Brainstem Control Of Three-dimensional Eye Movements

This thesis examines the neural mechanisms that generate torsional and vertical eye movements, and how they handle the kinematics of three-dimensional rotations. Three-dimensional eye rotations were recorded in alert monkeys, and the midbrain was studied using single unit recording, electrical microstimulation, and pharmacological inactivation.;The direction of VOR slow phases was usually opposite to that of head rotation, as required for optimal visual stabilization. However, direction errors sometimes occurred because of low gain about a head-fixed torsional axis. This axis was orthogonal to Listing\u27s plane of saccadic eye positions, which suggests that saccades and the VOR share the same coordinate system. As predicted by the laws of rotational kinematics, even slow phase axes with zero torsional components produced torsional violations of Listing\u27s law. This proves that mechanics of the plant are not responsible for Listing\u27s law. Furthermore, the final positions were held. This signifies that the eye velocity signal is multiplied by position feedback before entering the oculomotor integrator. Finally, these violations of Listing\u27s law were corrected by VOR quick phases.;Microstimulation and inactivation of the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) demonstrated two right riMLF burst neuron populations that generate clockwise-upward-rightward and clockwise-downward-leftward rotations respectively, and two left riMLF populations for counterclockwise-upward-leftward and counterclockwise-downward-rightward rotations. The torsional axes of eye rotation evoked by riMLF stimulation were orthogonal to Listing\u27s plane, whereas axes that remained after riMLF inactivation aligned with Listing\u27s plane. This suggests that the neural coordinates of motor systems reflect their behavioral constraints.;The neural integrator, which generates the eye position signal, is central to oculomotor control. Post-saccadic drift during inactivation of the midbrain interstitial nucleus of Cajal (INC) showed that this nucleus is the integrator for vertical and torsional eye positions. Stimulation showed that the right INC controls clockwise rotations and the left INC controls counterclockwise rotations. Simulations of the drift suggested that the INC distributes integration over parallel independent neural compartments. This makes the integrator more computationally robust, and suggests a similar role for the modular connectivity observed throughout the brain