fNIRS

Functional near-infrared spectroscopy: measuring brain activity from outside the head by tracking blood oxygen with light.

• Near-infrared light passes a few centimetres through skin and skull, and oxygen-rich and oxygen-poor blood absorb it differently.
• Active brain regions draw in more oxygenated blood, so changes in the returning light track local activity.
• It is safe, silent and wearable, unlike MRI, so it suits studies of movement, children and everyday settings.
• It only reaches the outer 1–2 cm of cortex, and the blood response lags neural activity by several seconds.