Adaptive Changes of Vertical Vestibulo-Ocular Reflex Induced by Tenectomy of Vertical Recti Muscles and Neuronal Behavior Related to Vertical Eye Movement in the Region of the Interstitial Nucleus of Cajal in Alert Cats

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Abstract
Experiments were performed in alert cats to examine adaptive changes of the vertical vestibulo-ocular reflex (VOR) induced by tenectomy of the vertical recti muscles of one eye and behavior of neurons in the region of the interstitial nucleus of Cajal (INC) during this adaptation. Continuous pitch rotations (0.11 Hz) of a constant amplitude (± 10°) with only the operated eye uncovered produced a gradual and significant increase in VOR gain that started within 15 min, reaching a plateau within 1 h. When only the normal eye was allowed to view again, VOR gain quickly decreased to the control values within 10 min. Such reversible gain change was not observed when visual input was not allowed to the operated eye, indicating that these changes are adaptive ones to reduce visual-vestibular conflict. A total of 18 neurons were recorded from the INC region during the VOR adaptation. These include 9 eye-movement-related neurons (8 vertical burst-tonic neurons and 1 vertical tonic neuron) and 9 other neurons that did not show close correlation with eye movement but were activated by pitch rotations (tentatively called pitch cells). During adaptive gain increase of the vertical VOR, all the vertical burst-tonic and tonic neurons showed a significant gain increase, whereas a significant change in response gain was observed in only 1 of the 9 pitch cells, indicating that vertical burst-tonic neurons in the INC region are involved in the adaptive gain increase in the vertical VOR. Furthermore, the gain change (i.e. not only the increase but also the decrease) in response of these neurons occurred in parallel with the VOR gain change, suggesting that they are involved in the generation of vertical eye position signal.