Adaptive mechanisms of VOR compensation after unilateral peripheral vestibular lesions in humans.

To further elucidate possible central plastic adaptive processes during the recovery from a unilateral peripheral vestibular lesion, we investigated vestibular functions in humans over a period of 2 months after an acute unilateral labyrinthine lesion. A unilateral peripheral vestibular lesion creates both a tonic imbalance that causes spontaneous nystagmus and a decrease and directional asymmetry of dynamic vestibular responses. We establish that the tonic imbalance expressed by the spontaneous nystagmus rapidly decreased (similar to other species), whether the lesion remained complete or not. This rebalancing, in the case of complete lesions, is at least partly due to restoration of central vestibular tone on the lesioned side. This restoration of tone also explains, in the case of a complete lesion, the recovery of dynamic vestibular responses for high-velocity inhibitory stimulation of the remaining labyrinth. A clear recovery of the dynamic response for excitatory stimulation of the remaining labyrinth cannot be proven, as has been shown in monkeys during the first 4 days after a unilateral vestibular lesion. This is probably due to the fact that in our patients the first recording could not be performed before day 3 after the onset of symptoms. Therefore, any fast dynamic recovery may have been missed.