Reliability and Validity of a Novel, Portable Thermal Grill Illusion Apparatus Using Two Graphene Probes: An Initial Psychophysical Validation Study

BACKGROUND: The thermal grill illusion (TGI) is a psychophysical phenomenon in which combined heat and cold stimulation evokes an illusory burning or painful heat sensation. Although TGI has potential for sensory assessment, conventional multi-probe apparatuses are often bulky and costly. This cross-sectional observational study conducted an initial psychophysical validation of a novel portable TGI device using two graphene probes. METHODS: Fifty-two healthy participants were evaluated using a novel 2-probe device and a previously used 8-probe apparatus. Concurrent validity and agreement were assessed using correlations, intraclass correlation coefficients (ICC), Bland-Altman analysis, and Bayesian equivalence testing for Visual Analogue Scale (VAS) ratings of heat, cold, pain, and discomfort. Sensory-quality profiles were compared using item-level Short Form McGill Pain Questionnaire-2 (SF-MPQ-2) descriptors. Within-session reliability and measurement precision were evaluated using ICC, standard error of measurement (SEM), and trial-averaging analyses across ten repeated trials. RESULTS: The 2-probe device showed moderate-to-excellent correspondence with the 8-probe apparatus for VAS ratings. Pain and discomfort ratings were lower with the 2-probe device, but both devices commonly elicited hot-burning sensations. Repeated measurements with the 2-probe device showed improved reliability and reduced measurement error when multiple trials were averaged; averaging four trials provided high reliability for pain ratings. CONCLUSION: The portable 2-probe device showed feasible psychophysical performance for inducing and repeatedly assessing TGI sensations under experimental conditions. Further validation with formal control conditions and clinical populations is warranted. SIGNIFICANCE STATEMENT: This study provides initial psychophysical validation of a pocket-sized two-probe thermal grill illusion (TGI) device using graphene probes. The device showed moderate-to-excellent correspondence with an eight-probe apparatus, preserved a core hot-burning sensory quality, and improved reliability when multiple trials were averaged. These findings support its feasibility as a compact tool for standardized repeated assessment of TGI sensations under experimental conditions, warranting further validation with formal control conditions and clinical populations.