A 30-year-old classic experiment that is often used to argue against free will might have been misinterpreted.
In the early 1980s, Benjamin Libet at the University of California in San Francisco, used electroencephalography (EEG) to record the brain activity of volunteers who had been told to make a spontaneous movement. With the help of a precise timer that the volunteers were asked to read at the moment they became aware of the urge to act, Libet found there was a 200 millisecond delay, on average, between this urge and the movement itself.
But the EEG recordings also revealed a signal that appeared in the brain even earlier – 550 milliseconds, on average – before the action. Called the readiness potential, this has been interpreted as a blow to free will, as it suggests that the brain prepares to act well before we are conscious of the urge to move.
This is one of those experiments that’s made it into science fiction, gets regularly invoked in TED talks, and probably has made it into a David Brooks column. And now there’s evidence that we’ve misunderstood the phenomenon:
“Libet argued that our brain has already decided to move well before we have a conscious intention to move,” says [National Institute of Health and Medical Research scientist Aaron] Schurger. “We argue that what looks like a pre-conscious decision process may not in fact reflect a decision at all. It only looks that way because of the nature of spontaneous brain activity.”
Schurger’s group designed an experiment around the fact that
when we have to make a decision based on visual input, for example, assemblies of neurons start accumulating visual evidence in favour of the various possible outcomes. A decision is triggered when the evidence favouring one particular outcome becomes strong enough to tip its associated assembly of neurons across a threshold.
Schurger’s team hypothesised that something similar happens in the brain during the Libet experiment. In this case, rather than make a spontaneous movement, subjects were told to act as soon as they heard a click. What they found was that the fastest responders were “those in whom the accumulation of neural noise had neared the threshold – something that would show up in their EEG as a readiness potential.”
So what does this say about free will? “If we are correct, then the Libet experiment does not count as evidence against the possibility of conscious will,” says Schurger.