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Modern interpretation of Libet data

  1. Aug 15, 2015 #1
    Many may be familiar with the Libet experiments that suggested that the brain begins the process of preparing for a motor action before the subject decides to make the action. This has been discussed and argued over a great deal in the past several decades. I came across this in reading about consciousness and brain processes and wondered if indeed that data and its interpretation still held true today.

    I found that Aaron Schurger had done some further experiments more recently that cast a different light on the matter.


    I like Schurger's argument. Briefly, my take from this is that he argues that random fluctuations in neural activity will tend to approach a threshold over time; once the threshold is exceeded a motor action is decided. Thus in respect to voluntary, non temporally constrained decisions to move, the decision to move and the action are in fact much closer together than Libet's data indicated.

    In other words, the Libet experiment suggests that for voluntary actions the brain prepares for action BEFORE the conscious decision to act. Hence the urge to act is a post facto addition to experience. Schurger suggests that typical neuronal activity buzzes away and it is not until a threshold point is reached that the decision to act occurs (and the act itself).

    That is, when we are thinking of doing a voluntary act, the choice of when to act is precipitated by the neural activity exceeding the threshold. The RP curve typical of the Libet experiment simply describes normal activity with the actual motor response grafted on (because a motor response occurred).

    I think this still suggests that the motor act occurs without 'conscious' direction as such, it serves merely to bring decision point and motor response closer together temporally. But it does show that the decision to act and the act are not separate as such, rather they both arise from the same underlying neural activity.

    But I also may be misreading the paper. Is the proper interpretation that when neural activity reaches close to the threshold it's already primed by an expectation to act at some time, and thus at that point it tips over the edge and hence causes the conscious decision from which the act arises.

    Even then though I do not think it rescues us from the sense that the 'voluntary, conscious' act arises unconsciously from a potential.

    I'd be interested in anyone else's take on this paper and its findings. Or whether there are more and later findings on this?
  2. jcsd
  3. Aug 15, 2015 #2
    I haven't read Schurger's work, but your statement as written is incorrect. Motor action is definitely not related to gross physical excitation. Motor expression is relegated to a mechanism in the medial forebrain bundle that we call the "go" mechanism.

    As far as Ben Libet's work, this is old news and doesn't really have a place in contemporary discourse. Libet's claim was that there was a "readiness potential" that pre-ceded a voluntary action bu that the "conscious" mind could "veto" that act if it wanted to. Everything is correct about this model except for the veto act.
  4. Aug 16, 2015 #3
    DiracPool I am just a curious onlooker and certainly have no familiarity with the subject matter, but I gathered a sense from my various readings that the Libet model was still under some debate (though perhaps more in philosophical circles).
    The Schurger paper dates from 2012 so is relatively recent. As I noted, I drew my own inference of the meaning of Schurger's findings and I guess I don't really understand much of the detail. But I did gather that they were arguing for the 'readiness potential' not reflecting specifically a buildup to a neural decision to move, but rather a spontaneous fluctuation in activity that approaches a threshold.

    This extract from a New Scientist article probably summarises it better than I have:

    "Previous studies have shown that when we have to make a decision based on visual input, for example, assemblies of neurons start accumulating visual evidence in favour of the various possible outcomes. A decision is triggered when the evidence favouring one particular outcome becomes strong enough to tip its associated assembly of neurons across a threshold.

    Schurger’s team hypothesised that something similar happens in the brain during the Libet experiment. Volunteers, however, are specifically asked to ignore any external information before they make a spontaneous movement, so the trigger to act must be internal.... Schurger’s team reasoned that movement is triggered when this neural noise accumulates and crosses a threshold."

    A couple of extracts from the Schurger paper to expand on this:

    "A large body of work, spanning more than four decades, has examined the properties of the readiness potential: its temporal profile, topography at different latencies, variability in different task contexts and disease states, potential cortical “generators,” and even its relevance to “free will”. However, we still lack a precise mechanistic account of what the RP reflects, beyond descriptive phrases such as “planning and preparation for movement.” Here we have offered such an account—in terms of ongoing spontaneous fluctuations in neural activity, a neural accumulator, and a threshold—that is both plausible and parsimonious. Our account departs from the prevailing assumptions about the nature of the RP and thus suggests that some very basic questions be revisited from a different perspective."


    "We propose that the neural decision to move corresponds to a commitment to produce a movement now and that this commitment is associated with a threshold crossing of the accumulator underlying the response decision, a lateralization of the premovement potential, and an abrupt increase in excitability in primary motor cortex ∼100 ms before the onset of muscle flexion (or ∼150 ms before the button press, for the hardware that we used). We propose that the precise time of the “neural decision to move now” is partly determined by spontaneous fluctuations that are temporally autocorrelated."


    "Why then should there be such a long and variable gap between the time of a motor decision and the subjective estimate of the time of the motor decision, whereas no such gap exists for sensory decisions? In fact, this question arises only when we assume that the motor decision coincides in time with the onset of the RP. We have argued that this need not be the case and that the neural decision to move may come much closer in time to the movement itself (e.g., −150 ms). We propose that the neural decision to move coincides in time with average subjective estimates of the time of awareness of intention to move and that the brain produces a reasonably accurate estimate of the time of its movement-causing decision events."


    I understood all of this to be suggesting that a voluntary decision to move arises from spontaneous fluctuations accumulating enough to cross a threshold - that is, that the RP is not strictly speaking an RP arising directly from and prior to the intention to move but rather a fluctuating neural pattern that at some point initiates the voluntary action.
  5. Aug 16, 2015 #4
    I'm sure it is. It was a very contentious study back in the day. My first encounter with Libet's work was the review article in Behavioral and Brain Sciences in the late 80's. Here it is if you haven't perused it yet..


    The seminal study was published in the journal Brain a couple years prior to that:


    Libet got a lot of heat because he speculated that there was a psychological "backward referral" involved with the introspective report of the conscious will to initiate a voluntary act. Those studies struck a nerve because it hit at the heart of "free will." As you've demonstrated, it's still being debated. I guess it's still striking a nerve. I reconciled my position on it a long time ago. Basically, the studies are valid, you just have to draw your own personal inference from it.

    There used to be this girl that would show up at the brain-consciousness conferences I used to attend, like the ASSC and the bi-annual Tucson conference..


    Her name was Susan Pockett. I think she was from Australia. Not a very pleasant person, she always seemed mad and grumpy. Kind of had an elitist attitude for some reason. She attended a full day workshop that my neuroscience idol Walter Freeman was giving one year. I tried to befriend her, but to no avail. Too grumpy.

    In any case, she took it it upon herself to battle Libet in the literature 20 years after those seminal studies came out. Here's a bit of the transaction..


    At the end of the day, from what my experience tells me, the initiation of a voluntary act is indeed preceded by neuroactivity, call it a "readiness potential" if you want. "Free will" is an illusion that exists because it is survivally advantageous for us to believe that we have free will. I'm not going to go into detail why I believe this, this is just my opinion on the Libet debate. Whether a "backwards referral" is actually happening or not I don't think makes much difference.
  6. Aug 16, 2015 #5
    Thanks for those links DiracPool, I'm always very curious to learn more about both the function of the brain and human cognition – I wish I'd gone for a career in that sort of field. That said, I actually know very little as I've never had the time or opportunity to really sit down and tackle the topic.

    I find your comments about "free will" interesting, although I suppose my direct interest is not in the philosophical implications, more in the actual process and how that plays out in terms of consciousness. Although it's not entirely clear to me what people mean by 'consciousness', I think I subscribe to the view that consciousness/awareness are purely states of the brain (or more exactly processes in the brain).

    When I read about the Libet model, the central idea that the brain might simply do its thing and that conscious awareness or volition might arise from that process as a sort of post hoc narrative seemed intuitively correct. I found Shurger's results to provide a more illuminating interpretation and of greater explanatory power, which is why I was curious how that had since played out. I guess I can do my own research on that – Schurger seems very active in the field with a lot of publications.

    Nonetheless his proposition still leaves me with a sense of the brain's processes being in large part not directly conscious (again, whatever that means). I think that the general model exposed by both Libet and Schurger's work point in the same direction. I hope to find the time to read some more on this remarkably fascinating topic.
  7. Aug 17, 2015 #6


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    I think most people use that framework. I guess the only question is if there is another framework (what? Penrose?), whether Libet's data favours one over the other. But in a sense, we believe in the framework regardless of data, so for the modern point of view, the Libet experiments are essentially irrelevant.
  8. Aug 17, 2015 #7
    If we, for the sake of discussion, were to take the Penrose model seriously, how does that change things from your perspective?
  9. Aug 17, 2015 #8


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    There are more modern experiments that lend credence to the idea that free will is more of a feeling than a fact. My favorite is the one that predicts which, of two choices, a subject makes spontaneously. The brain activity of the subjects is analyzed as they randomly choose to click a button in their left or right hand and is able to predict (with some confidence) which hand they will pick based on brain activity. They are instructed to click the button as soon as they decide, but the algorithm can predict up to 10 seconds before hand, which hand they will choose.

  10. Aug 17, 2015 #9
    That's an interesting finding Pythagorean, although I don't think I can access the article to read it. I suppose the obvious question is what exactly is the experiment showing. The same applies to Libet and Schurger. It seems that as soon as you've given someone an instruction that they have understood and agree to act upon, you've set in motion a conscious act.

    Schurger's case illustrates a mechanism for how the actual 'now' event arises, that is, what leads to the neural decision for the motor act to occur, but there seems no way to argue that it wasn't precipitated by a conscious decision. The brain activity cannot precede the internal acceptance of the instruction. So the resulting act is a direct consequence of a conscious choice - to accept the direction to act. I can't see how we can say otherwise.

    Of course, part of the problem for me as an interested layperson in following this stuff is the definition of consciousness - what exactly do we mean by that? Is there an accepted scientific definition?
  11. Aug 17, 2015 #10
    Nope. At least, nothing compelling that I've ever heard. There are many (most?) that will argue that consciousness is nothing but the epiphenomenon of self awareness that is purely and utterly the deterministic biochemical activity of neurological action.

    Others will argue that it is something else... something more fundamental. Roger Penrose might be such a person, and is not someone whose opinion I would take lightly.

    I would very much like to hear Atyy's opinion on this, as he is one of our top neuroscientists on the forum, as well as one of the more informed contributors in the quantum physics threads.
  12. Aug 17, 2015 #11


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    The actual interesting definition of consciousness is not self-awareness persay, but the ability to have a subjective experience in the first place (of course, they may be mutually inclusive). How do clumps of matter experience things and unify those experiences? A robot recently passed a classic self-awareness test (the three wiseman test) but is that robot actually experiencing the process? Doubtful. Is it that impressive? Only for engineers working on artificial intelligence.

    If we understood how subjective experiences arises in (presumably) the human brain, that would be the holy grail (or at least a monumental pillar) of neuroscience.
  13. Aug 18, 2015 #12
    Pythagorean, this isn't meant to be an obtuse question, but when you say a 'subjective experience' how would you define that?

    From what little reading I've done on the subject science seems to have a pretty good idea generally of how the brain functions. If the brain in essence takes sensory input and combines it with existing states to generate some kind of response (even if that is in effect no response), then I can imagine that subjective experience is simply that process.

    That is, subjective experience would just be what it 'feels like' to be inside that process, in which case there is really nothing to explain in terms of how it 'arises' - it just is what it is. It seems to me that science can uncover the way these processes work and how the various functional units interact, but if subjective experience is simply the unfolding of these processes then 'consciousness' is irreducible.

    Why do we think there is something more there to be exposed and explained?
    Last edited: Aug 18, 2015
  14. Aug 18, 2015 #13


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    What you describe is computation. We can have robots do that (process data, compute, generate output). Is the robot experiencing anything? Probably not.

    If we knew how subjective experience can even occur in the first place, we could create a robot that actually did experience things (or at least know why we couldn't). The computational part you describe is known as the "easy problem" (it's not actually easy, but it's theoretically tractable, whereas we still have no clue how to robustly measure the experiential aspect of consciousness (i.e., the "hard" problem).
  15. Aug 18, 2015 #14
    You're right Pythagorean... While subtle, there is a very real difference between the subjective experience of "qualia" and the experience of "self awareness", but it would seem that the latter would be an emergent result from the former.

    As to your larger point, I would totally agree. The subjective experience of qualia is the aspect of consciousness that is difficult to clearly define in a reductionist/mechanistic sort of way, and does not seem to be necessary for the purposes of reflexive deterministic neurological action.

    I'm not sure if you meant to say "irreducible" here Graeme. If the mental experience of consciousness is only the epiphenomenon of "what it feels like" during the unfolding of neurological activity, then it would appear that consciousness has been reduced to the specific neurological mechanism described. But it seems (to me anyway) that it's not that clear cut or obvious.

    If you haven't read it yet, I'd recommend a book by A.G. Cairns-Smith... "Evolving the Mind... on the nature of matter and the origin of consciousness"

    Smith is no pseudoscientific light weight. He's a respected authority, and his views are taken very seriously.
    Last edited: Aug 18, 2015
  16. Aug 18, 2015 #15
    I'd also add a few more books to the list...
    "Vital Dust" by Christian de Duve, "The Emergence of Everything" by Harold Morowitz, and maybe even "The Matter Myth" by Davies and Gribbin.




    All are readily understandable (with the requisite degree of effort) by the interested layperson reader, but not short on scientific information. All are secular scientific expositions without quasi-religious/new age silliness (Morowitz does make some references to theological/philosophical perspectives, but those are not material to the larger scientific discussion).

    To me, there is a common running theme. The emergence of consciousness appears to be "built into" the physics of the universe. Yet, that same physics precludes a causative "Ghost in the Machine". Either the machine is "real" or the ghost is "real", but not both. I would argue that which it is is a very reasonable debate.

    To the chagrined monitors, I promise to not take this line of discussion any further as I am aware that it leads to philosophical concepts. I am merely pointing out that there frequently seems to be an essentially default assumption that consciousness is purely a physical process of neurological activity, but it is not clearly demonstrated that this is entirely true.
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  17. Aug 18, 2015 #16


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    Your last post sounds like panpsychism to me, which is an interesting consideration. It's possible that a consciousness-like property is fundamental to all matter, like mass or charge, but it only does interesting things in complex configurations. There are also theories proposing that it may be a property of electromagnetic fields (and thus emerges from the electrical activity of neurons) which I find to be similar in concept.

    I think, to some extent, most of the evidence so far (while it's not a cut clear case) suggests that consciousness is a passive consequence of deterministic neurological action (thus why we haven't been able to "find" free will; free will is already kind of a contradictory idea, that seems to imply that humans should be able to evade cause and effect of the physical universe). Other evidence is genetic and environmental factors on behavior

    Of course, QM theories do seek to relieve neuroscience of determinism, but QC isn't really my bag.
  18. Aug 18, 2015 #17
    Sadly, I'm really unable to pursue that discussion while upholding my promise to the moderators to maintain the prohibition against philosophical rhetoric.

    Suffice it to say that (as I'm sure you're well aware) "cause and effect" at the quantum level becomes a very dicey thing... as does the ontological "nature" of physical existence.

    That said, I want to make it very clear that I am definitely not suggesting that mental/conscious experience should not be described with reference neurological activity. My only contention is that it is questionable as to what that neurological activity represents at the fundamental "physical" level, and what the causative relationships are in the process.

    But, again, I want to respect the PF policy regarding philosophical debate, so I really don't want to add anything further to that position.
    Last edited: Aug 18, 2015
  19. Aug 18, 2015 #18
    I shan't dig too deeply into this bearing in mind the prohibition against philosophical matters although I must admit to being unclear where philosophy intrudes in this particular topic.

    While I don't have anything like the background you both have, my personal view is that consciousness is no magic thing. Pythagorean you suggest that neurological activity is just computation and then reject consciousness as simply being that, but I suggest there is not the slightest evidence there IS more than that. Why is it not reasonable to adopt the simplest possible explanation that we cannot work out what consciousness is simply because there isn't anything to find?

    Is a robot conscious? Is a computer conscious? Perhaps it is. Perhaps this is the 'built in' nature of consciousness you argue for Feeble Wonk.

    The simplest early life operated on very simple cause and effect principles that are completely accessible to explanation. Why should evolution have supplied something extra to the mix beyond a much more complex arrangement? Why should some magic 'something' appear at some point? To me this is invoking some metaphysical quantity for which I personally see no evidence.

    Anyways, clearly that is to go down the philosophical path and no doubt I am just displaying my ignorance of the deeper issues. In terms of my original question, the various links and comments have been most interesting but I return to one earlier point I made. Why in the case of the Libet experiments is the time of commencement of the conscious aspect of the act not taken to be the point of receiving the instruction? Why was the RP preceding the act itself taken as representing some backward referral when it could as easily be taken to represent a part of the causal chain originating with the original instruction?
  20. Aug 18, 2015 #19
    "Philosophy" intrudes at the point where one begins to discuss the ontological nature of the quantum wave function.

    This is particularly true when one questions the role of consciousness in the act of "observation" as it relates to quantum state reduction.

    You might very well be correct in your assessment Graeme. If we accept a Bohmian "pilot wave"/hidden variable type of QT interpretation, it might be just as cut and dry as you suggest. Or, if we accept a "many worlds" type of interpretation, then all possible quantum states of the macroscopic organic activity are realized in one of worlds, so no explanation is necessary. But anything short of that requires some serious explanation as to how an observed outcome of a reduced quantum state is arrived at. And maybe, just maybe, consciousness is itself a hidden variable.

    But, again, this treads perilously close to metaphysical and/or philosophical conjecture, so...
    Last edited: Aug 18, 2015
  21. Aug 18, 2015 #20


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    You misunderstood me. That may be the case, the problem is we don't have evidence that computation us consciousness and equating them blindly as if we do is putting the cart before the horse.

    We simply don't know yet.
  22. Aug 18, 2015 #21
    Heh, I have no idea what you just said there Feeble Wonk! I'm afraid my understanding of that sort of thing hovers between "not much" and "none", depending on the time of day! I am not dismissing what you say, it just goes over my head is all.

    I understand, but I come back to the earlier question. What evidence do we have that it isn't? Or even better, what evidence are we looking for to show us that consciousness is something other?

    As I've noted, I am pretty ignorant of the deeper issues and the body of knowledge around this topic - I'm just a curious sort of guy but no more than that. But I've been digging around for a while now trying to find some idea of exactly what people are actually meaning when they talk of consciousness and while I've come across a lot of ideas, there doesn't seem to be any agreed definition.

    In terms of Libet, I was curious as to whether the data shows that the brain simply does what it does and the subjective sense of the act is a later addition arising from the underlying neurological processes (as has been pushed in a few articles I've read about it). Schurger's explanation makes better sense for me than what I have understood of Libet's model, but being pretty uninformed of the science I wonder at why the experiment doesn't consider the entire sequence from time of instruction.

    In fact, I'd be interested in what the subject's could report of thinking. If it were me, there'd be a sense of "OK, I'm gonna do it soon. Maybe now… ummm, no, maybe I'll wait till that dial reaches the 5th mark. Yeah… no, yeah, now!" I might not actually put that into words as such, but that's pretty much the 'feeling' I'd have.

    While that doesn't mean that my subjective sense of things is preceding and directing the action, it seems a better fit to the idea that an RP is seen up to some time before the actual decision to act.

    We don't simply sit around with nothing going on in our heads and then like a bolt from the blue do something (at least, not for voluntary acts in response to a direction).

    I suppose I'm seeking some contextual information about why Libet would have assumed that the decision to act as noted subjectively is a kind of an unrelated one-off event that isn't part of some larger process begun when the instruction was first given. I'd be more impressed if the RP arose BEFORE the instruction! :)

    So while I follow the general outline of the issue, I am not sure I understand its significance as I have understood it from what I've read. I have no axe to grind (though personally I'd be quite happy with subjectivity arising after the underlying process begins).
  23. Aug 18, 2015 #22


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    I'll leave this one to Hermione Granger:

    “But that’s – I’m sorry but that’s completely ridiculous! How can I possibly prove it doesn’t exist? Do you expect me to get hold of – of all the pebbles in the world and test them? I mean you could claim that anything’s real if the only basis for believing in it is that nobody proved it
    doesn’t exist!”

    The context is a little different, but the fact remains. You can't prove a negative, but that doesn't affirm the positive. You still have to find the evidence for the positive. Certainly we have evidence that some neural processes are associated with consciousness. But some are not (most actually) and some processes are present during both consciousness and unconsciousness. Add to that that we have no way to assay causation and we don't really have a lot to go on.

    That's why it's the "hard" problem!
  24. Aug 19, 2015 #23
    Thank you DiracPool for the earlier links. I have just read the paper by Benjamin Libet in the Beavioural and Brain Sciences journal link (although not as yet the commentary and the author's response) and I see that Libet did endeavour to screen for any prior planning, although it isn't clear to me whether accepting an instruction and therefore having an intention to act at some later time causes neurological activity not present otherwise. Though I suppose we are always in the stages of *some* level of intention.

    One thing I didn't understand is how subjects reported an urge to act time ('W') and an act itself time ('M'). If I do this myself I am not aware of any 'urge to act' - that is, I cannot distinguish between W and M. It wasn't apparent from the paper how subjects assessed the urge to act - is there some kind of internal signal one can report? I notice this with most voluntary acts in life. That is, I am not conscious of the urge to act - it just happens. What exactly were Libet's subjects reporting for W?
  25. Aug 20, 2015 #24
    I skimmed over this part of your post previously, and forgot to ask you about it.

    Are you saying that there is no inhibitory feed back mechanism to abort a "go" signal?
  26. Aug 20, 2015 #25


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    I'd love to hear DiracPool and Pythagorean's (or anyone else's) comments on

    Canceling actions involves a race between basal ganglia pathways.
    Schmidt R, Leventhal DK, Mallet N, Chen F, Berke JD.
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