Role of the visual input in recovery of function following unilateral vestibular lesion in the goldfish. II. Long-term behavioural changes.

Previous behavioural studies in our laboratory have demonstrated that the visual input is both necessary and sufficient for the acquisition of the acutely compensated state following hemilabyrinthectomy (HL) in the goldfish. Here we examine the role of the visual input in the maintenance of the compensated state. Exposure of acutely compensated animals to illumination from below (IFB) or infra-red illumination (IRI) elicited a decompensation: whereas IRI was no longer effective 4 days after HL, the susceptibility to IFB disappeared slowly over a number of weeks. Exposure of acutely compensated animals to unilateral illumination (UI) induced a highly asymmetrical dorsal light response 1 day after HL: tilt towards the ipsilateral side was extreme, whilst tilt towards the contralateral side was similar to preoperative values. This pronounced side difference decreased rapidly over the next 3 days and then more slowly over the following weeks and months. The findings show (1) that the maintenance of the acutely compensated state is temporarily dependent not only on the presence of light but also on its direction of incidence; and (2) that the visual-vestibular integration governing posture and locomotion is strongly biased in favour of the visual input to the lesioned side during the early postoperative period and subsequently returns to near preoperative values. The present results are compatible with the hypothesis that acute vestibular compensation in the goldfish is based on a visual substitution process. The latter is not permanent, however, the chronic course of compensation being characterized by a progressive decrease in reliance on visual cues. The observed changes in visual-vestibular integration with time are assumed to reflect modifications in inter- and/or extra-vestibular commissural systems by which the intact labyrinth gradually strengthens its control over the deafferented nuclear complex.