Voluntary modulation of the vestibuloocular reflex in humans and its relation to smooth pursuit

Human subjects attempted to modify their vestibuloocular reflex (VOR) in the dark by fixating imagined targets while experiencing predictable (SIN) sinusoidal (0.01-2.5 Hz) and unpredictable (SSN) sum of sines rotational stimuli (0.02-1.9 Hz). Modification was attempted under 2 instructional sets: VOR enhancement, ie tracking an imaginary earth-fixed target; VOR suppression, ie fixation of a chair fixed target. When compared to gain characteristics exhibited during the relax state with the same stimuli, subjects were able to alter VOR gain under both experimental conditions, raising it during the enhance paradigm and lowering it during the suppress paradigm. While ability to suppress the VOR was dependent on stimulus frequency, decreasing as frequency of rotation increased, subjects were equally able to modify their responses to the unpredictable and the predictable stimuli. Response phase did not change and was maintained close to 180 deg, regardless of instructional set, predictability, or frequency of stimulation for frequencies greater than 0.1 Hz. At frequencies below 0.1 Hz, a phase lead developed that was similar for all paradigms and rotational stimuli. In contrast, when subjects attempted to pursue visual targets that matched closely the velocities and frequencies of the chair rotation during predictable (SIN) and unpredictable stimulation (SSN), success was dependent on predictability of the stimulus. SSN target motion caused a significant decrease in pursuit velocity as compared to results using SIN target motion. Phase characteristics for both types of stimuli were similar, demonstrating a slight lead at lower frequencies and lagging as frequency of target oscillation increased. The results suggest that voluntary modulation of the VOR is not mediated by a neural control mechanism that is based on prediction. In addition, pursuit does not appear to contribute significantly to ability to cancel VOR. Instead, VOR modulation may be a cognitive event that involves use of a mechanism that produces simple parametric gain changes.