Characterizing Cerebral and Extracerebral Components in TMS-Evoked Near-Infrared Spectroscopy Signals

Near-infrared spectroscopy (NIRS) can be used to study changes in blood volume and oxygenation level due to transcranial magnetic stimulation (TMS). In previous studies, no attention has been paid to the fact that TMS also activates superficial tissue, which may confound the analysis of the NIRS signal originating from the brain. In addition, stimulation-related changes in, e.g., blood pressure and heart rate may induce global interference in the NIRS signal. We delivered TMS trains to the left primary motor cortex of healthy subjects and recorded NIRS from both ipsi- and contralateral hemispheres. In addition, the shoulder of one subject was stimulated and NIRS was recorded simultaneously above the stimulation site. Extracerebral contribution was estimated from the shoulder stimulation data, using principal component analysis (PCA) and from NIRS data corresponding to short source detector (SD) distances (multidistance method). Motor cortex stimulation resulted in pronounced reductions in the concentration of oxygenated hemoglobin (HbO(2)) on the contralateral (non-stimulated) hemisphere. On the ipsilateral (stimulated) hemisphere, a less pronounced decrease of HbO(2) was observed. Also the shoulder stimulation resulted in reduction of HbO(2). Applying PCA resulted in smaller HbO(2) reductions on the contralateral hemisphere and increased signal-to-noise ratio on both hemispheres. The multidistance method also resulted in smaller HbO(2) reductions on the contralateral hemisphere. The present results suggest that some reductions of HbO(2) may be due to extracerebral effects. These effects have to be taken into account when analyzing and interpreting TMS-evoked NIRS signals.