Stimulus-Driven and Voluntary Saccades Are Coded in Different Coordinate Systems

We make fast, "saccadic" eye movements to view our surroundings, "voluntary" saccades when saccade targets are deliberately selected, and "stimulus-driven" saccades when a target suddenly appears. Saccades of patients with spatial neglect have been studied to identify the coordinate systems guiding such behavior. However, previous reports disagree on whether neglect involves an eye-centered deficit of (delayed and hypometric) saccades specifically when performed in the direction opposite the brain lesion or not. We show that this inconsistency is due to independent mechanisms underlying voluntary and stimulus-driven saccades. We used a new experimental procedure comparing identical saccades performed either during an exploratory search task or a stimulus-driven task, both of which required similar cognitive functions (Figure 1). Only the patients' stimulus-driven saccades showed the eye-centered deficit. The same saccades were intact when voluntarily performed. However, here the patients showed a head-centered deficit; their saccades ignored the left part of space. In none of our control subjects with or without brain lesions did the neglect patients' pattern of deficits occur. The results argue that the brain flexibly uses a system of distinct but interrelated neural circuits for visual orienting to optimally encode its sensorimotor functions in multiple behavioral situations.