Optokinetic response in monkey: underlying mechanisms and their sensitivity to long-term adaptive changes in vestibuloocular reflex.

Publisher Website
Google Scholar
Abstract
A behavioral approach to study the mechanisms underlying optokinetic responses, and the location (relative to those mechanisms) of the variable gain element(s) subserving long-term adaptive modification of the [rhesus] monkey vestibuloocular reflex (VOR). Horizontal optokinetic nystagmus (OKN) and afternystagmus (OKAN) were elicited by a full-field visual stimulus consisting of a striped drum rotating at constant velocity around the monkey. Optical devices were used to increase or decrease the gain of the VOR (measured in darkness as smooth eye velocity divided by imposed head velocity). OKN and OKAN consist of smooth tracking eye movements in the direction of stimulus motion and rapid, resetting saccades in the opposite direction. During the OKN produced by sudden illumination of the moving stimulus, the velocity of the tracking eye movements shows an initial rapid rise (short-time-constant component) followed by a much more gradual increase (long-time-constant component). When the illumination is subsequently extinguished, smooth eye velocity shows a rapid drop, followed by a much more gradual decay (OKAN). Eye acceleration (AE) in the first 100 ms of the response to drum illumination was taken as a measure of the open-loop response of the short-time-constant component. AE increased as a function of the velocity of the preceding retinal image motion, but was relatively insensitive to 6-fold changes in VOR gain. The initial eye velocity of OKAN was assumed to be an index of the level achieved by the long-time-constant component of the prior OKN. The values achieved after 15 s of exposure to high drum velocity were very sensitive to changes in VOR gain, ranging from 37.degree.-120.degree./s as VOR gain ranged from 0.29-1.80. Other measures of the optokinetic response changed in these experiments. The maximum eye velocity achieved during OKN was quite sensitive to changes in VOR gain, at least for high stimulus velocities or low VOR gains. The time course over which OKAN developed (charged) became somewhat faster as VOR gain increased. The time course of decay of OKAN in the dark decreased continuously during the several weeks needed to complete each experiment, but was not a function of VOR gain. Measures of the optokinetic response that reflect the gradual changes in eye velocity were very sensitive to changes in VOR gain, providing new support for the suggestion that the 2 components of OKN are generated by 2 independent mechanisms. A model is suggested in which the variable gain elements underlying VOR adaptation lie between the sites at which the long- and short-time-constant mechanisms access the final oculomotor pathways. Computer simulation reproduced all the essential features of these data.