Article

Functional Near-Infrared
Spectroscopy (fNIRS) for
Assessing Cerebral Cortex
Function During Human
Behavior in Natural/Social
Situations: A Concise Review

Organizational Research Methods
1-23ª The Author(s) 2016
Reprints and permission:
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DOI: 10.1177/1094428116658959
orm.sagepub.com

Valentina Quaresima1 and Marco Ferrari2

Abstract
Upon adequate stimulation, real-time maps of cortical hemodynamic responses can be obtainedby functional near-infrared spectroscopy (fNIRS), which noninvasively measures changes inoxygenated and deoxygenated hemoglobin after positioning multiple sources and detectors
over the human scalp. This review is aimed at giving a concise and simple overview of the basic
principles of fNIRS including features, strengths, advantages, limitations, and utility for evaluating
human behavior. The transportable/wireless commercially available fNIRS systems have a time
resolution of 1 to 10 Hz, a depth sensitivity of about 1.5 cm, and a spatial resolution up to 1
cm. fNIRS has been found suitable for many applications on human beings, either adults orinfants/children, in the field of social sciences, neuroimaging basic research, and medicine. Some
examples of present and future prospects of fNIRS for assessing cerebral cortex function during
human behavior in different situations (in natural and social situations) will be provided.
Moreover, the most recent fNIRS studies for investigating interpersonal interactions byadopting the hyperscanning approach, which consists of the measurement of brain activity
simultaneously on two or more people, will be reported.
Keywords
functional near-infrared spectroscopy, functional near-infrared topography, hemodynamic response,
optical imaging, cortical activation

1
2

Department of Life, Health and Environmental Sciences, University of L’Aquila, Italy
Department of Physical and Chemical Sciences, University of L’Aquila, Italy

Corresponding Author:
Valentina Quaresima, Department of Life, Health and Environmental Sciences, University of L’Aquila, Italy.
Email: valentina.quaresima@univaq.it

2Organizational Research Methods

Introduction
The rapid advancement and availability of functional neuroimaging methodologies have transformed the field of cognitive neuroscience research. The processes of autoregulation and neurovascular coupling maintain cerebral blood flow (CBF) adequate for brain activity. The coupling
between the neuronal activity and the CBF is fundamental for brain functions. In fact, when aspecific brain region is activated, CBF increases in a temporally and spatially coordinated manner
tightly linked to changes in neural activity through a complex sequence of coordinated events
involving neurons, glia, arteries/arterioles, and signaling molecules. The detection of theincreased cerebral oxygenation, secondary to cortical neuronal activation (neurovascular coupling) in response to specific stimuli, has been successfully used to create human brain maps.
Those brain maps allow the visualization of stimulated areas in task-related brain ‘‘activation’’studies. Usually, the CBF increase lasts over the period of the stimulus/task and then subsides
when the stimulus/task is terminated. The mechanism that regulates the induced focal hyperemic
responses has been recently reviewed (Filosa, Morrison, Iddings, Du, & Kim, 2015). So far,functional magnetic resonance imaging (fMRI) (Magistretti & Allaman, 2015), electroencephalography (EEG) (Michel & Murray, 2012), magnetoencephalography (MEG) (Hall, Robson, Morris,& Brookes, 2014), positron emission tomography (PET) (Magistretti & Allaman, 2015), andfunctional near-infrared (NIR) spectroscopy (fNIRS) (Scholkmann et al., 2014) represent themost widely used functional neuroimaging modalities. However, fMRI and fNIRS are the only
functional neuroimaging modalities based on the neurovascular coupling. The discovery of EEG
in 1929 could be considered as the founding of the functional neuroimaging. fMRI in its 24thyear has grown largely because of its noninvasiveness, relative ease of implementation, andreasonably good spatial (1-10 mm) and temporal resolution (about 1 second). In contrast, theelectrophysiological responses measured by EEG, MEG, and electrocorticography have effectually a better temporal resolution (i.e., <1 millisecond). However, with respect to EEG, MEG,
and fNIRS, fMRI has the substantial advantage to resolve small and/or deep structures in thebrain. Moreover, the robust fMRI signal is highly reproducible and reliable even though it isslow. fMRI overcomes the spatial resolution of high density EEG (HD-EEG) and MEG (<2 cm),the high production cost of radioisotopes for PET studies, as well as the ethical limitations fortheir use. Nevertheless, fMRI, as well as MEG, imply large-sized equipment and huge purchase/
operating costs. In addition, fMRI and PET require the subject in a supine position that may yieldaltered hemodynamic changes rather than in seating or standing positions.
It has been proven that the functional activation of the human cerebral cortex can be successfully explored also by fNIRS, optical topography, NIR imaging, diffuse optical imaging (DOI), or
diffuse optical tomography (DOT) (Ferrari & Quaresima, 2012; Scholkmann et al., 2014). Basically, fNIRS is a noninvasive vascular-based functional neuroimaging technology that measures,
simultaneously from multiple measurement sites, concentration changes of oxygenatedhemoglobin (O2Hb) and deoxygenated-hemoglobin (HHb) at the level of the cortical microcirculation blood vessels. For the measurement of the two fNIRS parameters, the spectral features of
O2Hb/HHb in the NIR range are exploited (Figure 1). Thus, fNIRS has become an importantcomplement to the fMRI and HD EEG. The most significant advantages of fNIRS can besummarized as follows: (a) the use of low-cost, silent, and transportable/portable instrumentation(e.g., mobile EEG), making long-term monitoring and repeated measurements of cortical activities possible in various situations, including the natural ones; (b) the measurement of the changes
in O2Hb and HHb, delivering additional information with respect to the fMRI signal, which inturn is based on the blood-oxygen-level dependent (BOLD) signal (that is related only to HHbchanges); and (c) the possibility to move freely during the fNIRS measurements, allowing
measurements even during an outdoor activity in a real-life situation like bicycle riding (PiperQuaresima and Ferrari

3

Figure 1. Absorption spectra of oxygenated (O2Hb) and deoxygenated (HHb) hemoglobin. The difference intheir absorption spectra allows the measurement of their respective concentration. Most of the commerciallyavailable functional near-infrared spectroscopy (fNIRS) systems utilize wavelengths in the wide range (685-850 nm)
in order to maximize the separation between O2Hb and HHb. Beyond 900 nm, the majority of NIR photons areabsorbed by water (peak absorption centered around at 980 nm).

et al., 2014) or walking around in order to accomplish a real-world prospective memory task
(Pinti et al., 2015) since the subject’s body is not fixed as during fMRI/PET measurements. It isnoteworthy to mention that the most recent mobile EEG systems have the great advantage, withrespect to fNIRS, to be miniaturized (Bleichner et al., 2015; Debener, Emkes, De Vox, &Bleichner, 2015; Debener, Minow, Emkes, Gandras, & de Vos, 2012). Although their miniaturization does not interfere with capturing active behavior in social environments, even thesemobile EEG systems are not completely limitations free. On the contrary, social behaviors could
be strongly affected (Pinti et al., 2015) by the visibility of the currently available mobile fNIRS
systems (for an example, see Figure 5).
Considering the well-recognized benefit of using a multimodal approach, recently many concurrent EEG-fNIRS, fNIRS-fMRI, and EEG-fMRI studies have been carried out for investigating
the brain at multiple spatial and temporal scales simultaneously. Especially, concurrent EEG-fNIRS
measurements can be collected without difficulty (Chen, Sandmann, Thorne, Bleichner, & Debener,2016) compared with concurrent fNIRS-fMRI and EEG-fMRI measurements.
Interestingly, Montague et al. (2002) first performed the measurement of the brain activation
simultaneously on two subjects by using two fMRI systems. This hyperscanning approach, also
adopted later in the EEG and fNIRS fields, has opened new perspectives for investigating interpersonal interactions in a social context in the framework of the social cognitive neuroscience (forreview, see Babiloni & Astolfi, 2014; Koike, Tanabe, & Sadato, 2015).
In summary, fNIRS is a very significant alternative/complementary functional neuroimaging
methodology for investigating human cortical function under more realistic, ecologically valid
parameters and in settings not compatible with fMRI, PET, and MEG.
This review is aimed at giving a concise and simple overview of the basic principles of fNIRS
including features, strengths, advantages, and limitations summarized in terms that can be understood even by nonspecialists. Moreover, the main characteristics of the commercially available
fNIRS systems and some examples of the present and future prospects of fNIRS for assessing

4

Organizational Research Methods

cerebral cortex function during human behavior in natural and social situations, also adopting thehyperscanning approach, will be reported.

Search Strategy and Selection Criteria
Articles were identified using the stepwise approach. Papers were retrieved by the authors
through different strategies. First, a search on the two databases PUBMED and SCOPUS was
performed using the keywords functional near-infrared spectroscopy, functional near-infrared
imaging, optical imaging, optical topography, NIR imaging, diffuse optical imaging, diffuse
optical tomography, and hyperscanning. The references were screened, and the full texts ofrelevant publications were retrieved. Additional published reports were identified through amanual search of references in retrieved articles and in review articles. The research was
restricted to literature suitable for human studies published or made available up to January
2016. Research groups known to be active in the field were contacted for gathering further
information. The websites of the commercial systems were searched and visited for exploring
the specifications of the instruments. After collecting all the documentation, a consensus wasmade by the authors to properly select material eligible for inclusion in this review. The
material was sorted according to the type of fNIRS instrumentation. Tables were generated
to report the origin and the properties of each instrument.

Features, Strengths, Advantages, and Limitations of fNIRS
This brief review article does not comprehensively cover all the advancements and pitfalls in all
the aspects of fNIRS methods because they have been reported in detail in two recent technicalreview articles (Scholkmann et al., 2014; Torricelli et al., 2014). The most relevant topics and therelated recent references about fNIRS (from its history to its different technical aspects) are
reported in Table 1 (which includes 18 review articles). The features, strengths, advantages, andlimitations of fNIRS, according to nowadays knowledge, are summarized in Box 1 and Box 2. A
schematic representation of the NIR light traveling through the head is reported in Figure 2. The
NIR laser or light emitting diode illuminates a selected scalp surface. The propagation of the NIR
light in tissue is sufficiently complex; photons bounce off the many boundaries of cellular andsubcellular substructures, changing randomly the direction of propagation. In most tissues,
photons are scattered many times (even 10 times per centimeter of tissue). The light passing
through the scalp/skull/brain surface is highly scattered and attenuated. Then, the emerging light
can be measured by detectors positioned at different distances from the sources on the scalp
surface. The penetration depth of the NIR light depends on the light scattering, absorption(mainly due to O2Hb and HHb), as well as the separation distance of the light source and detector.
For a given source-detector pair, the detected photons are known to more likely have traveledthrough the central area of a banana shape than through its outer portions. Therefore, the fNIRS
measurements are most sensitive to O2Hb and HHb molecules found in that area. Then, theintensity changes of the emerging light can be measured from 1.5 to 2.5 cm below the skull.
The intensity changes revealed at the selected light wavelengths are used for calculating theconcentrations of O2Hb and HHb. Because of the NIR light propagation in tissues, the exact
volume of the illuminated tissue is unknown, and the spatial resolution is at around 1 cm. The
poor spatial resolution depends on the geometry of the array of the sources-detectors, where theseparation distance between source-detector dictates the ‘‘banana-shaped’’ sensing volume intrinsic to diffuse light transport.

Quaresima and Ferrari

5

Table 1. Most Relevant References About fNIRS: From History to Different Technical Aspects.
Topic

Pub Type Reference

History of fNIRS
Basics of near-infrared photon propagation in tissues
Principles, techniques, and limitations of NIRS andfNIRS

State of the art of instrumentation and methodology
of continuous wave fNIRS/fNIRIState of the art of time-domain fNIRS
instrumentation
State of the art of diffuse optical imaging

Factors influencing fNIRS data andrecommendations
Selection of the optimum source-detector distanceCaps for long-term measurements
Reference channel(s) approach for excluding
extracranial contamination
Statistical analysis of fNIRS data
False positives and false negatives in fNIRS
Methodological strategies for making fNIRS data
shareable in the neuroimaging community.
Integration of fNIRS with:
 Electroencephalography
 Functional magnetic resonance imaging
 Transcranial magnetic stimulation

R
R
R
R
R
R
R

Ferrari and Quaresima (2012)
Martelli (2012)
Ferrari, Mottola, and Quaresima (2004)
Hoshi (2007, 2011)
Lloyd-Fox et al. (2010)
Obrig and Villringer (2003)
Scholkmann et al. (2014)

R

Torricelli et al. (2014)

R
R
R
R

Boas et al. (2001)
Gibson, Hebden, and Arridge (2005)
Liao and Culver (2014)
Oriheula-Espina et al. (2010)

A
R
A

Brigadoi and Cooper (2015)
Kassab, Le Lan, Vannasing, and Sawan (2015)
Gagnon, Yücel, Boas, and Cooper (2014)

R
A
R

Tak and Ye (2014)
Tachtsidis and Scholkmann (2016)
Tsuzuki and Dan (2014)

R

Biessmann, Plis, Meinecke, Eichele, and Müller(2011)
Steinbrink et al. (2006)
Parks (2013)

R
R

Note. fNIRS ¼ functional near-infrared spectroscopy; A ¼ recent outstanding article; fNIRI ¼ functional near-infrared imaging;Pub ¼ publication; R ¼ review.

Commercially Available fNIRS Instrumentation
Since the middle of the 1990s, multi-channel fNIRS systems, which utilize arrays of multiple NIR
sources and detectors arranged over the scalp, have been introduced (Ferrari & Quaresima, 2012).
Different commercially available fNIRS instruments with the related key features, advantages anddisadvantages, and parameters measurable by using different techniques have been recently
reviewed in detail (Scholkmann et al., 2014). Briefly, three different NIRS techniques can be used,each based on a specific type of illumination: (a) the continuous wave (CW) modality, which basedon constant tissue illumination, simply measures light attenuation through the head; (b) the
frequency-domain (FD) method, which, illuminating the head with intensity-modulated light, measures both attenuation and phase delay of emerging light; and (c) the time-domain (TD) technique,which, illuminating the head with short pulses of light, detects the shape of the pulse after propagation through tissues. Therefore, the O2Hb/HHb quantitation depends on the fNIRS technology
adopted. The most commonly used CW-based fNIRS instrumentation measures changes of O2Hb
and HHb (with respect to an initial value arbitrarily set equal to zero) calculated using a modificationof the Lambert-Beer’s law (Scholkmann et al., 2014). Considering that the tissue optical path length
is longer than the distance between the source and the detector (because the scattering effects of6

Organizational Research Methods

Box 1. Features, Strengths, and Advantages of Functional NearInfrared Spectroscopy (fNIRS).

















fNIRS is a noninvasive and safe optical technique that uses light emitting diodes or laser
diodes as NIR light sources and different NIR detectors to measure human cerebral cortex
oxygenation changes in response to certain stimuli/tasks (neurovascular coupling). fNIRS
measurements can be performed in a natural environment without restraint and different
posture.
Human tissues are highly scattering media relatively transparent to light in the range 650
to 1,000 nm (Figure 1) where the dominant absorbers are oxygenated (O2Hb) and deoxygenated (HHb) hemoglobin (the oxygen transport red blood cell protein) located in small
vessels (<1 mm diameter) such as the capillary, arteriolar, and venular bed. NIR light isable to penetrate human tissues because the dominant factor in its transport is represented
by scattering, which is typically about 100 times more probable than absorption. As O2Hb
and HHb have different absorption spectra (Figure 1), the changes in the intensity of the
NIR light emerging from the head are utilized to measure O2Hb and HHb concentration
changes.
As consequence of the complex light scattering effect by different tissue layers, the lengthof NIR light path through tissue (optical path length) is longer than the physical distance
between the source and the detector. The spatial distribution of NIR light through thedifferent tissue layers is a banana-shaped region, as sketched in Figure 2.
Adequate NIR light penetration depth (almost half of the source-detector physical distance) can be achieved using a source-detector distance of 2 to 3 cm and 4 to 5 cm oninfants and adults’ head, respectively. The selection of the optimal source-detector separation depends on NIR light intensity, subject’s head region, and age.
fNIRS allows for semiquantitative/quantitative monitoring of important physiologicalmeasures: (a) O2Hb; (b) HHb; (c) total hemoglobin (tHb), (tHb¼O2HbþHHb) (tHb isstrictly related to cerebral blood volume). fNIRS parameters quantitation depends on the
adopted NIRS technology. The commonly used continuous wave-based instrumentations
provide only concentration changes in O2Hb and HHb (with respect to an initial value
arbitrarily set equal to zero) calculated by using the modified Lambert Beer law andexpressed in mmolar  cm. fNIRS measurements are repeatable and reproducible.
Real-time maps of cortical hemodynamic response (optical topography) can be obtained
applying an array of sources/detectors over the scalp (spatial resolution *1 cm). fNIRS ischaracterized by a relatively high temporal resolution (sampling rate up to 100 Hz;
typically between 1 and 10 Hz). Locations of sources/detectors can be captured using
neuronavigation systems.
fNIRS signals fluctuate during the resting state reflecting physiological phenomena (i.e.,
fluctuations attributable to systemic arterial pulse oscillations (*1 Hz) and respiration(*0.2-0.3 Hz) as well as localized blood pressure fluctuations (<0.05 Hz) and functional
connectivity.
fNIRS instrumentation is relatively low/moderate costly, transportable or portable (Table2). fNIRS systems can be miniaturized and even made wearable and wireless (Table 3).
fNIRS is characterized by a very high experimental flexibility with respect to the otherneuroimaging modalities. Compared with functional magnetic resonance imaging (fMRI),(continued)

Quaresima and Ferrari

7

Box 1. (Continued)

fNIRS: (a) is silent and more tolerant to subtle movement artifacts (e.g., overt speech isallowed), (b) measures O2Hb besides HHb providing a more complete evaluation of thecortical hemodynamic response, (c) allows long-time continuous measurements andrepeated measurements within short intervals, and (d) has a higher temporal resolution.
fNIRS technique is compatible with other electrical or magnetic monitoring systems andtherapeutic devices (i.e., pacemaker, hearing aids, cochlear implants, etc.).
fNIRS measurements can be integrated with fMRI, positron emission tomography (PET),
electroencephalography (EEG), or event-related potentials.

different tissue layers are unknown), the O2Hb and HHb signal changes are expressed as mmolar 
cm or mmolar  mm. Only the FD and TD techniques offer the possibility to absolutely characterize
the optical properties of tissues (absorption and reduced scattering coefficients), from which it ispossible to retrieve absolute O2Hb and HHb concentration changes. CW-based systems offer theadvantages of being low cost and easily transportable. In ascending order, CW-, FD-, and TDfNIRS–based instruments increase in cost and technological complexity. The different complexityof the fNIRS instruments is witnessed by the diversity of their cost, which varies from $10,000 up to
over $300,000. There is a wide variety of commercial fNIRS instruments, as reported in Tables 2 and3. Most of the fNIRS systems reported in Table 2 utilize fiber optic bundles to transport NIR light to/from tissues.
The fNIRS measurements can be carried out, for example, while a subject is sitting in anarmchair (Figure 3) or lying on a table. The fNIRS cap is placed on the top of the head. The NIR
light is not harmful and does not provide any sensation. In general, an fNIRS measurement lasts 30
to 60 minutes. The subjects can respond, for example, by pressing a button when they hear or see
certain sounds or words during auditory and visual tasks; the subjects may play games or be
occupied in other activities such as cognitive tasks on the computer or with an experimenter. A
typical activation revealed by fNIRS in a cerebral cortex area is sketched in Figure 4. The increasein O2Hb and the concomitant relatively smaller decrease in HHb reflect an increase in the local
arteriolar vasodilatation, with the consequent rise of the local CBF and cerebral blood volume.
However, the augmented amount of oxygen transported to the activated cortical area typicallyexceeds the local neuronal rate of oxygen utilization, resulting in an overabundance of cerebral
blood oxygenation in the activated area. The precise neuronal origin of the hemodynamic
responses measured by fNIRS and fMRI is still relatively unknown, and it is beyond the scopeof this article to cover these important issues.
So far, the fNIRS systems used for monitoring cortical activation vary in complexity from dual
channels to ‘‘whole-head’’ arrays of several dozen channels. fNIRS data processing/analysis methods permit topographical assessment of real-time regional cortical O2Hb/HHb changes. Data frommultiple simultaneous measurement sites are displayed by fNIRS systems in the form of O2Hb/HHb
map or image over a specific cortical area. So far, fNIRS has lacked the combination of spatial
resolution and wide field of view sufficient to map in detail the distributed brain functions. The
recent emergence of high-density 3-dimensional DOT (using more than 50 fiber optic bundles
covering the head) represents the last generation of fNIRS systems, which opens up more potential
clinical applications by offering mapping higher-order, distributed brain functions either at rest or in
response to interventions (Liao & Culver, 2014). For instance, high-density DOT imaging array has
been recently used to map multiple resting-state networks including the dorsal attention and default
mode networks and cortical responses during language tasks (Eggebrecht et al., 2014).

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Organizational Research Methods

Box 2. Limitations of Functional Near-Infrared Spectroscopy (fNIRS).



















fNIRS is unable to provide information about brain structure for anatomical reference.
A stable contact between source/detector and skin is critical. The layering and dark colorof hair attenuate NIR light.
Cortical hemodynamic responses to cognitive stimuli that involve deep brain regions, suchas basal ganglia and amygdale, cannot be investigated. Therefore, fNIRS measurements
are restricted to outer cortex and have low spatial resolution (around 1 cm). Depth sensitivity of NIRS signal to cortical hemodynamic response depends on many factors (i.e.,source-detector separation, source power, detector sensitivity, optical properties of theskin/skull layers, degree of white matter myelination). For most fNIRS systems, thetypical depth sensitivity is about 1.5 cm. The amplitude of the changes of fNIRS signal
is susceptible to the variation of source-detector position/distance.
The separation of the hemodynamic changes originating either from cerebral tissue or
extra-cerebral tissues/structures (scalp, temporal muscle, skull, frontal sinus, cerebrospinal fluid, and dura) is difficult.
The establishment of the exact spatial origin of the cortical hemodynamic response and theprecise identification of brain areas beneath the fNIRS probes are impossible without theuse of three-dimensional MRI.
The placement of multiple sources/detectors (in particular on the hairy scalp) is time
consuming. Improvements of headgear and attaching are necessary. Sources/detectors
cannot cover the entire head surface, making difficult the simultaneous exploration of all
the possible cortical neural systems. Increasing the number of fNIRS measurement points,
the weight, and the size of the headgear increase with subsequent greater movement
artifacts and larger data exclusion from the data analysis.
Continuous wave-based fNIRS systems cannot measure optical path length, then do notprovide absolute quantification value of fNIRS measures. Optical path length differences
across diverse head regions should be taken in consideration for interpreting fNIRS data.
Technological development of time-domain and frequency-domain–based instrumentations (being able to measure the spatial and temporal variations in optical path length) will
improve the fNIRS sensitivity and the quantitation of fNIRS parameters.
In the case of experimental tasks inducing large systemic vascular changes and breathing
pattern, the monitoring of some systemic parameters (i.e., heart rate, blood pressure, skull
blood flow, partial end-tidal carbon dioxide) is desirable.
No standardization is available yet for fNIRS instrumentations/signal processing/data
analysis and statistics procedures.
The software of fNIRS systems does not provide data stream mining, namely, real-time
input that allows an interactive user interface to adapt his or her behavior.
fNIRS signal artifacts (due to head motion and reduction of the grip of the sources/
detectors on skull) are not automatically corrected by software.
Most of the commercially available fNIRS systems are not approved by the USA Food andDrug Administration.

The methods of analysis of the fNIRS/DOT/DOI signals have been recently reviewed (Scholkmann et al., 2014). In particular, the methods to separate different components, caused by cerebral
and extra-cerebral systemic activity in fNIRS signals—the latter revealed by the shallow lightdetector (Figure 2), have been reported. Over the years, the complexity of fNIRS systems has grown

Quaresima and Ferrari

9

Figure 2. Schematic representation of the near-infrared (NIR) light traveling through the head. Spatial distribution of light flux through the different tissue layers of the head (due to complex light scattering) is simulatedusing the optical properties of scalp, skull, CSF, GM, WM, and so on. The penetration depth of NIR lightdepends on light scattering, absorption, as well as the separation distance of the light source and detector. In
particular, the NIR light goes deeper and with a higher light intensity in the case of: (a) lower skull thickness
compared with a higher skull thickness and (b) a greater source-detector separation. The detected emerging
signal comes mainly from hemoglobin located in small vessels (<1 mm diameter) such as the capillary, arteriolar,
and venular bed. The arrows represent the simplified spatial distribution of the NIR photons through thedifferent head layers. The shorter distance between light source and detector is utilized for measuring thesystemic hemodynamic fluctuations occurring in the superficial layers. These measurements can then be used as
regressors in the post-experiment analysis to remove the systemic contamination and isolate the brain signal.
CSF ¼ cerebrospinal fluid; GM ¼ gray matter of the cerebral cortex; DLD ¼ deep light detector; LS ¼ lightsource; SLD ¼ shallow light detector; WM ¼ white matter.

so that 2- or 3-dimensional topographic images can be obtained. Proprietary reconstruction systems
as well as many different open source software products are available. The disadvantage of using
fiber optic bundles, whose fNIRS systems are equipped, is that the fibers are often heavy and oflimited flexibility and length, perhaps provoking discomfort to the subjects. To overcome thesedisadvantages, different battery-operated multichannel wearable/wireless systems, suitable for performing fNIRS measurements on adult forehead, have been commercialized since 2009 (Table 3).
An example of a 22-channel wearable fNIRS system is shown in Figure 5. This represents the mostadvanced version of fNIRS systems suitable for evaluating brain activation in response to cognitive
tasks performed in normal daily activities (Pinti et al., 2015).

fNIRS to Assess Cerebral Cortex Functions During Active Behavior in Natural and Social
Situations
In 2014, the prestigious journal NeuroImage dedicated a special issue to commemorate the first 20
years of fNIRS research (Boas, Elwell, Ferrari, & Taga, 2014). Its 9 review articles and 49 contributed papers offer a comprehensive survey of the exciting advances driving the field forward andof the myriad of applications that will benefit from fNIRS. The main fNIRS applications in somebranches of the social sciences, including psychology/education, economics, and linguistics, arereported in Table 4. In addition, the main fNIRS applications in functional neuroimaging basic
research and in medicine are included too. So far, fNIRS has been applied successfully to various
research domains in healthy populations (newborns, children, and adults) and in patients withneurological and psychiatric disorders. It’s noteworthy to mention that fNIRS and/or EEG rather

10
CW
CW

Artinis, The Netherlands
fNIRS Devices LLC, USA
Gowerlabs, UK

Hitachi, Japan
Hitachi, Japan

ISS, USAFD
MRRA Inc., USA
CW
MRRA Inc., USA
CWNIRx Medical Technologies, LLC, USA CWNIRx Medical Technologies, LLC, USA CW
Rogue Research Inc., Canada
CW
Rogue Research Inc., Canada
CW
Shimadzu, JapanCW
Shimadzu, JapanCWTechEn, USA

Oxymon Mk III
fNIR Imager 1200SaNTS Optical Imaging
Systema,b
ETG-4000a,c,d
ETG-7100a,c,d,e

Imagent
fNIRS FLEXb
CAV-IIIb
NIRScouta,b
NIRScout Extendeda,bBrainsight NIRS-8f
Brainsight NIRSfLABNIRSa,dSMARTNIRSa,d,e

CW6a,b

200

20
5
5
100
100
100
100
167
167

10
10

250
4
10

80-10224, 48, 52
48, 72, 96,120
32-1,024512
496
Up to 384
Up to 2,048
Up to 144
Up to 2,304
Up to 142
Up to 142

Up to 128
48
Up to 256

Number of
Channels

Yes

Yes
Yes

Yes
Yes
Yes
Yes (2)
Yes (4)

Yes
Yes

Yes
Yes (2)
Yes

www.iss.com
www.mrrainc.com
www.mrrainc.com
www.nirx.net
www.nirx.net
www.rogue-research.com
www.rogue-research.com
www.shimadzu.com
http://www.an.shimadzu.co.jp/
bio/nirs/light_top.htm
www.techen.com

www.hitachi-medical.co.jp
www.hitachi-medical.co.jp

www.artinis.com
www.fnirdevices.com
www.gowerlabs.co.uk

Hyperscanning
Configuration(Subjects)
WebsiteNote. fNIRS ¼ functional near-infrared spectroscopy; CW ¼ continuous wave; FD ¼ frequency domain; Hz ¼ Hertz; EEG ¼ electroencephalography.
aInfant probe available. bHigh-density optical tomography system. cCombined with EGI’s Geodesic EEG System for simultaneous EEG/fNIRS measurements. dVideo interface for realtime recordings of fNIRS-data and video. eCommercially available only in Japan. fIntegrated with the Brainsight navigation system.

CW

CW
CW
CW

Company, Country

DeviceMaximum
Time-Resolution(Hz)
Technique

Table 2. Main Commercially Available Transportable fNIRS Systems.

11

Artinis, The Netherlands
Artinis, The Netherlands
Astem, Japan
DynaSense Inc., Japan
fNIRS Devices LLC., USA
Hitachi, Japan
Hitachi, Japan
Hitachi, Japan
Hitachi, Japan
MRRA Inc., USA
NIRx Medical Technologies, LLC, USA
Obelab, Korea
Shimadzu, Japan

Shimadzu, Japan

Spectratech Inc., Japan
Spectratech Inc., Japan
Spectratech Inc., Japan
Spectratech Inc., Japan
TechEn, Inc., USA

OctaMonPorta Lite
Hb-13N
Pocket NIRS HMa
fNIR Imager 1200Mb
WOT-100
WOT-220c
WOT-HS
HOT-1000
GeniedNIRSportb,e
NIRSIT
LIGHTNIRSc

SPEEDNIRSc,f

OEG-16
OEG-16 MEe
OEG-17APDc
OEG-SpO2a
Techen Wireless

0.76
6.1
6.1
6.1
75

13.3

10
50
1
62
4
5
5
10
10
5
7.8
33
13.3

16
16
16 to 57
16
8

22

8
1
8
2
12
16
22
34
2
496
64
204
20-22

Yes
n.a.
Yes
Yes
Yes

Yes

Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
n.a.
Yes
Yes
Yes

Yes (5)
Yes (5)
Yes (5)
Yes (5)
Yes (2)n.a.

Yes (4)
Yes (7)n.a.
n.a.
Yes (2)
Yes (4)
Yes (4)
Yes (4)n.a.
n.a.
Yes (2)
Yes (4)n.a.

www.artinis.com
www.artinis.com
www.astem-jp.com
www.dynasense.co.jp
www.fnirdevices.com
www.hitachihightech.com
www.hitachihightech.com
www.hitachihightech.com
www.hitachihightech.com
www.mrrainc.com
www.nirx.net
www.obelab.com
http://www.an.shimadzu.co.jp/
bio/nirs/light_top.htm
http://www.an.shimadzu.co.jp/
bio/nirs/light_top.htm
www.spectratech.co.jp/En
www.spectratech.co.jp/En
www.spectratech.co.jp/En
www.spectratech.co.jp/En
www.techen.com

Note. fNIRS ¼ functional near-infrared spectroscopy; Hz ¼ Hertz; n.a. ¼ not available.
aApparent arterial oxygen saturation (SpO2) and pulse wave measurements. bInfant probe available. cCommercially available only in Japan. dHigh-density optical tomography system.
eMedical use in Japan (depression discrimination auxiliary). fMedical use in Japan.

Company, Country

DeviceHyperscanning
Maximum
Time-Resolution Number of Wireless data Configuration
Website(Hz)
Channels Transmission (Subjects)

Table 3. Main Commercially Available Continuous Wave Portable Wireless fNIRS Systems.

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Organizational Research Methods

Figure 3. Example of a functional near-infrared spectroscopy (fNIRS) experimental setting. The subject issitting in a comfortable high-backed chair. The illuminating optic fiber bundles and the collecting ones areassembled into a flexible probe holder fixed to the head by a Velcro brand fastener, adapting it to the size andshape of the head. The fNIRS instrumentation (Oxymon Mk III, Artinis, The Netherlands) is isolated from thesubject by a white screen.

Figure 4. Illustration of a typical cortical hemodynamic response in an event-related paradigm, as revealed byfunctional near-infrared spectroscopy (fNIRS). The evoked hemodynamic response is subsequent (with about a2-second delay) to the earlier electrical activity, which can be measured in real time by EEG/MEG. Indeed, theonset of the neural firing is almost immediate after the beginning of the paradigm, and the succeeding increase in
the oxygenated hemoglobin (O2Hb, red line) and the concomitant smaller decrease in the deoxygenatedhemoglobin (HHb, blue line) (as measured by fNIRS) reflect an increased local arteriolar vasodilatation in thecortex (neurovascular coupling). As a result, the local cerebral blood flow and the total hemoglobin volume[tHb ¼ (O2Hb þ HHb), green line] (strictly related to the cerebral blood volume) raise. EEG ¼ electroencephalography; MEG ¼ magnetoencephalography.

Quaresima and Ferrari

13

than fMRI offer the great advantage to understand the infant brain. This would allow a clarificationof the early development of human mind including the development of the social brain (Geddes,2015). Significant fNIRS studies dedicated to the understanding of infant cognition, typical andatypical development, and language and its development are nicely reviewed in recent articles(Table 4). Interestingly, the fNIRS approach has been adopted for addressing some noteworthy
developmental issues: object processing (Watanabe, Homae, Nakano, & Taga, 2008), social communication (Minagawa-Kawai et al., 2009), social situations across cultures (Lloyd-Fox et al.,2014), and human action processing (Lloyd-Fox et al., 2015b). More recently, fNIRS has been also
used on infants in natural and social situations with live people (Lloyd-Fox et al., 2015a; Urakawa,
Takamoto, Ishikawa, Ono, & Nishijo, 2015). Urakawa et al. (2015) performed their study on 7month-old infants during the social interactive play ‘‘peek-a-boo,’’ in which communicative ostensive signals in both auditory and visual modalities were presented by a real human presenter. A more
prominent cortical activation in the dorsomedial prefrontal area was found in response to social playwith a partner’s direct gaze compared to an averted gaze, suggesting that a partner’s direct gazeshifts an infant’s attention to the partner’s eyes for interactive communication. Lloyd-Fox et al.
(2015a) investigated 6-month-old infants’ sensitivity to ostensive signals for speech and gestureprocessing during naturalistic communicative interactions. The results of this study have indicatedthat only the multimodal presentation of combination of ostensive signals (infant-directed gaze andinfant-directed speech) is able to enhance the activation of the cortical regions known to be involvedin processing auditory and visual aspects of social communication. Both studies have demonstrated
how fNIRS can give a contribution in assessing cerebral cortical functions in infants during naturaland social situations.
Considering the interest of the journal Organizational Research Methods in promoting a more
effective understanding of methodologies and their application in organizational settings, the potential of fNIRS to assess brain functions during active behavior in social environments is nicely
documented by some recent studies (related to speech production, neuroeconomic research, neuromarketing, daily cognitive function—virtual shopping—using virtual reality, and human interactions). In fact, overt speech production and its neural correlates cannot be investigated by PET or
fMRI due to their high sensitivity to movement artifacts. Suda et al. (2010) investigated face-to-faceconversation by a 52-channel fNIRS system. The subjects, while sitting in front of an interviewer,
were asked to talk alternately about a certain topic for a predefined amount of time. The conversation
periods were accompanied by an activation observed in the superior frontal and superior temporalgyri and not observed in the Broca’s area. Kopton and Kenning (2014) have recently reviewed thefNIRS studies with relevance to (economic) decision making and presented a decision table forneuroeconomic researchers that may enable a better determination of the suitability of fNIRS for
studying neuroeconomics. Misawa, Shimokawa, and Hirobayashi (2014) focussed on the price topic(which is a purchasing decision-making factor) and measured the prefrontal cortex activation (16channel OEG-16, Spectratech Co., Japan) induced by the evaluation of the product price. Theirresults suggest that prefrontal cortex activation can be used as an indicator of the extent to which
consumers feel a product price to be ‘‘expensive’’ or ‘‘inexpensive.’’ Okahashi et al. (2014) applied
virtual shopping with three different difficulty levels for investigating in two groups of convalescent
brain-damaged patients and healthy young adults, the difference on prefrontal cortex activation, as
measured by fNIRS (16-channel OEG-16, Spectratech Co., Japan). They reported that both groups
had different task performances and different prefrontal cortex activation patterns as well as different subjective assessment of the rising difficulty of the task. Very recently, Pinti et al. (2015)
evaluated the feasibility of using a portable fNIRS 22-channel wireless system (WOT-220, Hitachi,
Japan) for monitoring the prefrontal cortex activation during a prospective memory task conducted
outside in a typical London street location and mimicking the demands of everyday life. Significant
prefrontal cortex activations in response to social and nonsocial prospective memory cues were recorded.

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Organizational Research Methods

Table 4. Main fNIRS Applications on Human Beings (Either Infant/Child or Adult) in the Field of Social Sciences,
Neuroimaging Basic Research, and Medicine.
Field of Application

Topic

Psychology/education

Attention
Cognition and foodCognition in infants
Development of social brain
functions in infancy
Development (typical andatypical)EmotionExecutive function in children
Gender differences
Interhemispheric organization
Neuroergonomics

Psychology general review article

Pub Type Reference
R
R
R
R
R

Wiley and Riccio (2014)
Val-Laillet et al. (2015)
Aslin (2012)
Aslin, Shukla, and Emberson (2015)
Grossmann (2015)

R

Minagawa-Kawai, Mori, Hebden, andDupoux (2008)
Vanderwert and Nelson (2014)
Wilcox and Biondi (2015)
Doi, Nishitani, and Shinohara (2013)
Schneider et al. (2014)
Moriguchi and Hiraki (2013)
Cheng, Li, and Hu (2015)
Homae (2014)
Ayaz et al. (2013)
Derosière, Mandrick, Dray, Ward,and Perrey (2013)
Cutini, Basso Moro, and Bisconti(2012)
Ferreri, Bigand, Perrey, and Bugaiska(2014)
Nagamitsu, Yamashita, Tanaka, andMatsuishi (2012)
Hofmann et al. (2014)
Costantini et al. (2013)
Ding, Gao, Fu, and Lee (2013)
Holper et al. (2013)
Kopton and Kenning (2014)
Misawa, Shimokawa, and Hirobayashi(2014)
Dieler, Tupak, and Fallgatter (2012)
Minagawa-Kawai, Cristià, andDupoux (2011)
Quaresima, Bisconti, and Ferrari(2012)
Dieler et al. (2012)
Dieler et al. (2012)
Rossi, Telkemeyer, Wartenburger,
and Obrig (2012)
Naseer and Hong (2015)
Strait and Scheutz (2014)
Strait and Scheutz (2014)
Canning and Scheutz (2013)
Liu, Pelowski, Pang, Zhou, and Cai(2015)
Cheng et al. (2015)
Cui, Bryant, and Reiss (2012)
Funane et al. (2011)

R
R
R
R
R
A
R
R
R
R
R

Economics

Linguistics

Psychosomatic and behavioral
disorders in children
Reading comprehension
Social brain
Social interaction
Teacher-student interaction
Neuroeconomic research
Neuromarketing
Language and its development

R
A
A
A
A
R
A
R
R
R

Neurological language disorders
Verbal fluency in psychiatry
Word and sentence processing

R
R
R

Functional neuroimaging Brain computer interface
basic research
Cognitive states measurements
Human-robot interaction
Driving research

R
R
R
R
R

Hyperscanning (multi-subject
measurements)

A
A
A(continued)

Quaresima and Ferrari

15

Table 4. (continued)
Field of Application

Topic

Pub Type Reference
R
A
R

Motor tasks/gait control

Medicine

R
R
R

Resting-state functional brain
connectivity
Alzheimer’s disease

A

Anxiety disorders

A

Attention deficit/hyperactivity
disorderAuditory cortex plasticity after
cochlear implanta

A
A
R

Eating disorders
Epilepsy
Job-related stress
Mood disorders

Neurology
Pain assessment
Parkinson’s disease
Psychiatry

R
R
A
A
R
A
A
R
A
A
R
R

Schizophrenic disorders

R

Stroke recovery/rehabilitation

Traumatic brain injury

R
R
R
A
A
A

Visual cortex ageingA

Substances related disorders

Koike, Tanabe, and Sadato (2015)
Jiang et al. (2015)
Scholkmann, Holper, Wolf, and Wolf
(2013)
Leff et al. (2011)
Perrey (2008, 2014)
Niu and He (2014)
Zeller, Herrmann, Ehlis, Polak, andFallgatter (2010)
Takizawa, Nishimura, Yamasue, andKasai (2014)
Marx et al. (2015)
Chen, Sandmann, Thorne, Bleichner,and Debener (2016)
Saliba, Bortfield, Levitin, and Oghalai(in press)
Val-Laillet et al. (2015)
Obrig (2014)
Kawasaki et al. (2015)
Matsubara et al. (2014)
Onitsuka, Oribe, and Kanba (2013)
Ono et al. (2015)
Takizawa et al. (2014)
Obrig (2014)
Yücel et al. (2015)
Eggebrecht et al. (2014)
Ehlis, Schneider, Dresler, andFallgatter (2014)
Ernst, Schneider, Ehlis, and Fallgatter(2012)
Koike, Nishimura, Takizawa, Yahata,
and Kasai (2013)
Mihara and Miyai (2012)
Obrig and Steinbrink (2011)
Obrig (2014)
Ernst et al. (2014)
Roberts and Montgomery (2015)
Merzagora, Izzetoglu, Onaral, andSchultheis (2014)
Ward, Aitchison, Tawse, Simmers,
and Shahani (2015)

Note. fNIRS ¼ functional near-infrared spectroscopy; Pub ¼ publication; A ¼ recent outstanding article; R ¼ review.
aCochlear implant users are not suitable for functional magnetic resonance imaging (fMRI) investigation.

Applying functional neuroimaging modality simultaneously in two or more people, a recent
technique known as hyperscanning, it is possible to calculate interbrain neural effects that appear
only in interactions between individuals (human interactions) (for reviews, see Babiloni & Astolfi,2014). The hyperscanning approach in fMRI studies is advantageous for determining precisely the16

Organizational Research Methods

Figure 5. Mapping of the prefrontal cortex activity during face-to-face conversation by two wearable functional
near-infrared spectroscopy (fNIRS) systems, each equipped with 22 channels (WOT-220, Hitachi HighTechnologies Corporation, Japan). Each flexible probe unit covers the head in correspondence of the underlying dorsolateral and the rostral prefrontal cortex. The processing unit is worn on the waist. Multiple subject
measurement (up to four people) is allowed. Photo courtesy of Hitachi.

region(s) involved in inter-brain effects. However, it is almost impossible to record inter-brain
effects in daily life using fMRI. By contrast, the hyperscanning approach in fNIRS studies, characterized by a reasonable high temporal resolution, captures moment-to-moment interactions in anatural context (for reviews, see Koike et al., 2015; Scholkmann, Holper, Wolf, & Wolf, 2013). An
example of hyperscanning fNIRS measurement is shown in Figure 5. In particular, a couple ofheadsets, which can be used for neuromarketing studies, offers the potential to gauge consumer
reactions in relatively normal situations. The first hyperscanning fNIRS study goes back to 2011
(Funane et al., 2011). For that study, a 22-channel wireless system (WOT-220, Hitachi, Japan) wasused to monitor cortical responses in two subjects while sitting down face-to-face and synchronously
pressing a button (after counting 10 seconds in their mind) when it was requested. Higher prefrontal
cortex inter-brain coherence was significantly associated with shorter time intervals between buttonpresses. The authors concluded that the performance of a cooperative behavior is associated withinter-brain coherence.
Face-to-face interaction is one of the most common types of social interaction in everyday life;
uncertainties about other people’s intention influence interpersonal interactions. Very recently, Tanget al. (2015) investigated how face-to-face interaction impacts interpersonal brain synchronization
during two-person economic exchange by combining an economic exchange game paradigm with
fNIRS hyperscanning approach (ETG-4000, Hitachi Medical Company, Japan). In their study, pairsof strangers interacted repeatedly either face-to-face or face-blocked while their cortical activation
was simultaneously measured in the right temporo-parietal junction and the control region (rightdorsolateral prefrontal cortex). The related fNIRS results have indicated an increased interpersonal
brain synchronization during face-to-face interactions in right temporo-parietal junction region witha greater shared intentionality between partners. Gender-related social interactions (two-persongame) have been also investigated by Cheng, Li, and Hu (2015). They have observed a coherent
brain activity across dyads in relation with the cooperation specifically with opposite-sex partners.
The effects of this synchronization were observed roughly in frontopolar, orbitofrontal, and left
dorsolateral prefrontal cortex. The authors have suggested the utility of using task-related coherenceQuaresima and Ferrari

17

for differentiating the neural processes that underlie mixed-sex cooperation compared to same-sex
cooperation.
Another important social interaction is represented by teaching, which involves a pupil and ateacher. Previously, Holper et al. (2013) recorded prefrontal brain oxygenation during dialog execution simultaneously in 17 teacher-student pairs using a four-channel wireless fNIRS prototype.
Their main finding was that in the students who successfully transferred the knowledge, a lower
activation (with respect to the students without transfer) was observed. Those results could encourage future studies aimed at investigating brain networks involved in real educational setups whereknowledge is acquired in a complex entangled process involving an interaction between a pupil anda teacher.
Although leadership is considered a notable feature of the human society, little is known about theneural basis of leader-follower communication. Very recently using the fNIRS-based hyperscanningapproach (ETG-4000, Hitachi Medical Company, Japan) in a realistic three-subject interpersonalcommunication context, Jiang et al. (2015) found evidence that human leaders cooperated with theirfollowers to achieve group decision by synchronizing their brain activities with those of the followers through their tactful communication skills and competence. In addition, they found that it
was possible to predict leadership based on the interpersonal neural synchronization data as well as
communication behaviors early in their interactions. Their findings give an important contribution to
the theoretical discussion about the relevance of communications in leader emergence. Finally, theauthors have suggested that the fNIRS hyperscanning approach might be used in neuro-feedback orneuro-intervention during leadership training.
In a multiperson interaction environment, the information exchange process is much more complex than that in a two/three-person situation. Therefore, single-brain level analysis and paired-brain
level analysis may not support comprehensive study of the characteristics of multi-brain interaction
processes. Confronting these challenges, Duan et al. (2015) have recently proposed a generalframework for hyperscanning and modeling multiple interacting brains called cluster imaging ofmulti-brain networks. As a demonstration of this method, the authors have conducted a simple
preliminary experiment in which nine participants drummed together and tried their best to maketheir beats consistent with each other while the oxygenation of the prefrontal cortex and left
temporal parietal junction were simultaneously recorded by two fNIRS systems (ETG-4000, Hitachi
Medical Company, Japan; LABNIRS, Shimadzu, Japan).

Conclusions
The feasibility and the success of applying fNIRS in some branches of the social sciences,
neuroimaging basic research, and medicine have been well documented. The increasing numberof publications (about 280 in 2014; source: SCOPUS 2014, Elsevier B.V.) is promising for an
expansion of fNIRS in these fields. The development of fNIRS has strongly gained from theadvances in microelectronics, computer technology, and optical engineering. With the adventof further miniaturization and integration such as integrated optics, wearable and even disposable
fNIRS technology can be envisioned. The ongoing significant developments in time-domain
diffuse optics could generate compact and even wearable time-domain fNIRS systems (Dalla
Mora et al., 2015). The fNIRS systems that will emerge from these developments would further
enlarge the number of fNIRS applications and facilitate the comparison of the fNIRS findings. The
prediction of the future directions of fNIRS for assessing brain function during human behavior innatural and social situations is not easy. However, fNIRS will give an outstanding contribution inrevealing cortical responses to social stimuli in infant brains and in investigating real social
interactions in adult brains.

18

Organizational Research Methods

Acknowledgment
The authors would like to thank Rosanna De Carolis for the technical support and Stefania Lancia for making
available Figure 3.

Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or
publication of this article.

Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research leading to this article was made possible, in part, by the 2014 grant from the‘‘Fondazione Cassa di Risparmio della Provincia dell’Aquila’’ and the ‘‘Abruzzo earthquake relief fund’’(Toronto, ON, Canada).

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Author Biographies
Valentina Quaresima is a biochemistry associate professor at the Department of Life, Health andEnvironmental Sciences, L’Aquila University, Italy. Her research interest currently focuses on the investigationof the cerebral cortex metabolic/hemodynamic changes in response to cognitive and motor tasks of different
complexity revealed by functional near-infrared spectroscopy (fNIRS).
Marco Ferrari is biochemistry professor at Department of Physical and Chemical Sciences, L’Aquila
University, Italy. He has pioneered near-infrared spectroscopy brain applications. His research activity is
focused on the study of the cerebral cortex metabolic/hemodynamic changes on cognitive/motor tasks ofdifferent complexity by using functional near-infrared spectroscopy (fNIRS).