Vestibular-evoked reflexive head movements and their dependence on the body's orientation in space

When the body is passively moved in space the mass of the head imposes force on the cervical spinal column. Reflexive activation of the neck musculature during whole body motion is hypothesized to function to counteract the inertial forces of the head. However, even when the body is stationary in space gravitational acceleration imposes force on segments of the body. Counteracting this force is complicated because it changes as the body's orientation changes with respect to gravity. In this study, we have focused on a subset of vestibulo-collic reflexes that act to stabilize the head when the whole body is rotated. The kinematics of reflexive head movements produced in response to whole body rotation were recorded from Squirrel Monkeys. The orientation of the body was sequentially manipulated to determine how gravity influences the kinematics of the reflexive head movements. Only small changes in reflexive head movement gain were observed at low stimulus frequencies. In contrast, larger changes in gain were observed at higher stimulus frequencies. These results suggest the vestibular system compensates for the body's orientation in space while regulating the postural stability of the head with respect to the trunk during low frequency perturbations of the body.