A comparison of the horizontal and vertical vestibulo-ocular reflexes of the rabbit

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Abstract
The gain and phase of the horizontal (H) and vertical (V) vestibuloocular reflexes [VOR] were measured in rabbits. The VVOR was evoked by sinusoidal rolls about the longitudinal axis of the rabbit. This axis was maintained at different orientations with respect to the earth horizontal axis: VVOR 0.degree., prone; VVOR 90.degree., nose-up; VVOR 180.degree., supine; VVOR, 0.degree.L, left side down. In contrast to the HVOR, the VVOR 0.degree. had a higher gain (eye velocity/head velocity) and a smaller phase lead (eye position + 180.degree. with respect to head position) at low frequencies of sinusoidal vestibular stimulation (.+-. 10 deg, 0.005-0.05 Hz). At higher frequencies (0.05-0.8 Hz), the HVOR and VVOR 0.degree. were equivalent in both gain and phase. The low-frequency gain of the VVOR was smallest in the nose-up orientation. The VVOR 90.degree. was equivalent in both gain and phase to the HVOR over the entire range of frequencies tested (0.005-0.8 Hz). Threshold angular accelerations for the HVOR and VVOR 90.degree. C were < 0.04.degree./s2. The compensatory eye movements of the HVOR were frequently interrupted by anticompensatory re-setting eye movements. These anticompensatory re-setting eye movements were present in the VVOR 90.degree., but not in the VVOR 0.degree.. An estimate gain and phase of the otolithic component of the VVOR 0.degree. was derived by subtraction of the VVOR 90.degree. (semicircular canal signal) from the VVOR 0.degree. (semicircular canal signal + otolith signal). This procedure was based on the assumption that the signals from the otolith organs and vertical semicircular canals combine linearly. The VVOR 180.degree. provided an interesting test of the assumption of linear combination of otolithic and semicircular canal signals by reversing the phase of the modulated otolithic signal. The VVOR 180.degree. is apparently non-linear.