Look away: the anti-saccade task and the voluntary control of eye movement

Key Points The anti-saccade task, in which subjects are required to make a saccadic eye movement away from a target, rather than towards it, is a useful task for investigating the voluntary and flexible control of movement. Anti-saccades have a longer latency than pro-saccades and subjects are more likely to make errors on anti-saccade trials. These errors usually consist of a rapid saccade to the target, which is often corrected within a short latency by a second saccade away from the target. Two processes are needed for the anti-saccade task: suppression of the automatic pro-saccade, and inversion of the stimulus vector into the correct saccade vector. Monkeys and humans perform similarly on this task. In electrophysiological studies of monkeys doing the anti-saccade task, saccade neurons in the superior colliculus (SC) and frontal eye fields (FEF) seem to be inhibited before the target appears, to suppress the automatic pro-saccade. This is proposed to prevent activity in these neurons from crossing a threshold that would allow a saccade to be initiated. The suppression could arise from several sources, including other neurons in the FEF and SC, the supplementary eye fields, the dorsolateral prefrontal cortex (DLPFC) and the substantia nigra pars reticulata. The generation of the anti-saccade requires vector inversion and a build-up of activity in a different set of saccade neurons. Monkey electrophysiology studies indicate that vector inversion involves the lateral intraparietal area and/or the FEF. In humans, functional imaging and event-related potential (ERP) studies have been used to study the anti-saccade task. These studies have identified differences in brain activity between pro-saccades and anti-saccades that are consistent with data from electrophysiological studies in monkeys. Young children struggle to perform the anti-saccade task, apparently because they have difficulty in suppressing the automatic pro-saccade. This might relate to the protracted maturation of the frontal lobes, which have been proposed to mediate top-down inhibition of saccade neurons. Patients with lesions of the DLPFC have a similar deficit in performance. By contrast, lesions of the FEF impair the ability of patients to generate the anti-saccade. A number of clinical conditions affect performance on the anti-saccade task. Patients with schizophrenia show increased error rates and prolonged reaction times. Patients with attention-deficit hyperactivity disorder struggle