Mechanisms of human vertical visual-vestibular interaction.

1. The purpose of this study was to infer the properties of the mechanisms contributing to visual-vestibular interaction (VVI) of human beings during vertical motion. Predictable trains of single-frequency sinusoids; poorly predictable sums of sinusoidal harmonics; and unpredictable random impulses of passive, whole-body rotation about a horizontal, interaural axis were produced by the use of a servo-driven chair at frequencies from 0.4 to 3.2 Hz. The vestibuloocular reflex (VOR) was studied in darkness with the use of the magnetic search coil technique to record eye movements during head rotation. Telescopic spectacles of varying magnifications and visual field areas were used as a challenging stimulus to induce substantial gain enhancement by VVI. Real and imagined targets moving with the head were used to induce gain reduction. VVI was compared with vertical smooth pursuit and small field optokinetic nystagmus (OKN) for similar stimulus motion. 2. The vertical VOR and visually enhanced VOR (VVOR) were directionally symmetrical. Viewing with telescopic spectacles of powers from x1.9-4 was associated with significantly increased gain at frequencies up to 2.0 Hz as compared with the VOR (P < 0.01). Gain enhancement was not strongly influenced by stimulus velocity for either predictable or poorly predictable head motion, and there was a trend toward greater VVOR gain at higher head velocities. Phase was compensatory at all frequencies for predictable sinusoids. For poorly predictable and unpredictable head motion, gain enhancement with telescopic spectacles was significantly less than during predictable head motion. During poorly predictable head motion, phase lags were observed that increased with frequency and telescopic spectacle power. 3. The perseverance of VVI during disappearance of the visual environment was evaluated by blanking it during various proportions of the cycle of predictable head rotation at frequencies from 0.8 to 2.4 Hz. Below 2.0 Hz, a trend toward gain enhancement was observed with x1.9 telescopic spectacles when the visual environment was present for as little as 6% of the sinusoidal cycle. This effect was statistically significant (P < 0.01) at 0.8 and 1.2 Hz when the visual environment was present for 50% of the cycle. 4. Suppression of the VOR was evaluated for visual fixation of real and imaginary head-fixed targets during predictable, poorly predictable, and unpredictable rotations. Fixation of a real target was most effective at low frequencies of predictable rotation and was significantly effective in reducing gain relative to the VOR at frequencies of < or = 2.4 Hz.(ABSTRACT TRUNCATED AT 400 WORDS)