Impaired identification of faces after a left hemisphere lesion.

A 82 year-old right-handed man, without any intellectual impairment, suffer ed from an acute neurological deficit consisting in letter-by-letter readin g, right superior quadrant hemianopia with achromatopia in the lower quadra nt, and anemia. Cerebral MRI showed an infarct involving the ventral struct ures of the left hemisphere sparing the splenium of the corpus callosum and the thalamus. Neuropsychological examination revealed that the patient eas ily identified the objects, the animals and the famous places he could not name: his comments attested normal visual recognition. Conversely, when he was presented with famous faces, he always had a strong feeling of familiar ity, but could not provide accurate information about the corresponding ind ividual. Biographic information about personnalities was not impaired in th e semantic-biographic store, because it could be accessed from the names. A ctivation of face recognition units (where the visual decription provided b y the structural encoding and the stored sets of descriptions of familiar f aces are compared), was effective, since the patient could distinguish famo us faces from unknown ones. In a modular-sequential model of face recogniti on, this deficit is interpreted as a disconnection between face recognition units and person identity nodes (which are considered to contain semantic- biographic information about individuals). This kind of disturbance differs from classic prosopagnosia in which, characteristically, the patients are unable to experience a feeling of familiarity when viewing famous faces, an d to perform a categorization between famous and unknown faces. Right hemis phere has a preponderant role in structural analysis of faces and in activa tion of face recognition units. The integrity of this hemisphere in this pa tient could explain the preservation of these two steps of processing. Left -hemisphere specific function in facial recognition enabled access to seman tic-biographic store in a conscious, verbal and explicit way, after the rig ht hemisphere had achieved basic visual analysis and activation of facial r epresentation in memory. We compare the cognitive impairment in our patient to theses encountered in classical prosopagnosic patients. This case illustrates the validity of th e modular-sequential model considered. In addition it throws a light on the poor-known role of the left hemisphere in face recognition.