Korbinian Brodmann was a Neurologist who revolutionised the study of the brain by developing a brain ‘map’. This brain map was based on the microscopic properties of the brain. Brodmann dissected the brains of humans and other species and described the unique properties of brain areas under the microscope. Brodmann described 52 areas which could be distinguished from each other microscopically. This is an important point to remember. These brain areas came to be known as Brodmann Areas. He related them to anatomical landmarks on the surface of the brain including gyri and sulci. His findings were published in an iconic text in 1909 which laid the foundations for a more precise study of the brain.
Figure 1 – Cytoarchitectonics of human brain according to Brodmann (1909), The Top Diagram is the Lateral Surface of the Cortex, The Bottom Diagram is the Medial Surface, Image Public Domain
Figure 2 – Three drawings by Santiago Ramon y Cajal, taken from the book “Comparative study of the sensory areas of the human cortex”, pages 314, 361, and 363 showing the microscopic properties of the brain Staining Methods Differ in the Sections – Left: Nissl-stained visual cortex Middle: Nissl-stained motor cortex Right: Golgi-stained cortex, Image – Public Domain
Video 1 – Knife-Edge Scanning Microscope Used to Visualise the Hippocampus
Fast forward 100 years and in the early 21st century we see the use of Brodmann Areas permeating neurosciences studies. The use of the Brodmann Areas has also found application in some areas of clinical practice including procedures such as Deep Brain Stimulation. Looking more closely at the neuroscience research, the use of the Brodmann Area terminology has been particularly helpful in studies using an imaging modality known as Magnetic Resonance Imaging (MRI). MRI makes use of large magnetic fields applied to the body to image small variations in the magnetic signatures of the body’s tissues. Structural MRI refers to the use of MRI to image the structures in the brain whereas functional MRI typically makes use of changes in blood oxygenation to infer brain activation states over time.
Video 2 – An Explanation of fMRI by Dr Jeff Dunn
By using the Medline database to look at research papers which combined MRI and Brodmann Areas, a quick search (using the search term ‘MRI Brodmann Area*’) resulted in the retrieval of 434 papers. In many of the papers the Brodmann Areas are used to communicate the location of brain activity seen during the study. The assumptions in these studies are that marked changes in blood oxygenation in a brain region accompany increased blood flow and prior increased activity in that brain region. There is also the assumption that this activity can be precisely localised by mapping the activity to a clearly delineated Brodmann Area. I would like to challenge this latter assumption.
In the translation of Brodmann’s 1909 publication approximately 21 pages were devoted to the section on Brodmann Areas in humans (see here). There are three interesting observations about Brodmann’s descriptions
1. Brodmann’s description of the Brodmann Areas is essentially a description of the microscopic properties of the brain. The description of parts of some Brodmann Areas are independent of gross anatomical landmarks.
2. Brodmann described significant individual variation for some Brodmann Areas. This variation by definition could be ascertained only by microscopic examination of the brain.
3. Brodmann recognised that his peers sometimes shared his interpretation of the brain’s structure but at other times differed significantly in the areas they considered distinct.
So the problem is simply stated ‘How can MRI scans visualise the microscopic properties of the brain?’. Well the simple answer is that they can’t and they don’t!
MRI scans are interpreted using software. There are various types of software. One approach maps a brain to the idealised brain used in the Talairach Atlas. Interestingly the Brodmann Areas were approximated on the Talairach Atlas based on anatomical landmarks. Another approach is to use dissections of brains albeit of small sample size as the basis for the mapping software. However all of these approaches suffer the same problem – individual variation. If a person’s brain anatomy differs significantly from the averaged subject within a standardised database used in imaging software then the software generated map may have a significant error.
Indeed from this discussion it will be apparent that Brodmann Maps generated by this method will need to be described using probabilities rather than certainty. The problems run much deeper than this though. We must ask why 21st century Neuroscience is so dependent on a 1909 publication only recently translated into English, spanning only 21 pages and containing only the results of the analysis with a dubious relationship to anatomical landmarks at times.
Is it time for neuroscientists to return to the foundations and refine Brodmann’s work for the benefit of all?
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