Accessory optic system and pretectum of birds: comparisons with those of other vertebrates.

We compare the functional and anatomical organization in birds and other vertebrates of the accessory optic nuclei and of those pretectal nuclei implicated in optokinetic responses. In all vertebrate groups, the neurons in these nuclei respond most strongly to slow large-field visual motion in particular directions; the several nuclei differ in the direction of stimulus motion that evokes the best response. These nuclei are essential for optokinetic nystagmus (OKN) in all species examined; the pretectum is necessary for horizontal OKN and the accessory optic nuclei for OKN in other directions. At least in the accessory optic system of birds, the directional parcellation is not well-developed at hatching and requires visual experience to develop normally. There is evidence that the accessory optic system may play a role in transforming the visual motion signal from retinal coordinates into vestibular or oculomotor coordinates. In regard to anatomical connections, in all vertebrate groups studied, the accessory optic and pretectal nuclei project either directly or indirectly to the cerebellum; in addition, the accessory optic system and pretectum are extensively reciprocally connected. In some groups, but not in others, projections have been discovered from the accessory optic system and pretectum to the extraocular motor nuclei and from the accessory optic system to both the vestibular complex and the interstitial nucleus of Cajal.