Regions of the cerebral cortex of the brain have been defined according to the structure and organization of the neurons that they contain. These regions are called Brodmann areas. It is important to note that the term—Brodmann area—is not possessive but refers to the numbering system that Brodmann used to describe the regions. Recently, it was found that Brodmann areas also contain brain regions that are now thought to share similar functions. Though there is ongoing debate about the definition and border of the regions, Brodmann areas are still useful in describing regions of the brain in terms of both function and susceptibility to pathology. Advances in imaging techniques will continue to allow for a more refined description of Brodmann areas.
Brodmann areas were originally described by German anatomist Korbinian Brodmann (1868—1918). He became interested in neuroscience due to the influence of Alois Alzheimer (1864—1915), a German scientist after whom Alzheimer’s disease is named. Brodmann used a histological method known as the Nissl staining method to examine the structure and organization of cells in different regions of the brain. This stain was developed by the German neuropathologist Franz Nissl (1860—1919). It labels neuronal cell bodies, specifically a particular kind of RNA in the rough endoplasmic reticulum, which are also known as Nissl bodies. Brodmann applied this stain to very thin sections of brain specimens that he had acquired. This allowed him to visualize the structure and organization of cells, which is often referred to as their cytoarchitecture. Brodmann published his original cortical maps in 1909 and described the layout of the brains of humans, monkeys, and other organisms. Additionally, neuroanatomists such as Georg Koskinas (1885—1975), Constantin von Economo (1876—1931), and Alfred Walter Campbell (1868—1937) followed in his footsteps. Though Brodmann’s original cortical maps labeled 52 individual areas, several of them are found only in nonhuman primates and thus there are fewer areas in the human brain. His original descriptions included 43 areas in the human cortex. Drs. von Economo and Koskinas followed up on Brodmann’s original mapping of the brain in 1925, 16 years after it was published.
More than 50 years later, in the 1980s, the Brodmann area map experienced a resurgence. This was because several new imaging techniques, such as magnetic resonance imaging (MRI), had been invented and could be used to view the brain. These new modalities utilized variants of Brodmann’s maps to detail structures of the brain. Specifically, the imaging could define regions of patients’ brains that were undergoing some sort of pathology, such as a tumor or tissue loss following a stroke. Furthermore, as some of these new imaging techniques allowed for visualization of brain activity under various conditions, scientists and physicians could begin to correlate the cytoarchitecture of brain regions defined by Brodmann with the functions that they coordinated.
Examples of Brodmann Areas
There are several Brodmann areas that are more widely known or more often referenced. This is because they correspond to a common action, such as movement or sensory perception, or they are involved in certain pathologies.
Brodmann areas 1, 2, and 3 (often referred to in the order 3, 1, and 2) are collectively known as the primary somatosensory cortex. They are located in the parietal lobe of the brain in a region known as the postcentral gyrus. The postcentral gyrus corresponds roughly to a location starting at the top of one’s ear and ending at the top of the head. There is a primary somatosensory cortex in both the right and left halves of the brain. The primary somatosensory cortex serves as the final destination of sensory inputs, such as touch or limb position, from the entire body. An interesting characteristic of the primary somatosensory cortex is that different regions along the postcentral gyrus correspond to sensory input from specific regions in the body. For instance, sensory input about touch perceived in the leg is processed by neurons closer to the top of the head, whereas input from the mouth or tongue activates neurons on the side of the head. This arrangement of inputs that maintains a regional separation according to the location of the sensory or motor input is known as somatotopy or as a homunculus. Somatotopy is Greek for “body” and “place” while homunculus is a Latin word meaning “little man.”
Whereas the primary somatosensory cortex is devoted to the perception of touch, there are other regions in the brain that are responsible for detection of other senses. For example, a region designated by Brodmann areas 41 and 42 is known as the primary auditory cortex. Located in the upper part of the temporal lobe on both the right and left sides of the brain, the primary auditory cortex is responsible for processing auditory information that originates in the ears and is what dictates the perception of sound. Similar to the homunculus of the primary somatosensory and motor cortices, the primary auditory cortex is also organized in a particular manner. Different regions of the primary auditory cortex correspond to sounds of different frequencies; this has been termed a tonotopic organization.
See also: Brain Anatomy; Cerebral Cortex; Homunculus; Somatosensory Cortex; Somatosensory System
Augustine, James R. (2008). Human neuroanatomy: An introduction. Burlington, MA: Elsevier Science.
Dubin, Mark. (n.d.). Locational descriptions of human Brodmann areas, edited from NeuroNames. Retrieved from http://spot.colorado.edu/~dubin/talks/brodmann/neuronames.html
Zilles, Karl, & Katrin Amunts. (2010). Centenary of Brodmann’s map—conception and fate. Nature Reviews Neuroscience, 11, 139—145.