A common mammalian plan of accessory optic system organization revealed in all primates

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Abstract
The accessory optic system (AOS), which was described as ear ly as 1870 by Gudden1, constitutes a distinct midbrain visual pathway in all classes of vertebrates2. In non-primate mammals, retinal fibres of this system project to a set of three nuclei3,4: the dorsal (DTN), the lateral (LTN) and the medial (MTN) terminal nuclei. Whereas all AOS cells respond to the slow motion of large visual stimuli, the neurons are tuned to complementary directions of movement5: horizontal temporo-nasal direction for the DTN, vertical up and down for the LTN and vertical down for the MTN. It has thus been suggested that these nuclei establish a system of retinal coordinates for the detection of whole field motion6. As the AOS provides direct and indirect pathways to both oculomotor and vestibular structures7,8, each of these nuclei is thought to be an essential link in the co-ordination of eye and head movements in relation to movement within the visual field. One problem for the generalization of this theory is that the medial terminal nucleus has never been found in primates. In this report we establish both the existence of this nucleus and its afferent input from the retina in all major groups of primates (prosimians, New and Old World monkeys and apes), indicating a common anatomical plan of organization of the AOS in mammals.

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