[Study of functional neuro-anatomy of perception using positron emission tomography]

The investigation by means of functional neuroimaging techniques, notably positron emission tomography, of the neural networks involved in perceptual processes is presently carried out according to two different though complementary approaches. In one of them, the relationships between impairments in perceptual processes and alterations in resting brain metabolism are studied in brain-damaged subjects, according to the classic paradigm of neuropsychology, but here the metabolic alterations reflect the functional disruption of neural networks directly or indirectly affected by the focal brain lesion. In the other approach, one records the changes in local synaptic activity in response to a given stimulation or task, as compared to a reference condition; thus far this second approach has been mainly applied to the normal human subject. Though different, both approaches highlight the neural networks involved in a given perceptual or cognitive task, which appear as a set of brain areas more extensive than that revealed by classic neuropsychology as necessary to perform the task. PET activation studies in the healthy subject have allowed to investigate, within each domain of perception, the successive "bottom-up" steps of information processing, from the most elementary ones carried out passively, to the most complex ones involving e.g., the semantic representations. The attentional aspects of perception have also been extensively investigated, notably selective "top-down" attention (which modulates the response of those brain areas that process the given perceptual attribute), sustained vigilance (which activates a parieto-prefrontal network in the non-dominant hemisphere), and mental imagery (which activates in a "top-down" fashion the entire perceptual network, including the primary cortices). The wide-scale application of this approach to brain-damaged subjects will seemingly soon provide important clues regarding the neural mechanisms of impaired perception and recovery therefrom.