Rapid compensation of horizontal optokinetic nystagmus in hemilabyrinthectomized rats: a fast return to symmetry.

Abstract

In the rat, after unilateral labyrinthectomy the loss of unit responses in the deafferented ipsilateral vestibular nucleus should lead to a complete postlesional asymmetry in H-OKN. The compensation process of this oculomotor deficit were recorded by means of a search coli technique using amplitude detection in 30 pigmented rats, 8 animals as control and 22 submitted to left hemilabyrinthectomy. Recording sessions were performed from day 1 to 8 after the lesion. Velocity steps of visual stimulation were delivered in clockwise (CW) and counterclockwise (CCW) directions at velocities from 2 to 40 degrees/s. The H-OKN was not observable on postlesional day 2. It reappeared asymmetrical on day 2 with depressed responses to CW stimulation while the response to the CCW stimulation were almost as large as in intact animal. This asymmetry was quantified by the gain values (eye velocity/stimulus velocity) of the nystagmic responses which were, at 5 degrees/s of stimulus velocity, 0.37 for CW stimulation and almost twice as large, 0.72 for CCW stimulation. Later this asymmetry was consistently reduced by a progressive increase of the CW responses and a parallel decrease of the CCW. This process led to the responses to reach almost symmetrical values on day 8 with a gain of 0.52 and 0.63 for CW and CCW responses respectively at the same stimulus velocity. However the gain of intact animals was never attained. The initial eye velocity was symmetrically altered on day 2 and remained unchanged until postlesional day 8. These results demonstrate that the deficit appeared to be compensated more by a restoration of a symmetry than by a restitution of a gain of responses equivalent to that of an intact animal.