The Sensory Side of Movement

Expecting to be met by peaceful silence, I was shaken awake this morning by the sound of something hard hitting the floor. I had just reached over to my bedside to silence the chirp of my cell phone alarm, but things went awry as I fumbled the device instead of picking it up and hitting the snooze button. My arm, you see, had been positioned beneath my body when I went to bed last night. My hand was asleep.

Most people perform actions without thinking about what they feel like. When we walk, we generally don’t think about the feeling of the inside of a shoe, or about the changes in pressure applied along the bottom of the foot throughout the process of taking a step. But while our minds are elsewhere, the brain is using information about what the body is feeling to coordinate our movements. 

In a way, the brain “knows” what it feels like to move; information about any, and all, sensations detected by our body—somatosensory information—goes up to the somatosensory areas of the brain, where it gets seamlessly interwoven with motor output—the messages that the motor areas of the brain send to our muscles to tell them to move. So without even thinking about it, we can navigate around obstacles in space and make our way from one place to another without being impeded by things like changes in the surface of the ground beneath us. When we’re thirsty, we can think about getting a drink from the kitchen and leave it up to our bodies to get us there.

But consider what it would feel like to walk from one place to another if our legs had just been crossed for so long that one of them had fallen asleep. To stand on that leg and get it to move the way we want it to would not be so effortless. Similarly, if one of our hands was to fall asleep, the once straightforward task of reaching and grasping an object could become much more challenging, and we might find ourselves waking up to the sound of a smartphone smacking the floor instead of drifting off in the fleeting silence of a snoozed alarm. The brain relies on being able to use somatosensory information to shape our motor output, and when it can’t do that, we have difficulty moving the way we want to. Effective movement control thus involves both somatosensory and motor areas of the brain, and requires them to cooperate with each other.

That’s why Dr. Paul Gribble’s research in movement neuroscience doesn’t just focus on motor areas. In fact, it’s geared towards somatosensory areas of the brain. 

Dr. Gribble is a professor at Western University’s Brain and Mind Institute, where he studies how the human brain learns motor skills. Whether it’s learning to walk, play an instrument, or hit a tennis ball—as the brain learns to perform new movements, it makes new connections (and alters existing ones) in areas that are involved in controlling how we move. This process, called motor learning, changes the way these areas function and communicate with each other so that we can move better. 

Motor learning happens all the time. As we grow, age, get injured, recover from injury, try new things, or otherwise interact with the ever-changing world, the human brain is constantly adapting and re-learning how to move in different conditions.

Much of the neuroscientific research on motor learning has focused on motor areas of the brain. After all, a big part of these areas’ jobs is to tell the body how to move, so if parts of the brain are going to change how they operate to allow us to move better (i.e., motor learning), one would think that motor areas would be the parts doing the changing. This is the truth; but it’s not the whole truth.

Though they are frequently overlooked, somatosensory areas also change when we learn. And in a recent study, Dr. Gribble and two other researchers—Dr. Hiroki Ohashi and Dr. David Ostry—showed that when humans learn new motor skills, changes in the somatosensory cortex (the major somatosensory area of the brain) actually happen before changes in the motor cortex (the major motor area of the brain).

Somatosensory and motor cortices. Image adapted from “Brain Lateral View” by Chiara Mazzasette.

In the study, human volunteers did a task that involved learning to use their finger to press on a device in a specific way. At first, people were very bad at doing the task. But as they kept trying, they got better and better at it, which meant they were learning. It took about 60–90 tries for people to get so good at the task that they couldn’t really improve anymore, which meant that most of their motor learning was complete. 

To look at changes in the brain during learning, the researchers measured something called cortical excitability in both the somatosensory cortex and the motor cortex of volunteers. Measuring cortical excitability can provide information about how an area of the brain functions, and in this study, changes in excitability can be taken to mean that an area is functioning differently now than it was before.

The researchers measured excitability before learning, and then again at multiple points in the learning process. What they found was that the somatosensory cortex had changed very early on in learning, during the stages when people were getting better at the task. The motor cortex, on the other hand, did not change until after the 90th try, when performance had already plateaued. 

The study also found that people who had bigger changes in the somatosensory cortex went on to learn more overall. This was not true for changes in the motor cortex, which bore no relation to how much people ended up learning.

These results suggest that changes in the somatosensory cortex happen as part of the early stages of learning, actually preceding learning-related changes in the motor cortex. “It really points to the involvement of the somatosensory cortex as a key part of motor skill learning,” says Dr. Gribble, “and that’s a relatively understudied part of the literature.”

Implications for stroke rehabilitation

If the somatosensory cortex plays a critical role in motor learning, then it should also be important for processes in which motor learning is involved—like the recovery of motor function after a neurological injury such as a stroke.

“After a stroke, if part of your brain is damaged, it's like your brain is re-learning how to move without that part of the brain helping you out,” says neuroscientist and physiotherapist Dr. Sue Peters. 

Dr. Peters has years of first-hand experience helping stroke patients work towards regaining their mobility. She began her career as a physiotherapist and went on to earn a PhD in neuroscience after feeling like more could be done to help her patients. “For me,” she says, “one of the big things that’s currently missing from rehab is this real integration of neuroanatomy and neurophysiology into what's happening in practice.” 

Dr. Peters’ research looks at how the brain controls movement in naturalistic environments. In both physiotherapy and in her field of research, she says there is little emphasis on the role of somatosensory areas of the brain in movement control and stroke rehabilitation. “Everything is motor,” she explains. “But just because a patient’s motor output is affected, that doesn’t mean we can assume the only problem is in the motor cortex. We’ve got to look further.” 

Think about somatosensation

Somatosensory input should integrate with motor output so seamlessly that we’re able to do the things we want to do without really thinking about it. But often, for someone who’s just had a stroke, that type of smooth integration doesn’t happen so well. It’s a bit like what happens when a body part falls asleep: if the brain is having trouble using sensation to shape motor output, we have trouble moving properly. Damage to any of the circuitry involved in this process, including the somatosensory cortex, could interfere with our ability to move—and we’d have to regain that ability through motor learning. But if the somatosensory cortex is integral to the learning process, that means stroke-related damage to this part of the brain could have the potential to impair the recovery of motor function after stroke. 

According to Dr. Peters, patients who have damage to the somatosensory system are less likely to do well in rehab programs. “They’re more likely to report falls. And there are a lot of clinical things you can measure—like number of falls, balance ability, and even just distance walking—that, if you have damage to the somatosensory system, you’re not going to do as well. Clinicians know this.” But as for whether the somatosensory system is currently considered in approaches to improving mobility after stroke, Dr. Peters says, “if it is, it’s very little.”

While she is convinced that the somatosensory cortex plays an important role in the recovery of motor function after stroke, Dr. Peters says there isn’t yet enough research to support how that insight should inform clinical practice. In her opinion, there is a gap for patients who have damage to the somatosensory system, and therapies that target the somatosensory cortex could have the potential to impact stroke rehabilitation in the future. But a lot more research needs to be done before anything like that can be implemented in practice.

Her take home message for readers? 

“Think about somatosensation more,” says Dr. Peters. “Somatosensation is important for learning. And it’s often neglected. And it shouldn’t be.”