Lights, Brain, Action: How fNIRS and Movies Are Changing How We Study the Mind
“I don't know who you are. I don't know what you want […] But […] I will look for you, I will find you...”
That’s the iconic line by Liam Neeson in Taken, but it could just as well describe the drive of cognitive neuroscientists studying how the brain makes sense of the world in our daily lives. A key component of their research “arsenals” is the tool they use to measure brain activity – what are they using to “look for” and “find” the answers in the brain? How good is this tool? In a recent study, researchers at Western University set out to study one such tool called functional Near-Infrared Spectroscopy, or fNIRS – a relatively novel and under-used brain-imaging method that holds a lot of potential, especially in clinical populations. Specifically, they aimed to show that fNIRS can provide reliable measures of how our brains respond to rich, real-life scenarios, such as those experienced when watching a movie like Taken.
But what is fNIRS?
fNIRS is a tool that utilizes near-infrared light to measure brain activity. “Near-infrared” (NIR) here refers to the range of wavelengths of light that lies just above the visible portion of the light spectrum. The wavelengths in this range are especially well-suited for the purposes of the tool; unlike ultraviolet or visible light, NIR light penetrates through biological tissues like the skin, bones, and the brain. At the same time, NIR light is absorbed well by hemoglobin molecules in the blood, to varying extents depending on the amount of oxygen being carried by the hemoglobin. Importantly, when parts of the brain are hard at work, the hemoglobin molecules deliver oxygen to those parts. So when researchers shine the NIR light into the head, it travels through the scalp, the skull, and the brain tissue, and reaches the blood vessels in the brain, where it gets absorbed in different amounts depending on brain activity level. By measuring how much of the light is absorbed in different regions of the head, the researchers can thus tell which parts of the brain are working harder during a certain task – the “functional” aspect of this method.
How does fNIRS compare to the other tools in the researchers’ arsenal? For one, fMRI (functional Magnetic Resonance Imaging) and PET (Positron Emission Tomography) are some common brain-imaging methods that require people to lie rigidly in noisy, expensive machines. On the other hand, the fNIRS device is quiet, portable, and wearable. This makes fNIRS ideal in research settings involving children, who may have difficulty staying still, or individuals with illnesses, who may experience difficulty communicating through language and/or behaviour. The equipment is also far more affordable to own and to operate, where a single fMRI session can cost up to several thousand dollars.
Compared to the fMRI scanner (left), fNIRS devices (right) are highly portable, wearable, and quiet. Image Sources: https://www.ndcn.ox.ac.uk/divisions/fmrib/what-is-fmri/introduction-to-fmri (left) https://www.artinis.com/blogpost-all/comparison-fnirs-vs-fmri (right)
Lights, Brain, Action!
Even though the technology that uses NIR to make biomedical measurements has been around since the 1970s, it’s mostly been used to monitor brain oxygenation levels, and to a lesser extent as fNIRS to study the cognitive processes involved in how we think or feel. Researchers like Kolisnyk and colleagues at Western University are hoping to change this, especially given its potential impact in pediatric and clinical settings where the other tools are less viable.
Thus in their recent study, Kolisnyk and colleagues aimed to validate fNIRS as a tool for measuring brain responses to complex real-life scenarios. Participants watched a suspenseful short film (Bang! You’re Dead) and listened to audio clips from Taken while they wore the fNIRS device. This kind of setup that uses movies, in addition to being highly entertaining to the participants, helps mimic how we experience the real world; it allows scientists to study the brain in action while we pay attention, understand language, follow stories, feel emotions, etc. By using fNIRS to measure brain activity during these highly engaging tasks, the researchers specifically aimed to test whether this tool can reliably measure consistent brain signals across different individuals. Moreover, participants were shown scrambled versions of the video and audio clips, which lacked clear storylines or language. This helped researchers test whether fNIRS is sensitive enough to clearly distinguish between brain responses to meaningful and scrambled information.
These evaluations of consistency and sensitivity were crucial to ensure the validity of fNIRS as a tool for studying the brain’s activity during complex situations – just like you would expect a good thermometer to give precise and accurate readings every time. And indeed, Kolisnyk and colleagues found that fNIRS could be a good tool. First of all, it successfully measured brain activity in regions involved in attention, language processing, and story-following, in patterns that matched previous findings using fMRI. More importantly, the measured brain activity patterns were consistent across individuals and were clearly distinguishable between the real and scrambled clips.
A tool for the real world
One limitation of fNIRS is that it can only measure brain activity near the surface, because there is a limit to how deeply the NIR light can penetrate. There are other structures beneath this depth that serve critical functions such as relaying information, processing emotion, storing memory, etc. Nevertheless, its many strengths – non-invasiveness, portability, comfort, and affordability – make it a highly accessible tool. This, combined with the results of Kolisnyk and colleagues’ study, suggests that fNIRS may open up new avenues to understand complex brain functions in diverse populations better than previously thought possible. Imagine understanding what’s going through the minds of infants and toddlers that can’t speak yet, of the individuals who may be unable to communicate due to autism or stroke-related language disorders, or of those who are in critical health condition and require life support at all times that are not compatible with large fMRI or PET machines.
Rather than replacing other brain-imaging tools, fNIRS can complement them, especially in situations where traditional methods fall short. By showing that fNIRS can take consistent, meaningful measurements, this research lays the foundation for broader use in both science and medicine. It’s a step toward understanding the brain not just in labs, but in classrooms, clinics, and homes — wherever brains are at work.
Featured article: Kolisnyk, M. et al. (2024). Assessing the consistency and sensitivity of the neural correlates of narrative stimuli using functional near-infrared spectroscopy. Imaging Neuroscience.