Hesse & Tsao just published this great study. It’s a binocular rivalry study (BR). In BR, incompatible stimuli are presented to each eye –– for instance: Obama’s face, and a Taco. Conscious perception alternates between the two every few seconds. So, researchers can compare conscious perception of a stimulus (e.g. Obama’s face) with unconscious processing of the same stimulus (e.g. Obama’s face when it is suppressed and the subject is conscious of the taco instead).
What’s new about Hesse & Tsao’s study? They developed a new no-report paradigm. It’s probably better than other no-report paradigms. But I’m not so sure the no-report aspect of the study is so important. They focus mostly on neuronal activity in the inferotemporal (IT) cortex, which shouldn’t exhibit so much report-related activity –– compared to PFC, for instance. So I won’t talk about the “no-report” aspect of this study (despite the hype, no-report is not new, and quite often unnecessary, in my humble opinion).
From a methodological perspective, the main new thing with this study – compared for instance to previous work by Logothetis et al. addressing similar questions – is more advanced recording and decoding. Hesse & Tsao recorded from multiple (mostly face-selective) IT neurons simultaneously during BR, and could decode the representational contents carried by those populations on a trial-by-trial basis.
Now the result. As expected IT neurons modulate their activity with BR alternations. But the main new and exciting finding is that these neurons encode both the conscious percept and the suppressed percept at the same time. That’s a case of mixed selectivity. IT neurons do track the conscious percept, but they also track the unconscious percept.
Since we can decode unconscious contents from IT neurons, it means that a special kind of population code in IT neurons correlates with the conscious content, and not any neuronal activity. That’s a really cool finding.
On Twitter Doris Tsao opposed these findings to a common – but incorrect – interpretation of earlier single-neuron recording findings, by Sheinberg & Logothetis.
Here’s, for instance, what Koch et al. (2016) write about this: “Single-neuron recordings in monkeys, carried out during paradigms such as binocular rivalry, suggest that activity in most V1 neurons is linked to the identity of the physical stimulus rather than the percept. This contrasts with the activity of neurons higher up in the visual hierarchy, which correlates with the percept rather than the stimulus.”
To the best of my knowledge, Logothetis et al. never claimed that IT neurons track only the conscious percept. In any case, Hesse & Tsao have now proved this story wrong. They also convincingly argue that there’s no subset of IT neurons dedicated only to the representation of the conscious percept.
So that’s it? We’ve found that the neural correlate of consciousness (of faces) is a special kind of population coding in IT neurons?
Probably not. Binocular rivalry can happen for unconscious percepts. There’s also evidence that binocular rivalry – at least for vertical vs. horizontal gratings – operates at an earlier level than other forms of suppression like dichoptic metacontrast masking. (And metacontrast masking likely operates earlier than crowding, which itself probably operates earlier than object substitution masking)
So, the mechanism responsible for rivalry is upstream from the mechanism responsible for consciousness of contents. Which means that whatever contrast you get from comparing conscious vs. suppressed contents encompasses neural correlates of consciousness and a set of pre-requisites of consciousness.
Now, there’s a big difference between finding the neural correlate of a content of consciousness, and finding the neural correlate of consciousness of that content.
Hesse & Tsao have convincingly shown that the conscious perceptual content correlates with a specific population code in IT. They have also shown that the same neuronal population can carry both conscious contents and unconscious contents at the same time. That’s a nice discovery.
Does it mean that it is in virtue of being coded in a specific manner by IT neurons that the content is conscious rather than non-conscious? We don’t really know. We still don’t know whether this kind of population coding in IT would be sufficient on its own for consciousness of the representational content carried by this neuronal population. But with more studies like Hesse & Tsao’s we’ll surely get there.
I thank Janis Hesse & Doris Tsao for discussing their study with me and for reading a draft of this blog post.
