[Impaired recognition of faces: implicit recognition, feeling of familiarity, role of each hemisphere]

We report three observations of patients who suffered from impaired face recognition following cerebral lesions. Two had classical prosopagnosia, resulting from bilateral in one case and right unilateral occipito-temporal in the other. They could not differentiate famous face from unknown ones, and did not feel any familiarity. The third patient has a normal feeling of knowing, could distinguish between familiar and unfamiliar faces, but was unable to evoke any biographical information about the personalities. Prosopagnosic patients demonstrated, in an experimental condition of learning face-name pairs, implicit knowledge. We assume that these capacities were dependent of the activation of networks coding familiar faces in memory. Mental imagery of faces were normal in theses two cases. In addition, stimulation of mental imagery in the first patient improved implicit knowledge in forced choice tasks. These cases throws a light on the respective role of each hemisphere in face recognition. The right hemisphere is advantaged in perceptual analysis, and activates, from the perceived faces, mnestic systems which codes for previously encountered faces. It generates feeling of familiarity, probably by the way of specific systems which differs from, and completes, those allowing identification. The left hemisphere enable access to semantic-biographic knowledge in a conscious, verbal and explicit way.