Refining signal localization: a transcranial magnetic stimulation motor mapping approach to functional near-infrared spectroscopy.

Functional near-infrared spectroscopy (fNIRS) has emerged as a promising brain imaging tool due to its cost-effectiveness and balance between spatial and temporal resolution. However, its reliance on the 10-20 EEG coordinate system for probe placement introduces potential inaccuracies in cortical localization. Despite concerns regarding its spatial precision, the integration of transcranial magnetic stimulation (TMS) with fNIRS for validating signal localization has not been systematically explored. This study aimed to demonstrate the interindividual variability in hand motor representations and how it influences the precision of fNIRS recordings during motor tasks.Neuronavigated TMS was employed on 18 neurotypical adults to map the motor representations of the first dorsal interosseous (FDI) and fourth dorsal interosseous (4DI) muscles. Center-of-gravity (CoG) coordinates from TMS-evoked motor maps were compared with fNIRS channel locations, including the theoretical hand channel defined by the 10-20 EEG system. FNIRS signals were recorded during a hand-grasp motor task, and the subject-specific hand channel was determined by identifying the fNIRS channel closest to the individual's TMS CoG.TMS motor mapping revealed substantial interindividual variability, with 56% of participants demonstrating deviations from the theoretical fNIRS hand channel. TMS motor maps showed that the FDI and 4DI representations were closely positioned, with the 4DI representation slightly anterior to the FDI (= 0.022). Analysis of fNIRS signals indicated that subject-specific hand channels exhibited significantly higher hemodynamic response amplitudes compared to the theoretical hand channel (= 0.0004), suggesting enhanced signal sensitivity when using individualized cortical mapping. Additionally, fNIRS signal variance was significantly higher in the theoretical channel, indicating greater signal variability and lower signal robustness.These findings highlight the limitations of rigidly applying the 10-20 EEG system for spatial localization in fNIRS-based motor studies and show the benefits of integrating TMS-derived cortical mapping for improved signal accuracy and robustness.