PREVIEW CONTROL OF GAZE SACCADES - EFFICACY OF PREDICTION MODULATES EYE-HEAD INTERACTION DURING HUMAN GAZE SACCADES

Healthy human subjects made orienting saccades towards visual target s timuli, either with the head fixed or during intended time optimal hea d movements. Four experimental paradigms were used to study the influe nce of target predictability on eye-head coordination. They represente d different sequences of horizontal target steps, that were varied in amplitude, direction and frequency. In some subjects midflight perturb ations of the active head movements were applied to examine the intras accadic vestibulo-ocular reflex (VOR). In coordinated gaze saccades, l atencies and dynamics of the eye saccade and the additional head traje ctory demonstrated specific task-related changes with respect to the h ead fixed condition. Highly predictable target steps result in the rel atively earlier onset of the head movement and an increase of the intr asaccadic head contribution to the overall gaze displacement. Differen ces in the level of VOR suppression became significant when gaze ampli tudes exceeded 60-degrees. Consequently, an effective speed up of larg e gaze saccades was found with increased target predictability. We con cluded, that eye-head coordination during human gaze saccades underlie s high level preview control mechanisms. A parametric modulation of th e intrasaccadic VOR maintains gaze accuracy, although the actual contr ibution of the more flexible head motor system varied, depending on ga ze amplitude and prediction. The efficacy of preview control depends o n interaction of these factors.