Libet plays ball

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.

 

Beyond Libet

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!)

Libet was wrong…?

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.