Immediate effects of thermal tactile oral stimulation on swallowing-related prefrontal cortical activation in healthy adults

Abstract Thermal-tactile oral stimulation (TTOS) is commonly used to facilitate swallowing; however, its effects on cortical activity remain unclear. We investigated the immediate effects of tactile stimulation (TS) and TTOS on swallowing behavior and prefrontal cortex (PFC) activation. Twenty healthy right-handed adults participated in three randomized conditions: (1) TTOS, (2) TS, and (3) rest. After each condition, the participants performed a 30-second Repetitive Saliva Swallowing Test (RSST). The number of swallows, latency to the first maximal laryngeal elevation, and subjective ease of swallowing (5-point scale) were recorded. Cortical hemodynamic responses were measured using a 16-channel functional near-infrared spectroscopy (fNIRS) system that covered the PFC. Both TTOS and TS significantly increased the number of swallows. Only TS significantly shortened the latency to the first maximal laryngeal elevation relative to the rest condition ( p = 0.035), whereas TTOS showed only a trend toward significance ( p = 0.055). Notably, subjective ease of swallowing was significantly higher in the TTOS condition than that in the TS condition. The fNIRS measurements revealed a significant main effect of the task on oxygenated hemoglobin concentrations, suggesting broad PFC activation during both stimulation conditions. Condition-specific correlational analyses showed that while PFC activation was positively correlated with swallowing frequency in the TS condition, this correlation did not reach statistical significance in the TTOS condition. These findings suggest that orosensory stimulation facilitates swallowing and activates the broad prefrontal network. While both TS and TTOS trigger prefrontal hemodynamic responses, the distinct brain–behavior correlation patterns across conditions provide exploratory insights into how sensory inputs modulate swallowing-related cortical networks, offering a valuable theoretical foundation for future swallowing rehabilitation strategies. This trial was registered in the UMIN Clinical Trials Registry (UMIN-CTR) (registration no. UMIN000051241) on 2 June 2023.