Tag Archives: Readiness Potential

Libet Unreadied

Posted on by
21

Benjamin Libet’s famous experiments have been among the most-discussed topics of neuroscience for many years. Libet’s experiments asked a subject to move their hand at a random moment of their choosing; he showed that the decision to move could be predicted on the basis of a ‘readiness potential’ detectable half a second before the subject reported having made the decision. The result has been confirmed many times since, and even longer gaps between prediction and reported decision have been claimed. The results are controversial because they seem to be strong scientific evidence against free will. If the decision had been made before the decider knew it, how could their conscious thought have been effective? My original account (14 years ago, good grief) is here.

Libet’s findings and their significance have been much disputed philosophically, but a new study reported here credibly suggests that the readiness potential (RP) or Bereitschaftspotential has been misunderstood all along.

In essence the RP is a peak of neural activity, which seemed to the original researchers to represent a sort of gathering of neural resources as the precursor of an action. But the new study suggests that this appearance is largely an artefact of the methods used to analyse the data, and that the RP really represents nothing more than the kind of regular peak which occurs naturally in any system with a lot of background noise. Neural activity just does ebb and flow like that for no particular reason.

That is not to say that the RP was completely irrelevant to the behaviour of Libet’s subjects. In the rather peculiar circumstances of the original experiment, where subjects are asked to pick a time at random, it is likely that a chance peak of activity would tip the balance for the unmotivated decision. But that doesn’t make it either the decision itself or the required cause of a decision. Outside the rather strange conditions of the experiment, it has no particular role. Perhaps most tellingly, when the original experiments were repeated with a second group of subjects who were asked not to move at all, it was impossible to tell the difference between the patterns of neural activity recorded; a real difference appeared only at the time the subjects in the first group reported having made a decision.

This certainly seems to change things, though it should be noted that Libet himself was aware that the RP could be consciously ‘over-ruled’, a phenomenon he called ‘Free Won’t’. It can, indeed, be argued that the significance of the results was always slightly overstated. We always knew that there must be neural precursors of any given decision, if not so neatly identifiable as the RP. So long as we believe the world is governed by determined physical laws (something I think it’s metaphysically difficult to deny) the problem of Free Will still arises; indeed, essentially the same problem was kicked around for centuries in forms that relied on divine predestination or simple logical fatalism rather than science.

Nevertheless, it looks as though our minds work the way they seem to do rather more than we’ve recently believed. I’m not quite sure whether that’s disappointing or comforting.

Libet was wrong…?

Posted on by
90

Picture: clock on screen. One of the most frequently visited pages on Conscious Entities is this account of Benjamin Libet’s remarkable experiments, which seemed to show that decisions to move were really made half a second before we were aware of having decided. To some this seemed like a practical disproof of the freedom of the will – if the decision was already made before we were consciously aware of it, how could our conscious thoughts have determined what the decision was?  Libet’s findings have remained controversial ever since they were published; they have been attacked from several different angles, but his results were confirmed and repeated by other researchers and seemed solid.

However, Libet’s conclusions rested on the use of Readiness Potentials (RPs). Earlier research had shown that the occurence of an RP in the brain reliably indicated that a movement was coming along just afterwards, and they were therefore seen as a neurological sign that the decision to move had been taken (Libet himself found that the movement could sometimes be suppressed after the RP had appeared, but this possibility, which he referred to as ‘free won’t ‘, seemed only to provide an interesting footnote). The new research, by Trevena and Miller at Otago, undermines the idea that RPs indicate a decision.

Two separate sets of similar experiments were carried out. They resembled Libet’s original ones in most respects, although computer screens and keyboards replaced Libet’s more primitive equipment, and the hand movement took the form of a key-press. A clock face similar to that in Libet’s experiments was shown, and they even provided a circling dot. In the earlier experiments this had provided an ingenious way of timing the subject’s awareness that a decision had been made – the subject would report the position of the dot at the moment of decision – but in Trevena and Miller’s research the clock and dot were provided only to make conditions resemble Libet’s as much as possible. Subjects were told to ignore them (which you might think rendered their inclusion pointless). This was because instead of allowing the subject to choose their own time for action, as in Libet’s original experiments, the subjects in the new research were prompted by a randomly-timed tone. This is obviously a significant change from the original experiment; the reason for doing it this way was that Trevena and Miller wanted to be able to measure occasions when the subject decided not to move as well as those when there was movement. Some of the subjects were told to strike a key whenever the tone sounded,  while the rest were asked to do so only about half the time (it was left up to them to select which tones to respond to, though if they seemed to be falling well below a 50-50 split they got a reminder in the latter part of the experiment).  Another significant difference from Libet’s tests is that left and right hands were used: in one set of experiments the subjects were told by a letter in the centre of the screen whether they should use the right or left hand on each occasion, in the other it was left up to them.

There were two interesting results. One was that the same kind of RP appeared whether the subject pressed a key or not. Trevena and Miller say this shows that the RP was not, after all, an indication of a decision to move, and was presumably instead associated with some more general kind of sustained attention or preparing for a decision. Second, they found that a different kind of RP, the Lateralised Readiness Potential or LRP, which provides an indication of readiness to move a particular hand, did provide an indication of a decision, appearing only where a movement followed; but the LRP did not appear until just after the tone. This suggests, in contradiction to Libet, that the early stages of action followed the conscious experience of deciding, rather than preceding it.

The differences between these new experiments and Libet’s originals provide a weak spot which Libetians will certainly attack.  Marcel Brass, whose own work with fMRI scanning confirmed and even extended Libet’s delay, seeming to show that decisions could be predicted anything up to ten seconds before conscious awareness, has apparently already said that in his view the changes undermine the conclusions Trevena and Miller would like to draw. Given the complex arguments over the exact significance of timings in Libet’s results, I’m sure the new results will prove contentious. However, it does seem as if a significant blow has been struck for the first time against the foundations of Libet’s remarkable results.