The classic motor cortex map is wrong

[sue_frantz]

Need another example of the American Psychological Association’s integrative theme #1: “Psychological science relies on empirical evidence and adapts as new data develop”?

The motor cortex map that psychology instructors have been teaching since the 1940s is now known to be wrong. In retrospect, that’s not too surprising. Wilder Penfield and Edwin Boldrey mapped it by electrically stimulating different areas of the motor cortex and seeing which muscles moved. Today’s brain imaging techniques are, let’s say, more refined.

In this open access Nature article, Evan Gordon and colleagues (2023) report using fMRI to map the motor cortex. And it sure looks different.*

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Figure a is, of course, the classic homunculus. Figure b is what researchers found when they looked at the motor cortex with fMRI. There are two big differences. First, there are three main areas centered by toes, fingers, and tongue. The areas on either side of these core areas, researchers believe, control opponent muscle groups. For example, one elbow area may control the biceps whereas the other elbow area may control the triceps (E. Gordon, personal communication, July 18, 2024).

The second big difference are the three action/body areas. These action/body areas connect to many areas of the brain including those associated with free will, executive control, and pain. The action/body area between the areas centered on the fingers and centered on the tongue has especially strong connections to the visual cortex, perhaps creating a mechanism for eye-hand coordination (Gordon et al., 2023).

What about the somatosensory cortex? Researchers have found a few differences (Willoughby et al., 2021), but we should be able to stick with our existing somatosensory cortex map for the time being. The motor cortex mapping team are working on solving the challenges associated with mapping the entire somatosensory cortex (E. Gordon, personal communication, July 18, 2024). We can let students know that research is ongoing on the specifics, but that the general principle that the greater the sensitivity of the body part, the more area devoted to it in the somatosensory cortex still holds.   

As for the map of the motor cortex, it’s going to take a bit of time for the textbooks to catch up to this new information because of the publishing timeline, but they will get there.

Reference

Gordon, E. M., Chauvin, R. J., Van, A. N., Rajesh, A., Nielsen, A., Newbold, D. J., Lynch, C. J., Seider, N. A., Krimmel, S. R., Scheidter, K. M., Monk, J., Miller, R. L., Metoki, A., Montez, D. F., Zheng, A., Elbau, I., Madison, T., Nishino, T., Myers, M. J., … Dosenbach, N. U. F. (2023). A somato-cognitive action network alternates with effector regions in motor cortex. Nature. https://doi.org/10.1038/s41586-023-05964-2

Willoughby, W. R., Thoenes, K., & Bolding, M. (2021). Somatotopic arrangement of the human primary somatosensory cortex derived from functional magnetic resonance imaging. Frontiers in Neuroscience, 14, 598482. https://doi.org/10.3389/fnins.2020.598482