In situations where the body and head can move independently, the VCR and CCR must work together to stabilize the head with respect to space and with respect to the body. The dynamic and kinematic properties of these two reflexes appear to be organized to promote relatively simple, linear interactions between them over a wide range of head movement frequencies. Neck EMG outputs produced by both reflexes exhibit nearly identical second-order lead characteristics as stimulus frequencies increase from 0.5 to 5.0 Hz. Their input/output curves are matched so that the threshold of the VCR is offset by the high sensitivity of the CCR at low stimulus amplitudes. In addition, the patterns of muscle activation produced by the two reflexes in the decerebrate cat are quite similar. Direct evidence of linear summation of the VCR and CCR has been obtained in situations where the head rotates with respect to a stationary trunk [16, 22] and where animals make head motions to compensate for rotation of the trunk [3, 22], In the latter situation, the two reflexes appear to play an important role in stabilizing the head of the cat at frequencies below about 3 Hz . In primates, however, voluntary head movements may override this reflex stabilization [1, 25].
ASJC Scopus subject areas