Identification of scalp blood flow in NIRS data based on Granger causality

NIRS (Near infra-red spectroscopy) is a spectroscopic device to assess the dynamic changes in the hemoglobin concentration evoked by brain activity non-invasively. It has been said that NIRS data reflects brain activities of cortical surface. Recently, it is reported that NIRS data reflects not only cortex blood flow but also the scalp blood flow. To discuss about this matter, we applied Granger causality for the NIRS data to detect the relationship between scalp and cortex blood flows in motor execution. Five healthy subjects took part in the experiment. We measured scalp blood flow and conventional NIRS data simultaneously using a high density probe holder. As a result, in four of five subjects, we detected the Granger causality strongly from the scalp blood flow data to the conventional NIRS data. This method can be useful to quantify the scalp blood flow component included in the conventional NIRS data.