In summary on a 1985 experiment by Bejamin Libet at this website.
http://scienceweek.com/2004/sa040813-1.htm
1) In 1983, Benjamin Libet and his colleagues at the University of California San Francisco published a profoundly influential paper on the source of human control. In this study, participants watched a small clock hand that completed one full revolution in 2.56 seconds. While fixated on the clock, a participant voluntarily flexed his wrist at a time of his choosing. After the movement, the clock hand continued to rotate for a random time and then stopped. Then, a participant reported the position of the clock hand at the time when she first became aware of the will to move. Libet and his colleagues called this subjective judgment W, for "will". In other parts of the experiment, participants judged when they actually moved, and Libet called this judgment M, for "movement". The timing of the W and the M told Libet and his collaborators when -- subjectively speaking --a participant formulated a will to move and actually moved.
2) In addition, Libet's team measured two objective parameters: the electrical activity over the motor areas of the brain, and the electrical activity of the muscles involved in the wrist movement. Over the motor areas, Libet recorded a well-known psychophysiological correlate of movement preparation called the "readiness potential" (RP), which Hans H. Kornhuber and Lueder Deecke first described in 1965. The RP is measured using electroencephalographic recording electrodes placed on the scalp overlying the motor areas of the frontal lobe, and appears as a ramplike buildup of electrical activity that precedes voluntary action by approximately 1 second. By also recording the electrical activity of the muscles involved in the wrist movement, Libet precisely determined the onset of muscle activity related to the RP.
3) Libet and his colleagues examined the temporal order of conscious experience and neural activity by comparing the subjective W and M judgments with the objective RP and muscular activity. First, the investigators found that, as expected, W came before M. In other words, the subjects consciously perceived the intention to move as occurring before a conscious experience of actually moving. This suggests an appropriate correspondence between the sequence of subjective experiences and the sequence of the underlying events in the brain. But Libet also found a surprising temporal relation between subjective experience and individual neural events. The actual neural preparation to move (RP) preceded conscious awareness of the intention to move (W) by 300 to 500 milliseconds. Put simply, the brain prepared a movement before a subject consciously decided to move. This result suggests that a person's feeling of intention may be an effect of motor preparatory activity in the brain rather than a cause. As Libet himself indicated, this finding ran directly contrary to the classical conception of free will.
4) Considering all the existing data, the brain is apparently going full speed ahead well before a person experiences the conscious intention of moving. Consequently, no role appears for conscious processes in the control of action -- or so it might seem. Although research casts doubt on whether conscious processes cause actions, the data remain consistent with the idea that conscious processes could still exert some effect over actions by modifying the brain processes already under way. The fact that conscious awareness of intention precedes movement by a few hundred milliseconds means that a person could still inhibit certain actions from being made.(1-5)
Can anyone here expand on this or explain the meaning of free will in light of this evidence?
A centipede was happy quite, until a toad in fun
Said, "Pray, which leg comes after which?'
This raised his doubts to such a pitch
He fell distracted in the ditch
Not knowing how to run.
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