Perception of threshold-level whole-body motion during mechanical mastoid vibration.

BACKGROUND: Vibration applied on the mastoid has been shown to be an excitatory stimulus to the vestibular receptors, but its effect on vestibular perception is unknown. OBJECTIVE: Determine whether mastoid vibration affects yaw rotation perception using a self-motion perceptual direction-recognition task. METHODS: We used continuous, bilateral, mechanical mastoid vibration using a stimulus with frequency content between 1 and 500 Hz. Vestibular perception of 10 healthy adults (M±S.D. = 34.3±12 years old) was tested with and without vibration. Subjects repeatedly reported the perceived direction of threshold-level yaw rotations administered at 1 Hz by a motorized platform. A cumulative Gaussian distribution function was fit to subjects' responses, which was described by two parameters: bias and threshold. Bias was defined as the mean of the Gaussian distribution, and equal to the motion perceived on average when exposed to null stimuli. Threshold was defined as the standard deviation of the distribution and corresponded to the stimulus the subject could reliably perceive. RESULTS: The results show that mastoid vibration may reduce bias, although two statistical tests yield different conclusions. There was no evidence that yaw rotation thresholds were affected. CONCLUSIONS: Bilateral mastoid vibration may reduce left-right asymmetry in motion perception.

Balancing sensory inputs: Sensory reweighting of ankle proprioception and vision during a bipedal posture task.

During multisensory integration, it has been proposed that the central nervous system (CNS) assigns a weight to each sensory input through a process called sensory reweighting. The outcome of this integration process is a single percept that is used to control posture. The main objective of this study was to determine the interaction between ankle proprioception and vision during sensory integration when the two inputs provide conflicting sensory information pertaining to direction of body sway. Sensory conflict was created by using bilateral Achilles tendon vibration and contracting visual flow and produced body sway in opposing directions when applied independently. Vibration was applied at 80Hz, 1mm amplitude and the visual flow consisted of a virtual reality scene with concentric rings retreating at 3m/s. Body sway elicited by the stimuli individually and in combination was evaluated in 10 healthy young adults by analyzing center of pressure (COP) displacement and lower limb kinematics. The magnitude of COP displacement produced when vibration and visual flow were combined was found to be lesser than the algebraic sum of COP displacement produced by the stimuli when applied individually. This suggests that multisensory integration is not merely an algebraic summation of individual cues. Instead the observed response might be a result of a weighted combination process with the weight attached to each cue being directly proportional to the relative reliability of the cues. The moderating effect of visual flow on postural instability produced by vibration points to the potential use of controlled visual flow for balance training.