Remote Subthreshold Stimulation Enhances Skin Sensitivity in the Lower Extremity

Abstract

Foot sole skin interfaces with the ground and contributes to successful balance. In situations with reduced sensitivity in the glabrous foot skin, stochastic resonance (SR) improves skin sensitivity by adding tactile noise. Some situations, however, involve an interface comprised of hairy skin, which has higher thresholds for sensitivity. For example, in lower extremity amputation the residual limb is comprised of hairy leg skin. The main objective of this study was to determine if SR improves skin sensitivity in hairy skin, and whether a specific intensity of noise is most effective. Secondary objectives were to compare the effect between locations, ages and modalities. In 60 healthy participants a vibrotactile (test) input was delivered at the lower extremity concurrently with a second, noisy stimulus applied more proximally. The presence of a remote SR effect was tested in 15 young participants using electrotactile noise at the calf. Secondary objectives were tested in separate groups of 15 subjects and differed by substituting for one of the three variables: vibrotactile noise, heel site, and with older participants. A forced-choice protocol was used to determine detection ability of the subthreshold vibration test input with varying noise levels applied simultaneously (0, 20, 40, 60, 80, and 100% of perceptual threshold). An SR effect was identified when increased detection of the input was obtained at any level of noise versus no noise. It was found that all four test groups demonstrated evidence of SR: 33–47% of individuals showed better detection of the input with added noise. The SR effect did not appear consistently at any specific noise level for any of the groups, and none of the variables showed a superior ability to evoke SR. Interestingly, in approximately 33% of cases, threshold values fluctuated throughout testing. While this work has provided evidence that SR can enhance the perception of a vibrotactile input in hairy skin, these data suggest that the ability to repeatably show an SR effect relies on maintaining a consistent threshold.

The Effect of Subliminal Electrical Noise Stimulation on Plantar Vibration Sensitivity in Persons with Diabetes Mellitus

“…We hypothesized an improved vibration perception following plantar SEN stimulation. Contrary to our hypothesis and to the literature on electrical noise stimulation in healthy younger and older adults [ 4 , 6 , 16 , 17 ], our measurement results did not demonstrate statistically significant effects of SEN stimulation on vibration perception.…”

“…However, the applied bandwidths and noise signal forms are highly variable in the literature. For example, Plater et al [ 16 ] used electro-tactile non-uniform white noise with a bandwidth of 0–50 Hz and different noise intensities to investigate the perception of 30 Hz vibrations on the hairy skin of the calf. Toledo et al [ 17 ] employed a bandwidth of 5–1500 Hz and used Gaussian bandpass filtered noise with zero mean.…”

“…Conversely, improving VPT could possibly delay the development of diabetic foot ulceration. While it has already been demonstrated in healthy participants of different ages that sensory perception can be improved by means of SEN stimulation [ 4 , 6 , 16 , 17 ], to the best of the authors’ knowledge proof of efficacy in patients with diabetes mellitus is still lacking. Therefore, the present study aimed to investigate possible effects of SEN on plantar VPT in a cohort of participants with diabetes mellitus.…”

Mechanoreceptor sensory feedback is impaired by pressure induced cutaneous ischemia on the human foot sole and can predict cutaneous microvascular reactivity

“…Due to evidence of perceptual threshold drift across an experiment ( Plater et al, 2021 ), PT for the testing site was re-established prior to each trial. In addition, it is likely the sensitivity changes due to ischemia are transient; therefore, only one site’s sensitivity recovery was tested per loading trial.…”

Effectiveness and safety of subthreshold vibration over suprathreshold vibration in treatment of muscle fatigue in elderly people

“…The effect of subthreshold vibration can extend to remote areas from the application site. Plater et al [ 56 ] investigated the effect of subsensory vibration applied away from the testing site. They applied subthreshold vibration at the posterior thigh and measured the vibrotactile threshold at the calf.…”