Figure 1 –Dr Korbinian Brodmann, German Neurologist, Frontpiece of ‘Localisation in the Cerebral Cortex’, 1909, Public Domain*
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The brain is a complex organ which enables us to think and feel, perceive and remember, laugh and cry. The German Neurologist Dr Korbinian Brodmann shown above undertook a
study of the microscopic study of the brain of humans and many other species. He published his findings in the 1909 German publication ‘Localisation in the Cerebral Cortex’. On the basis of his analysis, Brodmann divided the brain up into the eponymously named Brodmann Areas. In his book he describes the Brodmann Areas in terms of the large surface markings on the brain as well as other large structures in the brain but also in relation to his microscopic analysis. The microscopic analysis was obviously the driver for his conclusions and the relation to the large structures and surface markings was of secondary importance. This can be seen by Brodmann’s careful description of the microscopic properties of the brain even where the relationship with the large structures or surface markings is unclear. This point becomes clearer when Brodmann describes large individual differences in the microscopic properties of some Brodmann Areas. By inference we can only ever truly know the extent of the Brodmann Area in an individual by a microscopic analysis.
However Brodmann was a pragmatist and so he delivered a map of the brain defined by the large brain structures and surface markings on the brain. People who read his work came to interpret it as an easy to understand map of the brain defined by large features that they could readily identify. Before long this interpretation spread and became extremely popular. Even today the Talairach Atlas features information on the Brodmann Areas which result from a visual inspection of the macroscopic features of the brain. The Talairach Atlas is a valuable tool that has been used in brain imaging to standardise brain scans produced by Magnetic Resonance Imaging and other techniques. Below the diagrams illustrate the macroscopic brain map of the Brodmann Areas that made Brodmann’s work so accessible. There are also a number of drawings of histological sections of the brain by Professor Cajal the pioneer of the microscopic analysis of the brain.
Figure 2 – Cytoarchitectonics of human brain according to Brodmann (1909), Public Domain*, The Top Diagram is the Lateral Surface of the Cortex, The Bottom Diagram is the Medial Surface
Figure 3 – 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, Public Domain*
Left: Nissl-stained visual cortex Middle: Nissl-stained motor cortex Right: Golgi-stained cortex
What is quite remarkable is that Brodmann’s work on the human brain which has made such a lasting impression is covered in 21 pages in a recent English translation of his work. For this article, I am looking at the Insular Region. Brodmann’s 1909 description makes no reference to any Brodmann Areas in this region. I was a little surprised by this as he recognises it as a distinct region. We now know that the Insular Cortex is an incredibly important part of the brain that plays a role in emotions as well as the regulation of a number of autonomic activities and the integration of sensory stimuli. While the absence of any Brodmann Areas in the Insular Region should make this article fairly brief there are a few problems which again I found surprising.
During my examination of the literature on the Brodmann Areas I have come to find that the Insular Cortex is associated with Brodmann Areas 13, 14 and 52. Looking firstly at Brodmann Area 52, Brodmann mentions this area in his description of the Temporal Region (see Appendix). He reports that it encroaches upon the Insular Cortex and is a transitional region. So the description of Brodmann Area 52 as part of the Insular Region isn’t convincing although it does have a close relationship. Turning next to Brodmann Areas 13 and 14, I could find no mention of these in the description of the human brain. Brodmann instead mentions areas 13 and 14 in his description of
1. The Guenon monkey
2. The Lemur
3. The Kinkajou
4. The Rodents
What is interesting is that in his description of the Rodents, Brodmann includes the Ground Squirrel and the Rabbit. However the Rabbit is often wrongly considered to be a Rodent when in fact it is a member of the taxonomic order Lagomorpha. Therefore part of his analysis of Brodmann Areas 13 and 14 is based on considering members of two different orders as members of the same order in his analysis. This is unfortunate as Brodmann has not provided a description of these areas in humans.
Finally however Brodmann does describe the human Insular Cortex by dividing it into an Anterior Agranular area and a Posterior Granular area demarcated by a linear extension of the Central Sulcus. Brodmann mentions briefly without much detail some other analysis he has done on the Insular Cortex.
In summary, in his seminal 1909 work on localisation in the Cerebral Cortex, Brodmann description of the human Insular Cortex is incomplete. He assigns no Brodmann Areas to the Insular Cortex or Region but instead describes them in several other species. In one case he mistakenly groups the orders Rodentia and Lagomorpha together in his analysis of areas 13 and 14.
These findings suggest that the cytoarchitectural description of the human Insular Cortex should be revisited.
Brodmann’s Localisation in the Cerebral Cortex. 1909. Translated and Edited by Laurence J Garey. Springer. 2006.
*Public Domain in those countries where the Copyright term of the life of the author (Korbinian Brodmann 1868-1918) plus the additional country specific term has lapsed from Copyright at the time of writing
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