Chemical Approach to the Thermal Grill Illusion

This study proposes a novel approach to elicit the thermal grill illusion (TGI)–a phenomenon characterized by the concurrent application of hot and cold stimuli to the skin that causes sensations of pain and burning. Contrary to conventional techniques that manipulate heat physically through Peltier elements or water, our method employs chemicals, namely, capsaicin and menthol, to activate transient receptor potential ion channels. This approach perceptually conveys hot and cold sensations and synergistically induces TGI. Notably, the proposed technique offers both energy efficiency and compactness, enhancing its suitability for practical applications. We conducted three experiments to validate the effectiveness of the proposed technique. Experiment 1 aimed to ascertain the feasibility of inducing TGI by contrasting sensations produced individually and collectively by each chemical, targeting to elicit both hot and cold sensations. Experiment 2 focused on exploring the impact of temporal disparities in the application of chemicals inducing hot and cold sensations on TGI induction. Experiment 3 investigated the correlation between the positioning of stimulus sites, delivering warmth and cold sensations, and the intensity of the resultant TGI. The findings demonstrate that adjacent applications of capsaicin and menthol can successfully generate TGI. Moreover, the intensity of TGI can be modulated by varying the time interval between the applications of these two chemicals and altering the locations of the stimuli. Specifically, a more pronounced TGI was achieved by applying capsaicin on the proximal side and menthol on the distal side of the forearm. These results hold promise for practical implementations, particularly in developing itch-suppressing patches.