Subthreshold electrical stimulation with pink noise enhances feedback control as evaluated by scaling exponent of postural sway.

Subthreshold somatosensory stimulation with pink noise has been shown to attenuate postural sway better than stimulation with white noise. This might be due to the different frequency structures of the noise signals. However, their effects on the underlying somatosensory feedback pathway are still unknown. Thus, we aimed to determine whether pink noise enhances the somatosensory feedback pathway more effectively than other noises with different frequency structures, such as white and red noises. Sixteen young adults stood quietly for 65 s under four stimulation conditions: no stimulation and stimulations with white-, pink-, and red-noise-like signals. Based on a stabilogram-diffusion analysis, we calculated the long-term diffusion coefficient and scaling exponent in the radial direction to evaluate the effects of these noise signals on their somatosensory feedback control. The root mean square (CoP) and mean velocity of the foot center of pressure were also computed to assess the amount of postural sway. The results showed that the stimulation condition had a significant effect on the scaling exponent, with the value under the pink-noise-like signal significantly lower than that under the no-stimulation condition. We also found that among the participants, the percentage of reduction in CoP by the pink-noise-like signal was positively correlated with the CoP value under the no-stimulation condition. Altogether, the somatosensory feedback control for balancing for quiet standing posture was improved by pink noise, and its effect on the variability of postural sway correlated with inherent postural sway variability.