Left Brain, Right Brain, Whole Brain

There's more communication between the two sides of the brain than we thought.

Posted Dec 05, 2013

“If the brain were so simple we could easily understand it we would be so simple we couldn’t.”— Lyall Watson (1939-2008)

It is widely appreciated that there is a certain lateralization of function in the brain. We read often about thinking “right brain” or “left brain” and so on. When thinking about movement and motor systems, the concept of lateralization is captured in the idea that one hemisphere activates—via the spinal cord—muscles on the opposite side of the body. For example, left brain controls right arm and vice versa.

Or is it really that simple? Like many concepts in science we must be careful that a generalization we make in order to grasp a general concept does not replace the details of the actual concept itself.

Canadian neurosurgeon Wilder Penfield (1891-1976) was prominent in epilepsy surgeries. As part of pre-surgical mapping of the brain, different brain areas were electrically stimulated which produced twitches in muscles on the other side of the body. Based on this in this and other work, Penfield showed in the 1940s that certain brain regions that were particularly involved in producing motor actions were lateralized. This was pretty good evidence that there was a strong organization of left hemisphere controls right, and right hemisphere controls left side.

Since those early observations, many related studies have provided additional information and generally confirmed that the one side of the brain controls the other side of the body. As information was refined over decades, we’ve come to think of the motor system in the brain as having 3 major areas in the frontal cortex—M1, supplementary motor area (SMA), and premotor area (PMA). Taken together these 3 areas make up just over 2/3 of the fibers of the corticospinal tract that go and make direct connections with spinal cord neurons to activate your muscles on the opposite side. Less than 10% of the connections from M1 but almost twice as many (more than 20%) from SMA stay on the same side of the body.

Speaking generally, the earlier stimulation studies like those of Penfield and friends were concerned with activating M1. Typically this produces distinct twitch contractions of muscles on the opposite side of the body. Moving more “upstream” to SMA, the stimulation might produce more complex actions at a related part of the body. So, stimulating in the hand region of M1 might produce twitches of single fingers at each joint, SMA stimulation might produce more coordinated actions of neighbouring fingers and multiple joints. Basically the movements are more complex and coordinated.

This is something I have often thought about. Martial arts training typically involves use of the entire body. Despite that, there is often a preference for one side over the other. For example, fighting drills and movement patterns are often learnt from the perspective of one hand forward and so on. Extensive and equal practice on both sides is not always expected. Years ago, though, I adopted training both sides equally for most of my martial arts practice and for lots of other activities. I have tried to make myself ambidextrous.

Which brings me back to some of the repercussions from rupturing my biceps tendon this summer (you can read about it here if you want). My left arm was out of commission for a while and so I taught myself to do lots with my other hand—my right hand. Since I’m “right handed” but I’ve done lots of things with both hands you’d think that would be easy, right?

Unless one of the things you were thinking about was throwing a baseball.

That wasn’t easy. Catching with my right hand? Pretty good. Almost as good as catching with my left hand. But throwing? It was hard to get the movement pattern right. I had to constantly switch back and forth between my arms to figure out how to do it. And I’m supposed to be an expert in motor control neuroscience--my motors weren’t listening to me!

After months of doing that it still remains difficult and I really have to think about it to throw the ball hard—with my whole body behind it like I can do with my right hand throw. But I’ve kept plugging away. My plan is to continue using both hands for catching and throwing. I enjoy the challenge of working on skill training with both sides of my body.

Tristan Thompson shooting left and right handed. (Mike McGinnis/Getty Images Sport)

In October 2013, Sports Illustrated wrote that Thompson, a self-described “left hander” who always shot, not surprisingly, with his left hand, had taught himself to shoot—with accuracy—with his right hand. Tristan himself has described the experience as becoming “a whole new person”. On December 4, 2013, Tristan Thompson had a career-high game, finishing with 17 points and 21 rebounds.

He makes an interesting study on the ability to transfer training from one side of the body to the other. In some ways it’s also reminiscent of “cross-education” of strength—where training muscles on one side of body produces a smaller strength gain in the untrained muscles on the other side—but related more to “skill”.

All of which was nicely underlined by a recent study published in the journal Experimental Brain Research. Scientists at Ohio State University measured activity in muscles in both arms of monkeys M. fascicularis while they stimulated the primary (M1), supplementary (SMA), and premotor (PMA) areas. This is one of the most detailed papers yet to look at responses across the body. There were a lot more responses on the same side of the body or on both sides than they had anticipated. Also, the likelihood that stimulation of the brain would produce responses on the same side of the body or on both sides at the same time was much higher in SMA than M1. That is, in areas related a bit more to coordination.

While currently this level of detailed study is not available for us H. sapiens, we can expect a similar functional organization in our brains. While the overall anatomical and functional organization of our nervous system does have lateralization, emerging evidence suggests it’s not as cut and dried as we once imagined.

Instead of left-brain, right-brain, my take-away message is try to use your whole brain whenever possible. You can practice this in your motor system by using both arms and legs to kick, throw, and catch. Or even just for brushing your teeth.

© E. Paul Zehr (2013)