The influence of optode pressure on the quality of functional Near-infrared Spectroscopy signal
Near-infrared spectroscopy (NIRS) is a cutting-edge and optical Brain-Computer Interfaces (BCIs) that measures the concentration changes of oxygenated (Oxy-Hb) and deoxygenated hemoglobin (deoxy-Hb) in the cerebral blood flow. The use of fNIRS as a neuroimaging technique has a rapid growth over the last 20 years due to the significant advantages, including non-invasively, portability, and safe procedure. However, fNIRS signals are influenced by multiple factors that create an ongoing challenge for the quality signal distinction among the studies. The effective factors on the fNIRS signal quality contain the number and placement of the optodes, motion artifacts, heartbeat, and respiration. Besides, the effect of applied optode pressure on the fNIRS signal quality by the preservation of user convenience remains largely unanswered. The presented research contains two phases. The first phase of this master study aims to find some of the efficient metrics for the fNIRS signal quality through the reliable open-access database. Furthermore, the impact of optode pressure variation on the signal quality is distinguished through the user experiments in the second phase. Three pressure levels were applied to the fNIRS optode, and two pressure metrics of Partial Pressure of CO2 (pCO2), and Laser Doppler flowmeter (LDF) were used in the experiments. The user experiments contained four healthy subjects that were asked to do the mathematical calculation task for 120 seconds with 60 seconds of initial baseline. The main conclusions, drawn from the quality metrics analyzed through the reliable datasets indicate four metrics, including Running Correlation (RC) between oxy-Hb and deoxy-Hb, visual checking of the time series, heartbeat extraction, and Moving Variance per channel. Among the founded metrics, RC has been chosen as a quality metric for the second phase due to the stability and quantitative variable. The results indicate a negative correlation between RC and signal quality. The results from the user experiments in the second phase represented that increasing the optode pressure impacts negatively on the signal quality at first and then improves the quality on the maximum pressure level. However, enhancement of the optode pressure to the maximum level creates inconvenience for the subjects and is not an efficient solution to increase the fNIRS signal quality. The study advances future studies especially for investigations of the influential pressure metrics in the fNIRS experiments.
Khezri, Shokooh Alinaghizadeh