Habituation of the vestibulo-ocular reflex (VOR) in the monkey during sinusoidal rotation in the dark

In experimentally naive monkeys the horizontal vestibulo-ocular-reflex (VOR) has a time constant which is in the range of 40--60 s. It can be measured as the nystagmus decline after pulses of angular acceleration, or from the transfer functions obtained from sinusoidal rotation with different frequencies. When frequencies below 0.1 Hz are applied, sinusoidal rotation leads to a pronounced phase advance, a decrease in gain and a shortening of the pre- and post-rotatory nystagmus time constant. Even very low frequencies (e.g., 0.002 Hz) are effective where the phase advance of eye relative to head velocity is already 90 degrees in the naive animal. Exposing the animal to stimulation only at a single frequency shifts the whole frequency curve towards a greater phase advance. These results are consistent with habituation experiments in which steps of angular velocity have repeatedly been applied. In these experiments nystagmus duration is shortened, whereas the initial response at the end of acceleration does not change. This corresponds to a phase shift and a gain reduction in the low frequency range (below 0.1 Hz) which we have also observed during sinusoidal rotation.