Cortical and subcortical influences on the nucleus of the optic tract of the opossum

In the present work we propose a new phylogenetic hypothesis for the role p layed by cortical and subcortical efferents to the nucleus of the optical t ract, the main visual relay station of the horizontal optokinetic reflex in mammals. The hypothesis is supported by anatomical and physiological data obtained in the South American opossum (Didelphis aurita) using the followi ng experimental approaches: (i) single-unit recordings in the nucleus of th e optic tract and simultaneous electrical stimulation of the contralateral nucleus of the optic tract; (ii) single-unit recordings in the nucleus of t he optic tract and simultaneous electrical stimulation of the ipsilateral s triate cortex; (iii) injection of cholera toxin subunit B into the striate cortex and subsequent immunohistochemical reaction to reveal the presence o f the marker in the thalamus and mesencephalon; and (iv) single-unit record ings in the nucleus of the optic tract both before and after ablation of th e ipsilateral visual cortex. The main results are: (i) there is a strong in hibitory reciprocal effect upon the nucleus of the optic tract following st imulation of its contralateral counterpart; (ii) electrophysiological and a natomical data imply that the visual cortex does not project directly to th e nucleus of the optic tract. Rather, cortical terminals seem to target the nearby anterior and posterior pretectal nuclei and orthodromic latencies i n the nucleus of the optic tract following stimulation of the visual cortex were twice as large as in the superior colicullus; and (iii) ablation of t he entire visual cortex did not have any effect upon binocularity of cells in the nucleus of the optic tract. These results strengthen the model propo sed here for the role of the interactions between the nuclei of the optic t ract under optokinetic stimulation. The hypothesis in the present work is that the cortical influences upon the nucleus of the optical tract, in addition to the subcortical ones, appeare d only recently in phylogenesis. In more primitive mammals, such as the opo ssum, subcortical interactions are thought to play a relatively important r ole. With the emergence of retinal specializations, such as the fovea, one might suppose that there followed the appearance of new ocular movements, s uch as the smooth pursuit and certain types of saccades, that came to join the pre-existent optokinetic reflex. (C) 1999 IBRO. Published by Elsevier S cience Ltd.