response has been much more difficult to measure, and more controversial, than hyperoxygenation. (See Buxton, 2001 and Obrig et al., 1996, 2000a,b, for more detailed discussion of this topic.)

Because oxygenated and deoxygenated hemoglobin (oxy-Hb and deoxy-Hb) have characteristic optical properties in the visible and near-infrared light range, the change in their concentrations during neurovascular coupling can be measured optically (Chance et al., 1998; Villringer and Chance, 1997). Most biological tissues are relatively transparent to light in the near-infrared range of 700-900 nm largely because water absorption is relatively low at these wavelengths. However, the chromophores oxy-Hb and deoxy-Hb absorb specific wavelengths in that range. Thus, that spectral band is often referred to as the optical window for the noninvasive assessment of brain activation (Jöbsis, 1977). Photons introduced at the scalp pass through most of the tissue and are either scattered by it or absorbed by oxy-Hb and deoxy-Hb. Because a relatively predictable quantity of photons follows a banana-shaped path back to the surface of the skin, the photons can be measured at the scalp with photodetectors (Gratton et al., 1994). Changes in the chromophore concentrations cause changes in the intensity of detected light and are quantified according to a modified Beer-Lambert law, essentially an empirical description of optical attenuation in a highly scattering medium (Cope and Delpy, 1988; Cope, 1991). If absorbance changes at two (or more) wavelengths, one of which is more sensitive to oxy-Hb and another to deoxy-Hb, are measured, changes in the relative concentrations of these chromophores can be calculated. Using those principles, researchers have demonstrated that it is possible to assess brain activity through the intact skull in adult humans (Chance et al., 1993; Gratton et al., 1995; Hoshi and Tamura, 1993; Kato et al., 1993; Villringer et al., 1993). Other chromophores, including cytochrome-c oxidase, can also be assessed optically. Cytochrome-c oxidase, a marker of metabolic demands, holds the potential to provide more direct information about neuronal activity than hemoglobin (Heekeren et al., 1999; Jöbsis, 1977). However, because cytochrome-c oxidase is used much less often than the hemoglobin-based measures, it will not be discussed further here (see Heekeren et al. [1999] for more detail).

Typically, an fNIRS apparatus comprises a light source, which is coupled to the participant’s head via either light-emitting diodes (LEDs) or fiber-optic bundles, and a light detector that receives light after it has interacted with tissue. Light scatters after entering tissue, and a photodetector placed 2-7 cm away from the optode, an optical sensor device that optically measures a specific substance usually with the aid of a chemical transducer, can collect light after it has passed through tissue. When the distance between the source and the photodetector is set at 4 cm, the fNIRS signal is sensitive to hemodynamic changes within the top 2-3 mm of the cortex and extends 1 cm to either side perpendicular to the source-detector axis (Chance et al., 1988). Studies have shown that the gray matter of the cortex can be imaged with interoptode distances as short as 2 cm (Chance et al., 1988; Firbank et al., 1998). Several types of brain activity have been assessed

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