Adaptation of saccadic eye movements: neurological evidence for different mechanisms controlling the amplitude of reactive and voluntary saccades

Abstract

How the central nervous system generates accurate motor behaviour across the life span is a fundamental issue in Neuroscience. Sensory-motor adaptation processes play a central role, and yet their underlying neural substrates and task-specificity bases are still poorly understood. We addressed these issues here by studying adaptation of saccadic eye movements, a well-established model of sensorimotor plasticity. The cerebellum plays a major role in saccadic adaptation but it has not yet been investigated whether this role can account for the known specificity of adaptation to the saccade type (e.g. reactive versus voluntary). In this study, we used the double-step target paradigm to measure the adaptive abilities in 9 neurological patients at a chronic stage after a stroke affecting different cerebellar and/or pre-cerebellar areas. Each patient was tested during two separate sessions, for reactive saccades (RS) triggered by the sudden appearance of a visual target, and scanning voluntary saccades (VS) performed when exploring a more complex scene. The results revealed that 6 out of 9 patients showed a deficit in saccadic adaptation: 2 patients for RS, 3 for VS and one patient for both RS and VS. Thus, a dissociation of adaptive abilities between reactive and voluntary saccades was seen in 5 subjects, suggesting the involvement of separate cerebellar modules for the two saccade types. The deficit mostly concerned ipsilesional saccades rather than controlesional saccades. These findings extend for the first time the demonstrated role of the cerebellum in RS adaptation to adaptation of VS and hence suggest a cerebellar involvement in task-switching processes for oculomotor plasticity. They are consistent with our recent proposal that the adaptive update of saccadic commands involves both a brainstem site common for RS and VS and partially separated areas in the cerebellum specific to the reactive or voluntary task (Alahyane et al., Brain Res., 2007).

Footnotes

 This work was supported by ANR grant to DP, a FRM fellowship to NA, and was promoted by the Hospices Civils de Lyon, project n° 2002.303.