Electrotactile stimulation on the tongue: Intensity perception, discrimination, and cross-modality estimation

Abstract

Due to its high sensitivity and conductivity, electrotactile stimulation (ETS) on the tongue has proven to be a useful and technically convenient tool to substitute and/or augment sensory capabilities. However, most of its applications have only provided spatial attributes and little is known about (a) the ability of the tongue's sensory system to process electrical stimuli of varying magnitudes and (b) how modulation of ETS intensity affects subjects’ ability to decode stimulus intensity. We addressed these questions by quantifying: (1) the magnitude of the dynamic range (DR; maximal comfortable intensity/perception threshold) and its sensitivity to prolonged exposure; (2) subjects’ ability to perceive intensity changes; and (3) subjects’ ability to associate intensity with angular excursions of a protractor's handle. We found that the average DR (17 dB) was generally large in comparison with other tactile loci and of a relatively constant magnitude among subjects, even after prolonged exposure, despite a slight but significant upward drift (p < 0.001). Additionally, our results showed that as stimulus intensity increased, subjects’ ability to discriminate ETS stimuli of different intensities improved (p < 0.05) while estimation accuracy, in general, slightly decreased (increasing underestimation). These results suggest that higher ETS intensity may increase recruitment of rapidly adapting mechanoreceptor fibers, as these are specialized for coding stimulus differences rather than absolute intensities. Furthermore, our study revealed that the tongue's sensory system can effectively convey electrical stimuli despite minimal practice and when information transfer is limited by memory and DR drift.

• Dynamic range
• electrocutaneous
• sensory substitution
• sensory augmentation
• psychophysics
• human–machine interface

Notes

Notes

1. There are two ways to quantify DR: (1) stimulus DR—ratio of the physical MAX and THR values; and (2) perceived magnitude estimation of only the MAX level, since THR is perceived in a similar manner (magnitude-based DR; Kaczmarek et al. 1992). Although the latter is a better psychophysical measure because it does not co-vary with waveform manipulations, we used the stimulus DR (that we simply referred to as DR) as we were interested in perception at subdivision levels of the DR.

2. A more direct comparison of psychophysical sensitivity to intensity discrimination and identification can be obtained by using a methodology introduced by Durlach and Braida (1969). However, this methodology limits responses to be ordinal and verbal, which would have restricted the objectives of our study.