At some time or another, we have all witnessed someone in public doing something, either foolish or brilliant, and wonder: What is going on in that person’s head right now? Or, when it comes to some people, we may consider whether there is anything at all occurring beneath their skulls. Although it is certainly not difficult to imagine, for instance, the social advantages from which one might stand to benefit if one could understand the inner workings of others’ minds, it is clear that we lack this ability; however, science need not to.
If the general public was asked whether it is possible to look at neural activity in the brains of people while they perform various tasks in a laboratory, there would be a consensus that, yes, neuroscientists already can do this. Indeed, neuroscientists have been using functional neuroimaging to examine how the human brain responds to a wide range of stimuli for over three decades.
Neuroimaging methods typically include positron emission tomography (PET), functional magnetic resonance imaging (fMRI), electroencephalography (EEG), magnetoencephalography (MEG), and functional near-infrared spectroscopy (fNIRS), and, relative to one another, each of these techniques has unique spatial and temporal advantages and disadvantages. However, all but one of these methods are limited by their ability to investigate the neural underpinnings of cognitive processes in naturalistic, real-life situations: fNIRS is the exception, and it is becoming exceedingly so.
fNIRS is a safe, noninvasive optical imaging technique. It does not use isotopes like PET, contact agents like EEG, or magnetic fields like fMRI; rather, it is a relatively small head device, resembling something comparable to a bicycle helmet, which participants wear on their heads. Similar to fMRI, fNIRS measures changes in concentrations of blood oxygenation to index neural activity in the brain. However, fNIRS uses near-infrared light—instead of a magnetic field—to observe these changes.
Specifically, human tissue and bone are largely transparent to near-infrared light, and so this form of light is shined into the brain through sources and collected from detectors; a source and detector form a channel, and fNIRS is typically a multi-channel system (Bakker, Smith, Ainslie, & Smith, 2012). When this light is sent into the brain, some of it gets absorbed and scattered, and some continues through the brain unobstructed. The intensity of the light that makes it back to the detectors is used to compute changes in concentrations of oxygenated and deoxygenated hemoglobin. However, it is worth noting that fNIRS is limited in that this light cannot penetrate deeper than about 4cm into the brain. fNIRS cannot, therefore, investigate activation in subcortical regions (Lloyd-Fox, Blasi, Elwell, 2010).
The advent of using near-infrared spectroscopy to assess functional activation in the human brain was 25 years ago, and there have been considerable technological advancements to fNIRS systems since this inception (Ferrari & Quaresima, 2012). Of particular significance is the recent development of fiberless, battery-powered fNIRS devices. These systems enable participants to freely perform tasks without the constraints common to other neuroimaging methods, providing an unprecedented opportunity to study cognition in a way that is more ecologically valid—that is, outside the laboratory.
Wireless fNIRS is able to investigate situations that are difficult to contrive in a laboratory setting, namely novel, open-ended tasks, and is therefore an appropriate technique to explore the cognitive processes recruited by such situations. Researchers at University College London (UCL) have recently attempted to show that wireless fNIRS can assess the neural underpinnings of everyday-life tasks. For example, one study, entitled “Using fiberless, wearable fNIRS to monitor brain activity in real-world cognitive tasks,” was conducted in a naturalistic environment—Queen Square Gardens in London—and required participants to remember to respond in certain ways when they encountered social and non-social cues (Pinti et al., 2015). In particular, when encountering a confederate (another experimenter positioned in various locations), they were asked to remember to greet the person with a fist bump; they fist bumped mailboxes for the non-social condition.
Prospective memory refers to our ability to remember to carry out an intention at a particular time in the future, or when a certain event occurs (McDaniel & Einstein, 2007). Thus, prospective memory is recruited when remembering to greet someone, or a mailbox, upon an encounter. Moreover, retrieving a future intention is largely self-initiated because we have to decide when it is appropriate to stop our current activities to realize the intention.
To capture this in the real world, the study asked participants to engage in an ongoing task during the time between forming the intention to greet something and realizing that intention. For example, participants were required to count the number of unobstructed stairways of the Queen Square buildings as they walked. The wireless fNIRS system was successful in observing differences in prefrontal activation between the social and non-social conditions. Specifically, differences were found not only when an intention was retrieved but also when it was being maintained during the ongoing tasks. These findings suggest that cognitive processes such as prospective memory can be studied outside the confines of a laboratory and that fiberless fNIRS is a viable neuroimaging method.
So is fiberless fNIRS the future of cognitive neuroscience? In some respects, no, but in others, yes: These systems are limited in terms of the scientific problem in question, in that, for example, they are not suitable for questions regarding subcortical brain regions; however, fiberless fNIRS presents a unique, and perhaps more sensitive, approach to investigating the processes underpinning everyday-life activities in real-world settings. As Professor Paul Burgess, one of the principal investigators of the aforementioned UCL study, explained at the annual conference of the British Neuropsychological Society on March 17, 2017, “If you are going to study these kinds of processes, [fNIRS] is pretty much tailor-made.”
The future applications of fiberless fNIRS are extensive. Engineers will continue to improve and refine fNIRS equipment, and these technological advancements will enable researchers to study the brain in a diverse set of contexts in the natural world. Imagine exploring what is going on in a brain surgeon’s brain while doing brain surgery. Social interactions between groups of people could be explored—situations in which multiple fNIRS devices are used on participants. The prospects of investigating neural activity in athletes, pilots, astronauts, and so forth are also promising. What is more is that clinical science is perhaps the field in which participants stand to benefit the most from fNIRS, namely neuropsychologists might use this method to study the efficacy of neurorehabilitation interventions.
Thus, future studies will help further establish wireless fNIRS as a valid method for exploring complex cognitive processes in the natural world. We are indeed one step closer to understanding what is going on in people’s heads, as it were, and it seems there may finally be an answer to the call for a more ecological psychology (Neisser, 1976).
Bakker, A., Smith, B., Ainslie, P., & Smith, K. (2012). Near-Infrared Spectroscopy. Applied Aspects of Ultrasonography in Humans (Second Edi, Vol. 16). Elsevier. https://doi.org/10.1016/B978-0-08-097086-8.55061-2
Ferrari, M., & Quaresima, V. (2012). A brief review on the history of human functional near-infrared spectroscopy (fNIRS) development and fields of application. NeuroImage, 63(2), 921–935. https://doi.org/10.1016/j.neuroimage.2012.03.049
Lloyd-Fox, S., Blasi, A., & Elwell, C. E. (2010). Illuminating the developing brain: The past, present and future of functional near infrared spectroscopy. Neuroscience and Biobehavioral Reviews, 34(3), 269–284. https://doi.org/10.1016/j.neubiorev.2009.07.008
Pinti, P., Aichelburg, C., Lind, F., Power, S., Swingler, E., Merla, A., … Tachtsidis, I. (2015). Using Fiberless, Wearable fNIRS to Monitor Brain Activity in Real-world Cognitive Tasks. Journal of Visualized Experiments, (106), 1–13. https://doi.org/10.3791/53336
McDaniel, M. A., & Einstein, G. O. (2007). Prospective memory: An overview and synthesis of an emerging field. Thousand Oaks, CA: Sage.
Neisser, U. (1976). Cognition and reality: Principles and implications of cognitive psychology. San Francisco, CA: Freeman.