The influence of gravity on horizontal and vertical vestibulo-ocular and optokinetic reflexes in the rabbit.

The influence of the linear acceleration of gravity on the vertical and horizontal vestibulo-ocular reflexes (VVOR, HVOR) as well as the vertical and horizontal optokinetic reflexes (VOKR, HOKR) has been examined in rabbits. Rabbits were mounted in a biaxial rate table in front of a rear projection tangent screen. Eye movements were measured with a light projection technique. The HVOR, VVOR, HOKR and VOKR were measured in rabbits which were maintained both prone and supine. The gain of the HVOR for the supine orientation was reduced at all frequencies tested (0.01-0.80 Hz). Similarly there was a reduction in the gain of the HOKR. By contrast, the gain of the VVOR in the supine orientation was enhanced over a lower range of frequencies (0.02-0.04 Hz) and reduced at higher frequencies (0.10-0.80 Hz). The gain of the VOKR was not reduced in the supine orientation. The range of eye positions over which compensatory eye movements occurred was restricted in the supine orientation. The altered orientation of the medio-laterally polarized hair cells of the utricular maculae with respect to gravity in the supine orientation may cause postural instability and facilitate 'righting reflexes'. A reduction in the gains of the HVOR, VVOR and HOKR caused by linear accelerations in the sagittal plane during locomotion may decrease automatic postural responses during certain movements in which these automatic postural adjustments would not necessarily be adaptive.