Corvid consciousness – computation, cognition, or comprehension?
A really nice paper came out recently that claims tohave discovered a neural correlate of sensory consciousness in a corvid bird (the carrion crow). The authors use an elegant set up involving barely perceptible visual stimuli to distinguish the delivery of a stimulus and the subjective percept that it engenders. The experiment clearly demonstrates that crows can maintain an internal representation for a period of time before taking an action based on a rule that is subsequently presented to them. This kind of task has been used in primates to distinguish what happens in the brain when an animal consciously detects a stimulus versus when it doesn’t. But is this really a correlate of conscious subjective experience or simply a marker of ongoing neural activity that mediates working memory? What do we even mean by conscious subjective experience? Does maintaining an active neural state necessarily entail a mental state?
The experimental set up is really powerful. The authors trained two crows to perform this behavioral task while recording from hundreds of neurons in a particular part of the bird brain that is thought to be analogous to the mammalian prefrontal cortex. The birds are either shown a stimulus or not, for a very brief time – so brief that they sometimes detect it and sometimes don’t. Whether they do or not is determined in some way by the current internal state of the crow’s brain (this may reflect the precise phase of rhythmic patterns of ongoing neural activity and excitability at the moment of stimulus presentation).
After a brief period, the crows are then presented with a rule, which tells them how they should act in response to either the presence or absence of a stimulus. This means that the crow must maintain some sort of internal representation for some period of time while waiting to be told what to do with that information.
The important thing about this set up is that it dissociates the presence of the stimulus with the percept of the animal. In particular, the crows can be wrong in two ways – they can fail to see something when a stimulus was actually presented, or they can think they saw something when no stimulus was presented, leading to a distinction between direct visual stimulation and subjective experience.
In primates, including humans, you get quite different brain responses on trials when the subject reports (either by behavior or by language) that they consciously saw something versus when they didn’t. Neurons in primary visual cortex respond to a visual stimulus whether the animal “sees” it or not. Trials where the animal reports a percept involve subsequent activation of many other areas of the cortex, especially frontal regions. This suggests that for a visual stimulus to rise to the level of conscious awareness, it must be broadcast to the rest of the brain.
The experiment with the crows was looking for the same kind of marker or neural correlate of conscious perception. And indeed they found it. Neurons in the posterior pallium of the crows (the nidopallium caudolaterale, for those keeping score at home) responded in two ways: some neurons showed an immediate response to the visual stimulus whether it was perceived or not. Others showed prolonged activity after the stimulus was removed, which correlated much better with the subsequent behavior of the animal than with the stimulus itself.
Importantly, because the crows don’t know what they’re supposed to do with the information during this waiting period, this neural activity should not be interpreted as merely preparation for a particular motor action. Instead, the authors interpret it as reflecting a subjective conscious signal that is maintained over this period. This interpretation is supported by the fact that the patterns match the animal’s percept (as reported by its behavior), even when it was mistaken.
So, what does all this tell us? The similarity with the situation observed in primates prompts the authors to conclude that crows are similarly having some kind of conscious experience. At least, they take these data as evidence for “access consciousness”, meaning that some internal neural state representing something in the outside world can be maintained over time even in the absence of that stimulus and that this information can be made available or broadcast to the rest of the brain and subsequently acted upon.
They are more cautious about taking it as evidence for “phenomenal consciousness”, which would entail it feeling like something to have that experience. Maybe it does, maybe it doesn’t – it’s not obvious at all that it has to. And of course for animals without language, it’s almost impossible to answer that question – if they can’t tell you that it feels like something, it’s hard to find some other way to tell whether it does or not.
The question is whether it’s justified calling this consciousness at all. Maybe it’s just working memory – maybe some neural circuits can maintain reverberating patterns of activity over certain periods of time, can communicate that activity to other parts of the brain, and can use it to inform action. Does any of that have to entail subjective experience? I would guess that lots of animals can probably do that kind of task, possibly even very simple ones (though readers may enlighten me otherwise). Indeed, it seems like you could design circuits that would work like that in a robot without expecting it to subjectively perceive anything. Could this all be achieved by mindless computation?
Or maybe the alternative is true – maybe that kind of internally sustained representation defines mental experience. Maybe you don’t need to add anything else to that – maybe having such representations and being able to act upon them in response to further information merits being called cognition. But does that necessarily entail subjective experience? We can have cognition without consciousness so is there anything else here that justifies the grand conclusion from this paper: that crows are conscious?
Well first there’s the question of what is being represented. You might think it’s the visual stimulus but it isn’t just that (it may not be that at all, after the initial brief moment) – it’s the belief of whether such a stimulus was present or not. (Remember, the crow can be mistaken). The crow knows that it will receive instructions to behave in one way or another depending on whether it perceived the stimulus. It doesn’t have to maintain a trace of the visual stimulus itself –which happens to be a grey square – just a record of its own perceptual decision.
As an aside, many people would quibble with the word “representation” here, and for good reason. That term is used in so many different and often only vaguely defined ways across neuroscience, cognitive science, psychology, computer science, and philosophy. Here I think it is justified, as it refers to a sustained pattern of activity that reflects a belief about the world and that can be communicated or “re-presented” to other parts of the brain to inform action.
This seems to go beyond a simple kind of working memory in that what is being remembered or maintained is an inference, not merely a stimulus. Does this thereby qualify as mental activity? Would it qualify as comprehension? Does the crow know what it knows? Or is it just higher-order neural activity? Again, it seems like you could program that kind of perceptual decision-making and retention of an abstracted signal of the decision for some time in a robotic system, without entailing mentality.
The other characteristic relevant to questions of consciousness is that this representation is made available to the rest of the brain. This is reminiscent of the Global Neuronal Workspace theory of conscious awareness developed by Stan Dehaene and colleagues. The idea is that perceptual stimuli have to reach some threshold of neural activity in order to ignite activity across a much wider network of the brain. In such cases the stimulus is actually perceived consciously. When this ignition fails to occur the stimulus is not perceived, though we can see that lower areas of visual cortex respond the same in either case.
(There is, incidentally, congruent work on conditions like face blindness and tune deafness, which suggests that the problem in these conditions is not the primary responsiveness of face or music processing areas of the cortex, but rather the communication of signals from these areas to frontal regions, necessary for conscious processing of those stimuli).
More broadly, in this hypothesis, percepts are in somesense competing with each other for access to this global workspace, this unified consciousness. It is in this global workspace that not just percepts but also memories, ideas, beliefs, goals, plans, etc., are integrated, selectively attended to, and acted upon to drive coherent and sophisticated behaviour.
Is that the case in these crows? Who knows? The experimental set up does not provide much scope for sophisticated behaviour – they have been trained to respond in simple ways to this isolated information, not to integrate it with other information and operate upon it in any more complex way. However, the well-known problem-solving abilities of crows (e.g., here) suggest a highly developed capacity for reckoning and figuring of various kinds.
By themselves, the data in this paper can’t really address the question of subjective conscious experience in these birds, limited as they are to this very simple set up. But they’re certainly consistent with some kind of mental life. Perhaps the ability to derive and maintain and broadcast these kinds of perceptual beliefs (or other cognitive features that can be abstracted away from direct current experience) is a necessary but not sufficient function for mental experience.
Maybe that kind of activity would only entail subjective experience in an organism with a broader sense of self, phenomenologically calibrated through its own history of experience with a wider world. It seems most likely that such capabilities exist in graded fashion across the animal kingdom, not all-or-none. It’s probably an ill framed question to ask: are crows conscious? Perhaps better to ask what their consciousness is like.