To follow a slowly moving visual target, correct for shifts in the visual background, or compensate for movements of the head, the eyes make smooth eye movements to maintain visual fixation. Many neuronal populations participate in these smooth eye movements; probably different sets for different types of eye movements. We propose to anatomically trace the sources of these oculomotor signals. This will be done by placing horseradish peroxidase, which is transported from synaptic terminals to neuron cell bodies, in the vicinity of neronal populations in the vestibular complex and the nucleus prepositus hypoglossi that project into the abducens nucleus. Some of these premotor neurons are interneurons in the vestibuloocular reflex, some may have their responsivenes modified during plastic changes in the vestibuloocular reflex and some may operate as a neural integrator that maintains a continuous signal proportional to eye position. We will concentrate on two large, preabducens neural populations. The first is in the ventrolateral vestibular nucleus and adjacent parts of the rostral pole of the medial vestibular nucleus. The second is a compact population in a small nucleus in the common margins of the medial vestibular nucleus and the nucleus prepositus. We also plan to study the intrinsic structure of the medial vestibular nucleus and the nucleus prepositus hypoglossi with a view to studies of other subnuclei and the interpretation of the results of the principal studies. Disorders of the smooth eye movement system can be profoundly disturbing and disorienting. A variety of types of pathological nystagmus are due to interference with the smooth eye movement system. Normal reflex nytagmus evoked by visual, vestibular or thermal stimuli is used routinely in clinical diagnosis. Disturbances of the neural integrator may lead to nystagmus, drifts, and dysmetric eye movements. The anatomical basis of these various normal and pathological eye movements is almost unexplored.
