Perception of amplitude, pulse intervals, and location of electrocutaneous stimulation

Abstract

Our long-term goal is the development of a wearable warning system that uses electrocutaneous stimulation. Here, we investigated how stimulation pulse amplitude, pulse intervals, and location were perceived in order to select which of these three parameters are practical for designing a warning signal. The upper right arm was stimulated in 81 healthy volunteers with biphasic rectangular current pulses with a pulse width of 150 µs. Eight electrode pairs were placed around the upper right arm. Nine varying amplitudes 1 to 9, equally spaced between the individual perception and intolerance thresholds were presented to the participant in a pseudo-random order (equal for each participant). After a training phase, the participant was asked to rate the presented stimulation amplitude between 1 and 9. Further, five consecutive stimulation pulses with varying intervals (pauses) from 200 ms down to 0.5 ms were presented to the participant. The participant was asked to rate the perceived stimulation regarding the perception as ’Individual pulses’, ’Pulsating’, ’Vibrating’, or ’Continuous’. Next, five consecutive stimulation pulses were presented in a pseudo-random order at the 8 electrode pair positions. After a training phase, the participant was asked to report the stimulated electrode pair. The median reported amplitudes [25%-75%-percentile] for presented amplitudes 1 to 9 are: 1[1-1], 2[2-3], 3[2-4], 3[3-4], 4[3-5], 5[4-6], 6[4-7], 7 [5-8] and 7.5 [6-8]. It shows an underestimation of the perceived amplitude for the presented amplitudes ≥ 4. We conclude that a fine grained variation of the stimulation amplitude is not primarily practical for the design of a warning signal. Participants perceived electrocutaneous pulses as ’Individual pulses’ for median intervals above 74 ms, as ’Pulsating’ between 44 ms and 74 ms, as ’Vibrating’ between 12 ms and 44 ms, and as ’Continuous’ below 12 ms. The majority of the electrode pairs were reported correctly or at the two neighboring electrode pairs. The determined intervals in combination with the small location error offer the basis for the design of a spatio-temporal warning pattern. Our investigations provide the basis for appropriate warning signals for electric warning wearables.