A commonplace symptom of vestibular endorgan damage is postural dysequilibrium. Vestibular compensation eventually restores balance to intact levels. At issue in describing the compensation process is what role other sensory input plays in balance behavior. Visual and other sensory cues may substitute for the decreased vestibular input or they may contribute to a developing reflexive mechanism that allows for balance at a range of arousal levels. To determine which of the two mechanisms underlies compensated behavior, body sway of squirrel monkeys was measured prior to and following unilateral labyrinthectomy. Visual fixation and alertness were avoided by recording sway during the animals' sleep. In three monkey subjects, body sway was analyzed from epochs of slow-wave sleep. Sway (anteroposterior and lateral), EEG, vertical eye movements, ECG, and respiration were recorded on magnetic tape; simultaneous strip-chart records also illustrated sway episodes.
A characteristic sway pattern emerged following labyrinthectomy: slow deviation to the side of the lesion, followed by a rapidly corrective righting movement. This pattern resembled spontaneous eye nystagmus; therefore, the term body nystagmus was coined to refer to this behavior.
Frequency of body-nystagmic beats, slow-phase body velocity (SPBV), and power-spectral density of sway were evaluated. Lateral SPBV significantly increased (p < 0.05) after unilateral lesion, then declined to intact levels. Body-nystagmic beat frequency results were inconsistent. For power-spectral density, in the lateral dimension, for both 0.00 to 0.25-Hz and 0.00 to 1.00-Hz bandwidths, energy was significantly greater (p < 0.05) three days postlabyrinthectomy than either the pre-operative condition or the compensated stage (35 days). Differences for the anteroposterior dimension were not significant. When these indices were compared with a standard measure of compensation, spontaneous eye-nystagmic velocity, lateral SPBV described the compensation trend as well or better than the oculomotor measure.
These findings suggest that alternate sensory information contributes to the reestablishment of a reflexive balance mechanism. Rather than simply substitute for lost vestibular sensation, the alternate inputs integrate over the compensation period to regulate balance behavior across a range of arousal levels.