Why do we intuitively believe we have free will?

Tom Stafford at the BBC:

It is perhaps the most famous experiment in neuroscience. In 1983, Benjamin Libet sparked controversy with his demonstration that our sense of free will may be an illusion, a controversy that has only increased ever since.

Libet’s experiment has three vital components: a choice, a measure of brain activity and a clock.

The choice is to move either your left or right arm. In the original version of the experiment this is by flicking your wrist; in some versions of the experiment it is to raise your left or right finger. Libet’s participants were instructed to “let the urge [to move] appear on its own at any time without any pre-planning or concentration on when to act”. The precise time at which you move is recorded from the muscles of your arm.

The measure of brain activity is taken via electrodes on the scalp. When the electrodes are placed over the motor cortex (roughly along the middle of the head), a different electrical signal appears between right and left as you plan and execute a movement on either the left or right.

The clock is specially designed to allow participants to discern sub-second changes. This clock has a single dot, which travels around the face of the clock every 2.56 seconds. This means that by reporting position you are reporting time. If we assume you can report position accurately to 5 degree angle, that means you can use this clock to report time to within 36 milliseconds – that’s 36 thousandths of a second.

Putting these ingredients together, Libet took one extra vital measurement. He asked participants to report, using the clock, exactly the point when they made the decision to move.

More here.