This test addresses the capability of instruments and/or methods of data analysis to distinguish between absorption changes occurring in different compartments of the head, in particular, in the cortex versus the overlying tissues. It compares the sensitivity of the measurand $M$ to a small absorption change $\Delta \mu a$ in the lower compartment (index 2, corresponding to the cortex) with the sensitivity to an absorption change in the upper compartment (index 1, corresponding to extracerebral, superficial tissue) by considering the ratio Display Formula
$S2,1=SM,2SM,1=[M(\mu a,1,\mu a,2+\Delta \mu a,2)\u2212M(\mu a,1,\mu a,2)]/\Delta \mu a,2[M(\mu a,1+\Delta \mu a,1,\mu a,2)\u2212M(\mu a,1,\mu a,2)]/\Delta \mu a,1.$(4)