Exploring the cortical hemodynamic response of excitatory brain stimulation using concurrent TMS/fNIRS: a pilot study

Abstract Background: Many studies have convincingly demonstrated the therapeutic efficacy of transcranial magnetic stimulation (TMS) on depression. Diverse protocols (i.e., different intensities) were also explored to optimize the therapeutic effects of TMS. Previous studies indicated a linear relationship between stimulation intensity and behavioral outcomes. However, the mechanisms of the brain effects of different intensities remain unclear. Therefore, this study aimed to systematically investigate the effects of different intermittent theta-burst stimulation (iTBS) intensities over the left dorsolateral prefrontal cortex (lDLPFC) on brain activity by using functional near-infrared spectroscopy (fNIRS) simultaneously. Methods: Three different intensities (resting motor threshold, rMTs at 50%, 70%, and 90%) of iTBS were delivered over lDLPFC of 24 healthy participants on three visits separated by at least one week each. The TMS coil was put above the NIRS sensor, with a ‘bridge’ placed between these to avoid direct contact. The NIRS measured the hemoglobin value simultaneously immediately before stimulation (3 minutes), during the stimulation (3 minutes), and after stimulation (3 minutes). One-way ANOVA was used to examine the difference in the hemoglobin level change among different stimulation intensities. Results: Compared to baseline, the oxygenated hemoglobin (HbO) was increased in both 70% and 90% intensity conditions, while decreased in 50% intensity during stimulation. Notably, the HbO was increased in all three conditions after the stimulation, and a significant difference was found only at 90% intensity (p = 0.027) when compared to baseline, while no difference was found between different intensities. During the stimulation, the change of deoxygenated hemoglobin (HbR) was increased in all three conditions, significant differences were observed at 50% (p = 0.039) and 90% intensity (p = 0.001), but no difference was found between these conditions. Discussion: Concurrent TMS/fNIRS allow us to explore brain activity during stimulation. Higher subthreshold intensity might show more effects on the brain. Research Category and Technology and Methods Clinical Research: 10. Transcranial Magnetic Stimulation (TMS) Keywords: concurrent TMS/fNIRS, subthreshold intensities, left dorsolateral prefrontal cortex, hemodynamic response