Posts tagged ‘Libet’

New light on Libet’s challenge to free will; this interesting BQO piece by Ari N Schulman focuses on a talk by Patrick Haggard.

Libet’s research has been much discussed, here as well as elsewhere. He asked subjects to move their hand at a time of their choosing, while measuring neural activity in their brain. He found that the occurrence of a ‘Readiness Potential’ or RP (something identified by earlier researchers) always preceded the hand movement. But it also preceded the time of the decision, as reported by subjects. So it seemed the decision was made and clearly registered in brain activity as an RP before the subjects’ conscious thought processes had anything to do with it. The research, often reproduced and confirmed, seemed to provide a solid scientific demonstration that our feeling of having free conscious control over our own behaviour is a delusion.

However, recent research by Aaron Schurger shows that we need to re-evaluate the RP. In the past it has been seen as the particular precursor of intentional action; in fact it seems to be simply a peak in the continuing ebb and flow of neural activity. Peaks like this occur all the time, and may well be the precursors of various neural events, not just deliberate action. It’s true that action requires a peak of activity like this, but it’s far from true that all such peaks lead to action, or that the decision to act occurred when the peak emerged. If we begin with an action and look back, we’ll always find an RP, but not all RPs are connected with actions. It seems to me a bit like a surfer, who has to wait for a wave before leaping on the board; but let’s suppose there are plenty of good waves and the surfer is certainly not deprived of his ability to decide when to go.

This account dispels the impression that there is a fatal difficulty here for free will (of course there are plenty of other arguments on that topic); I think it also sits rather nicely with Libet’s own finding that we have ‘free won’t’ – ie that even after an RP has been detected, subjects can still veto the action.

Haggard, who has done extensive work in this area, accepts that RPs need another look; but he contends that we can find more reliable precursors of action. His own research analysed neural activity and found significantly lowered variability before actions, rather as though the disorganised neural activity of the brain pulled together just before an action was initiated.

Haggard’s experiments were designed to address another common criticism of Libet’s experiments, namely the artificiality of the decision involved. Being told to make your hand for no reason at a moment of your choosing is very unlike most of the decisions we make. In particular, it seems random, whereas it is argued that proper free will takes account of the pros and cons. Haggard asked subjects to perform a series of simple button-pushing tasks; the next task might follow quickly, or after a delay which could be several minutes long. Subjects could skip to the next task if they found the wait tedious, but that would reduce the cash rewards they got for performing the tasks. This weighing of boredom against profit is much more like a real decision.

Haggard persuasively claims that the essence of Libet’s results is upheld and refreshed by his results, so in principle we are back where we started. Does this mean there’s no free will? Schulman thinks not, because on certain reasonable and well-established conceptions of free will it can ‘work in concert with decisional impulses’, and need not be threatened by Haggard’s success in measuring those impulses.

For myself, I stick with a point mentioned by Schurger; making a decision and becoming aware of the decision are two distinct events, and it is not really surprising or threatening that the awareness comes a short time after the actual decision. It’s safe to predict that we haven’t heard the last of the topic, however.

Another strange side light on free will. Some of the most-discussed findings in the field are Libet’s celebrated research which found that Readiness Potentials (RPs) in the brain showed when a decision to move had been made, significantly before the subject was aware of having decided. Libet himself thought this was problematic for free will, but that we could still have ‘Free Won’t’ – we could still change our minds after the RP had appeared and veto the planned movement.

A new paper (discussed here by Jerry Coyne) follows up on this, and seems to show that while we do have something like this veto facility, there is a time limit on that too, and beyond a certain point the planned move will be made regardless.

The actual experiment was in three phases. Subjects were given a light and a pedal and set up with equipment to detect RPs in their brain. They were told to press the pedal at a time of their choosing when the light was green, but not when it had turned red. The first run merely trained the equipment to detect RPs, with the light turning red randomly. In the second phase, the light turned red when an RP was detected, so that the subjects were in effect being asked to veto their own decision to press. In the third phase, they were told that their decisions were being predicted and they were asked to try to be unpredictable.

Detection of RPs actually took longer in some instances than others. It turned out that where the RP was picked up early enough, subjects could exercise the veto; but once the move was 200ms or less away, it was impossible to stop.

What does this prove, beyond the bare facts of the results? Perhaps not much. The conditions of the experiment are very strange and do not resemble everyday decision-making very much at all. It was always an odd feature of Libet’s research that subjects were asked to get ready to move but choose the time capriciously according to whim; not a mental exercise that comes up very often in real life. In the new research, subjects further have to stop when the light is red; they don’t, you notice, choose to veto their move, but merely respond to a pre-set signal. Whether this deserves to be called free won’t is debatable; it isn’t a free decision making process. How could it be, anyway; how could it be that deciding to do something takes significantly longer than deciding not to do the same thing? Is it that decisions to move are preceded by an RP, but other second-order decisions about those decisions are not? We seem to be heading into a maze of complications if we go that way and substantially reducing the significance of Libet’s results.

Of course, if we don’t think that Libet’s results dethrone free will in the first place, we need not be very worried. My own view is that we need to distinguish between making a conscious decision and becoming aware of having made the decision. Some would argue that that second-order awareness is essential to the nature of conscious thought, but I don’t think so. For me Libet’s original research showed only that deciding and knowing you’ve decided are distinct, and the latter naturally follows after the former. So assuming that, like me, you think it’s fine to regard the results of certain physical processes as ‘free’ in a useful sense, free will remains untouched. If you were always a sceptic then of course Libet never worried you anyway, and nor will the new research.

red lampWas Libet wrong? The question has often been asked since the famous experiments which found that a detectable “Readiness Potential” (RP) showed that our decisions were made half a second before they entered our consciousness. There are plenty of counter arguments but the findings themselves have been reproduced and seem unassailable.

However, Christian Jarrett reports two new pieces of research which shed fresh light on the issue (Jarrett’s book Great Myths of the Brain is very sensible, btw, and should be read by all science journalists).

The first piece of research  shows that although an action may be prepared by the brain before we know we’ve decided to act, we can still change our minds. Subjects were asked to press a button at a moment of their choice after a green light came on. However, if a red light appeared before they pressed, they were asked to refrain. The experimenters then detected the RP which showed the subjects were about to press the button, and used the red light to try to cancel the intention. They found that although there was a ‘point of no return’, so long as the red light appeared in time the subjects were able to hold off and not press the button after all.

That means that a significant qualification has to be added to Libet’s initial findings – but it’s one Libet himself had already come up with. He was aware that the emergent action could be suppressed, and memorably said it showed that while we might not have free will, we could still have ‘free won’t’. I don’t know much use that is. In the experiment the subjects simply responded to a red light, but if the veto is to make consciousness effective again we seem to need the veto decision to happen instantly and overtake the action decision somehow. That seems problematic if not paradoxical. I also get quite confused trying to imagine what it would be like to veto mentally a decision you’re not yet aware of having made. Still, I suppose we must be grateful for whatever residual degree of freedom the experiments allow us.

The second piece of research  calls into question the nature of the RP itself. Libet’s research more or less took it for granted that the RP was a reliable sign that an action was on the way, but the new findings suggest that it is really just part of the background ebb and flow of neural noise. Actions do arise when the activity crosses a certain threshold, but the brain is much quicker about getting to that level when the background activity is already high.

That certainly adds some complexity to the picture, but I don’t think it really dispels the puzzle of Libet’s results. The RP may be fuzzier than we thought and it may not have as rigid a link to action as we thought – but it’s still possible to predict the act before we’re aware of the decision. What if we redesigned the first experiment? We tell the subject to click on a target at any time after it appears; but we detect the RP and whip the target away a moment before the click every time. The subject can never  succeed. The fact that that is perfectly possible surely remains more than a little unsettling.

whistlePhysical determinism is implausible according to Richard Swinburne in the latest JCS; he cunningly attacks via epiphenomenalism.

Swinburne defines physical events as public and mental ones as private – we could argue about that, but as a bold, broad view it seems fair enough. Mental events may be phenomenal or intentional, but for current purposes the distinction isn’t important. Physical determinism is defined as the view that each physical event is caused solely by other physical events; here again we might quibble, but the idea seems basically OK to be going on with.

Epiphenomenalism, then, is the view that while physical events may cause mental ones, mental ones never cause physical ones. Mental events are just, as they say, the whistle on the locomotive (though the much-quoted analogy is not exact: prolonged blowing of the whistle on a steam locomotive can adversely affect pressure and performance). Swinburne rightly describes epiphenomenalism as an implausible view (in my view, anyway – many people would disagree), but for him it is entailed by physical determinism, because physical events are only ever caused by other physical events. In his eyes, then, if he can prove that epiphenomenalism is wrong, he has also shown that physical determinism is ruled out. This is an unusual, perhaps even idiosyncratic perspective, but not illogical.

Swinburne offers some reasonable views about scientific justification, but what it comes down to is this; to know that epiphenomenalism is true we have to show that mental events cause no physical events; but that very fact would mean we could never register when they had occurred – so how would we prove it? In order to prove epiphenomenalism true, we must assume that what it says is false!

Swinburne takes it that epiphenomenalism means we could never speak of our private mental events – because our words would have to have been caused by the mental events, and ex hypothesi they don’t cause physical events like speech. This isn’t clearly the case – as I’ve mentioned before, we manage to speak of imaginary and non-existent things which clearly have no causal powers. Intentionality – meaning – is weirder and more powerful than Swinburne supposes.

He goes on to discuss the famous findings of Benjamin Libet, which seem to show that decisions are detectable in the brain before we are aware of having made them. These results point towards epiphenomenalism being true after all. Swinburne is not impressed; he sees no basic causal problem in the idea that a brain event precedes the mental event of the decision, which in turn precedes action. Here he seems to me to miss the point a bit, which is that if Libet is right, the mental experience of making a decision has no actual effect, since the action is already determined.

The big problem, though is that Swinburne never engages with the normal view; ie that in one way or another mental events have two aspects. A single brain event is at the same time a physical event which is part of the standard physical story, and a mental event in another explanatory realm. In one way this is unproblematic; we know that a mass of molecules may also be a glob of biological structure, and an organism; we know that a pile of paper, a magnetised disc, or a reel of film may all also be “A Christmas Carol”. As Scrooge almost puts it, Marley’s ghost may be undigested gravy as well as a vision of the grave.

It would be useless to pretend there is no residual mystery about this, but it’s overwhelmingly how most people reconcile physical determinism with the mental world, so for Swinburne to ignore it is a serious weakness.

wiring a neuronA few years ago we noted the remarkable research by Fried, Mukamel, and Kreiman which reproduced and confirmed Libet’s famous research. Libet, in brief, had found good evidence using EEG that a decision to move was formed about half a second before the subject in question became consciously aware of it; Fried et al produced comparable results by direct measurement of neuron firing.

In the intervening years, electrode technology has improved and should now make it possible to measure multiple sites. The scanty details here indicate that Kreiman, with support from MIT, plans to repeat the research in an enhanced form; in particular he proposes to see whether, having identified the formed intention to move, it is then possible to stop it before the action takes place. This resembles the faculty of ‘free won’t’ by which Libet himself hoped to preserve some trace of free will.

From the MIT article it is evident that Kreiman is a determinist and believes that his research confirms that position. It is generally believed that Libet’s findings are incompatible with free will in the sense that they seem to show that consciousness has no effect on our actual behaviour.

That actually sheds an interesting side-light on our view of what free will is. A decision to move still gets made, after all; why shouldn’t it be freely made even though it is unconscious? There’s something unsatisfactory about unconscious free will, it seems. Our desire for free will is a desire to be in control, and by that we mean a desire for the entity that does the talking to be in control. We don’t really think of the unconscious parts of our mind as being us; or at least not in the same way as that gabby part that claims responsibility for everything (the part of me that is writing this now, for example).

This is a bit odd, because the verbal part of our brain obviously does the verbals; it’s strange and unrealistic to think it should also make the decisions, isn’t it? Actually if we are careful to distinguish between the making of the decision and being aware of the decision – which we should certainly do, given that one is clearly a first order mental event and the other equally clearly second order – then it ceases to be surprising that the latter should lag behind the former a bit. Something has to have happened before we can be aware of it, after all.

Our unease about this perhaps relates to the intuitive conviction of our own unity. We want the decision and the awareness to be a single event, we want conscious acts to be, as it were, self- illuminating, and it seems to be that that the research ultimately denies us.

It is the case, of course, that the decisions made in the research are rather weird ones. We’re not often faced with the task of deciding to move our hands at an arbitrary time for no reason. Perhaps the process is different if we are deciding which stocks and shares to buy? We may think about the pros and cons explicitly, and we can see the process by which the conclusion is reached; it’s not plausible that those decisions are made unconsciously and then simply notified to consciousness, is it?

On the other hand, we don’t think, do we, that the process of share-picking is purely verbal? The words flowing through our consciousness are signals of a deeper imaginative modelling, aren’t they? If that is the case, then the words might still be lagging. Perhaps the distinction to be drawn is not really between conscious and unconscious, but between simply conscious and explicitly conscious. Perhaps we just shouldn’t let the talky bit pretend to be the whole of consciousness just because the rest is silent.

Recent research at UCL provides corroboration for the claim that elite sports players often feel they have plenty of time to think about how they address a ball which is actually coming at them so fast that normal human beings probably shouldn’t even see it.

This effect, a perceived slowing down of time, is the kind of thing which I think would once have been shrugged off as unresearchable, outside the scope of proper science: how can we tell how things seem to people? These days we’re bolder, and the team in this case came up with an experiment in which some subjects were asked to tap a screen while others were merely asked to observe. Those who were asked to prepare action reported feeling the time they had available seemed longer.

The finding makes a certain obvious sense in that being granted extra mental time when a particularly crucial task is coming up is bound to be helpful. But it’s also a little puzzling: if the brain is capable of giving us more time, why doesn’t it do so routinely? It would never be a bad thing to have more time for reflection; if the facility only gets turned on in these special circumstances there must either be some downside or cost which makes the brain ration its use, or something else is wrong.

It’s not altogether implausible that revving up our mental processes might have an energy cost, or that our neurons might be able to speed up a bit only temporarily; but as always with consciousness there’s a deeper issue. Did mental processing actually speed up, or did it simply feel as if it did; or indeed, was the time merely remembered as passing more slowly? There’s an assumption here that when we are playing baseball or tennis control is fully conscious, but actually it’s far from clear that the deliberative  level of thought has much to do with it – it generally seems more a matter of gut reaction than strategic debate.

Another complicating factor is that we don’t actually perceive the passage of time directly, in the way we can observe spatial sizes. There is no speial organ devoted to measuring time, and it seems likely that if we do have to assess time (we’re pretty bad at it- try telling when two minutes have passed without looking at a clock) we pick up on several different indirect clues.

It may well be that one way the brain works out how much time has passed is by counting the number of ‘events’ it remembers and assuming that the gap between each of them is about the same. ‘Events’ would be mental ones, which explains why outwardly uneventful time may seem to pass very slowly – because we keep checking our watches or asking ourselves how much longer, so that there is a steady stream of mental acts, whereas when we’re caught up in something interesting there are none of these mental markers put down and time seems to fly.

It seems highly plausible to me that when we’re asked to prepare mentally for a given act (whether tapping the screen or hitting a perfect drive to the boundary) a sequence of this kind is set up, in which we keep asking ourselves ‘am I going to do it now?’ or ‘is it time yet?’. If that’s true the chances are the impression of having more time to react is actually an illusion, though it may reflect a genuinely improved state of readiness.

I couldn’t help reflecting that the conditions of these experiments rather resembled those in the famous series of experiments by Benjamin Libet which seemed to show that a decision to move one’s hand at a given moment had actually been taken significantly before that same decision entered consciousness. Libet’s subjects were asked to get ready to act in just the sort of way which might have led to the kind of time-dilation effect considered in the UCL research. Libet’s ingenious system for measuring the time of decision, with subjects reporting the position of a clock at the vital moment, should be fairly well proofed against subjective errors: but could it be that a sense of time slowing down caused Libet’s subjects to delay slightly in reporting their decision?

Just  a thought.


Picture: qualintentionality. I see that this piece on has drawn quite a bit of attention. It provides a round-up of views on the question of whether free will can survive in a post-Libet world, though it highlights more recent findings along similar lines by John-Dylan Haynes and others. The piece seems to be prompted in part by Big Questions in Free Will a project funded by the John Templeton Foundation, which  is probably best known for the Templeton Prize, a very large amount of cash which gets given to respectable scientists who are willing to say that the universe has a spiritual dimension, or at any rate that materialism is not enough. BQFW itself is offering funding for theology as well as science: “science of free will ($2.8 million); theoretical underpinnings of free will, round 1 ($165,000); and theology of free will, round 1 ($132,000)”. I suppose ‘theoretical underpinnings’, if it’s not science and not theology, must be philosophy; perhaps they called it that because they want some philosophy done but would prefer it not to be done by a philosopher. In certain lights that would be understandable. The presence of theology in the research programme may not be to everyone’s taste, although what strikes me most is that it seems to have got the raw end of the deal in funding terms. I suppose the scientists need lots of expensive kit, but on this showing it seems the theologians don’t even get such comfortable armchairs as the theorists, which is rough luck.

We have of course discussed the Haynes results and Libet, and other related pieces of research many times in the past. I couldn’t help wondering whether, having all this background, I could come up with something on the subject that might appeal to the Templeton Foundation and perhaps secure me a modest emolument? Unfortunately most of the lines one could take are pretty well-trodden already, so it’s difficult to come up with an appealing new presentation, let alone a new argument. I’m not sure I have anything new to say. So I’ve invited a couple of colleagues to see what they can do.

Bitbucket Free will is nonsense; I’m not helping you come up with further ‘compatibilist’ fudging if that’s what you’re after. What I can offer you is this: it’s not just that Libertarians have the wrong answer, the question doesn’t even make sense. The way the naturenews piece sets up the discussion is to ask: how can you have free will if the decision was made before you were even aware of it? The question I’m asking is: what the hell is ‘you’?

Both Libet’s original and the later experiments are cleverly designed to allow subjects to report the moment at which they became aware of the decision: but ‘they’ are thereby implicitly defined as whatever it is that is doing the reporting. We assume without question that the reporting thing is the person, and then we’re alarmed by the fact that some other entity made the decision first. But we could equally well take the view that the silent deciding entity is the person and be unsurprised that a different entity reports it later.

You will say in your typically hand-waving style, I expect, that that can’t be right because introspection or your ineffable sense of self or something tells you otherwise. You just feel like you are the thing that does the reporting. Otherwise when words come out of your mouth it wouldn’t be you talking, and gosh, that can’t be right, can it?

Well, let me ask you this. Suppose you were the decision-making entity, how would it seem to you? I submit it wouldn’t seem any way, because as that entity you don’t do seeming-to: you just do decisions. You only seem to yourself to have made the decision when it gets seemed back to you by a seeming entity – in fact, by that same reporting entity.  In short, because all reports of your mental activity come via the reporting entity, you mistake it for the source of all your mental activity. In fact all sorts of mental processes are going on all over and the impression of a unified consistent centre is a delusion. At this level, there is no fixed ‘you’ to have or lack free will. Libet’s experiments merely tell us something interesting but quite unworrying about the relationship of two current mental modules.

So libertarians ask: do we have free will? I reply that they have to show me the ‘we’ that they’re talking about before they even get to ask that question – and they can’t.

BlandulaNot much of a challenge to come up with something more appealing than that! I’ve got an idea the Templeton people might like, I think: Dennettian theology.

You know, of course, Dennett’s idea of stances. When we’re looking to understand something we can take various views. If we take the physical stance, we just look at the thing’s physical properties and characteristics. Sometimes it pays to move on to the design stance: then we ask ourselves, what is this for, how does it work? This stance is productive when considering artefacts and living things, in the main. Then in some cases it’s useful to move on to the intentional stance, where we treat the thing under consideration as if it had plans and intentions and work out its likely behaviour on that basis. Obviously people and some animals are suitable for this, but we also tend to apply the same approach to various machines and natural phenomena, and that’s OK so long as we keep a grip.

But those three stances are clearly an incomplete set. We could also take up the Stance of Destiny: when we do that we look at things and ask ourselves: was this always going to happen? Is this inevitable in some cosmic sense? Was that always meant to be like that? I think you’ll agree that this stance sometimes has a certain predictive power: I knew that was going to happen, you say: it was, as it were, SoD’s Law.

Now this principle gives us by extrapolation an undeniable God – the God who is the intending, the destining entity. Does this God really exist? Well, we can take our cue from Dennett: like the core of our personhood in his eyes, it doesn’t exist as a simple physical thing you can lay your hands on: but it’s a useful predictive tool and you’d be a fool to overlook it, so in a sense it’s real enough: it’s a kind of  explanatory centre of gravity, a way of summarising the impact of millions of separate events.

So what about free will? Well of course, one thing you can say about a free decision is that it wasn’t destined. How does that come about? I suggest that the RPs Libet measured are a sign of de-destination, they are, as it were, the autopilot being switched off for a moment. Libet himself demonstrated that the impending action could be vetoed after the RP, after all. Most of the time we run on destined automatic, but we have a choice. The human brain, in short, has a unique mechanism which, by means we don’t fully understand, can take charge of destiny.

I think my destiny is to hang on to the day job for the time being.

Picture: dials. Libet’s famous experiments are among the most interesting and challenging in neuroscience; now they’ve been taken further. A paper by Fried, Mukamel and Kreiman in Neuron (with a very useful overview by Patrick Haggard) reports on experiments using a number of epilepsy patients where it was ethically possible to implant electrodes and hence to read off the activity of individual neurons, giving a vastly more precise picture than anything achievable by other means. In other respects the experiments broadly followed the design of Libet’s own, using a similar clock-face approach to measure the time when subjects felt they decided to press a button. Libet discovered that a Readiness Potential (RP) could be detected as much as half a second before the subject was conscious of deciding to move; the new experiments show that data from a population of 250 neurons in the SMA (the Supplementary Motor Area) were sufficient to predict the subject’s decision 700 ms in advance of the subject’s own awareness, with 80% accuracy.

The more detailed picture which these experiments provide helps clarify some points about the relationship between pre-SMA and SMA proper, and suggest that the sense of decision reported by subjects is actually the point at which a growing decision starts to be converted into action, rather than the beginning of the decision-forming process, which stretches back further. This may help to explain the results from fMRI studies which have found the precursors of a decision much earlier than 500 ms beforehand. There are also indications that a lot of the activity in these areas might be more concerned with suppressing possible actions than initiating them – a finding which harmonises nicely with Libet’s own idea of ‘free won’t’ – that we might not be able to control the formation of impulses to act, but could still suppress them when we wanted.

For us, though, the main point of the experiments is that they appear to provide a strong vindication of Libet and make it clear that we have to engage with his finding that our decisions are made well before we think we’re making them.

What are we to make of it all then? I’m inclined to think that the easiest and most acceptable way of interpreting the results is to note that making a decision and being aware of having made a decision are two different things (and being able to report the fact may be yet a third). On this view we first make up our minds; then the process of becoming aware of having done so naturally takes some neural processing of its own, and hence arrives a few hundred milliseconds later.

That would be fine, except that we strongly feel that our decisions flow from the conscious process, that the feelings we are aware of, and could articulate aloud if we chose, are actually decisive. Suppose I am deciding which house to buy: house A involves a longer commute while house B is in a less attractive area. Surely I would go through something like an internal argument or assessment, totting up the pros and cons, and it is this forensic process in internal consciousness which causally determines what I do? Otherwise why do I spend any time thinking about it at all – surely it’s the internal discussion that takes time?

Well, there is another way to read the process: perhaps I hold the two possibilities in mind in turn: perhaps I imagine myself on the long daily journey or staring at the unlovely factory wall. Which makes me feel worse? Eventually I get a sense of where I would be happiest, perhaps with a feeling of settling one alternative and so of what I intend to do. On this view the explicitly conscious part of my mind is merely displaying options and waiting for some other, feeling part to send back its implicit message. The talky, explicit part of consciousness isn’t really making the decision at all, though it (or should I say ‘I’?) takes responsibility for it and is happy to offer explanations.

Perhaps there are both processes in involved in different decisions to different degrees. Some purely rational decisions may indeed happen in the explicit part of the mind, but in others – and Libet’s examples would be in this category – things have to feel right. The talky part of me may choose to hold up particular options and may try to nudge things one way or another, but it waits for the silent part to plump.

Is that plausible? I’m not sure. The willingness of the talky part to take responsibility for actions it didn’t decide on and even to confect and confabulate spurious rationales, is very well established (albeit typically in cases with brain lesions), but introspectively I don’t like the idea of two agents being at work I’d prefer it to be one agent using two approaches or two sets of tools – but I’m not sure that does the job of accounting for the delay which was the problem in the first place…

(Thanks to Dale Roberts!)

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.