Saccadic eye movement programming in unilateral neglect

Abstract

The present study examined the eye movements made by patients with unilateral neglect under fixation gap and overlap conditions. The prior offset of fixation in a + 100 msec gap condition did not produce an increase in the numbers of contralesional saccades made by 3 out of 4 patients. This finding is incompatible with the view that the deficit in producing contralesional saccades reflects an inability to disengage visual attention from fixation. A significant reduction in saccade latency was, however, obtained in the gap condition (‘gap effect’). The latency reduction in the gap condition is consistent with models which attribute the gap effect to warning signal effects and the release of an ocular rather than an attentional disengagement mechanism. Saccade latency was not increased when two targets were presented bilaterally and simultaneously in both hemifields (in contrast to the increase in latency shown by normal subjects). The lack of a normal ‘bilateral target effect’ in neglect is attributed to an imbalance in the level of activity in the saccadic system. Three patients showed visual ‘extinction’ and did not make saccades to the contralesional bilateral targets. In contrast R.R. who shows object-based neglect did not show extinction and made saccades to the contralesional bilateral targets. This suggests that visual extinction may be influenced by the form of neglect shown by the patient. The effects on saccade amplitude of presenting two targets in the same hemifield were also examined in a global effect task. One patient showed a much greater global effect than normal when pairs of targets were presented in his ipsilesional hemifield. In contrast R.R. showed a normal magnitude global effect. It appears that the form of neglect shown by a patient is a factor that influences their eye movement behaviour on simple saccade tasks and these eye movement abnormalities cannot be accounted for by a deficit of attentional disengagement.