Also, some of what happens in the brain is just storytelling. Like, when the doctor hits your patellar tendon, just under your knee, with a reflex hammer. Your knee jerks, but the signals telling it to do that don't even make it to the brain. Instead the signal gets to your spinal cord and it "instructs" your knee muscles.
But, they've studied similar things and have found out that in many cases where the brain isn't involved in making a decision, the brain does make up a story that explains why you did something, to make it seem like it was a decision, not merely a reaction to stimulus.