The neural basis of vertical and horizontal line bisection judgments: An fMRI study of normal volunteers

Bisection of horizontal lines is used as a clinical test of spatial cogniti on in patients with left visuospatial neglect after right hemisphere lesion s. Bisection of vertical lines has also been employed, albeit less frequent ly. Interestingly, normal subjects often bisect horizontal lines too far le ft, and vertical lines too high. We used fMRI to investigate whether vertic al/horizontal stimulus orientation interacts with the neural mechanisms ass ociated with line bisection judgments (the Landmark task). For control of o rientation per se, subjects performed a visual detection task with the same stimuli. Statistical analysis of evoked BOLD responses employed SPM99, The Landmark task increased neural activity (P < 0.05, corrected) in the super ior and inferior parietal lobes bilaterally, though predominantly on the ri ght; early visual processing areas bilaterally; and cerebellar vermis, left cerebellar hemisphere, anterior cingulate, and prefrontal cortex bilateral ly. Vertical lines (relative to horizontal lines and vice versa) increased neural activity in early visual processing areas, consistent with different ial retinotopic stimulation. In addition, vertical lines activated right pa rietooccipital and superior posterior parietal cortex bilaterally. No signi ficant interactions between the neural mechanisms associated with task and stimuli were observed. Increased neural activation in parietal and parietoo ccipital cortex associated with vertical lines may reflect increased attent ional demands associated with this stimulus orientation. The right hemisphe ric dominance observed in posterior parietal during the Landmark task irres pective of stimulus orientation is consistent with lesion studies. Our resu lts suggest that the behavioral patterns observed in normal subjects and ne urological patients result from different stimulus effects rather than diff erential task demands. (C) 2001 Academic Press.