The human horizontal vestibulo-ocular reflex in response to active and passive head impulses after unilateral vestibular deafferentation.

We studied the compensatory eye movements made by subjects with unilateral vestibular deficits in response to passive (unpredictable, manually generated) and active (predictable, self-generated) head impulses. A typical head impulse is a brief, low-amplitude (15-20 degrees ), high-velocity (150-350 degrees /s), high-acceleration (4000-6000 degrees /s(2)), yaw head-on-trunk rotation. In the initial 75 ms of the response, the vestibulo-ocular reflex gain was significantly higher during active head impulses to both ipsilesional and contralesional sides, than during passive impulses. Mean gains were 0.15 (ipsilesional passive), 0.44 (ipsilesional active), 0.5 (contralesional passive), and 0.76 (contralesional active). Differences between active and passive head impulses were present from near the onset of head rotation. The mechanism for producing this behavior is unclear, but the findings could be related to enhanced sensitivity of second-order neurons during active head impulses. However, even with active movements, there is still a large and statistically significant asymmetry in the eye-movement responses for ipsilesional as opposed to contralesional head rotations. After 75 ms, rapid corrective eye movements often were generated to reduce any remaining gaze error.