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If you watch a skilled pianist, you will see that their right hand moves quickly up and down the high notes while their left hand plays the lower notes more slowly. Given that about 90% of people are right-handed, it might not seem too surprising that most people can perform faster and more precise movements with their right hand than their left. Indeed, when right-handers perform any task, from hammering a nail to slicing bread, they use their right hand for the faster and more difficult action, and their left hand for slower or stabilizing actions.
But why? It could be that the left hand is simply less capable of performing skilled movements than the right. But another possibility is that the left hand is actually better than the right hand when it comes to slower movements. To test this idea, Pflug, Gompf et al. asked healthy volunteers to tap along to a metronome with both index fingers. On some trials, the volunteers had to tap along to every beat. On others, they had to tap in time with every fourth beat. While the volunteers performed the task, Pflug, Gompf et al. measured their brain activity.
The results showed that the volunteers, who were all right-handed, followed the fast rhythm more precisely with their right hand than with their left. But they tapped the slow rhythm more accurately with their left hand than with their right. Areas of the brain that process sounds showed increased activity during the task. This increase was greater on the left side of the brain – which controls movement of the right side of the body – when the volunteers tracked the faster rhythm. By contrast, sound-processing areas on the right side of the brain – which controls the left side of the body – showed greater activity when participants tapped the slow rhythm.
The findings thus suggest that the left half of the brain is better at controlling faster rates of movement, whereas the right half is better at controlling movements with slower rhythms. This could also help explain why in most people the left side of the brain controls speech. Speech requires rapid movements of the lips, tongue and jaw, and so it may be better controlled by the left hemisphere. Understanding how the two hemispheres control different actions could ultimately lead to new strategies for restoring skills lost as a result of brain injuries such as stroke.
