The following analysis was submitted by Brains reader Bill Skaggs, a neuroscientist whose published work is in electrophysiology, but who has been working for some time on a book about the relationship between consciousness and the brain. He blogs at https://weskaggs.net. — JS
I would like to examine a famous set of experiments carried out in the 1970s by Benjamin Libet. The results that he obtained are widely considered paradoxical, but what I would like to argue is that they are really only paradoxical if interpreted dualistically. When properly understood, they function as a compelling refutation of dualism — even though Libet himself did not understand them that way.
The aim of Libet’s experiment was to gain an understanding of how free will manifests itself inside the brain. Libet’s goal was to design an experiment in which human subjects would choose, of their own free will, the time at which to execute a certain action. He wanted to record their brain electrical activity using an EEG system, and look for changes in relation to the time at which the decision to act was made. Specifically, he decided to focus on the so-called “readiness potential”, a large EEG wave that typically begins several hundred milliseconds before a person executes a motor response such a pressing a button.
Here is the methodological problem that Libet faced: it is easy to measure the time at which a subject presses a button, but how can you measure the time at which a subject has made a decision? The approach that Libet took was quite innovative. His subjects were instructed that at some time during a long interval they should press a button, and that they were completely free to choose the moment to push it — in fact they should try to do it in as random a way as possible, not as a result of any sort of cuing. He also asked his subjects to watch a bright dot of light that swept around a circle on an oscilloscope screen, and to note its angular position at the moment they decided to act. This information allowed the “decision time” to be calculated with a precision of slightly better than one twentieth of a second.
And here is what Libet observed. The onset of the readiness potential could be seen in the EEG several hundred milliseconds before the calculated decision time, that is, several hundred milliseconds before the time calculated on the basis of the reported dot position.
It is important to be clear that everything I have said up to this point concerns data, not inference or interpretation. Each trial of the experiment generated three time measurements:
- The time at which the button was pressed, recorded mechanically.
- The reported time of decision, calculated from the oscilloscope angle reported by the subject.
- The time of onset of the readiness potential, calculated from the recorded EEG signal.
The result that Libet obtained was that the time of onset of the readiness potential was systematically several hundred milliseconds (that is, several tenths of a second) earlier than the reported time of decision. In other words, physiological evidence of the decision appeared well before the moment at which the subjects reported that they made the decision.
The question is, how should that result be understood?
To many people, Libet’s finding seems counterintuitive, or even impossible. How could physical traces of the decision appear before the decision had been made? The only way to make sense of the results, it seems to many people, is to conclude that the decision was not really made when the subject believed it was made — that the sense of having made a choice at that moment was an illusion. Either that, or something must be wrong with the experiment methodologically.
Indeed, it is not all that difficult to find problems with Libet’s methodology. From an experimental scientist’s point of view, his method of identifying the decision time is problematic. Normally in an experiment it is desirable for the experimenter to have control over the independent variable. In this case, though, the experimenter has no control over the time at which the decision is made, and therefore can’t rule out the presence of unobserved variables that may influence the decision. Other concerns have also been raised about the validity of the subject’s report as a measure of the decision time.
Really, though, that doesn’t actually matter very much. The aspects of the Libet experiment that make it philosophically important are immune to those sorts of objections. Here is what makes the experiment important:
- The result is impossible to make sense of in the framework of dualism.
- Outside the framework of dualism, the result is not only easy to understand, it is inevitable, at least at a qualitative level.
Let me first spell out why the result is impossible to understand in the framework of dualism. If we view the Libet paradigm in terms of a machine-and-operator metaphor, we will see the decision as taken by the operator (by “consciousness”, as many people put it), and the EEG signal as something that happens in the machine (the brain). Libet’s result, then, means that signs of the decision can be detected in the machine before the operator has actually made the decision. And that, of course, seems paradoxical. The only way to interpret it (within the framework of dualism) is that the operator is only under an illusion of having made the decision, and that the decision was really made by the machine. This is in fact the most common interpretation of Libet’s experiment, and many people (including Libet himself) have seen it as the only possible interpretation.
But let’s now look at what happens if we resist the temptation to impose a dualistic interpretation on the data. Suppose we look at the scenario simply in terms of brain activity and its relation to behavior. The crucial thing is the neural process by means of which the subject takes note of the decision time. This must, at some level, involve the execution of a brain activity sequence that causes an observation of the dot position to be made and the result to be encoded within short-term memory. Libet’s result shows that this observing-behavior is systematically preceded by brain activity of some sort. But isn’t that inevitable? Would it be credible for this brain activity to execute with nothing whatsoever coming before it in order to trigger it? That would be a miracle. The precise timing observed by Libet is not inevitable, of course, but the fact of something occurring in the brain to cause the making of the decision cannot be avoided.
Thus when we look at the data without forcing it into a dualistic framework, we see that it is not paradoxical at all.
The real importance of Libet’s experiment is that it is a compelling refutation of dualism. From a non-dualistic point of view the result is not problematic in any way, but from a dualistic point of view it is impossible to make sense of.
It is worth noting, ironically, that Libet, although his historical importance derives from having refuted dualism in this way, was himself a thoroughgoing dualist. He was what Daniel Dennett called a “Cartesian materialist”. Libet wrote many papers about his experiment and variations on it, but all of them viewed the data within the context of a machine-and-operator metaphor. (He used “consciousness” as his name for the operator.) He thought that his experiment proved that consciousness could not initiate behavior; however he also thought that it might be possible for consciousness to veto behavior in some cases.
The conclusion that is most commonly drawn from the Libet experiment is that free will is an illusion — that we do not really control our own behavior — that the actions we believe are freely willed are actually generated by unconscious processes occurring inside our brains. I don’t believe that this is the correct conclusion. The correct conclusion, I believe, is that the intuitive concept of free will is intrinsically dualistic, and that any attempt to apply it in a non-dualistic framework will result in paradox.
(Footnote: I make no claim to be the only person who has ever figured this out. Libet published a “target article” on his work in Behavioral and Brain Sciences that was followed by several dozen responses, and some of the commentators clearly grasped the underlying dualism in Libet’s way of thinking. But none of them really explained it very clearly.)