Wearable fNIRS

A portable optical method that shines near-infrared light through the scalp and skull and measures how much returns, inferring cortical oxygenated- and deoxygenated-haemoglobin changes from the differential absorption. Because it tracks the same neurovascular signal as functional MRI but with light sources and detectors mounted in a soft cap, it can be worn while a person walks, works, or plays, making it a natural partner to mobile EEG.

Its virtues and limits both follow from measuring blood, not spikes. fNIRS is relatively tolerant of the electromagnetic noise and even the motion that cripple wearable EEG, and it reaches signals EEG cannot, but it is slow — the haemodynamic response unfolds over several seconds — and it only sees the outer few millimetres of cortex. Its most serious real-world confound is systemic physiology: scalp blood flow, heart rate, and blood pressure changes contaminate the signal and can masquerade as brain activation unless short-separation channels or regression are used to remove them.