fNIRS Overview

Functional near-infrared spectroscopy (fNIRS) is an emerging brain imaging technology, less mainstream than techniques like MRI, CT, PET, or ultrasound. It's a portable, non-invasive method that uses low levels of non-ionizing light to record changes in cerebral blood flow through optical sensors placed on the surface of the scalp.

How it compares to other brain imaging

Like fMRI, fNIRS measures changes in blood oxygenation as a way of inferring brain activity, but it does so without magnetic fields or ionizing radiation. Compared with fMRI or PET, fNIRS is relatively inexpensive, small, and portable — signals travel through flexible fiber optic cables from a head cap worn by the participant to the recording instrument. That portability makes it possible to record brain activity during real-world tasks, such as walking, standing, or navigating balance perturbations, situations that are difficult or impossible to study inside an MRI or PET scanner. fNIRS is limited to cortical brain activity, since near-infrared light can only penetrate the outer layers of the brain, and it's sometimes combined with EEG for a fuller picture of brain activity.

Preparing for an fNIRS session

• Head measurement — participants are typically asked to measure their (or their child's) head with a measuring tape beforehand, so the correct cap size can be selected — not too tight, not too loose.
• Clothing — fNIRS has no clothing restrictions like MRI does, though hair is usually best left down, without braids or ponytails, so the cap can make good contact with the scalp. If an MRI is scheduled the same day, clothing without metal (no jewelry, buckles, or hair accessories) is recommended. Glasses are fine for fNIRS but not for MRI, so participants with an MRI scan the same day may need MRI-safe glasses or contacts instead.

What happens during a session

A cap containing light sources and detectors is placed on the head — a quick, painless process that can be adjusted for comfort. Once fitted, the system runs a signal optimization check to confirm each source and detector has a strong signal; this sometimes involves gently moving hair out of the way, or using a small amount of water-based, fast-drying gel if hair is especially thick or dark.

Once the signal is solid, the recording begins. The specific task varies by study — common examples include playing a computer game, having a conversation, or even riding a bike, since fNIRS can record data during active movement in a way other brain imaging methods can't. Sessions typically last 20 to 40 minutes, though this varies depending on the study.

What fNIRS can reveal

By tracking relative changes in oxygenated and deoxygenated hemoglobin, fNIRS lets researchers infer metabolic activity and, by extension, brain activation, since cerebral blood flow increases in the areas of the brain being used for a given task — a relationship known as neurovascular coupling. This has made fNIRS especially useful for studying ambulatory motor learning, balance, and vestibular function — questions that are hard to study with imaging methods that require the participant to stay still inside a scanner.