Soutenance de thèse : François STOCKART

What happens in the brain when we experience the world around us; when we see, hear, feel or smell? Perceptual consciousness consists of the subjective feelings associated with perception, together with subjective feelings of confidence. It has been suggested that perceptual consciousness and confidence can be explained by evidence accumulation, a form of sequential sampling of sensory evidence. They are studied by asking participants to report whether they perceived stimuli presented around perceptual threshold and comparing neural activity when they reported perceiving vs. not perceiving, with high vs. low confidence. Defining the neural substrates of perceptual consciousness is challenging, because (1) of the difficulty of recording brain activity with high spatio-temporal resolution in humans able to provide subjective reports and (2) confounding neural activity associated with the mere fact of providing these subjective reports. To overcome those limitations, I used stereo-electroencephalographic data recorded from people with epilepsy and isolated perceptual consciousness using several experimental task manipulations.

In a first, pre-registered study, I combined stereo-electroencephalographic data, multivariate decoding and computational modeling to investigate whether visual consciousness can be explained by evidence accumulation. Participants performed 3 experiments in which we assessed the neural markers of face visibility while controlling for report-related activity. In an immediate-response experiment, I found markers of evidence accumulation from individual sEEG channels and decoded multivariate latent variables in the visual, inferior frontal, and anterior insular cortices. In two further experiments, signals in the ventral visual cortex differentiated between perceived and unperceived stimuli in delayed detection, (2) high- and low-intensity stimuli during passive viewing, and (3) high and low confidence when stimuli were seen. These results indicate that evidence accumulation in the ventral visual cortex reflects perceptual consciousness irrespective of reports. They were consistent with predictions made by a computational model of leaky evidence accumulation. 

In a second study, I investigated the temporal dynamics of auditory consciousness by characterizing the neural representations of sounds and silences over time. Participants listened to continuous streams of babble noise that faded in and out of consciousness. They performed 3 counterbalanced experiments: (1) pressing on a button while they perceived sounds (report-sound experiment), (2) pressing while they perceived silences (report-silence experiment) or (3) listening passively to the stream (no-report experiment). Participants were more sensitive to sounds in the report-sound experiment, and also more likely to report hearing them. Preliminary analyses of stereo-electroencephalographic data indicate that neural populations in Heschl’s gyrus responded to sound intensity irrespective of task requirements, while neural populations in other brain regions including the insula were modulated by the experiment performed or by participants’ reports. Some contacts responded to silences by increasing their activity more in the report-silence experiment, indicative of a representation of content absence. An implicit measure of confidence was also collected based on the strength of participants’ button press and used as a regressor in electrophysiological analyses.

Overall, the two sEEG studies as well as ancillary studies performed during this PhD thesis provide an anatomical and temporal demarcation of the neural substrates of visual and auditory consciousness distilled from report-related neural activity. They provide support for the view that evidence accumulation can explain key aspects of perceptual experience, encompassing both perceptual consciousness and associated feelings of confidence.