Face recognition and lipreading. A neurological dissociation.

Two cases with medial occipitotemporal ischaemic infarction in the territory of the posterior cerebral artery are discussed; neither was aphasic. The patient with the right-sided lesion is prosopagnosic and topographagnosic and is impaired at recognizing or classifying facial expressive gestures but can lipread speech efficiently. The other patient, with only a left-sided lesion, shows no deficits in face recognition nor in the classification of faces in terms of nonverbal messages, but is alexic and impaired at lipreading. It is argued that processing faces for verbal information (lipreading) and processing faces for nonverbal information (face recognition and interpretation of emotive and gestural messages) are functionally dissociated in the human brain. Theoretical interpretations of prosopagnosia that stress a perceptual component to all types of face recognition failure may therefore be misleading, for similar stimulus processing mechanisms are likely to be required to identify a speech sound from a face as to identify a nonverbal gesture of the mouth. Only the associative properties of the task differ.