Rodolfo Llinás has a wonderful story about testing his own so called free will, hence as a consequence defines it as "those activities that happen that the brain know are about to happen."
The whole interview, conducted by Roger Bingham, can be found here [
Enter the "i of the vortex"]. A
pdf transcript is also available. The relevant part for the video is between 34:45 – 41:50 whereas the same conversation is outlined on pages 9 – 11 in the transcript.
In short, the test went something like this: He managed to find a good spot on his head where he attached a coil (generating a magnetic field which activates the brain directly) and when switched on it would always stimulate the brain in the same way. In this case it always moved his foot inwards. So he intentionally tried to move his foot outwards every time there was a stimulation. The result was of course that the foot always moved inwards.
The interesting thing was that even though his intention was to the contrary, it felt as if he volitionally moved the foot inwards. He just said he changed his mind. He could not distinguish between the external stimulus and the feeling of volitionally moving the foot inwards – as in free will – although the original intention was to move it outwards. Thus, talking about the
feeling of having free will seems rather irrelevant. When the system does something, it feels like you made a choice about it, but in reality you just recognize that it is about to happen/happening as if you willed it (but you really didn't).