Redefining Movement: The SCAN Revolution

By Maddy Peterson

In the early 20th century, pioneering neurosurgeon Dr. Wilder Penfield made a groundbreaking discovery. While in the operating room, Penfield stimulated various regions of the motor cortex of his conscious patients and mapped out a representation of the human body (Penfield & Boldrey, 1937). This representation, famously known as the homunculus model, was a model for adjacent brain areas called the somatosensory and motor cortices. Of these brain regions, the motor cortex is of particular interest, and as the name implies, this region plays a role in generating movements, such as those involving the hands, mouth, or legs. Penfield depicted the motor cortex as a meticulously organized map, with specific regions dedicated to controlling distinct parts of the body. This was an invaluable tool for neurologists and neurosurgeons alike. 

Yet, as is the nature of science, even the most established models are subject to evolution and reevaluation. This homunculus model, long considered the cornerstone of our understanding of the motor cortex, was recently challenged by the advent of a new paradigm: the somato-cognitive action network (SCAN) model (Gordon et al., 2023). Because of their role in coordinating bodily actions and cognitive processes, these regions were named the somato-cognitive action network, or SCAN. This shift from the traditional, region-specific homunculus model to the more holistic and interconnected SCAN model offers a more nuanced and accurate depiction of the motor cortex. This in turn not only enhances our comprehension of the brain’s inner workings but also opens new avenues for treating neurological disorders, shaping the future of neuroscientific research and education.

A New Organizational Model

The discovery of SCAN began with the analysis of MRI data from just seven healthy adults ages 24-38 years. At first, everything appeared as expected: brain regions controlling the face, hands, and feet were identified where the old homunculus model would have predicted. However, between these brain regions were areas that didn’t appear to control specific body parts or have a direct role in movement. To make this even more intriguing, these different areas were functionally coupled to each other and other brain regions that are involved in action, physiological control, arousal, and pain. To confirm these findings, the authors replicated their analyses in much larger MRI datasets, which together totaled over 9,000 participants, encompassing neonates, children, adolescents, and adults. Once more, they identified SCAN interspersed between motor regions associated with specific body parts. To take this a step further, they also analyzed MRI data from human infants and primates, once more finding evidence for this unique network. 

So how exactly does this new understanding of the motor cortex compare with Penfield’s model? In Penfield’s traditional view of the primary motor cortex, moving any specific body part is thought to activate a distinct region in the precentral gyrus, a part of the brain associated with motor movements. For instance, there would be a distinct “tongue region” for tongue movements and a separate “toe region” for toe movements in this gyrus. Moreover, Penfield’s model suggested a continuous, linear arrangement of these body-part-specific regions along the cortex, so that regions like the “toe,” “ankle,” and “knee” areas would be adjacent to each other, reflecting the layout of the body. However, the SCAN model revealed a surprise: three areas in the motor cortex didn’t correspond to any specific body part’s movement. Instead of a linear layout, the motor cortex showed a pattern of concentric rings controlling the foot, hand, and mouth, with these rings interspersed by SCAN regions, indicating a more complex and interconnected organization than previously thought.

Looking Forward

The redefinition of the motor cortex’s organization through the SCAN model extends its impact beyond just updating the classic homunculus model; it reshapes our understanding of brain organization as a whole. Importantly, SCAN provides a framework for integrating body control with the planning of actions. Consider complex tasks like playing the piano or walking; they require executive control that orchestrates movement through a detailed plan involving specific brain regions. The SCAN regions are thought to be the areas in the primary motor cortex where this intricate coordination occurs. This integration is crucial for smoothly translating our intentions into physical movements, helping us avoid injury and effectively achieve our desired actions. Furthermore, the SCAN model offers insights into the interplay between mental and physical states – for instance, the physical sensation of “butterflies” in the stomach when feeling nervous. This revised understanding about how different areas within the primary motor cortex collaborate with other various brain regions when performing complex motor tasks also has significant implications for clinical applications.

Beyond the development of advanced technologies such as neural prosthetics, the SCAN model could also inform how researchers approach movement disorders and rehabilitation. Its comprehensive view of motor control could inspire new, more effective, and personalized treatment strategies, particularly in the field of brain circuit therapeutics (Siddiqi et al., 2023). Specifically, this more accurate model of brain regions involved in motor control could enable more precise targeting for direct brain stimulation or transcranial magnetic stimulation. This is especially promising for conditions like stroke, cerebral palsy, and Parkinson’s disease, where motor functions are greatly affected.

While unexpected, this transition from the homunculus model of motor cortex organization to the SCAN model encapsulates the ever-evolving essence of neuroscience. However, Wilder Penfield himself once described his homunculus model as “… a cartoon of representation in which scientific accuracy is impossible” (Penfield & Rasmussen, 1950). His words echo the idea that scientific models are often oversimplifications of complex realities, designed to guide understanding while always leaving room for further discovery and refinement. The emergence of the SCAN model exemplifies this process of scientific progression, challenging and expanding upon our existing knowledge. Rather than being the end of the road, this development paves the way for additional avenues of exploration.

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Written by Maddy Peterson

Illustrated by Judy Luu

Edited by Chris Gabriel, Faustina Jayaprakash, and Keionna Newton

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References

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., Kaplan, S., … Dosenbach, N. U. F. (2023). A somato-cognitive action network alternates with effector regions in motor cortex. Nature, 617(7960), 351–359. https://doi.org/10.1038/s41586-023-05964-2 

OpenStax College. (2013). Sensory Homunculus-en [Vector icon]. Wikimedia Commons. Modified by Popadius & Preoptic (2013). https://commons.wikimedia.org/wiki/File:Sensory_Homunculus-en.svg

Penfield, W., & Boldrey, E. (1937). Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain, 60(4), 389–443. https://doi.org/10.1093/brain/60.4.389

Penfield, W., & Rasmussen, T. (1950). The cerebral cortex of man; a clinical study of localization of function. Macmillan.

Siddiqi, S. H., Khosravani, S., Rolston, J. D., & Fox, M. D. (2024). The future of brain circuit-targeted therapeutics. Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 49(1), 179–188. https://doi.org/10.1038/s41386-023-01670-9