Ultra-Low-Frequency Electrical Stimulation of the Tongue Mucosa and Tonic Pupil Size in the Dark: A Randomized Controlled Pilot Trial

Background: Tonic pupil size is a non-invasive, indirect index of arousal that partly reflects locus coeruleus–noradrenergic activity but is also influenced by autonomic, cholinergic, serotonergic, cognitive and emotional factors. We tested whether ultra-low-frequency transcutaneous electrical nerve stimulation (ULFTENS) of the dorsal anterior tongue mucosa, delivered through an intraoral stimulation electrode (tongue ULFTENS electrode, TUD), modifies tonic pupil size in the dark. Methods: In this randomized controlled pilot trial, healthy women aged 20–30 years were randomized 1:1 to TUD stimulation or to a no-stimulation control condition (CTRL). Binocular pupil size was recorded by infrared pupillometry in darkness for 15 s at baseline (T0) and at two subsequent time points 5 min apart (T1, corresponding to the stimulation period in the TUD group; T2, after stimulation). The primary outcome was the change in pupil size from baseline to T1 (T1 − T0), analyzed by ANCOVA adjusted for baseline; the Group × Time effect was tested with a mixed-design ANOVA. Effect sizes with 95% confidence intervals (CIs) are reported for all comparisons. Results: Out of 54 randomized participants, 48 were analyzed (TUD, n = 21; CTRL, n = 27). A significant Group × Time interaction was found (F(1.84, 84.6) = 5.73, p = 0.006, η2p = 0.11, 95% CI 0.01–0.23). Pupil size increased over time in the CTRL group (T1 − T0: +295 pixels, 95% CI 79 to 512) and decreased in the TUD group (−397 pixels, 95% CI −743 to −50). At T1 (stimulation period of the TUD; no intervention in CTRL), the baseline-adjusted between-group difference was −689 pixels (95% CI −1059 to −319; p < 0.001; d = −1.06, 95% CI −1.67 to −0.45, large effect), about 10% of baseline pupil size. After stimulation, the difference was attenuated (−398 pixels, 95% CI −782 to −14; p = 0.043; d = −0.61, 95% CI −1.19 to −0.03, medium effect). No adverse events occurred. Conclusions: In young healthy women, ULFTENS of the dorsal anterior tongue mucosa was associated with the absence of the progressive increase in tonic pupil size observed in the dark in controls, with a large effect during stimulation and an attenuated post-stimulatory effect on pupillary tone. Because the control group received neither a sham device nor stimulation, effects of the device, oral sensory input and expectation cannot be excluded. Whether this reflects a reduction in noradrenergic arousal requires confirmation in sham-controlled studies with concurrent physiological measures. Trial registration: ClinicalTrials.gov, NCT06790459 (24 January 2025).