The difference in the time course of $\mu s\u2032$ between the shorter-wavelength region (500 and 585 nm) and the longer-wavelength region (800 nm) is interpreted based on the change in the wavelength dependence of the scattering spectrum by CSD. Figure 16 shows the reduced scattering coefficient $\mu s\u2032(\lambda )$ before, during, and after CSD obtained using the proposed method. The slope of scattering spectrum during CSD becomes steeper than that before or after CSD. This change in the slope of $\mu s\u2032(\lambda )$ causes an increase in $\mu s\u2032$ at the shorter wavelength in the visible wavelength region (500 to 600 nm) and a decrease in $\mu s\u2032$ at the longer wavelength in the visible wavelength region (600 to 780 nm). The spectrum of the reduced scattering coefficient $\mu s\u2032(\lambda )$ of biological tissues can be treated as a combination of $\mu s\u2032(\lambda )$ for cellular and subcellular structures of different sizes.^{40} Generally, the sizes of the cellular and subcellular structures in biological tissues are distributed as follows: membranes, $<0.01\u2009\u2009\mu m$;^{41} ribosomes, $<0.01\u2009\u2009\mu m$;^{42} vesicles, 0.1 to $0.5\u2009\u2009\mu m$;^{41} lysosomes, 0.1 to $0.5\u2009\u2009\mu m$;^{43} mitochondria, 1 to $2\u2009\u2009\mu m$;^{40} nuclei, 5 to $10\u2009\u2009\mu m$;^{40} and cells, 5 to $75\u2009\u2009\mu m$.^{40}Figure 17 shows the spectra of the reduced scattering coefficient $\mu s\u2032(\lambda )$ calculated by the Mie theory for spheres of various sizes. In the Mie-theory-based calculation, the refractive indices of a sphere and the surrounding medium were set to be 1.46 and 1.35 at a volume concentration of 2%. The slope of $\mu s\u2032(\lambda )$ decreases as the diameter of the sphere $d$ increases. The entire spectrum of $\mu s\u2032(\lambda )$ increases as the diameter of the sphere $d$ increases in the range of $d=0.01$ to $0.2\u2009\u2009\mu m$, but decreases as the diameter of the sphere $d$ increases in the range of $d=0.6$ to $10.0\u2009\u2009\mu m$. Therefore, the dependence of $\mu s\u2032(\lambda )$ on the particle size in the visible wavelength region shown in Fig. 17 implies that the volume increases in structures $<0.2\u2009\u2009\mu m$ (membranes, ribosomes, and small vesicles) contribute to the increase in $\mu s\u2032$ at the shorter wavelength in the visible wavelength region, whereas the volume increases in structures $>0.6\u2009\u2009\mu m$ (mitochondria, nuclei, and cells) contribute to the decrease in $\mu s\u2032$ in the longer wavelength in the visible wavelength region. If the volume increases in all cellular and subcellular structures occur simultaneously, the net scattering spectrum will have a greater slope of $\mu s\u2032(\lambda )$. On the other hand, it has been reported that the slope of the scattering spectrum obtained from the cultured cells during apoptosis, in which the cellular and subcellular shrinkages occur, becomes more gentle than that for nonapoptotic cells.^{44} Therefore, the changes in the slope of $\mu s\u2032(\lambda )$ in the visible wavelength region during CSD obtained by the proposed method indicate the swelling of cellular and subcellular structures generated by water movement between intracellular and extracellular compartments induced by depolarization due to the temporal depression of the neuronal bioelectrical activity. On the other hand, the slope of $\mu s\u2032(\lambda )$ in the NIR region in Fig. 16 appears to be the same at baseline and during and after CSD. As shown in Fig. 17, the slope of $\mu s\u2032(\lambda )$ in the NIR region (800 to 900 nm) is almost independent of the diameter of the sphere except for the case of $d=0.2\u2009\u2009\mu m$. As we mentioned above, the spectrum of the reduced scattering coefficient can be expressed as a combination of $\mu s\u2032(\lambda )$ for the cellular and subcellular structures of different sizes. Therefore, it is likely that the slope of $\mu s\u2032(\lambda )$ in the NIR region is constant even if that in the visible wavelength region is changed due to the morphological changes in the cellular and subcellular structure during CSD. Kohl et al.^{45} assumed that any change in $\mu s\u2032(\lambda )$ of the tissue does not alter its wavelength dependence, which can be justified for $\mu s\u2032(\lambda )$ in the NIR region calculated by Mie and Rayleigh scattering theory for the size and relative refractive indices of the scattering components in tissue.^{46}