The effect of rubber hand illusion on sensory thresholds

The rubber hand illusion (RHI), first described by Botvinic et al., is a phenomenon that causes changes in body perception and awareness as a result of the integration of simultaneously perceived visual and tactile stimuli. In order for RHI to occur, the participant's real hand is placed out of view; whereas the rubber model hand is visible and in the appropriate position of the real hand. While the participant is looking and focusing on the rubber hand, the same areas of the real and rubber hand are brushed synchronously. After this process is continued for a while, the person perceives the rubber hand as his own and starts to feel as if the brush that is applied to the rubber hand is applied to his own hand. Quantitative sensory tests are standardized subjective clinical sensitivity tests that require cooperation of the examinee. In these tests, calibrated stimuli are applied to capture perception and pain thresholds, thus providing information about sensory thresholds. In order for RHI to be used as a potential treatment method, its effect on pain and perception threshold needs to be revealed. In our study, we aimed to evaluate the effect of RHI on cortical, deep and superficial senses by using quantitative sensory tests and measuring sensory thresholds. Healthy volunteers who applied to Gazi University Faculty of Medicine, Department of Physical Medicine and Rehabilitation Polyclinic between May 2021 and September 2021 were included in our study. Three of the 44 applicants were excluded because they did not meet the criteria. The RHI screening experiment, which included 2 minutes of brush stroke, was performed on 41 subjects. 23 individuals who developed RHI were included in the main experiment. The study was designed as cross-sectional, randomized controlled, and single-blind. Participants were randomly assigned to start with asynchronous and synchronous sessions. Demographic information, disease history, dominant hand preference, MoCA and HAD scale scores of all participants were recorded in the study. The evaluations were made by a blind researcher who did not know which session the participant was in. Pain pressure threshold measurement with an algometer, two-point discrimination with a discriminator, and tactile sensory threshold measurement with Semmes-Weinstein filaments were performed on the participants. These measurements were repeated for both sessions before the start of the brush stroke, during the brush stroke, after the brush stroke and after the hand was removed from the environment. In our study, we examined the change of sensory thresholds against time with the effect of RHI. We found a significant increase in pain pressure threshold during and after the brush stroke of the RHI session (p=0.014) and after removing the rubber hand from the environment (p=0.023). In the asynchronous stroke session, we found a significant decrease in the pain threshold after brush strokes compared to before (p=0.005). For two-point discrimination, the values measured during the illusion and when the hand was removed after the illusion ended were significantly different (p=0.047). The tactile sensory threshold measured at the beginning of the RHI session was found to be significantly lower than the tactile sensory threshold measured during the illusion (p=0.013). In addition, the tactile sensory threshold value measured when the rubber hand was significantly lower than during the illusion (p=0.033). In our study, no correlation was found between RHI severity measured by proprioceptive deviation and pressure pain threshold, two-point discrimination threshold, tactile sensory threshold measured during brush stroke. In conclusion, this study shows that RHI increased the pressure -pain threshold, the two-point discrimination threshold, and the tactile sensory threshold in healthy volunteers. In the control session, asynchronous brush strokes, significantly decreased the pressure pain threshold, but did not change the two-point discrimination and tactile sensory threshold. In both sessions, we found that the threshold values that changed during the brush stroke returned to the initial state after the brush stroke was terminated and the rubber hand was removed from the environment. Thus, we have shown that the effect of the illusion does not last for a long time with a single session application.