EP 68. Stochastic resonance in the vestibular system

Abstract

Stochastic resonance (SR) is a phenomenon, wherein the response of a nonlinear system to weak input signals can be optimized by addition of a particular non-zero level of noise. Here we tested for the presence of SR in the vestibular system by evaluating if a white noise galvanic vestibular stimulation (GVS) lowers the vestibular detection threshold. Sixteen healthy subjects (mean age 28.9 ± 2.0 years) participated in the study. Vestibular threshold detection was done by identifying the amplitude of a 1 Hz sinusoidal GVS (Digitimer, Hertfordshire, UK) at which subjects exhibit motion induced by the stimulation. Threshold detection was performed without and with an additional white noise GVS within a frequency range of 0–30 Hz and an amplitude set to 80% of the individual cutaneous threshold for GVS. Each threshold detection task consisted in ten bipolar sinusoidal GVS trials with amplitudes ranging from 0.1 μA to 1.9 μA (0.2 μA steps) while subjects sat on a chair with their eyes closed. Body motion was recorded via two inertial sensors (APDM, Inc., Portland, OR) fixed on the head and the trunk and motion responses were counted whenever the motion amplitude exceeded two standard deviations from mean calculated during a resting period preceding each stimulation trial. Body motion responses to stimulation were normalized with respect to the largest response value across all stimulation trials. A binomial distribution function was fit to the GVS amplitude and body response data and the detection was then determined as the stimulation amplitude at the point of subjective equality (i.e., a 50% chance of motion detection). The total of 20 stimulation trials was examined in a random order and subjects were blinded to the protocol. Individual detection thresholds without and with white noise GVS were compared using a paired samples t-test. White noise GVS significantly lowered vestibular detection thresholds from 1.02 ± 0.06 μA to 0.87 ± 0.04 μA (p = 0.011). The mean improvement was 7.9 ± 5.9%. White noise GVS has been previously found to improve cardiac function, postural control, and dynamic walking stability in healthy subjects and patients with bilateral vestibular dysfunction as well as Parkinson’s disease. The present findings indicate that the previously observed ameliorating effects of white noise GVS might result from an enhanced vestibular information processing and provide thereby first evidence for the presence of SR phenomena in the vestibular system.