Spectral entropy provides separation between Alzheimer’s disease patients and controls: a study of fNIRS

Functional near-infrared spectroscopy (fNIRS) is commonly used as a non-invasive tool to measure cerebral neurovascular dynamics. Its potential for diagnostics of various brain disorders has been already demonstrated in many recent studies, including Alzheimer’s disease (AD). fNIRS studies are usually based on comparing hemoglobin measurements at baseline and during a specific task. At present, many proposed methods using fNIRS to diagnose AD involve certain tasks, which may be challenging for the elderly and patients with cognitive decline. Here, we propose a method to characterize AD patients and control in resting state, by applying spectral entropy (SE) analysis on oxyhemoglobin and deoxyhemoglobin, HbO and HbR, respectively, and total hemoglobin (HbT) based on fNIRS signals measured from the left and right sides of the forehead. We applied SE to very low frequency (VLF) (0.008–0.1 Hz), respiratory (0.1–0.6 Hz), and cardiac (0.6–5 Hz) bands to find out which band delivered the optimum result. Next, a t test with 0.05 significant level was performed to compare SE values of AD patients and controls. Results from the VLF band looked promising as SE values from AD patients were always significantly higher than those from controls. In addition, this phenomenon was consistent for both sides of the forehead. However, significant differences in SE values in the respiratory band were found from the left hemisphere only, and in the cardiac band from the right hemisphere only. SE value from the VLF band supports a strong argument that it provides good predictability related to the development of AD. We demonstrated that SE of brain fNIRS signal can be an useful biomarker for Alzheimer’s disease pathology.