Head-position-dependent Adaptation of Nonconcomitant Vertical Skew

Vertical phoria can be trained to vary with either head position or orbital eye position. The present experiments show that subjects can simultaneously adapt their eye-position-specific (nonconcomitant) vertical phorias in different directions at different head positions. Eye-position-dependent and head-position-dependent adaptive pathways, therefore, are not independent. Rather, the adaptation of vertical skew takes into account both eye and head position. In additional experiments, the magnitude of the nonconcomitant adaptive response was shown to be related to otolith output, increasing with head tilt ipsilateral to the tilt position at which training was received and decreasing in the contralateral direction.