Context-specific adaptation of the vertical vestibuloocular reflex with regard to gravity.

We determined whether head position with regard to gravity is an important context for angular vestibuloocular reflex (aVOR) gain adaptation. Vertical aVOR gains were adapted with monkeys upright or on side by rotating the animals about an interaural axis in phase or out of phase with the visual surround for 4 h. When aVOR gains were adapted with monkeys upright, gain changes were symmetrical when tested in either on-side position (23 +/- 7%; mean +/- SD). After on-side adaptation, however, gain changes were always larger when animals were tested in the same on-side position in which they were adapted. Gain changes were 43 +/- 16% with ipsilateral side down and 9 +/- 8% with contralateral side down. The context-specific effects of head position on vertical aVOR gain were the same whether the gain was increased or decreased. The data indicate that vertical aVOR gain changes are stored in the context of the head orientation in which changes were induced. This association could be an important context for expressing the adapted state of the aVOR gain during vertical head movement.