DOES MONOCULAR VIEWING IMPROVE TARGET DETECTION IN HEMISPATIAL NEGLECT

Experimental animals with large posterior-cortical lesions develop dis turbances of visual perception and visually-guided behavior in the con tralateral space, resembling the syndrome of unilateral spatial neglec t (USN) in humans. The visuo-motor performance in the ignored space is recovered some time following additional lesion inflicted to the cont ralesional superior colliculus or section of the intercollicular pathw ays. Based on the model that explains this recovery by disinhibition o f the ipsilesional superior colliculus (the 'Sprague effect') Posner a nd Rafal [37] proposed that ipsilesional monocular occlusion could pos sibly reduce the 'tone' of contralesional collicular neurons thereby r emoving an important contributor of hemineglect. A group of twenty-six right-hemisphere-damaged patients with USN was given a cancellation t ask under binocular and monocular (left and right) viewing conditions. Thirteen patients showed amelioration of left hemineglect in conditio ns of left-monocular viewing, as compared to the baseline binocular st ate. Although this result superficially resembles the Sprague effect, two patients benefited from right-monocular viewing whereas eleven pat ients showed no significant change in either right or left viewing con ditions. The possible role of structures mediating interocular differe nces in target detection in cases of right hemisphere damage is discus sed. It is suggested that even if the Sprague effect contributes to th e salubrious influence of monocular viewing it is hardly the single fa ctor involved.