Prosopagnosia is a form of visual agnosia characterized by an inability to recognize previously known human faces or equivalent stimuli
(hence, a retrograde defect) and to learn new ones (anterograde defect). As with more pervasive visual agnosia (q.v.), this may be:
- Apperceptive: due to faulty perceptual analysis of faces; or
- Associative: a semantic defect in recognition.
Familiar individuals may be recognized by their voices or clothing or hair; hence, the defect may be one of visually triggered episodic memory. It is important to note that the defect is not limited solely to faces; it may encompass animals ("zooagnosia"), or cars.
Prosopagnosia is often found in association with a visual field defect, most often a left superior quadrantanopia or even hemianopia, although for the diagnosis of prosopagnosia to be made this should not be sufficient to produce a perceptual deficit. Alexia and achromatopsia may also be present, depending on the exact extent of the underlying lesion.
Anatomically, prosopagnosia occurs most often in association with bilateral occipito-temporal lesions involving the inferior and mesial visual association cortices in the lingual and fusiform gyri, sometimes with subjacent white matter. Unilateral nondominant (right) hemisphere lesions have occasionally been associated with prosopagnosia, and a syndrome of progressive prosopagnosia associated with selective focal atrophy of the right temporal lobe has been reported. Involvement of the periventricular region on the left side may explain accompanying alexia, and disconnection of the inferior visual association cortex (area V4) may explain achromatopsia.
Pathological causes of prosopagnosia include:
- Cerebrovascular disease: this is by far the most common cause
- Tumor, for example, glioma, extending from one hemisphere to the other via the splenium of the corpus callosum
- Epilepsy (paroxysmal prosopagnosia), due to bilateral foci or spread from one occipital focus to the contralateral hemisphere
- Right temporal lobe atrophy
- Herpes simplex encephalitis, usually as part of an extensive amnesic syndrome (although memory impairment may put this outwith the operational criteria for an agnosia)
- Rare cases of developmental (or "congenital") prosopagnosia have been described.
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Farah MJ. Visual agnosia: disorders of object recognition and what theytell us about normal vision. Cambridge: MIT Press, 1995
Nunn JA, Postma P, Pearson R. Developmental prosopagnosia: should it be taken at face value? Neurocase 2001; 7: 15-27