Smooth pursuit and saccades to moving targets in blind hemifields : A comparison of medial occipital, lateral occipital and optic radiation lesions

We tested smooth pursuit and saccades to targets within the contralateral homonymous visual field defects of 10 patients with unilateral cerebral hemispheric lesions. Four patients had medial occipital lesions that spared the putative motion area in lateral occipitotemporal cortex and the optic radiations proximal to this site. The other six had lesions involving this area or the proximal optic radiations. From current hypotheses, that surviving extra-striate cortex is responsible for 'blindsight', we expected that the patients with lesions restricted to medial occipital cortex would be more likely to have residual ocular motor responses to moving targets. We found, however, that these patients with sparing of the lateral motion area did not show significantly better correlations of either post-saccadic eye velocity with target velocity or initial saccadic amplitude with the position of the moving target, compared with the other six patients. With increased target velocities against a background of darkness, two patients had weakly positive ocular motor correlations. However, in one patient this finding was replicated by a normal control stimulating the patient's scotoma and, in the other patient, the correlation disappeared when the eye with a residual monocular temporal crescents was occluded. These features indicate an artifact from light scatter. Only one other patient demonstrated a consistent, but weak, correlation of ocular tracking with target motion in the blind region; paradoxically, this patient had a lateral occipitotemporal lesion. We conclude that sparing of the lateral motion area is not sufficient condition for residual tracking of moving targets in homonymous visual field defects, and may not even be necessary.