Motor cortex interactions during movement
Our motor cortex is a region in the brain the controls our movement. There is a motor cortex in the right part of the brain that mainly controls left hand movements. There is another motor cortex region in the left part of the brain that mainly controls right hand movements. The keyword is mainly. The two motor cortices communicate with each other. Signals from right to left and from left to right is mainly inhibitory. This means that activity is constrained. Why is this important? This is crucial for controling left and right hand independently. You can move your left hand, without moving your right hand and vice versa. You can do this because of this inhibitory signal.
As a quick side note, this inhibition is not perfect. For example, try to draw a circle with your left hand and simultaneously draw a square with your right hand. This is really difficult and you may end up with two drawing somewhere between a circle and a square. This is where this inhibition fails.
Anyway, for most other movements this inhibition does work. So, how about when I only make a movement with one hand? In that case inhibition must be increased towards the motor cortex that will NOT move. And th inhibition must be becoming less towards the motor cortex that is going to move. Basically, the brakes need to be taken off. In this study, we investigated the time course of this inhibition towards the moving hand. That is, how does inhibition develop before moving?
Participant performed a movement task, where they had to move a red square towards a green square, using a joystick with the left hand. While they were doing this we tested inhibition signals from left to right motor cortex.
Testing this inhibition between motor cortex of the two halves of the brain can be done by using two TMS machines (learn more about TMS in the Neuroscience Methods section). One on each side. First, you give a TMS pulse at the left motor cortex. And 10 milliseconds later you give another TMS pulse at the right motor cortex. Those 10 milliseconds reflect the time it takes for the inhibitory signal to go from left to right.
When doing this you can test the muscle output. With the two pulses, the muscle output should be smaller (with inhibition) compared to when you would only give one pulse (no inhibition). We did this for five different time points before the movement.
Admittedly, the methods were quite complex. In practice it was also quite difficult to get everything to work at the same time. The results are luckily more straightforward. Inhibition towards the moving hand becomes gradually smaller in the 500 milliseconds before movement. The brakes are gradually taken off.
Interestingly, when you have to make more difficult movements, a difference in inhibition is observed 200 milliseconds before movement (which is 300 milliseconds after the go-signal), which means that the inhibition signal at that movement is particularly important for movement precision.
Doing this study as my first research project during my master was certainly challenging. But also really cool! We found how signals between motor cortex in the left and right part of the brain change over time. This study gives more insights how the two motor cortices are related.
Why is this important? During my internship I also worked with stroke patients. After a stroke, one part of the motor cortex is damaged. Interestingly, there is evidence that the motor cortex on the other side can take over some of the lost function. So, understanding how the signals goes from right to left and from left to right is important to develop TMS treatments in stroke recovery. If you want to learn more about TMS in stroke recovery, read the Brain Stimulation Therapy section.
Wischnewski, Kowalski, Freeman, Rink, Belagaje, Haut, Hobbs & Buetefisch (2016). Demand on skillfulness modulates interhemispheric inhibition of motor cortices. J Neurophysiol, 115(6), 2803-2813. https://doi.org/10.1152/jn.01076.2015