Intracranial Neurophysiological Correlates Related to the Processing of Faces

Face perception and recognition is an intriguing ability, already present in neonates. Numerous studies in patients with brain lesions identified the temporo-occipital cortex as the crucial structure for this capacity. Analysis of electrical signals (EEG) inside the brain of patients implanted with intracranial electrodes for diagnostic purposes allows researchers to describe the temporal and spatial organization of responses to various aspects of face processing in human subjects. Several findings have emerged and appear relevant for cerebral organization in general: (1) Selective face responses were obtained from the basal temporo-occipital cortex at around 200 ms (N200); however, other structures such as the lateral temporal lobe and frontal cortex also participate in face recognition and perception tasks. (2) Each structure has a distinct "response profile"; that is, with respect to a given task certain structures respond strongly, others less or not at all. This profile might change with a different task, although the physical parameters of the stimuli remain the same. (3) The right hemispheric predominance of face processing, as suggested by patient data and studies in healthy volunteers, seemed to be restricted to its early stages (i.e., before 100-150 ms). (4) Recognition of faces might be associated with differential intracranial responses, despite an incorrect overt response, reflecting neurophysiological correlates of implicit memory. (5) The more the stimulus resembled a complete human face, the earlier and larger the N200 response was found, in particular over the basal temporobasal cortex. Analysis of electrical signals from intracranial electrodes might help to improve our understanding of the underlying physiological and anatomical constraints of cognitive processes.