Criticism of the Libet Experiment on Free Will
Both Wegner and Gazzaniga are inspired, in part, by a famous experiment performed by the late Benjamin Libet, a neurophysiologist.19 When someone makes a voluntary movement, an event-related potential appears in the EEG about 600 milliseconds beforehand: this is known as the readiness potential. In Libet’s experiment, subjects viewed a light that revolved around a circle at a rate of approximately once every 2.5 seconds; they were instructed to move their fingers anytime they wanted, but to use the clock to note the time of their first awareness of the wish to act. Libet discovered that the awareness of wish preceded the act by about 200 msec—not much of a surprise there. But he also discovered that the readiness potential preceded the awareness of the wish by about 350 msec (200 + 350 = c. 600 msec). So there is a second type of readiness potential, which Libet characterized as a predecisional negative shift. Libet concluded that the brain decides to move before the person is aware of the decision, which manifests itself as a conscious wish to move. Put another way, behavior is instigated unconsciously (Wegner’s “unconscious cause of action”); conscious awareness occurs later, as a sort of afterthought, and conscious control serves only as a veto over something that is already happening. In other words, conscious will really is an illusion, and we are nothing more than particles acting in fields of force after all.
Libet’s observation of a predecisional negative shift has been replicated in other laboratories, but that does not mean that his experiment is immune to criticism and his conclusions are correct.20 In the first place, there is considerable variability around those means, and the time intervals are such that that the gap between the predecisional negative shift and the readiness potential could be closer to zero. And there are a lot of sources of error, including error in determining the onset of the readiness potential and error in determining the onset of the conscious wish (as for the latter, think about keeping track of a light that is rotating around a clock face once every 2.5 seconds). Still, that difference is unlikely to be exactly zero, and so the problem does not go away.
At a different level, Libet’s experiment has been criticized on the grounds of ecological validity. The action involved, moving one’s finger, is completely inconsequential and shouldn’t be glibly equated with choosing where to go to college, or whom to marry, or even whether to buy Cheerios or Product 19—much less whether to throw a fat man off a bridge to stop a runaway trolley careening toward five innocents. The way the experiment is set up, the important decision has already been made—that is, to participate in an experiment in which one is to raise one’s finger while watching a clock. And that decision has been made out of view of the EEG apparatus. I find this argument fairly persuasive. But still, there remains the nagging possibility that, if we recorded the EEG all the time, in vivo, we would observe the same predecisional negative shift before that decision was made, too.
More recently, though, Jeff Miller and his colleagues found a way to address this critique.21 They noted that the subjects’ movements are not truly spontaneous, for the simple reason that they must also watch the clock while making them. They compared the readiness potential under two conditions. In one, the standard Libet paradigm, subjects were instructed to watch the clock while moving their fingers and report their decision time. In the other, they were instructed to ignore the clock and not asked for any reports. Subjects in both conditions still made the “spontaneous” decision whether, and when, to move their fingers. But Miller and his colleagues observed the predecisional negative shift only when subjects also had to watch the clock and report their decision time. If Miller is right, Libet’s predecisional negative shift is wholly an artifact of the attention paid to the clock. It does not indicate the unconscious initiation of ostensibly “voluntary” behavior, nor does it show that “conscious will” is illusory. Maybe it is, but the Libet experiment does not show it.
Miller’s experiment is important enough that I would like to see it replicated in another laboratory, though I want to stress that there is no reason to think that there is anything wrong with his original study. When Miller did what Libet did, he got what Libet got. When he altered the instructions, but retained voluntary movements, Libet’s effect disappeared completely—not just a little, but completely. The ramifications are pretty clear.
This does not mean that the problem of free will has been resolved in favor of compatibilism, though it does suggest that compatibilism deserves serious consideration. Personally, I like the implication of a paper by the philosopher John Searle, titled “Free Will as a Problem in Neurobiology.”22 We all experience free will, and there is no reason, in the Libet experiment or any other study, to think that this is an illusion. Free will may well be a problem for neurobiology, and if so it is a problem for neurobiologists to solve. I do not lose any sleep over it. But if free will is not an illusion, and we really do have a meaningful degree of voluntary control over our experience, thought, and action, then moral judgment is secure from this threat as well. We should be willing to make moral judgments, using all the information—rational and intuitive—that we have available to us.
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For extended discussions of Libet’s work, including replies and rejoinders, see William P. Banks and Susan Pockett, “Benjamin Libet’s Work on the Neuroscience of Free Will,” The Blackwell Companion to Consciousness, ed. Max Velmans and Susan Schneider (Malden: Blackwell, 2007) 657–70; and Benjamin Libet, “The Timing of Brain Events: Reply to the ‘Special Section’ in this Journal of September 2004, edited by Susan Pockett,” Consciousness and Cognition 15.3 (September 2006): 540–47.