Implicit access to knowledge derived from unrecognized faces in prosopagnosia.

Prosopagnosia is an acquired neurological impairment characterized by an inability to experience a feeling of familiarity at the view of faces of known individuals and to identify these individuals. The inability of prosopagnosic patients to recognize faces does not entail that perceived faces go unprocessed in their brains, and there are indications that cognitive operations are still performed whereby a perceived face reactivates pertinent memories, but either these operations cannot be completed or their outcome fails to reach consciousness. A series of experiments were conducted on three severe prosopagnosic patients in an attempt to understand better this phenomenon known as covert face recognition, the conditions for its occurrence, and its functional locus. The capacity of the patients implicitly to access pertinent knowledge related to overtly unrecognized faces was inversely related to the severity of their perceptual deficit, suggesting that some preserved ability to extract the physiognomic invariants of a face is a necessary condition for the occurrence of the phenomenon. However, the results indicated that covert recognition does not take place at a perceptual or structural level although it is initiated at such a level, that it may have more than one underlying mechanism, and that it is achieved through the reactivation of specific information related to the individual's identity. Under special conditions that restricted the relevant knowledge that needed to be activated, transient overt recognition of faces was experienced by one of the prosopagnosic patients.