Dependence of the torsional vestibulo-ocular reflex on the direction of gravity.

The torsional vestibulo-ocular reflex (tVOR), which is mainly driven by input from the semicircular canals, has been shown to be augmented by otolith input: its gain is increased in the upright as compared to the supine head orientation. We tested whether otolith input in right/left ear down and upside-down orientation also contributes to the tVOR. In contrast to current models of canal-otolith interaction, which predict equal responses in upright and upside-down orientations, we found that the tVOR in the upside-down position is significantly decreased, possibly due to an additive interaction of canal and otolith signals.