Visual search disorders in acute and chronic homonymous hemianopia: lesion effects and adaptive strategies

Abstract

Patients with homonymous hemianopia due to occipital brain lesions show disorders of visual search. In everyday life this leads to difficulties in reading and spatial orientation. It is a matter of debate whether these disorders are due to the brain lesion or rather reflect compensatory eye movement strategies developing over time. For the first time, eye movements of acute hemianopic patients (n= 9) were recorded during the first days following stroke while they performed an exploratory visual-search task. Compared to age-matched control subjects their search duration was prolonged due to increased fixations and refixations, that is, repeated scanning of previously searched locations. Saccadic amplitudes were smaller in patients. Right hemianopic patients were more impaired than left hemianopic patients. The number of fixations and refixations did not differ significantly between both hemifields in the patients. Follow-up of one patient revealed changes of visual search over 18 months. By using more structured scanpaths with fewer saccades his search duration decreased. Furthermore, he developed a more efficient eye-movement strategy by making larger but less frequent saccades toward his blind side. In summary, visual-search behavior of acute hemianopic patients differs from healthy control subjects and from chronic hemianopic patients. We conclude that abnormal visual search in acute hemianopic patients is related to the brain lesion. We provide some evidence for adaptive eye-movement strategies developed over time. These adaptive strategies make the visual search more efficient and may help to compensate for the persisting visual-field loss.