Characterization of subjects with congenital prosopagnosia by combined electrophysiological and behavioural data

Abstract

Recently a form of prosopagnosia has attracted attention that is not accompanied by any discernible brain lesion. Because subjects complain a lifelong impairment and because a familial clustering has been reported this condition has been termed congenital prosopagnosia (cPA) although sensu stricto “congenital” requires the molecular establishment of a genetic basis. In the absence of such determination of this neuropsychological condition as an entity on its own it is necessary to aim at a delineation by neuroimaging and behavioural data. In 14 subjects with cPA and 19 controls (screened by a specially developed questionnaire) evoked responses were measured by simultaneous EEG and MEG in a viewing task with a sequence of faces and houses. A double dissociation between methodology and category was revealed: only MEG, but not EEG, showed an M170 (a component that has been linked to structural encoding of faces and to intercategorical discrimination) that was significantly delayed over the left hemisphere and reduced over the right hemisphere in the cPA group. The M170 for houses was not altered, suggesting that the deficit is restricted to a face processing system. Additional analysis with accuracy data from three basic tests (face-familiarity, face recognition, face imagery) revealed a significantly negative correlation between hit rate and latency of M170 over the left hemisphere for subjects with cPA only. Peak amplitude over the right hemisphere was not correlated with accuracy, one possible reason being simply that amplitude measures (especially in MEG) are much more susceptible to error than measures of latency.