Effects of transcutaneous electrical nerve stimulation on evoked tactile sensation for fingertip force control.

• The TENS frequency and pulse width parameters were linearly co-modulated both in positive and negative correlation. • Co-modulated TENS can evoke better sensation than individually modulated TENS in a discrete paradigm. • Frequency and pulse width co-modulation generates stronger and more accurate responses but lead to faster sensory adaptation. The bidirectional communication between the hand and brain is profoundly reliant on tactile perception. Transcutaneous electrical nerve stimulation (TENS) can evoke tactile sensation, but its role in fingertip force control is not well understood. We examined the tactile sensation evoked by TENS, especially the effects of concurrent modulation of TENS parameters compared to individual-parameter modulation on fingertip force control in both continuous (with dynamic stimulation force) and discrete (with static stimulation force) paradigms. Co-modulation was accomplished by linearly altering TENS pulse width and frequency in fixed proportions either positively (both pulse width and frequency increased and decreased simultaneously) or negatively (one increased while the other decreased) at a given "stimulation" force level applied by an instructor. Subjects then generated a "response" force with their fingertips to demonstrate the strength of perceived tactile sensation. The mean, maximum, and duration of response forces and the ratios of response force to stimulation force were examined in different phases and consecutive trials of stimulation. The positive and negative co-modulations led to stronger and longer duration response sensitivity than the individual modulations. In addition, subjects could effectively discriminate the evoked tactile sensation variations with co-modulated stimulation parameters from the lower to the higher modulation intensities. The co-modulations were more likely to cause sensory adaptations than the independent modulations. The findings of this study suggested that the co-modulated TENS can result in effective responses to tactile sensation when stimulation trials are controlled. The TENS with co-modulation may serve as an effective tool for eliciting tactile sensations for precise fingertip force control. The findings of this study may play a role in developing bidirectional hand/digit prostheses or restoring hand sensorimotor function. [ABSTRACT FROM AUTHOR]