Whenever gaze is directed from a far target to a near target, vergence and accommodative eye movements ensure that a fused, well- focused image is achieved. Thus. vergence and accommodation are essential components of most eye movements. However, although disorders of ocular alignment and accommodation dysfunction are common clinical oculomotor complaints, little is known about the neurophysiological bases for vergence and accommodative eye movements. While a few studies have recorded motor or immediately premotor neurons in the midbrain and pretectum, the regions that provide inputs to these cells have not been identified. Identifying these regions, and the way in which vergence and accommodation signals are processed in them, should aid in our understanding of oculomotor complaints related to the near- response. Our preliminary studies in the alert primate have demonstrated that an additional region involved in the near-response is the cerebellum; specifically the posterior interpositus nucleus (PIN). The proposed project will therefore investigate the role of the PIN specifically, and the cerebellum in general in controlling vergence and accommodative eye movements. Single-units recording, lesion, microstimulation and anatomical techniques will be used in trained monkeys. Eye movements will be measured with the search coil technique, accommodation will be measured with an infrared optometer, and a visual stimulator will present targets with independent vergence and accommodative demands. Specifically, cells in the near-response region of the PIN will be tested to determine the aspects of vergence and accommodation that they are related to. Also, they will be tested to see if they are located before and after the links that cross-couple the vergence and accommodation systems. The functional significance of the near- response region of the PIN will be investigated using either reversible lidocaine or irreversible, ibotenic acid lesions. The regions of cerebellar cortex that project to the near-response region of the PIN will be identified using localized injections of WGA-HRP. Also, using microstimulation, the cerebellum will be surveyed to identify other regions related to vergence or accommodation. More detailed knowledge of this nature of the central neural processing of vergence and accommodative eye movements should aid in our understanding of strabismus, disorders of binocular vision, and accommodative dysfunction.
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