The effects of unilateral frontal eye field lesions in the monkey: visual-motor guidance and avoidance behaviour

Macaque monkeys were tested on a visual-motor guidance task and observations of avoidance behaviour were made following unilateral frontal eye field lesions. Visual-motor guidance was assessed by speed and accuracy of reaching to press recessed, illuminated buttons arrayed in a 90 degree arc in front of the monkey. Unilateral frontal eye field lesions produced a marked neglect of stimulus lights contralateral to the side of the lesions, as shown in greatly increased errors and response latencies. Responses to the most peripheral of the ipsilateral stimulus lights were also affected, although not to the same degree. Partial to complete recovery occurred, usually within a month. A unilateral lesion of the principal sulcus did not cause symptoms of neglect in the visual-motor guidance task. Section of the cerebral commissures partially restored the visual-motor guidance deficit in the frontal eye field-lesioned monkeys, with recovery again occurring. Commissurotomy and the unilateral frontal eye field lesion performed in a single-stage operation produced an initially more severe neglect than the two-stage operation, but recovery was no less rapid. The unilateral frontal eye field lesions resulted in a severe neglect of contralateral threat objects when the monkeys were threatened from both sides (avoidance-avoidance tests). Avoidance was normal when the threat was presented to one side. Commissurotomy fully restored the avoidance-avoidance deficit and made it permanent. These and other recent findings suggest that the contralateral visual field defect and the other symptoms resulting from frontal eye field lesions represent an impairment in directing and sustaining attention in regions of the visual field arising from disruption of a corticothalamotectal sensorimotor integration system.