How the Brain Responds When You Lose the Use of an Arm

One of the most interesting neuroscience articles I’ve read recently looked at the short-term impacts on motor function in the brain after putting a cast on people’s dominant arm for several weeks.

This study, led by researchers at Washington University in St. Louis, entitled “Plasticity and Spontaneous Activity Pulses in Disused Human Brain Circuits,” used a dense-sampling methodology (Newbold et al., 2020).

This means that rather than having one or two brain scans on numerous individuals, as is typical in neuroimaging research, researchers collected daily brain scans of only three volunteer participants over the course of two months (42-64 brain scans per participant). To appreciate the massive investment this study required, the typical neuroimaging study scans participants only once, with a median sample size of 25, and each MRI scan costs upwards of $500 (Marek et al., 2022). (Plus, however much money it took to convince people to have their arm bound for two weeks!)

Before casting participants’ arms, the mean motion of both arms was captured over the course of two weeks using an accelerometer that participants wore for the duration of the study period. Researchers also measured participants' grip strength, fine motor control ability, and baseline brain activity for the two weeks prior to casting.

Each participant had their dominant arm in a cast for a total of two weeks, during which time the motion of the casted arm decreased by 41-55 percent, and the motion of the non-dominant arm increased by 15-24 percent. Within two days of casting, brain scans revealed significant decreases in functional connectivity in the somatomotor cortex and cerebellum, areas of the brain that process motion, fine motor control, and balance.

Functional connectivity (FC) measures how much a given brain area communicates with the rest of the brain. Depending on the particular participant and brain subregion, functional connectivity decreased between as little as 7 percent and as much as 86 percent.

The largest declines in FC were between the left somatomotor cortex, which controls motor function and sensation in the right arm (the dominant and casted arm of all participants), and the right somatomotor cortex, which controls the left arm. These areas are normally highly interconnected to coordinate fine motor control between both hands, but FC declined dramatically when the dominant arm was in a cast. In other words, use it or lose it!

When the casts were removed, participants’ grip strength in their dominant hands decreased by 27-42 percent, and fine motor skills decreased by 12-29 percent. Interestingly, grip strength or fine motor skills did not improve in the non-dominant hand, despite the increased usage. Within three days, brain activity in all motor areas returned to normal; within two weeks, fine motor skills and grip strength returned to normal.

These results are a classic demonstration of brain plasticity: the ability of your brain to flexibly and rapidly change as your environment or behavior changes. Everyone expected motor function to decrease during the casting period and return to normal once the cast was removed. But another set of findings was entirely unexpected.

The two regions in the left hemisphere that saw significant decreases in FC with the rest of the brain, the somatomotor cortex and cerebellum, increased in FC with each other and began to fire rapidly. During the casting period, researchers noted “spontaneous pulses” of brain activity in these regions, which were higher amplitude than typical background brain activity. They did not look anything like typical motor signaling.

Researchers termed these signals “disuse pulses” and think they may be a protective mechanism. Rather than giving up and atrophying, unused brain regions decide to band together and spontaneously fire together. This serves one of two purposes. Either they aim to keep these regions active and healthy—like exercise—so that it’s easier to return to normal functioning once these brain regions are used again. Or, it could be the beginnings of activity attempting to re-specialize—if the left motor cortex can no longer control the right hand, perhaps it will find a new function.

Neuroscience Essential Reads

How the Brain Learns to Perform Quickly Without Overthinking

The Upcoming Solar Eclipse Is a Natural Wonder

Either way, the next time you find yourself in a bind, brain plasticity will help you out.

Facebook image: Studio Romantic/Shutterstock

LinkedIn image: SeventyFour/Shutterstock