Functional connectivity is defined as spatial correlation between slow cerebral hemodynamic changes (<0.1 Hz) at different regions, which is usually based on the measurement of fMRI. Functional connectivity analysis is a useful method for brain science, so it is desired to conveniently perform the analysis. A candidate is NIRS, which provide cerebral hemodynamic information as well as fMRI although its signals come partly from the extracranial layer. When this measure is estimated by NIRS, hence, the effects of the extracranial component should be considered. In this study, we examined the effects of extracranial layer on functional connectivity estimated from NIRS measurement by using a multi-distance probe.
Four healthy subjects participated in the experiments after providing written informed consent. They were instructed to be relaxed in a chair with eyes closed for about 5 minutes. Cerebral hemodynamic changes was measured at 12 channels (source-detector distance (SD) =7.5, 15 and 30mm) located around the left and right middle frontal gyri by ETG-7100 (Hitachi Medical Co.). The effective intracranial hemodynamic changes were obtained by adaptive subtraction or independent component analysis (Funane's method). For adaptive subtraction measurement channels with SD=7.5 and 30mm were used and for Funane's method those with 15 and 30 mm.
The results showed that the difference in connectivities with and without correction of extracranial contribution were significant for proximal measurement points (P<0.05), while the difference was not significant for distant measurement points, which suggested that the coherence length of extracranial hemodynamic changes might be short compared to that of intracranial hemodynamic changes.