Vergence-mediated modulation of the human horizontal angular VOR provides evidence of pathway-specific changes in VOR dynamics

The horizontal vestibulo-ocular reflex (VOR) evoked by passive, high-acceleration, head-on-body rotations (head thrusts) while viewing a far (124-cm) or near (15-cm) target was recorded (scleral search coil) In four subjects with normal vestibular function and in one subject with unilateral vestibular hypofunction. For responses in the subjects with normal vestibular function, the latency of responses relative to the onset of head movement was 7.5 +/- 1.5 ms for the VOR and 21.6 +/- 1.2 ms for the vergence-mediated increase in VOR gain. The gain of the VOR at the peak of the velocity response while viewing a far target was 1.01 +/- 0.06; while viewing a near target, it was 1.25 +/- 0.08 (p <0.003). The responses were modeled with two pathways based on the different latencies. The "far-viewing" pathway was represented by a constant gain term. The "near-viewing" pathway was represented by a first-order lead term, a gain that was dependent on viewing distance, and a delay. Analysis of the responses revealed that the lead term was greater for the adducting than the abducting eye. In the subject with unilateral vestibular hypofunction, ipsilesional responses showed no change in VOR gain with respect to viewing distance. Contralesional responses retained the vergence-dependent increase in gain. A bilateral model was developed based on the data from the subjects with normal vestibular function. Simulations of this model when inputs were eliminated from one side predict the changes observed in the subject with unilateral vestibular hypofunction. The response asymmetries arise because the near-viewing pathway is more susceptible to inhibitory cutoff than is the far-viewing pathway.