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Semantic dementia is a focal clinical syndrome, resulting from degeneration of the temporal lobes and characterized by progressive loss of conceptual knowledge about the world. Because of the highly circumscribed nature of the disorder it is a natural model for improving understanding of how semantic information is cerebrally represented. There is currently a lack of consensus. One view proposes the existence of modality specific meaning systems, in which visual and verbal information are stored separately. An opposing view assumes that information is represented by a unitary, amodal semantic system. The present study explores these alternatives in an examination of famous face and name knowledge in 15 patients with semantic dementia. The study of face recognition in patients with an established semantic disorder also permits an examination of the relationship between semantic dementia and the focal clinical syndrome of progressive prosopagnosia. The semantic dementia patients were profoundly impaired on both face and name identification and familiarity judgement tasks compared with amnesic patients with Alzheimer's disease and healthy controls. However, whereas the two reference groups performed better for names than faces, the semantic group showed the opposite pattern. This overall profile masked individual differences: semantic dementia patients with predominant left temporal lobe atrophy showed better recognition of names than faces, whereas patients with right temporal predominance showed the reverse pattern. Relative superiority for names or faces was mirrored by corresponding superiority for words or pictures on a standard semantic test. We interpret the findings as inconsistent with a unitary, amodal model of semantic memory. However, the data are not wholly compatible with a strict multiple system account. The data favour a model of semantic memory comprising a single interconnected network, with dedicated brain regions representing modality specific information. The data emphasize the importance of the anterior, inferolateral parts of the left temporal lobe for the representation of names and the corresponding parts of the right temporal lobe for faces. Dissociations between face and name knowledge provide a challenge for existing models of face processing. Moreover, they lead us to argue that the focal syndrome of progressive prosopagnosia is one of the clinical presentations of semantic dementia and not a separate clinical entity.

We report the case of a patient (C.O.) who showed a selective defect in the recognition of familiar people, with very mild disease progression during a period of 30 months resulting from focal atrophy of the right temporal lobe. On formal neuropsychological testing, C.O. obtained high scores on tests of general intelligence, episodic memory, language, executive functions, selective attention, visual recognition and visual-spatial abilities. On more specific tasks of familiar and unfamiliar face recognition, C.O. scored above the controls' means on perceptual tests, but obtained highly pathological results on amnesic-associative tests. His disorder of recognition of familiar people was not due to loss of person-specific information, since he obtained highly abnormal naming scores when presented with photographs of famous people but borderline or mildly abnormal scores in a tasks in which he had to name celebrities from verbal definitions. On the other hand, C.O.'s recognition disorder could not be considered to be a form of 'associative prosopagnosia' since a similar defect was observed when he was requested to access information about famous persons through their voice rather than their face. Two alternative interpretations are advanced to explain C.O.'s inability to access his relatively spared person-specific knowledge not only through the person's face but also through the person's voice. The first hypothesis is that, before accessing the person-specific information, unimodal recognition channels must converge into a multimodal, non-verbal person-recognition system and that the right anterior temporal cortices play a crucial role in this integrative activity. The second hypothesis is that the face-recognition units have privileged access to person-specific semantic knowledge and that other recognition subsystems require coactivation of the face-recognition units in order to access person-specific semantic information.

Right hemisphere dominance in face processing is well established and unilateral right inferior temporo-occipital damage can result in prosopagnosia. Here, we describe a 21-year-old right-handed woman with acute impairment in face recognition that selectively concerned unfamiliar faces, following a focal left lateral temporo-occipital venous infarct. She was severely impaired in discerning that unknown people seen in everyday life were unfamiliar, although she had no difficulty recognizing familiar people. Thus, she had no prosopagnosia, but abnormal 'hyperfamiliarity' for unknown faces. Her difficulty was not accompanied by delusions or deficits in discrimination, identification or memory for faces. Standard neuropsychological testing showed that her recognition of familiar faces was entirely normal. By contrast, her sense of personally knowing faces was severely impaired when unknown faces evoked weak signals of familiarity based on spurious cues, to the extent that she would misattribute fame to faces that were unknown but to which she had been incidentally exposed on a prior occasion. Priming experiments also revealed that, unlike normal subjects, she made familiarity judgements without accessing semantic identity representations. Moreover, in face recognition tests, she generally showed bias in that she relied more on right-hemisphere strategies to identify global traits and less on left-hemisphere processes compared with healthy subjects. This case provides novel evidence for a differential contribution of the two hemispheres to face recognition. Hyperfamiliarity for unknown faces might arise from an imbalance between reciprocal hemispheric functions in face recognition, with relative hypoactivation of left hemisphere processes but hyperactivation of right-hemisphere processes for retrieving stored associations about people, linking seen faces to representations of affective and personal relevance. Hence, abnormal bias in attributing some personal meaning to unknown faces could be evoked by spurious signals of familiarity based on irrelevant affective associations in the right hemisphere.