NIRS power spectrum at rest. Mean power spectra across all subjects show spectral inflections at approximately 0.08, 0.3, and 1.9 Hz in both the HbO2 and HbR signals. These peaks correspond to vascular, respiratory, and cardiac rhythms, respectively. Nonvascular (i.e., artifactual) signal constitutes a greater relative proportion of total power in the HbO2 signal than in the HbR signal.