SOME OBSERVATIONS ON THE NEURONAL MECHANISM OF OPTOKINETIC NYSTAGMUS

Abstract

Unitary activities were recorded from occulo-motor nucleus and optic nerve in rabbits by means of super-fine microelectrodes with the aim at clarification of neuronal mechanism concerning the reflex arc of optokinetic nystagmus.1) The unitary activities of occulo-motor nucleus when optic nystagmus was produced were as followsThe discharge rate increased or decreased with turning an optical cylinder till it reached to a certain rate, which continued to the beginning of nystagmus. This phenomenon made us consider that bulbs were kept in a deviated position.The impulses to the occular muscle which rested in quick phase decreased promptly to a minimum discharge rate just before the beginning of a quick phase and increased gradually with the beginning of slow phase. The change of discharge rate of the impulses to the occular muscle which contracted in quick phase corresponded to the course of nystagmus.2) The occular movement recorded with electronystagmography was accelerated towards the foveopetal direction of the image of a stripe when discharges of an optic nerve fiber which derived from the area close to the fovea were increased.Discharges of an optic nerve fiber with derived from the area close to the fovea showed constantly marked increase when the image crossed over the receptive field by foveo-fugal shifting; on the contrary this marked increase was not shown by foveo-petal shifting of the image.When the image left the fovea in slow phase, occular movement was so accelerated that the image could be caught on the fovea by increase of discharge from the nerve fibers derived from. the area close to the fovea. Even when the image drew nearer to the fovea in quick phase, occular movement was not accelerated and it decelerated intensively after the image crossed over and left the fovea.3) In order to clarify the cause that the dischasges of the unit derived from the area surrounding the fovea reacted a different way by shifting direction of the image, the unitary responses of optic nerve to shifting of stripes were examined in immobilized animals. An analysis of unitary discharges derived from periphery of the optic field revealed the existence of suppressive field on the eccentric side of and close to the receptive field. This unit showed the marked response by foveo-petal shifting of the image on the contrary to the unit derived from the area close to the fovea. These fact were considered advantageous for the formation of quick phase.In the area around the fovea, suppressive field was considered to be on the concentric side of the receptive field, which, however, was to be proved.