Open-loop versus closed-loop control of reflexive head movements

The vestibulo-collic reflex (VCR) produces compensatory head movements when the body is passively moved. The head movements reduce the vestibular signal driving the reflex, and therefore the reflex is considered to be a close-looped system. In this study, 73 units in the vestibular nuclei were recorded during the execution of the VCR. These included units which carry the vestibular signals thought to mediate the VCR. Nearly half of the recorded units (N=34; 47%) had responses that would be expected from a closed-looped system, such that they decreased as the head velocity decreased during the VCR. Other units had responses that would be expected from an open-leaped system and were either unaffected (24/73; 33%) or were larger (15/73; 21%) as head velocity decreased during the VCR. Units that had eye-movement related signals tended to have closed-loop responses that decreased during the VCR Non-eye-movement related units tended to have open-loop responses that were either unaffected or were larger. We conclude that sensory vestibular signals are transformed by the addition of inputs related to head rotation on the trunk.