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Stochastic resonance in elderly tactile sensation Wells, Cari

Abstract

A common symptom of ageing is an increase in vibrotactile detection thresholds, especially in the feet. A hypothesized result of this increase is dysfunction in those aspects of gait that rely on footsole cutaneous input. One way of ameliorating elevated thresholds is by the use of stochastic resonance (SR), a non-linear phenomenon where the addition of noise to a subthreshold signal renders the signal detectable. The effect of SR on elderly vibrotactile thresholds was investigated using 3 studies. In the first study, detection thresholds for vibration sensitivity were assessed at 4 frequencies, at 55 locations on the footsole, and in young and old participants. Results showed that there were 3 regions of sensitivity on the footsole: the ball/medial arch, the lateral border of the foot and the heel, and the toes. The ordinal pattern of regional sensitivity was age and frequency invariant. In the second study, the effectiveness of SR at lowering vibrotactile detection thresholds from the first study was investigated using a 2 Age x 2 Signal Level x 4 Frequency x 6 Noise Level protocol. Effects were quantified using a psychophysical measure, %Corr. At the 90% signal level, 33% noise optimized %Corr. At the 80% signal level, either 50% or 66% noise produced a maximum in the %Corr measure. In the third study, the effectiveness of SR in lowering vibrotactile differential thresholds was investigated using a 2 Age x 2 SR Condition x 2 Frequency Range x 6 Signal Level design. Results showed that the elderly have higher differential thresholds than the young, however only at hypothesized FAII mediated frequencies. Noise was found to be effective in decreasing differential thresholds in both young and elderly at the hypothesized FAI mediated range, at subthreshold and near-threshold signal levels. At the higher frequency range, noise was only effective in decreasing differential thresholds in younger subjects. It has been hypothesized that elevated detection and differential thresholds may be a factor in dysfunctional gait due to impoverished cutaneous reflex engagement. Together, the results of the three experiments suggest that SR may be useful as a rehabilitative aid for functions, such as gait and grasp, which rely on vibrotactile sensation.

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