Brodmann area

A Brodmann area is one of roughly 50 numbered patches of the cerebral cortex — the wrinkled outer sheet of the brain — that a German doctor named Korbinian Brodmann mapped out in 1909. He stained thin slices of brain tissue and looked at them under a microscope, noticing that the cortex is built from layers of nerve cells stacked like floors of a building. In some patches the layers are thick, in others thin; in some the cells are crowded, in others spread out. Wherever the pattern of cells changed, he drew a border and gave the region a number, from 1 up to about 52.

What makes these areas so useful is that the way the cells are arranged (called cytoarchitecture, literally the architecture of cells) often lines up with what that piece of brain does. Area 17, for example, has a distinctive striped layer and turns out to be the first place vision is processed; areas 44 and 45 sit where much of speech is produced. So Brodmann's numbers became a shared address system: a scientist anywhere in the world can say a brain scan lit up in area 4 and others immediately know they mean the strip that controls movement. The map links structure (how the brain is built) to function (what it does), which is why it is still used in textbooks and research more than a century later.