Benjamin Libet’s experimental finding that decisions had in effect already been made before the conscious mind became aware of making them is both famous and controversial; now new research (published in a ‘Brief Communication’ in Nature Neuroscience by Chun Siong Soon, Marcel Brass, Hans-Jochen Heinze and John-Dylan Haynes) goes beyond it. Whereas the delay between decision and awareness detected by Libet lasted 500 milliseconds, the new research seems to show that decisions can be predicted up to ten seconds before the deciders are aware of having made up their minds.
The breakthrough here, like so many recent advances, comes from the use of fMRI scanning. Libet could only measure electrical activity, and had to use the Readiness Potential (RP) as an indicator that a decision had been made: the new research can go much further, mapping activity in a number of brain regions and applying statistical pattern recognition techniques to see whether any of it could be used to predict the subject’s decision.
The design of the experiments varied slightly from Libet’s original ingenious set-up. This time a series of letters was displayed on a screen. The subject were asked to press either a right or a left button at a moment chosen by them; they then identified the letter which had been displayed at the moment they felt themselves deciding to press either right or left. In the main series of experiments, no time constraints were imposed.
Two regions proved to show activity which predicted the subject’s choice: primary motor cortex and the Supplementary Motor Area (SMA) – the SMA is the source of the RP which Libet used in his research. In the SMA the researchers found activity which predicted the decision some five seconds before the moment of conscious awareness, but it was elsewhere that the earliest signs appeared – in the frontopolar cortex and the precuneus. Here the subject’s decision could be seen as much as seven seconds ahead of time: allowing for the delay in the fMRI response, this tots up to a real figure of ten seconds. One contrast with earlier findings is that there was no activation of the dorsolateral prefrontal cortex: the researchers hypothesise that this was because the design of their experiment did not require subjects to remember previous button presses. Another difference, of course, is the huge delay of five seconds in the SMA, which one would have expected to be comparable with the findings of earlier, RP-based research. Here the suggested explanation is that in the new experiments the timing of button presses was wholly unconstrained so that there was more time for activity to build up. The time delay in the fMRI study apparently means that the possibility that there was additional activity within the last few hundred milliseconds cannot be excluded: I conjecture that this offers another possible explanation if the RP studies actually detected a late spike which the fMRI couldn’t detect.
The experimenters also ran a series of experiments where the subject chose left or right at a pre-determined time: this does not seem to have shortened the delays, but it showed up a difference between the activation in the frontopolar cortex and the precuneus: briefly, it looks as if the former peaks at the earliest stage, with the precuneus ‘storing’ the decision through more continuous activation.
What is the significance of these new findings? The researchers suggest the results do three things: they show that the delay is not confined to areas which are closely associated with motor activity, but begins in ‘higher’ areas; they demonstrate clearly that the activity relates to identifiable decisions, not just general preparation; and they rule out one of the main lines of attack on Libet’s findings, namely that the small delay observed is a result of mistiming, error, or misunderstanding of the chronology. That seems correct – a variety of arguments of differing degrees of subtlety have been launched against the timings of Libet’s original work. Although Libet himself was scrupulous about demonstrating solid reasons for his conclusions, it always seemed that a delay of a few hundred milliseconds might perhaps be attributable to some sort of error in the book-keeping, especially since timing a decision is obviously a tricky business. A delay of ten seconds is altogether harder to explain away.
However, it seems to me that while the new results close off one line of attack, they reinforce another – the claim that these experiments do not represent normal decision making. We do not typically make random decisions at a random moment of our choosing, and it can therefore fairly be argued that the research has narrower implications than might appear, or even that they are merely a strange by-product of the peculiar mental processes the subjects were asked to undertake. While the delay was restricted to half a second, it was intuitively believable that all our normal decisions were subject to a similar time-lag – surprising, but believable. A delay of ten seconds in normal conscious thought is not credible at all; it’s easy to think of cases where an unexpected contingency arises and we act on it thoughtfully and consciously within much shorter periods than that.
The researchers might well bite the bullet so far as that goes, accepting that their results show only that the delay can be as long as ten seconds, not that it invariably is. Libet himself, had he lived to see these results might perhaps have been tempted to elaborate his idea of ‘free won’t’ – that while decisions build up in our brains for a period before we are aware of them, the conscious mind retains a kind of veto at the last moment.
What would be best of all, of course, is further research into decisions made in more real-life circumstances, though devising a way in which decisions can be identified and timed accurately in such circumstances is something of a challenge.
In the meantime, is this another blow to the idea of free will generally? The research will certainly hearten hard determinists, but personally I remain a compatibilist. I think making a decision and becoming aware of having made that decision are two different things, and I have no deep problem with the idea that they may occur at different times. The delay between decision and awareness does not mean the decision wasn’t ours, any more than the short delay before we hear our own voice means we didn’t intend what we said. Others, I know, will feel that this relegates consciousness to the status of an epiphenomenon.