The vestibulo‐ocular reflex during human saccadic eye movements.

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Abstract
Eye and head movements were recorded in normal humans during rapid refixations with the head still (saccades) or moving (gaze saccades) to determine if the vestibulo-ocular reflex was operating at such times. Subjects made self-paced refixations between fixed targets. Eye movements were recorded with the electrooculogram for large saccades and with the eyecoil/magnetic field method for smaller movements. The putative function of the vestibulo-ocular reflex during a gaze saccade is to adjust the movement of the eye for the movement of the head by adding the saccadic command and the vestibular signal. This action, referred to here as linear summation, would maintain gaze-saccade accuracy by making gaze velocity (eye in space) independent of head velocity. It would also preserve the duration of the eye movement. The present experiments show that these predictions fail badly for gaze saccades of about 200 deg. When a subject increased his head velocity voluntarily, for example, from 420 to 805 deg/s, mean gaze velocity rose from 540 to 820 deg/s and duration dropped from 380 to 250 ms. Linear summation did not occur. Evidence for a vestibulo-ocular reflex was not found during saccades or gaze saccades of 40 deg or larger. Accuracy of saccadic gaze changes was preserved by some mechanism that adjusted duration; an action beyound the purview of the vestibulo-ocular reflex. Such a mechanism is proposed. It suggests that an internal copy of eye and head position combine to estimate gaze position. The latter is then compared to an internal signal of desired gaze position. The difference, or error, drives burst neurones to produce the saccade. When the error becomes zero, the eye is on target and the saccade stops. Thus, duration is automatically adjusted. This model is compatible with the behavior observed in these experiments.