Vestibuloocular reflex and its interactions with visual following mechanisms in the squirrel monkey. I. Response characteristics in normal animals.

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Abstract
The horizontal vestibuloocular reflex (VOR), horizontal optokinetic (OK) following, and their interactions were studied in 8 squirrel monkeys. Sinusoidal rotations of the animal and/or an OK drum were presented in the frequency range 0.01-4.0 Hz at constant peak velocity. Slow-phase eye movements were analyzed in terms of their gains and phases relative to rotation of the head or drum. VOR gain is relatively flat, averaging 0.86 .+-. 0.03 over the entire frequency bandwidth. Phase lead is near 0.degree. from 4.0 to 0.1 Hz, rising at lower frequencies to reach 40.3.degree. at 0.01 Hz. The latter phase is consistent with an effective first-order time constant of 19 s. The dynamic range of the VOR was assessed by varying peak head velocity over the range 40-360.degree./s at 0.2 and at 0.02 Hz. The VOR behaves linearly at 0.2 Hz, gain and phase remain nearly constant over a 9-fold increase in head velocity. In contrast, amplitude-dependent nonlinearities are apparent at 0.02 Hz. A vergence component is seen in the VOR during high-intensity sinusoidal rotations and may help to maintain visual image stabilization during binocular target fixation. The VOR was also studied in response to step changes in angular acceleration. Estimated gain and time constant are close to those determined from sinusoidal responses. Adaptation is observed in step responses as a peracceleratory response decline and a postacceleratory secondary response. In sinusoidal responses, adaptation is manifested as a steeper rise in phase lead below 0.1 Hz than 1st-order dynamics would predict. Comparison of the VOR with known characteristics of peripheral canal afferents indicates that the effective canal afferents indicates that the effective bandwidth of the VOR is extended to lower frequency than that of its afferent input; the VOR does not reflect the phase lead and gain enhancement seen in afferent responses at frequencies > 1 Hz, and the adaptation phenomenon observed in the VOR resembles a similar process described in afferent responses. OK responses to sinusoidal and constant-velocity drum rotations were assessed. Results are consistent with the notion that OK following reflects a combination of a relatively fast foveal pursuit system and a slower visual following system, presumably involving the peripheral visual field. OK afternystagmus (OKAN), observed on extinguishing the light after 30 s of constant-velocity OK drum rotation, decays with a time constant averaging 19 s. Visual-vestibular interactions were assessed during rotation of the animal in the presence of a stationary OK drum (visual enhancement) and during rotation of the animal and the OK drum in tandem (visual suppression). The eye movements obtained during either enhancement or suppression are consistent with a linear summation of VOR and OK responses. A model, modified from Robinson (1977), simulates the responses to separate and combined vestibular and OK stimulation.