Researchers writing in the journal PNAS this week have come a little bit closer to understanding how the brain chooses which hand to use for any particular action. They think it comes down to a mental competition between the hands.
Flavio Oliviera and his team (from the US and Belgium) collected together a group of 
right-handed people and instructed them to reach for images placed on a table. The participants could use either hand to pick the images up, and these images were in different places on the table.
While this was happening, the researchers applied transcranial magnetic stimulation (TMS) to the brains of the participants. This temporarily disrupts brain activity in the subjects' left or right posterior parietal cortex. As the name suggests, it's at the back of the brain and it's important for planning movements or thinking about 3D space around you.
When TMS was applied to the left side the right-handed participants tended not to use their right hands as much as they would normally. But applying TMS to the right side of the brain produced no change at all. The left side of the brain is usually involved in controlling the right side of the body, and vice versa, so you might expect this result in right-handed people.
The researchers think that when we have to make a decision on which hand to use to press a button, our brain is actually coming up with several plans on how to achieve this, but it selects whichever plan it thinks is the best - using this area of the brain: the posterior parietal cortex.
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