Experimental study and modeling of vestibulo-ocular reflex modulation during large shifts of gaze in humans.

An experimental study of head-free and head-fixed gaze shifts explores the role of the vestibulo-ocular reflex (VOR) during saccadic and slow phase components of the gaze shifts. A systematic comparison of head-free and head-fixed gaze shifts in humans revealed that while the VOR is switched off as soon as the saccade starts, its function is progressively restored during the terminal phase of the saccade. The duration of this restoration period is fairly constant; therefore, the faster the gaze saccade, the sooner the VOR function starts to be restored. On the basis of these experimental data, a new eye-head coordination model is proposed. This model is an extension of the one proposed by Laurutis and Robinson (1986) where VOR gain is a function of both the dynamic gaze error signal and head velocity. This extension has also been added to another eye-head coordination model (Guitton et al. 1990). Both modified models yield simulation results comparable to experimental data. This study pinpoints the high efficiency of the gaze control system. Indeed, a fixed period of time (approximately 40 ms) is needed to restore the inhibited VOR; the gaze control system thus must have a knowledge of its own dynamics in order to be able to anticipate the end of the saccadic movement.