In an interview with Marcel Brass (a very knowledgeable cognitive neuroscientist), Dr. Brass was describing studies that he has done and others have done regarding understanding free will through brain scans, The interviewer first asked about the famous Libet study (well, famous to us free-will-o-philes) that seemed to show that free will is a false impression that evolved to help us explain our own behavior. But there were some methodological flaws in the original study, so Brass describes a lot of follow up studies that have looked in more detail in different ways to try to overcome the flaws.
In each case, the results show that our feeling of free will is really just a reconstruction of what we have already done in a way that explains it rationally. In other words, if unconscious processes cause us to do something, our conscious brain then explains it by concluding that we did it “because we wanted to” and therefore we believe we have free will. But these neuroscience studies show that this decision that we “wanted to” occurs AFTER we already did it. So it is a memory phenomenon, not a decision phenomenon. We do it, then we decide we wanted to (past tense) – rather than we decide we want to do it (present tense) and then do it.
Some of the studies showed that the experimenter could change whether we perceived that we “wanted” do something AFTER we finished actually doing it. So our perception of the intention (the free will) is bogus. This doesn’t mean we don’t have free will; it just proves that the fact that we feel like we have free will is not evidence for it. We need to look elsewhere.