Using Brodmann's areas to analyze the human brain's neocortex:
In a module taken from The Brain from Top to Bottom, you can find the following excellent summary of Brodmann's Areas: "The cellular architecture differs sufficiently from one part of the neocortex to another to be used as a criterion for defining cortical areas that are functionally distinct. That is what the German anatomist Korbinian Brodmann did in the early 20th century, when he developed a map of the brain based on the differences in the cellular architecture of the various parts of the cortex. Brodmann assigned each part of the cortex that had the same cellular architecture a number from 1 to 52."
In the Wikipedia images above right, you can see how Brodmann labeled the cortical areas. These images link to Wikipedia's list of Brodmann's Areas for quick reference.
The Canadian internet resource referenced above went on to explain: "Brodmann's intuition, whose accuracy has been confirmed many times since, was that a particular anatomical structure corresponded to a particular function. For example, Brodmann's area 17, which receives information from a nucleus of the thalamus that is connected to the retina, turns out to correspond precisely to the primary visual cortex. And Brodmann's area 4, from which the axons of the large pyramidal cells project to the motor neurons of the spinal cord, corresponds broadly to the motor cortex."
The brain's motor and somatosensory cortical maps:
In the Brodmann's Areas illustration to the left, the primary motor cortex corresponds to the area labeled 4. The neurosurgeon Wilder Graves Penfield (1891-1976) explored this region while treating patients with severe epilepsy in Montreal. Penfield's aim was to destroy nerve cells in the brain responsible for seizures. While the patient was under local anesthesia, Penfield stimulated the brain with electrical probes and observed the patient's responses. He did this to identify areas requiring surgery and to avoid vital areas that should not be destroyed. In doing this, he observed that stimulation to certain areas of the cortex triggered highly localized muscle contractions on the opposite side of the body. According to a module in The Brain from Top to Bottom, the areas of cortex assigned to various body parts "are proportional not to their size, but rather to the complexity of the movements that they can perform. Hence, the areas for the hand and face are especially large compared with those for the rest of the body. This is no surprise, because the speed and dexterity of human hand and mouth movements are precisely what give us two of our most distinctly human faculties: the ability to use tools and the ability to speak."
A homunculus is defined as any representation of a human being. The homunculus to the right (image links to source) displays the proportion of cortex dedicated to controlling different parts of the body. Note the large size of the tongue relative to the size of the foot.
In Evolving Brains, Allman cites the work of Hughlings Jackson in determining how the brain controls muscle movement. In observing that epileptic patients sometimes had seizures confined to a particular location in the body, Jackson "concluded that the muscles were 'represented' in the brain in a particular location, which he deduced to be somewhere in the cerebral cortex or in a nearby structure called the corpus striatum. This theory was a radical departure from the prevalent clinical view of the time, which was that epileptic seizures were caused by a disturbance in the lowest level of the brain stem." Allman writes: "In 1870, Hughlings Jackson's topographic prediction was confirmed by the German physicians Gustav Fritsch and Eduard Hitzig, who discovered the motor cortex by stimulating the surface of the brain in dogs with weak electrical currents and observing discrete movements of the body."
Allman explains that Jackson's observations "relate to three fundamental properties of the neocortex. The first is that the neocortex contains topographic maps, the second is that the parts of these maps which are used the most have the largest representations, and the third is that the neocortex has a key role in the genesis of epilepsy." Allman continues, saying: "The cortical circuitry is highly plastic in that it can change its functional organization in response to experience, and it is crucial for memory formation and storage."
In the illustration to the left (image links to source), areas labeled 3, 1, and 2 represent the somatosensory cortex, another kind of topographical map. If your dog licks the bottom of your foot, tickling it, a specific area of the cortex will be activated. This mapping allows you to know that it is your foot being licked, not the back of your neck. In Evolving Brains, Allman tells us how such topographical maps were discovered. "With the development of electronic amplifiers and oscilloscopes in the 1930s it became possible to record the electrical activity of the cortex. Edgar Douglas Adrian, Clinton Woolsey, and their colleagues found that the region adjacent to the motor cortex was electrically activated by mechanical stimulation of the surface of the body and named it the somatosensory cortex, from the Greek soma, 'body.' When they recorded from a particular site in the somatosensory cortex, they were able to map out a receptive field on the body surface which activated that site. By systematically moving the recording electrode from point to point on the cortical surface they were able to determine the representation of the body surface in the somatosensory cortex; they also found a second map of the body surface nearby."
"In the 1970s," Allman explains, "by using microelectrode recordings, Michael Merzenich, Jon Kaas, and their collaborators were able to establish that there are at least four maps of the body surface in the somatosensory cortex of monkeys. Like the motor cortex maps, the somatosensory cortex maps in primates show a strong emphasis on the hand and face, indicating that the exquisitely sensitive surfaces of the hand, lips, and tongue are connected to much larger areas of cortex than are the less sensitive parts of the body. As with the motor cortex, the somatosensory cortical maps are plastic and the cortical representation expands for the parts of the body that are heavily used. The distinction between somatosensory cortex and motor cortex is not absolute. The motor cortex has some sensory functions and vice versa."
Regarding the role of experience in determining cortical representations, Allman writes: "Functional imaging experiments done in human subjects have also demonstrated that the hand representation expands as a result of performing complex finger movements. The expansion of the hand representation can be observed following short-term training, but it is most notable in Braille readers and in musicians who play stringed instruments. These findings demonstrating the role of experience build upon Hughlings Jackson's original observation: the finer the degree of control and use of a muscle, the larger its representation in the cortex."
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