Central control of postural orientation in flatfish. II. Optic-vestibular efferent modification of gravistatic input.

Abstract

ABSTRACT Visual influences on postural responses mediated by the otolith organs were studied in behavioural and physiological experiments on two species of flatfish, the bothid Citharichthys stigmaeus and the pleuronectid Hypsopsetta guttulata. Ocular compensation to lateral tilt is altered toward a directional light stimulus ; the light effect is independent of the tilt effect, and sums with it. Maximum light effect is roughly one-fourth the magnitude of the maximum tilt response. Single shocks to the optic tectum evoke up to three responses in the cut stump of the ipsilateral vestibular nerve, falling into three latency groups at roughly 5, 10 and 20 msec; all three follow stimulation of the anterior lateral tectum. The two later groups suggest by their delay and extinction at low stimulus rates that their path involves at least one synapse. Some units determined as primary otolith-organ afferents by their response characteristics to lateral tilts show a change in rate following a burst of shocks to the tectum, but no response to single shocks. The usual effect is excitatory, with a latency of 50 –100 msec, and lasting 200 –500 msec after the stimulus. Some of these units also show tonic alteration in rate while a light is shining into one eye. The alteration is either an increase with light to either eye, or a directiondependent increase or decrease, with a latency of 50 –100 msec, and lasting ionically during stimulation times of up to 1 min. The effect is independent of tilt. While the proportion of affected units is small, the elimination of mechanical, muscular, autonomic or antidromic effects and the specificity of many of the responses demonstrate that optic-vestibular integration can involve an efferent control system from the tectum to the otolith organs. The presence of such visual interactions with vestibular input in adult flatfish leads to the hypothesis that the central control of the 90 ° postural change occurring during flatfish metamorphosis may involve crucial optic influences during the period of eye migration.