Brainstem mechanisms in vertical eye movement

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In order to study a role of the interstitial nucleus of Cajal (INC) in vertical eye movements, single cell recording was obtained in alert cats. Neurons were classified into 4 groups; vertical burst-tonic (51.5 %), tonic (12.9 %),burst (3.8 %) neurons, and the remaining cells that did not belong to any of these three groups but that were activated by pitch rotations (31.8 %). Discharge frequencies of burst-tonic and tonic neurons were linearly correlated with vertical eye position, and more than a half of them projected near the vestibular nuclei, suggesting that vertical eye position signal is carried back to the vestibular nuclei from the INC.Neuronal activity was examined in the INC region during adaptive gain change of the vertical vestibulo-ocular reflex (VOR). The vertical recti muscles of one eye were weakened by tenectomy, and the weak eye was continuously covered with a metal patch to prevent vision. When the patch was switched to the normal eye so that the weak eye was viewing while pitch rotations were continuously applied, the vertical VOR gain showed a gradual and significant increase within 1 hour. When the patch was switched to the weak eye so that the normal eye started viewing again, the VOR gain quickly decreased to the control values. Since such reversible gain change was not observed without visual input, these changes are adaptive changes to reduce visual-vestibular conflict. During such adaptive gain change, all the vertical burst-tonic and tonic neurons tested showed significant changes in response amplitudes in parallel with the change in the VOR gain, suggesting that they may send the altered VOR gain signal to the vestibular nuclei. These results suggest that the central feedback which sends eye position signal from the INC to the vestibular nuclei is involved in the neural integration of the vertical VOR in normal and adapted cats.