Eye Instability in the Rabbit Induced by Vestibular Stimulation in the Vertical Plane

Sinusoidal vestibular stimulation in the horizontal plane induces periodic eye instability in the intact rabbit and the hypothesis of an intrinsically unstable velocity storage mechanism has been conceived. The present research examined the stability of the vestibulo-ocular reflex (VOR) in the vertical plane, considering that the time constant values of vertical and horizontal VORs differ and that separate regions of the vestibulo-cerebellum affect the horizontal and vertical slow VOR components differently.Normal pigmented rabbits were sinusoidally oscillated in the dark about their vertical and longitudinal axes to evoke horizontal and vertical eye responses.Frequency and peak-to-peak amplitude stimulation parameters ranged from 0.1 to 0.8 Hz and from 5 degrees to 20 degrees, respectively. During horizontal VOR, periodic alternating drift (PAD) was superimposed on the ocular response, and both peak velocity and period were directly correlated with stimulation amplitude. Vestibular stimulation in the vertical plane induced PAD: the period of vertical PAD was shorter and the amplitude smaller than the corresponding horizontal PAD values. A further difference in vertical PAD occurred in the lack of modulation of period and peak velocity by the stimulus amplitude.These results support the hypothesis of different instabilities of the velocity storage mechanism in the vertical and horizontal planes, possibly due to separate sensory-motor systems sub-serving the vertical and horizontal VORs.