A New Network for Mapping Movement in the Brain
Post by Christopher Chen
The takeaway
Traditionally, a region in the motor cortex called the homunculus has described a somatotopic map linked to movement of specific body parts. However, new research suggests that a parallel network (SCAN) in the motor cortex incorporating cognitive aspects of movement also exists.
What's the science?
For many, learning about the homunculus has become a rite of passage in biology and neuroscience courses. In short, the homunculus is widely-known as the somatotopic map in our brains that controls specific body movements. For example, when we move our pinkie, a specific region (i.e., an effector-specific region) in the homunculus corresponding to pinkie movement becomes activated. Indeed, the father of homunculus theory, Dr. Wilder Penfield, discovered that directly stimulating specific regions in the brain could elicit movement of specific body parts. However, since its characterization nearly 100 years ago, the homunculus theory has been under growing scrutiny, with new theories emerging that suggest body movement may in fact be more complex than Dr. Penfield imagined.
One of the biggest reasons views on the homunculus theory have changed is that 21st century big data analysis and brain imaging techniques have provided opportunities to more deeply investigate how the brain processes body movement. Specifically, precision-functional mapping (PFM) – which integrates fMRI imaging during resting and active states to generate more detailed images of brain connectivity – has allowed neuroscientists to visualize and analyze brain patterning and activity in unprecedented ways.
In a recent article in Nature, researchers discovered evidence across thousands of human subjects of the existence of a novel somato-cognitive action network (SCAN) that helps inform voluntary body movements in parallel with effector-specific regions in the homunculus, providing a compelling new framework to understand how our brain facilitates movement.
How did they do it?
This study analyzed public domain fMRI images from thousands of participants from large-scale projects like the Human Connectome Project to inform its conclusions. To find patterns and similarities across such vast amounts of data, researchers used advanced algorithms and big data analysis techniques to generate functional maps of the brain in both resting and active states. Ultimately, the strategy was to apply this repertoire of advanced techniques to determine how the motor cortex communicated with the rest of the brain – including regions linked to cognition, motor planning, and even emotion – during specific body movements.
On a smaller scale, researchers performed brain imaging studies at the University of Washington to characterize brain connectivity under specific conditions involving specific body parts and movements. They also performed neuroimaging on a wider range of human subjects including infants, to supplement the data generated from the larger neuroimaging studies as well as trace the developmental arc of movement processing in the brain. Researchers also looked at non-human primates (macaques) to determine how evolutionarily conserved the brain’s processing of movement was.
What did they find?
The investigation’s most intriguing finding was the discovery and characterization of regions in the brain termed “inter-effector regions.” Anatomically, these areas are sandwiched between the effector-specific regions (i.e., regions corresponding to a specific body part) in the motor cortex and characterized in the homunculus, but hold a very different function. Rather than correspond to an isolated movement of a specific body part (e.g. lifting your index finger), inter-effector regions are linked to more integrated movements that require coordination of multiple body parts (e.g. reaching for a cup of coffee). Compellingly, researchers found that inter-effector regions shared closer connectivity to a region of the brain called the cingulo-opercular network (CON), a region associated with arousal, error processing, and even pain. Thus, these more cognitive-related inter-effector regions operate in parallel with more motor-related effector-specific regions during movement, collectively making up what the researchers describe as a dual-system model of body movement.
As for the smaller, more focused fMRI studies, they bolstered as well as extended the study’s core findings. Under controlled conditions, the movements of a range of body parts such as the abdominals, elbows, and eyebrows requiring less specific motor control elicited inter-effector region activity, while more specific body movements elicited activity only in effector-specific regions, highlighting the consistency and specificity of this dual-system network. Furthermore, the authors noted that effector-specific activity appeared to be organized in a ring-like pattern, with distal body parts at the center of the ring and proximal ones at the edges. Finally, researchers found that macaques had similar effector-specific and inter-effector region patterning as humans, and that humans as young as eleven months old expressed the beginnings of this patterning.
What's the impact?
The characterization of the SCAN is a testament to the brain’s incredible connectivity and complexity. With its evidence of a parallel system of effector-specific and inter-effector regions informing body movement, this study highlights how much brain processing occurs during seemingly mundane movements like walking or raising our hand. Naturally, a looming question is whether similar studies can employ big data analysis to generate maps of the brain during more complex, higher-order cognitive tasks.