Determine the functional brain activity

PET is powerful means of assessing functional brain activity, although it does not directly measures neuronal events. Rather it indicates levels of (or change in) activity under different conditions. To do this 'image subtraction' is often employed, meaning that activity during a control condition is (literally) subtracted by computer from activity during the active test condition, and the remaining PET activity taken as the index of the activation specific to the test condition.