Alterations in rat horizontal vestibulo-ocular reflex phase as a function of orientation in gravity

The effect of changes in static and dynamic gravity signals on the phase accuracy of the horizontal vestibulo-ocular reflex (HVOR) was studied in rats using chronically implanted scleral search coils to monitor eye movements. Rats were sinusoidally rotated using a range of different frequencies (0.035-2 Hz) in a plane which always activated the horizontal semicircular canals but in one of three different orientations with regard to gravity which differentially activated the otolith organs: 1) upright-normal static gravity signal, no dynamic otolith activation; 2) inverted-inverted static gravity signal, no dynamic otolith activation; 3) on-side-dynamic activation of the otolith organs. In the upright orientation, the HVOR shows a phase advance at 0.2 Hz and below but not at 0.5 Hz and above. Phase accuracy of the HVOR was further degraded in the inverted orientation with rats showing large phase leads at 0.2 Hz and below. In contrast, accuracy of the HVOR was significantly improved at 0.2 Hz and below in the on-side orientation with phase accurate eye movements down to the lowest frequency tested. The results further support the idea that otolith organs play an important role in VOR generation by supplementing the semicircular canals' response to angular head movements.