| The vestibulo-ocular reflex (VOR) guarantees a clear, stable percept of the visual world during locomotion. Other mechanisms, such as visually mediated eye movements, also contribute to the generation of eye movements that compensate for head rotation under a range of viewing conditions. The current research attempted to determine the relative contributions of vestibular and non-vestibular mechanisms, such as smooth pursuit, to the overall response to head rotation (referred to as the enhanced VOR, or EVOR) during viewing of a near target that was either earth-fixed (EFT), head-fixed (HFT), or consisted of the subject's own face viewed through a close mirror. Ten normal human subjects and two individuals with impairment of vestibular responses were tested during sinusoidal en-bloc head rotations in the frequency range 0.2-2.8 Hz in yaw. By applying head perturbations (>1,000 deg/s/s) during sinusoidal head rotations, it was shown that the early VOR, measured within 70 msec after each onset of head perturbation, was no different, regardless of whether viewing an EFT or HFT, and contributed only ∼75% to the overall eye response during EFT. Thus, non-vestibular factors made a substantial contribution and comprised retinal image slip (as demonstrated in experiments using strobe illumination), predictive mechanisms (evident from pseudorandom head rotations), and visual cues of target proximity (indicated by results during mirror viewing), but not vergence angle (which was uncorrelated with responses during monocular viewing). No one mechanism completely accounted for the non-vestibular component which exceeded, for example, the contribution due to smooth-pursuit eye movements, as shown by modeling EVOR responses. Comparison with prior 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. |
