Experimental and model-based analysis of differences in perception of cutaneous electrical stimulation across the sole of the foot

During electrocutaneous stimulations, variation in skin properties across locations can lead to differences in neural activation. However, little focus has been given to the effect of different skin thicknesses on neural activation. Electrical stimulation was applied to six sites across the sole of the foot. The intensities used were two and four times perception threshold. The subjects (n = 8) rated the perception quality and intensity using the McGill Pain Questionnaire and a visual analog scale (VAS). A finite element model was developed and combined with the activation function (AF) to estimate neural activation. Electrical stimulation was perceived as significantly less sharp at the heel compared to all other sites, except one site in the forefoot (logistic regression, p < 0.05). The VAS scores were significantly higher in the arch than at the heel (RM ANOVA, p < 0.05). The model showed that the AF was between 91 and 231 % higher at the five other sites than at the heel. The differences in perception across the sole of the foot indicated that the CNS received different inputs depending on the stimulus site. The lower AF at the heel indicated that the skin thicknesses could contribute to the perceived differences.