Cortical processing of saccade-related efference copy signals in patients with cerebellar lesion

The updating of visual space across saccades is thought to rely on efference copies of motor commands. In humans, thalamic lesions impair performance on a saccadic double-step task, which requires the use of efference copy information, and the altering of saccade-related efference copy processing. This deficit is attributed to disruption of a pathway from the superior colliculus to the frontal eye field. However, the cerebellum is probably also involved in efference copy processing, due to its pivotal role for predictive motor control. The present study investigated the processing of efference copy information in eight patients with focal cerebellar lesions and 22 healthy controls by means of a saccadic double-step task with simultaneous event-related potential recording. Despite intact behavioural performance, a positive event-related potential component between 150 and 450 ms after first saccade onset in the updating condition, which has been interpreted in terms of the integration of efference copy signals with motor intentions for a subsequent saccade, was markedly reduced in the patients. These findings suggest that the cerebellum contributes to on-line saccade monitoring, and that cerebellar lesions alter saccade-related efference copy processing. However, given the intact behavioural performance, the reduced positivity in the patients may indicate that cerebellar damage is accounted for by either exploiting reduced saccade-related information, or making use of compensatory strategies to circumvent a deficit in using efference copy information procured by the cerebellum. The present study extends previous findings on the neural underpinnings of saccadic updating and further elucidates the mechanisms underlying cerebellar predictive motor control.