“…Several studies have shown that the anatomical structure affects the sensitivity of the fNIRS signal by using realistic 3D head models (Custo, Wells, Barnett, Hillman, & Boas, 2006; Fukui, Ajichi, & Okada, 2003; Hoshi, Shimada, Sato, & Iguchi, 2005; Strangman et al, 2003). In particular, by constructing subject‐specific head models, Nakamura et al found that variability of the partial optical pathlength in the brain was very high between subjects and between fiducial points, and that partial optical pathlength was strongly associated with the depth of the brain surface (Nakamura et al, 2016). Moreover, the partial optical pathlength of the fNIRS probe pair depends not only on the individual's head structure, but also on the local head structure to which the probe is attached, even for the same individual subject.…”