In this month's Nature Reviews Neuroscience, Zilles and Amunts (1) dedicated an article to Korbinian Brodmann and his map, celebrating its 100th anniversary (Brodmann's original work was published in 1909).
First, a little background. Brodmann's original map contains 52 areas; however, areas 12-16 and 48-51 are only found in nonhuman primate brains, so only 43 areas are actually labeled. How Brodmann constructed his "map" is quite complicated. He made numerous razor thin, horizontal slices of human brains. He then stained the cell bodies within those slices and attributed a number to an area if it was cytoarchitectonically distinct from its neighboring areas of the cortex.
Many others followed Brodmann's work with maps of their own. According to the article,
"During the next three decades, Otfried Foerster, Alfred Walter Campbell, Grafton Elliott Smith, Constantin Freiherr von Economo and Georg N. Koskinas argued for localizable anatomical and functional correlation and the segregation of cortical entities"Many of those names may be new to you, which highlight how influential Brodmann's work has been. The reason there are many different "maps" is because brain mapping is not an exact science. Trying to differentiate the cortex based on brain architecture can produce profoundly different results, depending on the staining technique that is used and on the researcher's subjectivity.
"The Vogts used myelin-stained histological sections to study brain architecture (that is, myeloarchitecture). Their myeloarchitectonic map has many more areas (a total of 200) than that of Brodmann, because the Vogts further subdivided the Brodmann areas on the basis of the regionally more differentiated architecture of intracortical nerve fibres."Below is a comparison of the various "maps" that have been produced since Brodmann's work in 1909.
(click to enlarge)
A little historical note for anyone who was forced to memorize all those Brodmann areas, but was hampered by its apparent lack of logic (areas 1,2,3, start in the mid-lateral areas, while the remaining numbers are distributed in a quasi-random order). Each area number was assigned based on the order in which he prepared a slide, hence the apparent randomness of number assignment.
In his time, testing whether each "area" was correlated to a specific function was quite difficult. Over time, as other "maps" were published and his original became criticized for lack of objectivity, his map fell out of fashion. That is until the 1980's, when various brain imaging techniques were developed. Being able to image a live human during the performance of a specific task, it became possible to associate functional data with cytoarchitectual data. It was Brodmann's map that become apart of many of the first software and sterotaxic atlases for these machines.
Brodmann's work helped to revolutionize modern neuroscience. While many other maps have followed Brodmann's, and even though contemporary research has shown that "his map is incomplete or even wrong in some of the brain regions," many of the areas do correlate very well with various functional areas of the cortex, which is why his work still has relevance 100 years later.
Zilles K, & Amunts K (2010). Centenary of Brodmann's map - conception and fate. Nature reviews. Neuroscience, 11 (2), 139-45 PMID: 20046193