Does monocular viewing improve target detection in hemispatial neglect?

Experimental animals with large posterior-cortical lesions develop disturbances of visual perception and visually-guided behavior in the contralateral space, resembling the syndrome of unilateral spatial neglect (USN) in humans. The visuo-motor performance in the ignored space is recovered some time following additional lesion inflicted to the contralesional superior colliculus (SC) or section of the intercollicular pathways. Based on the model that explains this recovery by disinhibition of the ipsilesional SC (the 'Sprague effect') Posner and Rafal [37] proposed that ipsilesional monocular occlusion could possibly reduce the 'tone' of contralesional collicular neurons thereby removing an important contributor of hemineglect. A group of twenty-six right-hemisphere-damaged patients with USN was given a cancellation task under binocular and monocular (left and right) viewing conditions. Thirteen patients showed amelioration of left hemineglect in conditions of left-monocular viewing, as compared to the baseline binocular state. Although this result superficially resembles the Sprague effect, two patients benefited from right-monocular viewing whereas eleven patients showed no significant change in either right or left viewing conditions. The possible role of structures mediating interocular differences in target detection in cases of right hemisphere damage is discussed. It is suggested that even if the Sprague effect contributes to the salubrious influence of monocular viewing it is hardly the single factor involved.