The effect of passive torque on the kinematics of vestibular-evoked reflexive head movements

When the body is passively moved in space, reflexive compensatory head movements (vesitbulo-collic reflex, VCR) are produced to help stabilize the head with respect to the trunk. In addition, cervical stretch reflexes are activated when the head moves on the trunk (cervico-collic reflex, CCR). Each of these reflexes is thought to contribute to the stability of the head with respect to the trunk by producing head movements during body perturbations. In humans, the VCR has been shown to rapidly adapt to changes in the head´s inertia by adding mass to the head during whole body rotations. In this study, a method was developed for modifying the torque acting on the head during rotation in squirrel monkeys in order to study adaptive properties of the VCR in animals. The VCR produced during horizontal rotation was studied in two squirrel monkeys while the torque exerted on the head was varied (0-5.5 oz-in). During low torque conditions the kinematics of the head movements (gain and phase) were anti-compensatory rather than compensatory. As the imposed torque was increased, the gain of the head movements gradually increased until the head was stable in space. These results illustrate the interactions between the VCR and CCR in small non-human primates free to move their head during whole body rotations. Further studies are necessary to determine what neural circuits underlie the observed interactions between the VCR and CCR