Spatial orientation of periodic alternating drift (PAD).

Sinusoidal vestibular stimulation induces in the intact rabbit in prone position a periodic alternating drift (PAD), evident in the earth horizontal plane when the animal is rotated about the vertical axis but weak in the vertical one when the animal is rotated about the longitudinal axis. It has been hypothesized that these oscillations are related to an intrinsic instability of the velocity storage, due to the length of its time constant. The velocity storage has the longest time constant aligned with the vertical axis, and it changes its orientation with the gravity vector. The present research examined the spatial orientation of PAD in relation to changes of the animal position with respect to gravity. Normal pigmented rabbits were sinusoidally oscillated about their longitudinal axes to evoke vertical eye responses. The stimulation was carried out with the animal in prone position and with the animal in nose-up condition. With the animal in prone position, PAD had a weak vertical component, but an evident horizontal component was visible. When the animal was in nose-up position, the horizontal component of PAD was clearly visible, while the vertical component was negligible. In both stimulation conditions PAD period and peak velocity were not modulated by the stimulus characteristics. These results are consistent with a model of PAD based on an interaction between velocity storage and the cerebellar adaptation-habituation circuit.