tradeoffs, and injury-related protective behavior—appear across the group. Given that, the default evolutionary expectation isn’t obviously that pain-like processing appears in crabs and lobsters but is absent in shrimp unless we have strong reasons to think a major functional transition or loss event occurred. And I’m not sure we do. So, yes, taxonomic differences matter, and suborder distinctions may well track meaningful neurobiological differences, but they don’t by themselves defeat inference from related taxa.
Again, none of this is to claim that the evidence for pain in PWS is already strong or that skepticism is unreasonable. Just trying to push back a little on how skeptical we should be.
Rob has a thoughtful reply here.
Hi Bob. Thanks for the clarifying comment, and linking to Rob’s reply. I had linked to your and Meghan’s paper in the middle of the crosspost (in the paragraph with for “Bob Fischer’s”), but it is great to have additional context.