[Flexibility of adaptation of vestibule-ocular reflex in human beings]

Vestibulo-ocular reflex (VOR) makes images remain relatively stable on the retina. To keep appropriate performance and minimize image slip throughout life, VOR is subject to long-term adaptive regulation by visual input. It has been reported that adaptive changes in VOR gain (eye velocity/head velocity) are evoked either by fitting subjects with magnifying, miniaturizing, or reversing spectacles during normal behavior or by moving a large visual field in or out of phase relative to the subject's head movement. These feature frequency-selectivity. We studied the flexibility of adaptive gain change in VOR required by a horizontal visual-vestibular mismatch in earth vertical axis rotation (EVAR), including adaptive gain change from EVA to off-vertical axis rotation (OVAR) and other velocities. The visual-vestibular mismatch was made by oscillating subjects in EVAR for 30 minutes at 0.3 Hz with the peak velocity of 30 deg/s and 60 deg/s, synchronized with both in-phase (gain decrease: x 0 experiment) and out-of-phase (gain increase: x 2 experiment) sinusoidal rotation of white-black stripe patterns. Subjects were 19 healthy adult volunteers with no history of neurological symptoms. Horizontal and vertical eye positions were recorded by bitemporal DC-coupled electrooculography. In the x 2 adaptation experiment with 0.3 Hz at the peak velocity of 30 deg/s, the percent change in gain (post-pre/ pre) was 110% at the same stimulation and 100% at 40 deg/s in EVA. In the x 0 adaptation experiment with 0.3 Hz at the peak velocity of 30 deg/s, the percent change in gain was -50% at the same stimulation in EVA. In the x 2 adaptation experiment with 0.3 Hz at the peak velocity of 60 deg/s, the percent change in gain was 66% at the same stimulation in EVA, 30% at the same stimulation in nose-up position, and 74% at the same stimulation in nose-down position. In the x 0 adaptation experiment with 0.3 Hz at the peak velocity of 60 deg/s, the percent change in gain was -34% at the same stimulation in EVA. No change in VOR gain was observed at other peak velocities. These results suggest that VOR adaptation depends frequency and maximum angular head velocity, and this characteristic is observed in OVAR.