Cross-coupled effects of vestibular and optokinetic stimulations on the dynamics of the vestibulo-ocular and optokinetic reflexes in the cat.

Cats were submitted to repeated step stimulations either vestibular or optokinetic. Regardless of which of the two stimuli was used, dynamic modifications were observed in both vestibulo-ocular response and optokinetic after-nystagmus (OKAN). The progressive changes in post-rotational nystagmus and OKAN were quantified by measuring the duration of their primary phase. A parallel evolution of these two parameters was found. When repeated unidirectional vestibular stimulations were used, the same asymmetry was induced in both vestibuloocular responses and OKAN. These results support the hypothesis that the vestibulo-ocular and the optokinetic reflex share a common velocity storage mechanism, although alternative hypotheses cannot be excluded

Absence of vestibular habituation of vestibulo-ocular reflex in the vertical plane in the cat

The effect of exposure to repeated angular velocity steps about the earth-vertical axis on the vertical vestibulo-ocular reflex (VOR) during onside pitch rotation was investigated in normal cats. By contrast with the VOR in the horizontal plane, the amplitude and duration of the vertical VOR did not progressively decrease throughout the repetition of velocity steps alternated in both directions. Instead, the amplitude of VOR decreased by about 40% during the very first trials in naive cats and then stayed unchanged with repeated stimuli. Habituation of the amplitude of the vertical VOR was observed when the velocity steps were always directed in the same direction. However, the duration of the vertical VOR did not show any signs of habituation. The habituation of the amplitude of the vertical VOR during unidirectional training was due to the progressive development of an initial inhibition of the VOR. This initial inhibition appeared much earlier during the bidirectional protocol, and was presumably responsible for the larger reduction in VOR amplitude observed during the very first session. These results support the model of two distinct mechanisms for VOR habituation, one producing an increasing inhibition of nystagmus, and the other depressing the response duration, and suggest that only the first mechanism is generated during repeated stimulation in the vertical plane. The low-frequency information provided by the velocity storage mechanism during onside pitch rotation, when the otoliths are positioned so they do not signal head tilt relative to gravity, could prevent a decrease in the overall response by the second mechanism

Anatomical segregation of different adaptive processes within the vestibulocerebellum of the cat

Bilateral surgical lesions of the flocculus or the nodulo-uvular lobes were performed in the cat. Effects of these lesions on optokinetic and optokinetic afternystagmus OKAN), vestibulo-ocular reflex (VOR), visual suppression, and adaptation and habituation of VOR were studied using an identical experimental protocol. After flocculectomy, all these functions were impaired, except for habituation. Long-term postoperative recordings only revealed a recovery of the suppression of VOR, suggesting a limited contribution of the flocculus to this function. After nodulo-uvulectomy, only habituation and OKAN were modified. When the lesion was restricted to part of the uvula, OKAN duration was decreased. For other lesions involving the uvula together with the nodulus and/or the lobules VII-VIII,OKAN duration was increased. Habituation was lost after destruction of the nodulo-uvular lobes. When this latter structure was damaged, the retention component of habituation was selectively impaired, sparing the acquisition. Additional lesions outside the vestibulocerebellum appeared necessary to suppress the two components. Comparison of results obtained after flocculectomy and after nodulouvulectomy confirms and extends to non-primate species the concept of a "differential control" of adaptation and habituation by distinct vestibulocerebellar structures