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+When a specific pool of neurons is active, the hemodynamic response is believed to be localized
+within 1–3mmof this neural activity (e.g., Shmuel et al. 2007, using 7T in human occipital cortex,
+V1) and likely represents the local field potentials rather than spiking activity of clusters of neurons
+(Goense&Logothetis 2008).However, the hemodynamic response unfolds slowly compared with
+the rapid change in neural activity. Even after a brief burst of stimulus-evoked neural activity
+(<1 s), the adult hemodynamic response peaks approximately 6 s later and the concentrations of
+Oxy-Hb and deOxy-Hb do not return to baseline until more than 10 s after the stimulus was
+initially presented (Malonek & Grinvald 1996) (see Figure 1). These spatial-temporal aspects of
+the hemodynamic response greatly constrain the designs and interpretations that can be drawn
+from neuroimaging methods that rely on neurovascular coupling.
+\\
+\\
+FMRI employs the presentation of radio wave pulses in a staticmagnetic field to record what is called the blood-oxygen-level-dependent (BOLD)
+response that varies with fluctuations in local concentrations of deOxy-Hb. By contrast, fNIRS
+simultaneously records changes in bothOxy-Hb and deOxy-Hb by employing two wavelengths of
+near-infrared light (e.g., 690 nm and 830 nm), which span the crossover point in their respective
+absorbance spectra.
+\\
+While the spatial resolution of fMRI varieswith the strength of the staticmagnetic field (B0; e.g.,
+.5T, 3T, or 7T) and the specific scanning parameters, the typical fMRI voxel size of 3mm3 is well
+matched to the resolution of the hemodynamic response itself. In contrast, current fNIRS systems
+have a much coarser spatial resolution. As shown in Figure 2, near-infrared light is delivered to
+the surface of the scalp via optical fibers (emitters). Surrounding each emitter are one or more
+optical fibers (detectors) that collect a small fraction of the photons that return to the surface of
+the scalp after passing through various layers of neural tissue between the emitter and the detector.
+Each pair of optical fibers that link an emitter and a detector is referred to as a fNIRS channel
+and samples the underlying tissue from which the hemodynamic response is being recorded.
+Photons leaving the emitter and reaching the detector are primarily absorbed in the surface layers
+above the cortex, including the scalp, skull, cerebral-spinal fluid, and surface vasculature (see
+Figure 2). The remaining photons travel along a banana-shaped trajectory that dips into the
+underlying cortex. The distance between the emitter and detector determines the lowest point of
+the banana-shaped trajectory; the greater the separation between the two, the deeper the trajectory
+through the cortex. However, greater separation also leads to greater attenuation of the signal.
+Current systems typically employ emitter-detector separations of 2–3 cm, which optimize cortical
+sampling and signal strength.However, photons must necessarily enter and exit the brain through the scalp and therefore are substantially modulated by absorption in the superficial layers of tissue
+between the surface of the cortex and the scalp. This introduces a substantial source of noise into
+what is a rather weak signal. Thus, signal averaging, as in the ERP, must be used to factor out
+noncortical noise (under the assumption that background systemic vascular effects are uncorrelated
+with the presentation of a stimulus).
+\\
+\\
+One final consideration regarding spatial resolution of fNIRS is skull thickness. The thickness
+of the skull is directly related to the amount of cortex that is being recorded from in a given fNIRS
+channel. This is important not only because the skull is quite thin in neonates (6mmon average) and
+becomes thicker with age (10 mm by age 7) but also because skull thickness varies across regions
+of the head within a given age (Beauchamp et al. 2011), creating a confound when examining
+absolute changes in cortical activation across ages or across brain regions (e.g., hemispheres). This
+underlying anatomy should be taken into consideration for studies that compare neural activity
+to a given stimulus across fNIRS channels without normalizing these activations to a second
+stimulus. In the future, anatomical data that compensate for this differential path length from
+fNIRS could help to prevent the false attribution of hemispheric and regional differences (unless
+only relative changes in activation comprise the dependent measure). In addition to variability in
+skull thickness, it is important to determine the distance from the tips of the optical fibers on the
+scalp to the depth of the cortical area that is being targeted in a given recording session.
+\\
+\\
+Finally, the temporal resolution of fMRI is typically 0.5 Hz (i.e., a whole-brain sample every
+s). This slow sample rate has proven to be sufficient for most applications because the underlying
+hemodynamic response is an order of magnitude slower. In contrast, fNIRS is typically recorded
+at 10 Hz and higher sampling rates are possible because detection of the optical response is not
+limited by the interaction between slice selection and gradient encoding in fMRI. Thus, fNIRS
+has much better temporal resolution than fMRI and in principle could provide a more accurate
+measure of the shape and timing of the hemodynamic response. In practice, however, that potential
+has not yet been realized, in part because of noise from noncortical, surface-vascular responses
+(which does not affect fMRI) and because infants cannot provide a sufficient number of stimulus blocks (or events) to average out the noise. Moreover, phased-array head coils have improved the
+sampling rates of fMRI (Keil et al. 2013) so that the intrinsic superiority of fNIRS in the temporal
+domain is not likely to be a significant advantage in the future.
+\\
+\\