On-off asymmetries of responses to caloric step stimuli

Rapid head movement was combined with continuous caloric irrigation of the external ear to evaluate the time courses of the on-step (null-to-supine repositioning) and off-step (supine-to-null repositioning) vestibulo-ocular reflexes (VOR) responses in eight healthy subjects. During continuous air-caloric irrigation, each subject's head was positioned with his/her lateral semicircular canal alternately placed in the null (horizontal) and activating (vertical) planes, and the time course of the slow phase eye velocity was evaluated. The resulting responses were characterized by estimates of activation and adaptation components, and their amplitudes and time constants were calculated. Comparisons of the on-step and off-step responses revealed the activation and adaptation components of the VOR to have similar amplitudes. On the other hand, the time constant for the activation component was prolonged for the off-step response, while that for the adaptation component was shortened. Null-to-supine and reciprocal vestibular stimulation during continuous caloric irrigation of the external ear canal elicited asymmetric VOR responses that exhibited differing time courses.