In response to passive high-acceleration head impulses patients with low vestibulo-ocular reflex (VOR) gains often produce covert (executed while the head is still moving) corrective saccades in the direction of deficient slow phases. Here we examined 23 patients using passive, and nine also active, head impulses with acute (< 10 days from onset) unilateral vestibular neuritis and low VOR gains. We found that when corrective saccades are larger than 10°, the slow-phase component of the VOR is inhibited, even though inhibition increases further the time to reacquire the fixation target. We also found that saccades are faster and more accurate if the residual VOR gain is higher, saccades also compensate for the head displacement that occurs during the saccade, and the amplitude-peak velocity relationship of the larger corrective saccades deviates from that of head-fixed saccades of the same size. We propose a mathematical model to account for these findings hypothesizing that covert saccades are driven by a desired gaze-position signal based on a prediction of head displacement using vestibular and extra-vestibular signals, covert saccades are controlled by a gaze feedback loop, and the VOR command is modulated according to predicted saccade amplitude. A central and novel feature of the model is that the brain develops two separate estimates of head rotation, one for generating saccades while the head is moving and the other for generating slow phases. Furthermore the model, while developed for gaze-stabilizing behavior during passively-induced head impulses, also simulates both active gaze-stabilizing and active gaze-shifting eye movements.
COLAGIORGIO, PAOLO;VERSINO, MAURIZIO;COLNAGHI, SILVIA;QUAGLIERI, SILVIA;MANFRIN, MARCO LUCIO;RAMAT, STEFANO
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