Intensity Discrimination of Electrocutaneous Haptic Feedback Is Enhanced at Low Frequencies and with Repeated Exposure

Electrocutaneous (EC) stimulation is a promising modality for noninvasive tactile feedback. Increasing the pulse frequency of EC stimulation has been shown to increase the perceived intensity in a nonlinear way. However, it is unclear why the relationship is nonlinear and how the nonlinearity impacts the theoretical number of discriminable sensory gradations that can be evoked by EC stimulation, as would be necessary to effectively convey the magnitude of tactile interactions. To address this, we systematically quantify just-noticeable differences (JNDs) and corresponding Weber fractions (WFs) through three psychophysical experiments in healthy adults to elucidate the perceptual resolution and mechanisms for frequency-based sensory encoding. First, we quantify the JNDs and WFs at 25-, 50-, 75-, and 100-Hz reference points and provide quantitative data supporting prior work that suggested intensity discrimination is worse at higher pulse frequencies. Second, we demonstrate that perceptual learning is frequency dependent; intensity discrimination at 50 Hz improved with greater exposure through additional JND experiments, whereas no improvement was observed with repeated exposure at 100 Hz. Third, by isolating frequency discrimination from intensity cues using pseudo-randomized pulse amplitudes, we show that temporal discrimination is impossible at higher frequencies. This suggests that temporal cues present at lower EC pulse frequencies contribute to enhanced perceptual sensitivity at lower frequencies. These findings serve as a quantitative benchmark for intensity discrimination across the working range of EC pulse frequencies, and provide new evidence that enhanced perceptual sensitivity at lower frequencies is likely due to the learnability of supplemental temporal cues. These findings highlight the importance of documenting the experience level of participants when reporting discriminability and may be used to optimize EC haptic feedback systems.