The vergence eye movement system adjusts the angle between the eyes during shifts in gaze between far and near objects. This system is primarily responsible for the normal alignment of the visual axes during binocular viewing. Although misalignment of the eyes is the most common human oculomotor complaint, the normal function of the neural circuits which control vergence is only now becoming understood. The overall aim of this project is a comprehensive understanding of the neural circuits which control vergence eye movements. This goal is approached in a series of experiments using electrophysiological recordings from the brains of alert, behaving monkeys. The animals are trained to maintain precise ocular fixation and make conjugate and vergence eye movements on demand. The position of both eyes is measured using an accurate search coil technique. Because the vergence system works in conjunction with brain circuits which adjust the dioptric power of the lens, procedures for controlling and measuring lens accommodation are also used. In most of the experiments, single unit recording is used to determine the signals carried by individual neurons in the midbrain and pons. In some experiments, the technique of antidromic activation is used to identify cells which project to the motor nuclei. The accurate measurement of responses, together with sophisticated behavioral paradigms, allows a precise, quantitative analysis of neuronal signals.
The aim of the first series of experiments is to identify midbrain and pontine neurons which project to the extraocular motor nuclei and determine the conjugate and vergence signals they carry. Previous work has shown hat such signals cannot always be inferred form the anatomical connections of the cells which carry them. Another experiment will test a recently proposed model of accommodation and vergence interactions at the neuronal level, using an adaptation paradigm. A final experiment will test the hypothesis that the saccadic and vergence systems are independent. This will be done by determining how vergence and saccadic velocity commands are combined at the level of the motoneurons.
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