Primary sensory cortices, top-down projections and conscious experience.

Beginning with a prominent article by Crick and Koch in 1995 (Nature 375, 121-123), cognitive neuroscience has witnessed an intensive debate about whether or not neural activity in primary visual cortex correlates with conscious visual experience. While some studies--especially those employing functional magnetic resonance imaging--imply that this is the case, others--particularly those recording from single neurons--suggest that it is not. In the light of this ongoing controversy, it is surprising that the analogous question in other sensory modalities has received far less attention. The first part of the present article reviews studies relevant to the role of primary auditory and primary somatosensory cortices in conscious auditory and tactile experience. As will become evident, the results of these studies, at least at first sight, appear no less contradictory than those obtained in the visual modality--in fact, they evidence discrepancies that resemble those found in the visual system to an impressive degree. The second part of the article attempts to reconcile the seemingly contradictory data by suggesting that only activity induced in the primary sensory cortices through cortico-cortical top-down signals can become consciously accessible, whereas activity induced by bottom-up signals from the thalamus cannot. This conclusion is in line with the earlier proposals of several prominent neuroscientists that portrayed conscious perception as the result of an active interpretative process by the brain, rather than a passive reflection of the environment.