What Is fNIRS, and How Does It Let Scientists Study the Brain Outside a Scanner?

Brain scanners like fMRI can produce detailed images of activity deep inside the brain, but they require a person to lie still inside a large, noisy machine. That makes them useless for studying the brain during an ordinary conversation, a walk, or a face-to-face interaction with another person, or a robot.

How it works

Functional near-infrared spectroscopy, or fNIRS, solves that problem with a lightweight cap or headband fitted with light sources and detectors. It shines a safe, near-infrared light through the skull and measures how much of that light bounces back after passing through blood in the outer layers of the brain. Active brain tissue uses more oxygen, which changes how it absorbs and scatters the light, so the pattern of returning light reveals which regions are working harder at a given moment.

What it can and can't do

Because the light can only penetrate a few centimeters, fNIRS mainly captures activity in the cortex, the brain's outer layer, and cannot see deeper structures involved in emotion and memory, such as the amygdala or hippocampus. It also has lower spatial precision than fMRI. Its advantage is portability: researchers can record brain activity while someone sits, talks, walks or interacts naturally with another person or a machine, conditions no traditional scanner allows.

Where it is used

fNIRS has become a common tool in studies of social interaction, human-robot trust, workplace ergonomics, infant brain development and sports performance, anywhere researchers need to track prefrontal activity in a realistic setting rather than on a lab bench inside a scanner.