Comparison of hemodynamic changes after repetitive transcranial magnetic stimulation over the anatomical hand knob and hand motor hotspot: A functional near-infrared spectroscopy study

Background: Low-frequency rTMS can induce upregulation of excitability in the contralateral hemisphere by interhemispheric interaction. Objective: The aim of this study was to compare the effects of interhemispheric modulation on hemodynamic changes after applying low-frequency rTMS over the anatomical hand knob (HK) and the hand motor hotspot (hMHS) in the dominant motor cortex. Methods: Ten healthy right-handed participants without a history of neurological or psychiatric symptoms (five males; 29.8 +/- 2.8 years) participated in this single-blind, randomized, cross-over study. rTMS was applied under three conditions over the dominant (left) hemisphere for 20 minutes: 1) 1Hz rTMS stimulation on the HK (HK-rTMS), 2) 1Hz rTMS stimulation on thehMHS(hMHS-rTMS), and 3) sham stimulation (Sham-rTMS). For all participants, functional near-infrared spectroscopy (fNIRS) was applied for measurement of cerebral oxyhemoglobin (oxyHb) and deoxyhemoglobin (deoxyHb) concentration over the non-dominant (right) hemisphere during a serial reaction time task (SRTT) with the non-dominant (left) hand before and after each condition. Results: The average coordinates of the hMHS (x = -39.60 mm, y = -17.11 mm, z = 66.40 mm) were anterior and lateral to the HK (x = -36.72 mm, y = -28.87 mm, z = 56.41 mm). In fNIRS time-series analysis, the integral value of oxyHb was significantly increased over the motor cortical region of the non-dominant hemisphere after the hMHS-rTMS compared with ShamrTMS. The HK-rTMS also showed slight increment of oxyHb concentration but without statistical significance. The SPM group analysis showed greater magnitude of the activity in hMHS-rTMS than that of HK-rTMS after stimulation (p < 0.05). Conclusions: These results demonstrated an interhemispheric modulation effect of hemodynamic changes by 1Hz rTMS. The hMHS produced a more robust modulation effect of 1Hz rTMS on the contralateral hemisphere than did the HK. Therefore, the rTMS can be considered a better stimulation target than the HK.