Frequency-specific functional connectivity in the brain during resting state revealed by NIRS

Analyses of spontaneous hemodynamic fluctuations observed on functional magnetic resonance imaging (fMRI) have revealed the existence of temporal correlations in signal changes between widely separated brain regions during the resting state, termed “resting state functional connectivity.” Recent studies have demonstrated that these correlations are also present in the hemodynamic signals measured by near infrared spectroscopy (NIRS). However, it is still uncertain whether frequency-specific characteristics exist in these signals. In the present study, we used multichannel NIRS to investigate the frequency dependency of functional connectivity between diverse regions in the cerebral cortex by decomposing fluctuations of oxygenated hemoglobin (oxy-Hb) and deoxygenated hemoglobin (deoxy-Hb) signals into various frequency bands. First, within a wide frequency range (0.009–0.1 Hz), we observed that both oxy-Hb and deoxy-Hb signals showed functional connectivity within local regions and between contralateral hemispheric regions of the cortex. Next, by decomposing measured fluctuations into narrower frequency components, we determined that only oxy-Hb signals showed frequency-specific functional connectivity between the frontal and occipital regions, emerging in a narrow frequency range (0.04–0.1 Hz). To clarify the coherency of functional connectivity, we calculated the average coherence values between selected channel pairs. This approach demonstrated that functional connectivity based on the oxy-Hb signals between homologous cortical regions of the contralateral hemisphere (homologous connectivity) showed high coherence over a wide frequency range (0.009–0.1 Hz), whereas connectivity between the prefrontal and occipital regions (fronto-posterior connectivity) showed high coherence only within a specific narrow frequency range (0.04–0.1 Hz). Our findings suggest that homologous connectivity may reflect synchronization of neural activation over a wide frequency range through direct neuroanatomical connections, whereas fronto-posterior connectivity as revealed by high coherence only within a specific narrow frequency range corresponding to the time scale of typical hemodynamic response to a single event may reflect synchronization of transient neural activation among distant cortical regions. The present study demonstrated that NIRS provides a powerful tool to elucidate network properties of the cortex during resting state.

Research Highlights
► Frequency-specific functional connectivity during resting state was revealed by NIRS.
► Inter-hemispheric homologous regions showed high coherence over a wide frequency range.
► Fronto-posterior regions showed high coherence only within a narrow frequency range.
► The results suggest that each connectivity has different interaction mechanisms.
► NIRS provides a unique clue to elucidate dynamic properties of the global network.