November 17, 2010
On free will-8: The 1983 and later experiments of Benjamin Libet
(For previous posts in this series, see here.)
In 1983, Benjamin Libet and his associates did some experiments that were similar to the 1963 Grey Walter experiment but with the added feature that the patients could observe the equivalent of a clock and thus note when they made the decision to act. This enabled a more objective determination of the time when they first had the conscious thought to carry out the action and not depend upon a possibly misleading feeling of surprise to infer the ordering of events.
One of the key original papers was published in the journal Brain (Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential): The unconscious initiation of a freely voluntary act, vol.106, p. 623-642, 1983) which does not seem to be available online but you can read online a later review published by Libet in 1999 (Do we have free will?, Journal of Consciousness Studies, vol. 6, No. 8–9, 1999, pp. 47–57) where he summarizes his findings and its implications for free will.
Using electrophysiological measurements of something called the readiness potential (RP) in the brain to detect unconscious brain activity, Libet and his co-workers asked people to move a finger (M) and also to indicate when they made a conscious decision to want to move the finger (W) by observing a clocklike device. The part of the brain where the readiness potential originates is called the supplementary motor area (SMA) and is the part involved in motor preparation, i.e., prior to taking an action. According to the free will model (D), there should be a definite temporal sequence in which an act of will (which cannot be detected experimentally) should occur first, followed by the conscious thought to do so (W, determined by the clock reading as noted by the patient), then unconscious brain activity (RP, measured using an EEG device), and behavior (M) last.
What they found was that while the RP time did precede M by an average of about 550 milliseconds (i.e., a little more than half a second), it also preceded W by about 350 milliseconds. The brain seemed to have made the unconscious decision to move the finger before the subject was aware of having made the decision to do so, suggesting that the actual temporal sequence of events was unconscious neural activity, followed by conscious decision to take an action, followed by the action. This was the same result as the Grey Walter experiment except for the crucial additional features that the Libet experiment was able to quantify the time intervals involved, had a more objective measure of when the conscious decision was made, and was able to locate the part of the brain where the precursor activity was occurring.
In other words, what we think of as our will (as manifested by our conscious thoughts) may be just an afterthought. What may be happening is that our unconscious neural activity makes a decision and then sends two signals out, one to create a conscious thought that we have decided to take an action and the other to actually take the action. Rather than the thought being the cause of our actions, our conscious thoughts are merely a passive recognition, after the fact, of decisions made unconsciously without a deliberate act of will.
In his 1999 paper, Libet spelled out what he thought was at stake in the question of whether we have free will or not.
The question of free will goes to the root of our views about human nature and how we relate to the universe and to natural laws. Are we completely defined by the deterministic nature of physical laws? Theologically imposed fateful destiny ironically produces a similar end-effect. In either case, we would be essentially sophisticated automatons, with our conscious feelings and intentions tacked on as epiphenomena with no causal power. Or, do we have some independence in making choices and actions, not completely determined by the known physical laws?
For example, actions by a person during a psychomotor epileptic seizure, or by one with Tourette’s syndrome, etc., are not regarded as actions of free will. Why then should an act unconsciously developed by a normal individual, a process over which he also has no conscious control, be regarded as an act of free will?
As one might expect with such a controversial result, others attempted to replicate Libet's results and while there seemed to be a general consensus that there was nothing faulty about his data or his methods, their inferences were challenged. Judy Trevena and Jeff Miller were the chief skeptics who have had an ongoing back-and-forth with Libet. In their recent 2010 paper, Trevena and Miller do not dispute the finding that brain activity occurs before an awareness of the decision to move the finger. (Brain preparation before a voluntary action: Evidence against unconscious movement initiation, Judy Trevena and Jeff Miller, Consciousness and Cognition, vol.19, Issue 1, March 2010, Pages 447-456.)
What they find is that the strength of the brain activity is independent of whether a decision is made to move the finger or to not move the finger. They argue that hence one cannot take the RP signal as an unconscious decision to move the finger but that it must signify something else. To quote their own words:
We tested that assumption by comparing the electrophysiological signs before a decision to move with signs present before a decision not to move. There was no evidence of stronger electrophysiological signs before a decision to move than before a decision not to move, so these signs clearly are not specific to movement preparation. We conclude that Libet’s results do not provide evidence that voluntary movements are initiated unconsciously.
Furthermore, they say,
As with the movement-preceding negativity, we found no evidence that prevailing conditions in the brain just before a spontaneous decision can predict the outcome of that decision—namely, the spontaneously selected response hand. Thus, our results appear to contradict the idea that our spontaneous conscious decisions merely consist of "going along" with whatever our brains were going to do anyway.
But Trevena and Miller did not give believers in free will much to cheer about. They were careful to say that, "nothing in our results suggests that conscious decisions are produced by anything other than neural activity", thus throwing cold water on the idea that there is an entity called the will that exists independently of the physical brain and makes the decisions. All they are saying is that the RP signal experiments of Libet only provide evidence of unspecific neural activity prior to an action and are not predictive of the actual action, and are hence not evidence of a decision.
Given the results of his experiments on free will, one might reasonably conclude that Libet is not a believer in it. What is interesting, as I will discuss in the next post in this series, is that it is Libet himself who, despite the evidence of his own experiments, defends the idea of free will and tries to find ways to retain it in the face of his own data.
Next: Trying to salvage free will