Modulation of vestibular sensitivity by passive motion

Abstract

Degree Project Thesis, Programme in Medicine. TITLE: Modulation of vestibular sensitivity by passive motion. Information from the vestibular system contributes to the interpretation of how the body is oriented in space. The purpose of this study was to investigate if perception of vestibular input is affected by passive motion. We hypothesized that vestibular afference is down regulated by a period of conditioning (10 minutes of passive, stochastic, rotating movement while blindfolded) and that the perception of movement based on vestibular input, therefore, is decreased after conditioning. By using galvanic vestibular stimulation to create illusionary movements, response to vestibular signals can be investigated independently from other sensory information. We studied sway response during standing on a stable surface, perception of rotation when seated and threshold for detection of motion. All tests were performed, before as well as after motion conditioning, with either GVS or real movement as stimulus. The results indicate that vestibular sensitivity is modulated by motion conditioning. After conditioning, the threshold for motion detection was increased to 248% ± 31% (mean ± SD) of that before (P = 0.001). Perception of real rotations (30° - 180° over 5 s), in which nonvestibular sensory cues were also available, were significantly reduced by motion conditioning (with 16.1% in average). When using GVS, subjects reported larger illusionary movements before conditioning compared with immediately after. After conditioning, reported rotation to a given stimulus intensity nearly halved (from 113 to 61 degrees when exposed to 1 mA over 10 s). Interestingly, we also found that rapid vestibulospinal balance reflexes (latency ~300 ms), evoked by GVS and recorded as lateral shear force exerted on a force-plate, were halved in amplitude. We conclude that, in healthy individuals, vestibular sensitivity is modulated by passive motion. The modulating process operates over short time frames and affects both perception of vestibular motion signals and automatic vestibular balance reflexes, suggesting sub-cortical or afferent regulation. Dysfunction in this process is likely to alter movement sensation and balance control.