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Neuroscience

A foundational new role for near-infrared spectroscopy (NIRS) simultaneously for two individuals who are engaged in an interactive
task. This particular system enables the acquisition of real-time NIRS
An emerging neuroimaging technology, functional near-infrared signals and eye-tracking acquisitions using SMI glasses with scene
spectroscopy (fNIRS), uses optodes secured in a cap worn on the and pupil cameras synchronized to the neural signals. In this
head and is suitable for simultaneous use on multiple subjects in example, each cap includes 42 channels divided into two
natural situations without intolerance to head movement. Like MRI, hemispheres for each subject. Cap configurations are flexible and
NIRS enables the observation of working neural systems in individual can be modified according to experimental aims. Acquisition rates
subjects without toxicity due to ionization. This technology takes for NIRS signals range from 10 to 33 ms with spatial resolution of
advantage of the physiological principle that active neural tissue approximately 3 cm. This temporal resolution is well-suited for
recruits oxygenated blood in greater proportions than non-active measures of connectivity between active brain regions within and
neural tissue. The paramagnetic effects of deoxyhemoglobin across brains, but compared to fMRI, relatively compromised with
(deOxyHb) are reduced within the local micro vasculature during this respect to spatial resolution.
recruitment process. The signal amplification in MRI referred to as
the blood oxygen level-dependent (BOLD) signal (Ogawa et al., Both fMRI and fNIRS signals reflect changes in brain blood flow and
1990) is due to the reduced proportion of deOxyHb and the blood oxygenation, which are coupled to underlying neuronal
resultant decrease in paramagnetic effects. The BOLD signal is also activity. The latter has been well established as recently
detected by NIRS using spectral absorption (Jöbsis, 1977) which demonstrated by Eggebrecht and colleagues (Eggebrecht et al.,
differentiates oxyhemoglobin, OxyHb, and deOxyHb signals. Pulsed 2012) during visual stimulation in healthy volunteers. The high
lasers (using the Shimadzu NIRS systems) emit 3 wavelengths of light positive correlation between fMRI BOLD with deOxyHb and OxyHb is
and detectors measure the changes in oxygenated hemoglobin now well-established (Sato et al., 2013; Scholkmann et al., 2014).
(OxyHb) and deoxygenated hemoglobin (deOxyHb) concentrations.
For each channel, the absorption of near-infrared light at 780, 805, “Raw” fNIRS signals obtained during a finger thumb tapping task for
and 830nm is measured and converted to corresponding a single subject, single run and single optode illustrate the
concentration changes for deOxyHb, total Hb (HbT), and OxyHb acquisition of both OxyHb and deOxyHb signals in response to a
(Matcher & Cooper, 1994) respectively according to the modified task-related time series (Figure 3). Note the anti-correlation between
Beer-Lambert Law (Fig. 1).
the OxyHb and deOxyHb signals, consistent with theoretical
expectations. Due to the known correspondence with neural (rather
Shimadzu Corporation (Kyoto, Japan) is a leading manufacturer of than cardiovascular) events, the deOxyHb signal is expected to be
fNIRS systems. Figure 2 shows a Shimadzu LABNIRS configuration most closely related to fMRI BOLD signal (Franceschini et al., 2006).
specialized for hyperscanning in which signals are acquired

Fig. 1 Absorption spectra for deoxyhemoglobin and oxyhemoglobin. The functions
illustrate a maximum absorption difference between OxyHb and deOxyHb at
780nm and 830nm. Oxygen concentrations in cerebral blood affect the
wavelengths of light that are reflected. The modified Beer-Lambert Law is
used to convert direct measurements of light attenuation at three Fig. 2 fNIRS system (LABNIRS, Shimadzu Corp.) in the Brain Function Laboratory
wavelengths that correspond to concentration changes in deOxyHb (780nm), at the Yale School of Medicine. Participants using the LABNIRS system
total hemoglobin (805nm), and OxyHb (830nm). are shown with SMI (ETG-2) eye-tracking glasses.

Fig. 3 Illustrated anti-correlation of OxyHb (red) and deOxyHb (blue) signals during a finger thumb-tapping task for a single run, single subject, single channel without
smoothing using the Shimadzu LABNIRS system (white arrow).

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