John-Dylan Haynes of the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany has looked into the matter, and published his findings, summarized recently in World Science:
Certain patterns of brain activity predict people's decisions up to 10 seconds before the people are aware of them... Researchers tracked brain activity while people viewed a stream of letters on screen, and then pressed a button. Each participant was asked to decide freely which of two buttons to press and when to press it.
Scanning the brains with a technique called functional magnetic resonance imaging, the investigators used a statistical method known as pattern recognition to examine brain activity associated with each choice. Activity in two brain regions, called the prefrontal and parietal cortex, predicted which button the person would press, they found... This activity occurred up to 10 seconds before subjects were consciously aware of having made a decision, according to the researchers.
Is it true that our decisions are made before we are even aware of them? Has all the trouble I've taken to choose entrees from menus been wasted time? Have I already decided on the answer to that question? At least the brain appears to be efficient.