Recognition of unfamiliar faces in prosopagnosia.

Prosopagnosia is clinically defined as a specific and extreme inability to recognize familiar faces. However, doubts have been expressed concerning prosopagnosics' preserved ability to recognize unfamiliar faces and to make other within category discriminations. The present study pursues these doubts. If recognition of unfamiliar faces and objects is intact, then prosopagnosics should demonstrate normal processing for all tasks that depend on the possession of intact stored visual descriptions at the category level. In particular, they should show normal face and object superiority effects. A detailed investigation was carried out on a well documented prosopagnosic (KD) and less extensively on three other (RB, AH and OA) well attested cases. Experiments 1 and 2 considered whether face and object superiority effects were observed in these patients. No difference in the pattern of recognition performance was found between normal and unusual arrangements of faces and objects. Their pattern of performance differed both from unilateral brain-damaged patients and normal controls. The results suggest both that these prosopagnosic patients were impaired on the recognition of unfamiliar faces and that their problem is not specific to faces. Experiment 3 showed that KD was impaired on a face/non-face decision task but appeared to benefit by the stimulus being presented in the normal orientation. For normal controls, mental rotation of a face appeared to be a separate process from face categorization. Experiment 4 found that KD, AH and OA were impaired, compared to normal controls, in their ability to recognize emotional expressions but not more than brain-damaged controls. The impairment of the prosopagnosics tested in the present study is placed at the interaction between stored object descriptions and the structural encoding stage of Bruce and Young. Consideration is given to an elaboration of the structural encoding stage in which boundary (category defining) information is separately processed from surface (detailed texture) information.