Effects of otolith stimulation in eccentric rotation on the vestibulo-ocular reflex in squirrel monkeys

When a subject is rotated around an axis located behind his head (eccentric rotation, ECR), he experiences a combination of angular acceleration and two kinds of linear accelerations, tangential and centrifugal. The effects of stimulation of the otolith organs by linear acceleration in ECR on gain of the vestibulo-ocular reflex (VOR) were examined in squirrel monkeys. The VOR gain in ECR with the animal's head facing away from the rotation axis was significantly higher than that in centric rotation (CR) at 1.0 Hz, but not at 0.5 Hz. However, the VOR gain did not increase in eccentric-lateral rotation when the animal faced tangentially, even at 1.0 Hz. After bilateral ablation of the otolith organs (sacculectomy and utricular neurectomy), the VOR gain in ECR did not increase, even at 1.0 Hz. These findings demonstrate that tangential acceleration along the interaural axis in ECR stimulates the utricular maculae, resulting in enhancement of the VOR gain. The results also suggest that ECR is a useful clinical test for the function of the otolith organs.