Effects of alertness on three-dimensional eye movements.

PURPOSE To investigate effects of alertness on three-dimensional (3D) eye movements. METHODS During drowsy and alert periods, 3D eye movements were recorded with dual search coils in three normal rhesus monkeys, and in two of these monkeys after placement of bilateral kainic acid lesions of the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF). RESULTS When the three monkeys were in the normal state, the average plane thickness (standard deviation of all rotation vectors from the regressed plane) during drowsy periods (range, 1.2-1.9 degrees ) increased significantly compared with the plane thickness during alert periods (range, 0.7-0.9 degrees ) in the light (P <.05). The plane thickness increased significantly in the monkey with asymmetric bilateral riMLF-lesions (P <.05); however, the increase was not significant in the monkey with more symmetric bilateral riMLF-lesions. After the bilateral riMLF-lesions, the plane thickness also increased with drowsiness in both monkeys (P <.02), as it had in the normal state. CONCLUSION The implementation of Listing's law is independent of the presence of vertical-torsional burst neurons in the riMLF. The increase in thickness of Listing's plane during drowsy periods is not due to an imprecise signal from saccadic burst neurons to the 3D velocity-to-position integrator. It is possible that this integrator itself controls Listing's law, depending on the state of alertness.