PhD Defense: Jaimeeganleong Soohoo Reynolds

The Effect of Inertia on Horizontal Reflexive Head Movements

Abstract

During unexpected movements of the trunk, reflexive head movements are influenced by passive properties of the head/neck plant (i.e. head inertia) and reflexive activation of the neck musculature due to the vestibulo-collic (VCR) and cervico-collic (CCR) reflexes. In order to examine the effect of head inertia on reflexive head movements, we applied various inertial loads to the head during passive movements of the trunk while the head was free to move in the horizontal plane. In the absence of the collic reflexes, head-on-trunk movements would be expected to increase in response to the increased inertial load applied to the head during passive movements of the body. However, our results suggest that non-human primates are able to compensate for small changes in head inertia by increasing neck muscle activation during whole body rotation in order to produce the same head kinematics in the unloaded and loaded condition.

We hypothesized that increased neck muscle activation was provided by the CCR, which acts to reduce head-on-trunk movements. To test this hypothesis, we isolated the CCR from the VCR by fixing the head in space during horizontal trunk rotation. In general, head kinematics were maintained when the head-fixed-in-space neck torque was greater than or equal to the inertial torque imposed by changes in head inertia during horizontal trunk rotation. These results suggest that the CCR could potentially increase its output in order to compensate for changes in head inertia.

Alternatively, neck muscle activation could increase due to neck stiffness, which also reduces head-on-trunk movements. This led us to hypothesize that reflexive activation of the neck muscles could inherently generate neck stiffness during whole body rotation by simultaneously contracting muscles on opposite sides of the neck. By modeling bilateral co-activation of the neck musculature due to the VCR and CCR, our simulations suggest that a reflexively-activated neck stiffness and an increase in the CCR could predict the effect of inertia on reflexive head movements. Overall, our data suggests that reflexive control mechanisms are able to compensate for changes in head inertia and provides a framework for studying the neural control of reflexive head movements.