PLOS Biology has a most interesting article from Stanislas Dehaene’s group on the neurodynamics of conscious experience. The researchers studied brain activation using EEG, while subjects rated visually presented stimuli on a scale from unseen to clearly seen. It was found that conscious experience of a stimulus was related to the engagement of a widespread network involving the frontal, parietal and temporal cortices.
SCR note: It is mentioning a recent study using fMRI (Christensen et al. 2006) provided comparable results, and adding two factors; (1) the conscious experience of a visual stimulus involved activation of both thalami, and (2) subjects consistently rated some experiences as vague, i.e., as “detected but not identified”. This experience was associated with both lower activation in those regions involved in conscious experience, and unique activation of additional regions, including some prefrontal regions.
Author description: Understanding the neural mechanisms that distinguish between conscious and nonconscious processes is a crucial issue in cognitive neuroscience. In this study, we focused on the transition that causes a visual stimulus to cross the threshold to consciousness, i.e., visibility. We used a backward masking paradigm in which the visibility of a briefly presented stimulus (the “target”) is reduced by a second stimulus (the “mask”) presented shortly after this first stimulus. (Human participants report the visibility of the target.) When the delay between target and mask stimuli exceeds a threshold value, the masked stimulus becomes visible. Below this threshold, it remains nonvisible. During the task, we recorded electric brain activity from the scalp and reconstructed the cortical sources corresponding to this activity. Conscious perception of masked stimuli corresponded to activity in a broadly distributed fronto-parieto-temporal network, occurring from about 300 ms after stimulus presentation. We conclude that this late stage, which could be clearly separated from earlier neural events associated with subliminal processing and mask-target interactions, can be regarded as a marker of consciousness.