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* To whom correspondence should be addressed. E-mail: kathleen.cullen{at}mcgill.ca.
Although numerous investigations have probed the status of the VOR during gaze shifts, its exact status remains strangely elusive. The goal of the present study was to precisely evaluate the dynamics of VOR suppression immediately before, throughout, and just following gaze shifts. A torque motor was used to apply rapid (100 deg/s), short duration (20 - 30 ms) horizontal head perturbations in three Rhesus monkeys. The status of the VOR elicited by this transient head perturbation was first compared during 15, 40, and 60 deg gaze shifts. The level of VOR suppression just following gaze shift onset (40 ms) increased with gaze shift amplitude in two monkeys, approaching values of 80% and 35%. In contrast, in the third monkey, the VOR was not significantly attenuated for all gaze shift amplitudes. The time course of VOR attenuation was then studied in greater detail for all three monkeys by imposing the same short duration head perturbations either 40 ms, 100 ms, and 150 ms following the onset of 60 deg gaze shifts. Overall, we found a consistent trend, in which VOR suppression was maximal early in the gaze shift and progressively recovered to reach normal values near gaze shift end. However, the high variability across subjects prevented establishing a unifying description of the absolute level and time course of VOR suppression during gaze shifts. We propose that differences in behavioral strategies may account, at least in part, for these differences between subjects.
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