Multidistance probe arrangement to eliminate motion artifacts in fNIRS

Functional near-infrared spectroscopy (fNIRS) has the potential of easily detecting cerebral activity. However, in practical fNIRS measurements, a subject's physical or physiological changes such as body movements have often caused serious problems. If such a change is evoked by the tasks being monitored, it strongly correlates with the task sequence, and its interference in fNIRS cannot be eliminated using conventional signal filtering techniques. Hence, further improvement is necessary to eliminate such interference if we intend to use fNIRS on subjects with little or no physical restraint such as infants. We introduced an additional detector (d(2)) between the source and detector (d(1)) positioned in a conventional arrangement. The distances from the source to the detector d(1) and d(2) were set at 30 mm and 20 mm, respectively. Concentration changes of oxygenated and deoxygenated hemoglobin (Delta HbO and Delta HbR) were calculated using the linear combination of absorbance changes at d(1) and d(2). Then tasks such as the upper-body tilting, the head nodding, the breath holding, and the finger opposition were performed by the participant. The statistical significance of the difference in concentration changes of Delta HbO and Delta HbR between task and rest periods was examined using the paired t-test. The results showed that interference due to upper-body tilting, head nodding, and breath holding was reduced by this method. Moreover, in the finger opposition task, a simultaneous increase of Delta HbO and decrease of Delta HbR was observed and these were significantly localized in the activation area by this method.