Average spectra of the full GRS data set (grey lines) and our final Gaussian decomposition results (blue lines). The three panels show different ways the average spectra were created: in (a) we use all voxels, in (b) we use only the voxels retained from moment masking, and in (c) we use only voxels whose intensity value is above a S/N threshold of 3. The red line in panel c shows an average of high negative values throughout the GRS data set that was obtained by using only voxels with a value below − 3 × S∕N. The inset inthe right panel shows a zoom-in of the red line to better display the individual smaller negative peaks. See Sect. 3.2 for more details.