Effect on horizontal and vertical optokinetic nystagmus of lateral terminal nucleus of the accesory optic system and midline lesions in the pretectum of monkeys

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The nucleus of the optic tract (NOT), an important visuo-motor relay between the retina and preoculomotor structures, is responsible for mediating horizontal optokinetic nystagmus (OKN) in monkeys, cats, rabbits and rats. In addition to its projection to the vestibular nuclei, the NOT has a prominent projection to the contralateral NOT via the posterior commissure. In order to evaluate the role of the commissural fibers between the NOTs in OKN, we cut the posterior commissure in three Macaca fuscata. [Results] (1) The gain reduction in the rapid rise and steady state slow-phase velocity of OKN can be explained by removal of an excitatory signal mediated by commissural fibers to the inhibitory interneurons in the contralateral NOT. However, interrupting the commissural fibers has noihing to do with the velocity storage mechanism, because time-constants of OKAN mostly remained unchanged by the lesion. (2) The lesions of the lateral nucleus of accessory optic system (LTN) abolished the nystagmus with slow-phase toward-up. The horizontal OKN toward lesioned side were developed well. The LTN may be responsible for not the vertical OKN but also horizontal OKN.