The sensitivity for each fNIRS channel (source-detector pair) was calculated as the voxel-wise product of the corrected photon fluence obtained for the source and the detector (adjoint field)29. This was normalized by the sum of sensitivity of all voxels, similarly as described by Brigadoi and Cooper, 201531, so each voxel was represented by a percentage sensitivity in respect to the whole volume. Figure 4A depicts the normalized sensitivity result for channel formed by source Fz and detector AFz.Figure 4(A) Illustration of a single-channel photon transport simulation after sensitivity normalization. (B) Normalized sensitivity results for all channels considered based on sources and detectors positions depicted in Fig. 3B. In both cases, the color scale has been set from 10−6 (black) to 3*10−3 (white) and the head atlas was Colin27.