From In the Realm of Hungry Ghosts, on p. 308-9 of my copy:
Impulses to act are generated in lower brain systems, but the job of the cortex is to censor some and permit others. As a prominent researcher expressed it, it's not a matter of free will but of "free won't."
How much time elapses between impulse and action? Electrical studies of brain function show that it's about half a second. For most of that time we are not aware of what our brain is proposing to do. In other words, there is a lag period between the impulse arising as a physical signal in the brain and our becoming aware of it as a conscious urge. In a well-functioning cortex the interval between awareness of the impulse and the activation of the muscles that will carry out the impulse is only one-tenth to one-fifth of a second.
Amazingly, it's only in this briefest of intervals that the cortex can suppress behavior it judges to be inappropriate. That's the gap where, for example, we can stop ourselves from raising our hand in anger or saying something hurtful. In that sliver of time we see ourselves about to perform the act, and, if necessary, we can stand between ourselves and the behavior in question.