Fixation changes after frontal eye-field lesions in monkeys

Abstract

Rhesus monkeys were trained to look through a peep-hole and fixate a 2° mirror which showed a reflexion of their own eye. Their direction of gaze was determined to the nearest 3.5° using corneal photography, and the accuracy with which they fixated the mirror was plotted pre- and post-operatively. Unilateral frontal eye-field lesions produced a consistent, though usually transient, shift in fixation of up to 10° to the ipsilateral side. On recovery, removal of the second eye-field produced a larger (up to 20°) and longer lasting shift in the opposite direction. Both unilateral and one-stage bilateral lesions also produced a downward shift in fixation up to 25°, with the monkey's eyes tending toward the primary, straight-ahead position. These fixation changes persisted after there was no oculomotor change evident on gross observation, and in the worst case there was no recovery over 4 months of testing. Additional unilateral lesions of the superior colliculus, or a unilateral lesion of sulcus principalis, had no effect on fixation. These changes in fixation suggest that the frontal eye-fields normally exert at least a tonic influence on the eye muscle nuclei.