oddly, you can detect signals in the premotor cortex before you can in the parietal cortex, irresistably suggesting that you start moving your arm before you decide to.

there is an out, though; it could well be that the main function of your "higher" brain areas is to inhibit the instinctive activity of the lower brain. so that the impulse to move your arm may begin in the premotor cortex, but the decision not to stop it arises in the parietal cortex and registers to you as the actual decision to move the arm.

we've got a long way to go to understand this stuff.