DISSOCIATION OF LOCAL AND GLOBAL PROCESSING IN VISUAL AGNOSIA

Subsequent to strokes in the right acid left inferomedial occipito-tem poral lobes, two patients became prosopagnosic and alexic, respectivel y. They also show a complementary dissociation of the analysis of hand written text. The patient with the right posterior stroke can read it but not recognize whose handwriting it is; the patient with the left p osterior stroke cannot read the text but knows who wrote it. The analy sis of spatial vision revealed that the prosopagnosic patient has no p roblem with seeing texture elements when presented in isolation. Yet s he performs poorly with Moire and texture perception, i.e. she suffers from a selective loss of global visual perception. The alexic patient performs well with Moire patterns but neither with (complex) texture elements nor with textures. She seemingly can locally and globally pro cess patterns composed of simple figural elements but fails with stimu li that require the integration of features. This finding of a concomi tant dissociation of local and global visual processes in the two pati ents supports the view that prosopagnosia as well as alexia are the mo st conspicuous aspects of more general alterations of visual perceptio n.