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 noncortical noise (under the assumption that background systemic vascular effects are uncorrelated
 with the presentation of a stimulus).
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+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.
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+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.
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+\\