Friday, 19 September 2008

Exposing some holes in Libet's classic free will study

Benjamin Libet's classic 1983 experiment purported to show that preparatory brain activity precedes our conscious decision to move - a controversial finding interpreted by some as evidence that free will is illusory.

In Libet's study, participants reported the time on a clock at the instant they had decided to move a finger. This is less straightforward than it sounds. Visual processing is sluggish whereas participants were presumably instantly aware of when they'd made a conscious decision to move. This would have led them to report a decision time that was too early (i.e. at the instant of their decision, the participants' brains would only just have been getting round to processing an earlier time on the clock).

Libet's team realised this, so in a separate control condition they also asked participants to report the timing of an electrical stimulus applied to their hand - the error in this time estimation was then used to apply a correction to participants' estimates of when they'd made a movement decision.

But in a new study, Adam Danquah and colleagues point out that our different sensory modalities operate at different speeds. They copied the control condition of Libet's experimental set-up, but they asked participants to report not just the timing of a mild electric shock, but also of a flash in the centre of the clock, and the sound of a click (delivered through headphones).

The researchers found that the participants' estimates were less accurate (i.e. even earlier) for the visual flash and auditory click than for the electric shock. In other words, Libet would have arrived at a different estimate of when participants had made a decision to move if he'd used a visual or auditory control task to make his adjustment.

"The degree of variability in bias across modalities and studies means that it is very difficult to know what correctional standard, if any, can be applied to awareness times of endogenous events [e.g. decisions]," the researchers wrote.

However, defenders of free-will shouldn't take comfort in these new results. Danquah and his colleagues added an important note about the implications of their work: "the magnitude of the biases reported here suggests that they [Libet's team] underestimated the degree to which... [preparatory brain activity] preceded the intention to move!"

In a second experiment, Danquah and his colleagues also identified another problem with the Libet paradigm. The clock used by Libet featured a dot that circled the clock-face, rather like a second-hand. Danquah's team showed that the speed at which the dot circled the clock face also affected participants' time estimations - the faster the dot, the more accurate participants' estimates became.

"The results reported here have implications for the whole tradition of having participants locate temporally subjective events using the clock paradigm," the researchers concluded.

ResearchBlogging.orgA DANQUAH, M FARRELL, D OBOYLE (2008). Biases in the subjective timing of perceptual events: Libet et al. (1983) revisited Consciousness and Cognition, 17 (3), 616-627 DOI: 10.1016/j.concog.2007.09.005

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

Disclaimer: I was a participant in this study several years ago, during my student days in Manchester.


sadunkal said...

Interesting. I think believing in free will is pretty naive anyway. How is a piece of meat supposed to be free from the universal laws? Or what makes us think that "We" are independent from that peace of meat?

Gerhard Roth from Germany is also focusing on this topic by the way, just so you know. But I don't know if any of his books are available in English or if he's making any experiments himself.

Anonymous said...

I think the results of Lebet's experiment are highly suspect.

As the subject knows that he (or she) is to flick his wrist at an undetermined point in time it seems likely to me that the brain and body begin to unconsciously prepare for the action, even if the subject is told to spontaneously make the decision.

I would be surprised with this type of experiment if it showed all of the preparatory brain/body work to flick the wrist occurred only after the subject made the conscious decision to lift the wrist NOW. Because the subject had prior knowledge of what he was to do the brain/body prepared to do it and all the subject then consciously did was decide to go ahead (the flick could be canceled up to that point) and trigger the movement.

Frank B

sadunkal said...

Well... There can always be a lot to criticize with this kind of complex experiments I guess.

But actually I'm thinking that maybe the burden of proof should be on the shoulders of the free-will-believers. Nobody really discusses whether or not single-celled organisms have free will, or insects and stuff like that... I mean where is it supposed to begin? At which state of complexity?

The belief in free-will can even be seen as superstition I suppose...

Anonymous said...

It is an interesting question whether (assuming that free will exist) self-consciousness is necessary to have free will.

I think the burden of proof is on science to prove free will does not exist. Man has perceived and acted as if he has free will for thousands of years and cultures have been developed on the basis of an assumption of volition.

Of course if there is no free will the discussion, in a sense, becomes moot, as we are simply automatons and will argue what we will argue - we can't help ourselves. :)

I for one, do not want to be an automaton. Another reason why the burden of proof, for me, is on science.


Frank B

Anonymous said...

Alexander Batthyany from the University of Vienna has offered a very fundamental, and I believe genius critique of Libet's experiment.
Batthyany argues that the Libet experiment only study the passive and not the active type of experience, and these experiments are therefore invalid in respect to the debate about the existence of freewill. The instruction in the Libet experiments to wait for an urge to act is argued to fall into the category of a passive desire, rather than a willed intention. On this basis, Libet's experiments are argued to merely confirm the intuitive feeling that passive desires such as hunger are not consciously willed; and this, says Batthyany, is something not even a hard-core dualist would claim. Therefore, Libet's experiment proves literally nothing beyond that which was not known beforehand (or could have been known by careful phenomenological analysis). I find this a very neat argument, found here:

TheOFloinn said...

Part of the problem is that this test does not test free will at all.

First, the will is the appetite for the products of the intellect, not of the senses. So, while the sensory appetites may be under intellective control -- we need not eat simply because we are hungry; we could diet, fast, wait until later, etc. -- it is also true that we do not will ourselves to be hungry or not.

Secondly, despite Nietzsche and his triumph of the will, the will is quite evidently subordinated to the intellect: we cannot want what we do not know. Therefore, insofar as we know imperfectly, the will is imperfectly determined to this or that. This is implicit in the instruction to the student-object in Libet's "experiment." Choose one. To which specific action does "one" refer? But when the object is known with certainty ("2+2=4") the consent of the will is determined precisely to it.

I'm not sure why anyone would suppose that the human body would not be involved in this process, esp. as regards a physical, bodily action. And especially as every action is preceded by a variety of preparatory thoughts and actions.

Ray Miller said...

The debate on free will and determinism is ultimately both pointless and inevitable.

It is pointless in the sense that either we have free will or it is pre-determined that we will think we have free will. (Or, indeed, pre-determined that we will think that we think!)

It is inevitable in that either we have free will and can thus engage in fascinating, if pointless, debates or it is pre-determined that we will have these debates.

There seems to be some assumption too that in order for there to be free will we have to know how it works. Most car drivers happily scoot along with only the most limited idea of how the engine works. It doesn't stop them driving. And then there's the Godel theorem...

The kinds of experiments that are carried out may throw light on how cognition operates but do not, in themselves, address the question of free will.

Personally, I'm happy to assume free will and choose to go off to the pub for a pint of Guinness.

But after half a pint, will the glass be half full or half empty...

Harvey Taylor said...

Never bought the idea that we don't have free will (a) because these kinds of experiment are subject to many different interpretations (b) the interpretations cited here are among the least likely because they tend to assume that unconscious processing is not somehow a consequence of free will (c) because there is no means of actually understanding the CONTENT of any such uncosncious procesing and (d) because it is just really UNUSEFUL to walk around believing we have no free will... People with a positive, internal locus of control tend to achieve what they believe they will achieve.People who believe they have free will, in my opinion, are more likely to exercise free will.

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