'Where' depends on 'what': A differential functional anatomy for position discrimination in one- versus two-dimensions

Line bisection is widely used as a clinical test of spatial cognition in pa tients with left visuospatial neglect after right hemisphere lesion. Surpri singly, many neglect patients who show severe impairment on marking the cen ter of horizontal lines can accurately mark the center of squares. That the se patients with left neglect are also typically poor at judging whether li nes are correctly prebisected implies that the deficit can be perceptual ra ther than motoric. These findings suggest a differential neural basis for o ne- and two-dimensional Visual position discrimination that we investigated with functional neuroimaging (fMRI). Normal subjects judged whether, in pr emarked lines or squares, the mark was placed centrally. Line center judgem ents differentially activated right parietal cortex, while square center ju dgements differentially activated the lingual gyrus bilaterally. These dist inct neural bases for one- and two-dimensional visuospatial judgements help explain the observed clinical dissociations by showing that as a stimulus becomes a better, more 'object-like' gestalt, the ventral visuoperceptive r oute assumes more responsibility for assessing position within the object. (C) 2000 Elsevier Science Ltd. All rights reserved.