Dynamics of directional plasticity in the human vertical vestibulo-ocular reflex.

Directional plasticity of the human vestibulo-ocular reflex (VOR) was studied in 10 subjects. The adaptation paradigm coupled 0.25 Hz, 19 degrees/s vertical pitch vestibular rotations with 28 degrees/s horizontal optokinetic oscillations. Electro-oculographic recordings in the dark were taken at 0.05, 0.1, 0.25, 0.5, and 1 Hz pitch rotations before and after training and at 15-minute intervals during 0.25 Hz adaptation. Peak head velocity was kept at 19 degrees/sec for frequencies above 0.1 Hz, while constant amplitude was maintained at +/- 24 degrees for 0.05 and 0.1 Hz. In all subjects, directional training produced slow phase horizontal VOR eye movements that were not present during vertical rotations before adaptation. During the 2-hour training period, the cross-axis VOR gain at 0.25 Hz increased up to 0.16. Adaptive VOR gain was highest at the lowest frequency and reached a tuned peak at the 0.25 Hz training frequency. Cross-axis VOR phase remained around 0 degrees at higher frequencies and lagged at lower frequencies. In all subjects, the cross-axis VOR gain was diminished when subjects were exposed to 0.25 Hz pitch rotations paired with a stationary visual field. The dynamics of the vertical VOR remained constant throughout the experiment. These results are further evidence that the frequency response characteristics of adaptive cross-axis VOR gain are similar in humans and cats, while phase behavior is less complex in humans. The high adaptive gain at low frequencies implicates otolith contributions during cross-axis adaptation.