A synthesis of evidence from various methodological approaches have informed on the cognitive neuroscience of free will. Undoubtedly, the most seminal experiment in this field was conducted by Benjamin Libet in the 1980s, in which he asked each subject to choose a random moment to flick her wrist while he measured the associated activity in her brain (in particular, the build-up of electrical signal called the readiness potential).
This figure provides Libet’s original published results.
This picture shows the readiness potentials (RP) preceding self-initiated voluntary acts. Each horizontal row is the computer-averaged potential for 40 trials, recorded by a system with an active electrode on the scalp, either at the midline-vertex (Cz) or on the left side (contralateral to the performing right hand) approximately over the motor/premotor cortical area that controls the hand (Cc). For subjects G.L., S.B. and B.D., this instruction was given at the start of all sessions. Nevertheless, each of these subjects reported some experiences of loose preplanning in some of the 40-trial series; those series exhibited type I RPs rather than type II.
Although it was well known that the readiness potential preceded the physical action, Libet asked whether the readiness potential corresponded to the felt intention to move. To determine when the subject felt the intention to move, he asked the subject to watch the second hand of a clock and report its position when she felt that she had the conscious will to move. Libet found that the unconscious brain activity leading up to the conscious decision by the subject to flick her wrist began approximately half a second before the subject consciously felt that she had decided to move. Libet’s findings suggest that decisions made by a subject are first being made on a subconscious level and only afterward being translated into a “conscious decision”, and that the subject’s belief that it occurred at the behest of her will was only due to her retrospective perspective on the event. He showed that the preparatory brain activity that occurs as you make a ‘free’ choice about something is actually made a few hundred milliseconds before the decision reaches your conscious awareness. In other words, your brain makes a decision before you do, and ‘free will’ is an illusion. (1)
This interpretation appears to leave little room for conscious processes in the control of action—or so it might seem. Whilst there is some debate as to conscious processes cause actions, these 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 couple of hundred milliseconds means that a person could still inhibit certain actions from being made. This therefore means that inhibition is potentially more important than action in the control of action.
Further evidence also suggests that our subjective experience of the beginning of the movement must also come from some premotor process—something that takes place before the muscles themselves contract. Although some investigators have questioned the validity of the timing as judged by subjects in Libet’s experiment, his results have had considerable impact and have been interpreted as casting serious doubt on the existence of a mind-body chain of causation. Moreover, numerous other studies now confirm the phenomenon of anticipatory awareness of action. In an important variation of this task, Haggard and Eimer asked subjects to decide not only when to move their hands, but also to decide which hand to move. In this case, the felt intention correlated much more closely with the “laterakused readiness potential” (LRP), an ERP component which measures the difference between left and right hemisphere brain activity. Haggard and Eimer argue that the feeling of conscious will therefore must follow the decision of which hand to move, since the LRP reflects the decision to lift a particular hand. (2)
Furthermore, a recent study by Soon and colleagues (Soon et al., 2008) has replicated Libet’s results by using modern brain imaging techniques, a more accurate way of measuring decision making in the brain: They found that the outcome of a decision can be encoded in brain activity of prefrontal and parietal cortex up to 10 s before it enters awareness. The authors argued that this delay presumably reflects the operation of a network of high-level control areas that begin to prepare an upcoming decision long before it enters awareness. (3) Finally, related experiments have shown that neurostimulation could affect which hands people move, even though the experience of free will was intact. Ammon and Gandevia (1990) found that it was possible to influence which hand people move by stimulating frontal regions that are involved in movement planning using transcranial magnetic stimulation in either the left or right hemisphere of the brain. (4)
References
(1) Libet, B., Gleason, C.A., Wright, E.W., Pearl, D.K. (1983). Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential). The unconscious initiation of a freely voluntary act. Brain. 106 (3):623–642.
(2) Haggard, P. and Eimer, M. (1999). On the relation between brain potentials and the awareness of voluntary movements. Experimental Brain Research 126, 128–133.
(3) Soon, CS, Brass M, Heinze, HJ,and Haynes, JD. Unconscious determinants of free decisions in the human brain. Nature Neuroscience 11, 543 – 545 (2008)
(4) Ammon, K. and Gandevia, S.C. (1990) Transcranial magnetic stimulation can influence the selection of motor programmes. Journal of Neurology, Neurosurgery and Psychiatry 53: 705–707.
