Motor cortex physiology in movement learning
In this study we investigated different forms of movement learning. Let's assume you try learning how to juggle. After a while you will be able to do something that somewhat resembles juggling. You learned it relatively quick, but the next day it doesn't work anymore. After a few weeks of training you finally got the hang of it. After you lost some interest you try it a year later. To your suprise you can still do it. This examples shows different forms of learning. On the one hand, there is fast but fragile learning that is quickly forgotten. On the other hand, there is slow but stable learning, that remains with you for a long time.
We studied if these two forms of learning are related to different mechanisms in the brain. For this we tested the output of the motor cortex towards the muscle with TMS (read more about TMS in the Neuroscience Methods section). We did this while participant learned a movement by manipulating a robotic joystick.
Participants moved a robotic joystic from left to right. But the joystick was programmed to push them away. So, participants had to learn to resist that force. While people performed the task we tested their motor cortex activity. Specifically, the output of the motor cortex to the biceps muscle.
We looked at two aspects of the motor cortex output. First, excitatory output. In simple terms these signals direct activate the muscle. Second, inhibitory output. These signals constrain activity of a muscle. Although constraining activity seems not necessary when learning a movement, it is crucial. It helps to finely control the muscle. Without this you would uncontrolably rip the joystick apart.
We found that the excitatory, activating, mechanisms of the motor cortex are predominantly important during fast by fragile learning.
Vice versa, inhibitory, constraining, mechanisms of the motor cortex are predominantly related to slow but stable learning.
Our results are in line with our expectations. When learning something new, first excitatory, activating, aspects are important. Something that is learned quickly may work out, but typically is not very subtle or fine. Only later in learning, when a movement pattern becomes more stable, you will be in full control. Finely controling movements are related to inhibitory, constraining aspects of muscle activity. This is precisely what we found.
Sarwary, Wischnewski, Schutter, Selen & Medendorp (2018). Corticospinal correlates of fast and slow adaptive processes in motor learning. J Neurophysiol, 120(4), 2011-2019. https://doi.org/10.1152/jn.00488.2018