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.

The tongue display unit (TDU) for electrotactile spatiotemporal pattern presentation

“…For example, guidance of a surgical tool manipulation has been demonstrated using only TDU-based feedback [94], and tactile feedback from robots to assist spinal cord injuries has been proposed [95], along with tactile feedback of seated pressure, to prevent pressure sores [96] and to gain greater awareness of ankle joint rotation in a fatigued state [97]. The TDU as a platform technology allows virtually any kind of information to be presented to the tongue, subject to the human limits of electrotactile perception on the tongue, which are just beginning to be explored [98]. …”

Perceived Intensity and Discrimination Ability for Lingual Electrotactile Stimulation Depends on Location and Orientation of Electrodes

“…Previous studies have been small (averaging eight participants each), and mostly centered on mapping the sensitivity of the tongue without analyzing electrotactile two-point discrimination ability (Lozano et al, 2009; Tyler et al, 2009; Wilson et al, 2012). These studies indicate that anterior-medial tongue regions have lower thresholds for electrotactile stimulation relative to posterior regions and sensitivity to electrotactile stimulation varies widely between individuals.…”

Sustained cortical and subcortical neuromodulation induced by electrical tongue stimulation

“…Previously, our group and others have shown that the tongue can be used as an effective interface for sending signals to the central nervous system (Chebat et al 2007; Bach-y-Rita and Kercel 2003; Sampaio et al 2001; Bach-y-Rita et al 1998). Compared to the skin, the tongue provides an inviting target for electrical neurostimulation due to the high electrolyte content of saliva, allowing for low-impedance electrical input, and the high density of sensory receptors permitting reasonably high throughput of incoming signals while preserving a relatively large dynamic range (Lozano et al 2009). Additionally, because tongue neurostimulation is non-invasive, it avoids a major drawback of other commonly-used types of electrical neurostimulation such as vagal nerve and deep brain stimulation while exhibiting less non-specific activation than other non-invasive modalities such as transcranial magnetic or direct current stimulation.…”