Selective horizontal dysmetropsia following prestriate lesion

We describe a patient (P.S.) who, following a right prestriate lesion, repo rted that objects in the left visual field appeared distorted and smaller t han those on the right. Other aspects of visual processing were remarkably unaffected, We carried out a series of size comparison tests using simple o r complex stimuli and requiring different types of behavioural responses. W e found that P.S. significantly underestimated the size of stimuli presente d in her left visual field. When comparison tasks involved stimuli placed a long the vertical axis or in the right visual field, P.S. performed well. T he vertical and horizontal components of size distortion were found to be d ifferentially affected. We conclude that size processing may be dissociated from other aspects of visual processing, such as form or colour processing , and depends critically on part of the occipital, prestriate areas (Brodma nn areas 18-19).