Vestibular and non-vestibular contributions to eye movements that compensate for head rotations during viewing of near targets.

Geometry dictates that when subjects view a near target during head rotation the eyes must rotate more than the head. The relative contribution to this compensatory response by adjustment of the vestibulo-ocular reflex gain (Gvor), visual tracking mechanisms including prediction, and convergence is debated. We studied horizontal eye movements induced by sinusoidal 0.2-2.8 Hz, en-bloc yaw rotation as ten normal humans viewed a near target that was either earth-fixed (EFT) or head-fixed (HFT). For EFT, group median gain was 1.49 at 0.2 Hz declining to 1.08 at 2.8 Hz. For HFT, group median gain was 0.03 at 0.2 Hz increasing to 0.71 at 2.8 Hz. By applying transient head perturbations (peak acceleration >1,000 degrees s(-2)) during sinusoidal rotation, we determined that Gvor was similar during either EFT or HFT conditions, and contributed only approximately 75% to the compensatory response. We confirmed that retinal image slip contributed to the compensatory response by demonstrating reduced gain during EFT viewing under strobe illumination. Gain also declined during sum-of-sines head rotations, confirming the contribution of predictive mechanisms. The gain of compensatory eye movements was similar during monocular or binocular viewing, although vergence angle was greater during binocular viewing. Comparison with previous studies indicates that mechanisms for generation of eye rotations during near viewing depend on head stimulus type (rotation or translation), waveform (transient or sinusoidal), and the species being tested.