Impaired recognition of faces following right and left hemispheric lesions: covert recognition, feeling of knowing, and respective function of each hemisphere

We report three observations of patients who suffered from impaired face re cognition following cerebral lesions. Two had classical prosopagnosia, resu lting 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 kno wing, could distinguish between familiar and unfamiliar faces, but was unab le to evoke any biographical information about the personalities. Prosopagn osic patients demonstrated in an experimental condition of learning face-na me pairs, implicit knowledge. We assume that these capacities, were dependa nt of the activation of networks coding familiar faces in memory. Mental im agery of faces were normal in these two cases. In addition, stimulation of mental imagery in the first patient improved implicit knowledge in forced c hoice tasks. These cases throws a light on the respective role of each hemi sphere in face recognition. The right hemisphere is advantaged in perceptua l 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 seman tic-biographic knowledge in a conscious, verbal and explicit way.