Your brain is made of maps
The cortex has a surprisingly simple organizing principle.
[Originally posted on Medium 01/27/2017; edited and references updated 02/05/2023; additional reference update 7/6/2023].
Neurons in the mitten-shaped part of the brain called the “cortex” are not arranged at random. In some parts of the cortex, neurons are laid out in maps based on the properties of stimuli to which they respond. The response patterns of the neurons change systematically as you move across the cortical sheet. Within one of these map-like regions, where a neuron is located predicts what information it will process best.
In short, the brain has a “topographic principle.”
Auditory Maps
The simplest example of a brain map is the primary auditory cortex, the first part of the cortex to process sound.
The auditory cortex’s “job” is to start translating sound waves into meaningful information about the world (loudness, pitch, distance, rhythm, timbre, presence of speech or music, etc.).
The auditory cortex receives signals from the cochlea in the inner ear. The cochlea gradually varies in thickness so that one end vibrates the most in response to high-pitched sounds, and the other end vibrates the most for low sounds. The stronger the vibration, the stronger the signal transmitted to the brain.
The auditory cortex organizes itself in the same way. It creates a map where the farther you move along the cortical sheet, the higher the pitch to which neurons respond best. [1] This organization of brain tissue by response to pitch is called a “tonotopic” arrangement.[2]
Visual Maps
The primary visual cortex is also arranged topographically. However, unlike the auditory cortex, it is a radial map, organized based on where a stimulus falls relative to the center of the circular retina.
Many cells in different parts of the retina transmit information to the visual cortex. The visual cortex determines, based on which cells relayed the signal, where in the retina a signal came from. So, the visual cortex is a “retinotopic” map.
Because the location of a signal in the retina corresponds to its position in the visual field, this retinotopic map maps the visual field.
Complication you may want to skip if you’re in a hurry: As shown in the image above, an added complication occurs: the information from the left retina goes to the right brain hemisphere and information from the right retina goes to the left brain hemisphere, creating two retinotopic maps. These adjoin each other where the visual cortex wraps around the back of the brain.
The retinotopic map illustrates a second principle of brain organization:
More cortical space will be devoted to processing important information than less important information.
In other words, the amount of space a brain map dedicates to a stimulus reflects its usefulness more than its real physical properties.
When the amount of cortical space devoted to processing an important signal expands, this is called cortical magnification.
In terms of real world maps, the equivalent might be a world map that bases the size of other countries on the friendliness of the United States’ relations with them. In such a map, the U.K. and Israel would be disproportionately large, and China and Russia would be disproportionately small.
In the visual cortex, cortical magnification is greatest for information coming from the center of the retina, and diminishes with “eccentricity,” or distance from the center.
Cortical magnification increases the space dedicated to processing information coming from the center of the retina because this is the area of most acute, detailed vision.
The most interesting maps in the brain
We normally think of maps as illustrating the space outside our bodies. However, the body is also a space that can be mapped, and your brain needs to chart it. That way, when somebody taps you on the shoulder, you can feel that the pressure comes from your shoulder, not your knee.
Your brain also uses information from your internal organs to create its body map, so you can tell whether, say, a sharp pain comes from your abdomen or your chest.
A region called the somatosensory cortex maps out your body space. It uses several senses, primarily touch.
The location of the body parts within the map is less continuous than you might expect after seeing the visual and auditory cortical maps, but there’s a certain logic. Notice that the overall shape of the body from feet to head to hands is arranged in a continuous order. However, important body parts with large numbers of nerve endings get their own separate place in the map.
These include the face and its features; the tongue and pharynx; the abdominal organs; and the genitals. These regions, too, are placed in a logical position relative to the body schematic and to each other. For example, the genitalia are located close to the legs and feet, just as in real life. The lips, teeth, gums, jaw, and tongue are together. They neighbor the pharynx, which works with them, and the rest of the face, where they are located. The intra-abdominal organs are next to the pharynx, perhaps because they work together when eating.
These important body parts have more nerve endings that send information to the brain. They also take up much more cortical space in the map, or undergo cortical magnification. Your fingertips need to be more sensitive than your back, so they have denser nerve endings, and they take up more space in your somatosensory body map.
If you use the proportions of body features in the somatosensory cortex body map to create a 3D figure, it looks like this:
That’s not an alien or a caricature of an ancient humanoid species. It’s your body according to your somatosensory cortex.
You have a second body map in your motor cortex, which commands your muscles to move. It has a slightly different, but equally ridiculous-looking, homunculus.
The motor map of your body follows the same principles as the somatosensory one.
There is an overall continuous body map with key organs separated, but placed as close as possible to their real position.
Key organs are cortically magnified. [3]
The more precisely a body part can move, the more space it takes up in the motor cortex body map. That’s why the hands are enormous and the feet are tiny.
The motor cortex’s body map looks even more distorted than the somatosensory one because the torso and arms barely feature at all.
Plastic Maps
A classic study of string musicians suggests that the brain’s maps change with experience. Specifically, cortical magnification shifts. Frequently-used parts of the map expand into neighboring space, while underused ones shrink.
Thomas Elbert and colleagues examined the somatosensory cortex of string instrument players, because they use their left hand much more extensively and precisely than their right hand. (The left hand, especially the second through fifth fingers, is used for “fingering,” or determining the exact pitch the instrument produces).
Thus, one might expect to see changes in musicians’ maps for their left hands, but not their right hands.
In this study, musicians were compared with non-musicians. The researchers located each person’s hand somatosensory map by applying light pressure to the first and fifth digit of each hand. They found that the musicians’ center of responsivity for the left hand was stronger than non-musicians’. Furthermore, its location had shifted along the cortical surface.
The researchers argued that the musicians devoted more space in their body map to the left hand, which strengthened and shifted the center of response.
For the right hand, as expected, the maps of musicians and non-musicians did not differ.
The age at which musicians started playing correlated with the magnitude of these brain changes: the more experience they had playing string instruments, the greater the cortical magnification. This correlation is more informative than an absolute difference between musicians and non-musicians, because it increases the likelihood that group differences came from different experiences using their left hands, and not from other differences in genetics or upbringing.
This study, conducted at the dawn of fMRI in 1995, used the primitive imaging and analysis techniques of the day. However, it spawned a line of research on the effects of long-term instrument playing on sensory and motor maps, using ever more sophisticated methods. Changes over time have been observed in children learning a musical instrument. The effects may be both structural and functional, and likely go beyond the simple changes in cortical magnification I’ve discussed here.
Three Key Principles of Brain Organization
1. Your brain creates “maps” where things that are more similar are located close together, and things that differ are located farther away.
2. Stimuli that are more important take up more space in the map than stimuli that are less important.
3. Experience can change the distribution of how neurons in the map respond to specific stimuli.
These principles matter because they draw a link between where things happen in the brain and what the brain actually does.
Where things happen is easy to measure, but uninformative when considered alone. Thus, neuroscientists consider figures showing blobs of activity on brains with descriptions like “your brain on x” to be meaningless.
On the other hand, we can’t directly measure what we really want to know — the processes the brain carries out (although we can measure the resulting behavior).
Topographic organization does not explain everything the brain does, and it describes some regions of the cortex better than others. However, topographic organization is a meaningful, predictable relationship between what the brain does and where this activity occurs. Better still, this relationship is intuitive and, with a little simplification, easy to visualize.
Next time you read a news article about the locations of important brain functions or neuroscientists’ critiques of this sort of study, keep in mind these three principles of mapping in the brain.
Notes
[1] I say “best respond” because neurons follow a “tuning curve.” A specific stimulus, such as a frequency of light or sound, evokes the maximal response in an individual neuron. However, a frequency that is only slightly higher or lower will also evoke a response, just a slightly weaker one. The closer a stimulus is to the one the neuron is “tuned” for, the stronger the response; the less similar, the weaker the response.
[2] As taught in undergraduate and graduate neuroscience textbooks and shown here, the whole primary auditory cortex is one single continuous tonotopic map. However, that is an oversimplification. There appear to be six separate tonotopic maps within the human auditory cortex, arranged in an intermingled and nonlinear way.
[3] Note that this map occurs at a very high level. Alan H.D. Watson observes that “although the region of motor cortex controlling the hand is relatively easy to define, the representations of the muscles moving different fingers or individual joints overlap to a considerable degree.”
Further Reading
Altenmüller, E., & Furuya, S. (2016). Brain plasticity and the concept of metaplasticity in skilled musicians. Progress in Motor Control: Theories and Translations, 197-208. Open access PDF.
Furuya, S., & Altenmüller, E. (2015). Acquisition and reacquisition of motor coordination in musicians. Annals of the New York Academy of Sciences, 1337(1), 118-124. https://doi.org/10.1111/nyas.12659. Open access PDF.