Towards optimum amplitude and frequency of electrotactile stimulation in amputee for effective somatotopic sensory feedback

The high rates of prosthesis abandonment can be attributed especially to the lack of functionality and the absence of a sensory feedback system capable of providing tactile information. Transcutaneous electrical stimulation (TENS) is a promising method for evoking tactile sensations in phantom limbs, despite the absence of well-defined protocols for determining stimulation parameters and methods for evaluating the evoked sensations. The present study conducts a psychophysical assessment of the sensations elicited by varying amplitudes and frequencies of stimulation in amputee volunteers.

Three volunteers with acquired amputations participated in three experiments where the current amplitude was kept constant, proportionate to their sensation and pain thresholds within each block. The block consists of 25 frequencies modulated between 5 and 125 Hz in an incremental and decremental manner. Sensory perception was assessed by evaluating (i) sensation location, (ii) sensation intensity, (iii) perception of frequency variation, and (iv) type of sensation.

The experiments show that stimulation amplitudes greater than 80% of the pain threshold are required to properly evoke referred sensations. The perception of intensity varies among individuals and is influenced by both the stimulation amplitude and the frequency variation. There is a tendency for better differentiation of low frequencies, which evoke discrete sensations, than high frequencies, above 50 Hz, which are related to continuous sensations.

The paper presents a psychophysical protocol that highlights significant individual differences in perception very correlated to stimulation amplitude and frequency, as well as the stimulation site. Such variations must be considered when designing protocols for sensory feedback.