Fig. 2.

(a) Two-layer tissue model of the head, which is composed of a semi-infinite bottom layer (i.e., corresponding to the cortical regions of the brain) with a distinct blood flow index, absorption coefficient, and reduced scattering coefficient of $F_c$, ${\mu }_{a,c}$, and ${\mu }_{s,c}^{\prime }$, respectively, and a superficial top layer (i.e., corresponding to extracerebral scalp and skull tissue) with thickness $\ell$, and distinct tissue properties denoted by $F_{ec}$, ${\mu }_{a,ec}$, and ${\mu }_{s,ec}^{\prime }$. The head is probed with a long source-detector separation, ${\rho }_l$ (yellow shading), and a short source-detector separation, ${\rho }_s$ (red shading), and the probe pressure against the head is varied. Increasing the probe pressure from $P^0$ (blue curves) to $P$ (red curves) induces a change in the diffuse correlation spectroscopy (DCS) signal [$g_2(\tau )$] at both the long separation (b) and the short separation (c). These signal changes arise entirely from pressure-induced changes in extracerebral flow.²⁸