The BOLD response: a new look at an old riddle.

Functional imaging has struggled for two decades to establish a quantitative link between the neuronal activity and the change in signal obtained. This review proposes a division of neuronal activity into a component relating to the electrical discharge activity and a component relating to the size of the activated neuronal population. These two components are argued to have opposing influences on the BOLD signal. The influence of these two components on the BOLD signal, are summarised in the standard model.