Vestibular catch-up saccades in labyrinthine deficiency

During rapid head rotations, saccades ipsiversive with compensatory vestibu le-ocular reflex (VOR) slow phases may augment the deficient VOR and assist gaze stabilization in space. The present experiments compared these vestib ular catch-up saccades (VCUSs) with visually and memory-guided saccades. To characterize VCUSs and their relationship to deficiency of the initial VOR , we delivered random, whole-body transients of 1000 and 2800 degrees/s(2) peak yaw acceleration around four different eccentric vertical axes in eigh t unilaterally and one bilaterally vestibulopathic subjects, as well as nin e age-matched normal subjects. Eye and head movements were sampled at 1200 Hz using magnetic search coils. Subjects fixed targets at either 500 or 15 cm distance immediately before unpredictable onset of rotation in darkness. Under all testing conditions, normal subjects exhibited only compensatory vestibular slow phases and occasional anticompensatory quick phases. This b ehavior was also typical of unilaterally vestibulopathic subjects rotated c ontralesionally. When rotated ipsilesionally, however, vestibulopathic subj ects had deficient slow-phase VOR gain with prolonged latency, and six of t he nine exhibited saccadic movements in the compensatory direction (VCUSs). Higher head accelerations preferentially evoked VCUSs, but there were no p referred combinations of target distances and eccentric rotation axes. Peak velocities and durations of VCUSs increased with saccade amplitude. The la tency distribution for VCUSs peaked around 70 ms, substantially shorter tha n reported for either visually guided express saccades or vestibular memory contingent saccades. The latency of each VCUS was highly correlated with t he gaze error prior to that VCUS. The amplitude of VCUSs was calibrated to gaze position error, such that VCUSs reduced gaze error by an average of 37 %. Thus when VOR slow-phase responses cannot compensate fully for head rota tion, vestibular gaze position error can nevertheless calibrate the program ming of VCUSs to augment the deficient VOR, much like catch-up saccades sub stitute for deficient visual pursuit.