Complementary gain modifications of the cervico-ocular (COR) and angular vestibulo-ocular (aVOR) reflexes after canal plugging

To determine whether the COR compensates for the loss of aVOR gain, independent of species, we studied cynomolgus and rhesus monkeys in which all six semicircular canals were plugged. Gains and phases of the aVOR and COR were determined at frequencies ranging from 0.02 to 6 Hz and fit with model-based transfer functions. Following canal plugging in a rhesus monkey, the acute stage aVOR gain was small and there were absent responses to thrusts of yaw rotation. In the chronic state, aVOR behavior was characterized by a cupula/endolymph time constant of ≈0.07 s, responding only to high frequencies of head rotation. COR gains were ≈0 before surgery but increased to ≈0.15 at low frequencies just after surgery; the COR gains increased to ≈0.4 over the next 12 weeks. Nine weeks after surgery, the summated aVOR + COR responses compensated for head velocity in space in the 0.5–3 Hz frequency range. The gains and phases continued to improve until the 35th week, where the combined aVOR + COR stabilized with gains of ≈0.5–0.6 and the phases were compensatory over all frequencies. Two cynomolgus monkeys operated 3–12 years earlier had similar frequency characteristics of the aVOR and COR. The combined aVOR + COR gains were ≈0.4–0.8 with compensatory phases. To achieve gains close to 1.0, other mechanisms may contribute to gaze compensation, especially with the head free. Thus, while there are individual variations in the time of adaptation of the gain and phase parameters, the essential functional organization of the adaption to vestibular lesions is uniform across these species.