Global neuronal workspace and Information Integration Theories
What are the minimal set of neuronal events and mechanisms sufficient for a specific conscious percept? During the last decades, there has been significant progress in the understanding of the so-called
neural correlates of consciousness (NCC). But it is still a matter of debate which mechanisms and which specific brain regions are involved in perceptual awareness. More specifically, what is the potential implication of the prefrontal cortex (PFC)?
According to the Global Neuronal Workspace (GNW) theory,
neurons of the PFC should encode, at any given moment,
the details of the current conscious experience. But other theories weaken its role. For instance, the
Information Integration Theory (IIT)
states that the major complex encoding perceptual experience should be primarily located in posterior areas. In that respect, the prefrontal modulation would reflect a consequence rather than a direct correlate of conscious experience.
Experimental setup
A team from
UNICOG (NeuroSpin department) set up an
experiment in order to decipher the role of the PFC in conscious perception.
Two non-human primates, which are good models to study consciousness processes, participated in that experiment. They were
never trained before to report their perception of the stimuli. Researchers recorded neuronal ensemble activity during briefly presented visual stimuli:
- either in isolated trials in which stimuli were clearly perceived
-
or in very fast sequences of rapid serial visual presentation (RSVP) in which perception and post-perceptual processing were challenged.
The study provides evidence that
neural representations at the level of the ventrolateral PFC are rich and occur shortly after stimulus onset. The identity of each of the presented images could be decoded form PFC population activity, at timescales that correspond to when feedforward computations are still being processed in most visual areas. Significant information is still present during RSVP when images are presented in streams of 10 images per second, a rate at which conscious access and post-perceptual processing are challenged.
The decoding accuracy of each consecutive stimulus, at its peak, was of the same magnitude during RSVP as when the same stimulus was presented in isolation and clearly visible. Therefore,
encoding of visual information in the ventrolateral PFC can occur prior to post-perceptual processes (i.e. thinking about the visual stimulus presented)
and before access consciousness. When stimuli were presented in isolation, a stable and sustained code emerged after the peak of maximal information at 150 ms. This stable representation is compatible with the neural correlates of consciousness in the GNW theory. Interestingly, the RSVP condition did not fully abolished this sustained activity but only significantly shortened and reduced it. RSVP stimuli could be consciously perceived, but too flimsily to lay down a short-term memory that would persist long enough to be later reportable in full. Another possibility is that RSVP stimuli may not all be consciously perceived during their representation: the impression of seeing them all would be, in part, an illusion, a consequence of our ability to consciously access any of them later on. The observed transient neural code would correspond to what has been termed a "pre-conscious" representation. Further studies including behavioral reports could help refine theories of consciousness.
Interestingly, these data indicate that
neuronal populations in the PFC reliably encode visual stimuli even under conditions that have been shown to challenge conscious perception and/or substantially reduce the probability of post-perceptual processing in humans.
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