**3. NIRS: Principles and methods in the assessment of the human cerebral cortex**

Biological tissue is relatively transparent to light in the near-infrared range. In NIRS, light at wavelengths of 700–900 nm is illuminated onto the scalp. Oxy-haemoglobin and deoxyhaemoglobin chromophores have different absorption spectra that are used to assess the attenuated light levels via changes in the chromophore concentration; these concentration changes can be used to determine the haemodynamic response.

Because oxygen consumption increases in the region of the brain with the greatest activity, changes in the oxygen content of the blood can be used to measure the relative levels of brain activity. The specific parameters of the response monitored with NIRS represent changes in the levels of oxygenated and deoxygenated haemoglobin at specific regions across the cerebral cortex during stimulation. The detailed principles of NIRS have been described previously (e.g. Gervain et al., 2011; Hoshi, 2007; Minagawa-Kawai et al., 2008; Strangman et al., 2002).
