Modern interpretation of Libet data

[DiracPool]

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?

It's been a while since I've encountered any debate on the Libet experiment, but after a quick review, the general idea is that, in the experiment, activity was recorded in scalp EEGs over the secondary motor cortex of the subjects 500 ms before the execution of a movement related to a particular cue (the position of a rotating red dot). Libet found that the subjects reported the "conscious intention to act" 300 ms after the 500 ms epoch (the readiness potential). 200 ms after that, the movement was executed. What Libet also found, though, was that the movement could be "vetoed" in the 100-150 ms between the time of the "subjective experience of the conscious will to act" (>300 ms) and the the actual act (>500 ms).

As I said in my previous post #4, Libet got into some heat on this issue because of his suggestion that there may be some neurally ambiguous "conscious will" involved in vetoing the act, as well in the backward or retro-causation implied in the antedating of the intention to move back to the beginning of it's neural causation.

https://en.wikipedia.org/wiki/Benjamin_Libet
"While consciousness plays no part in the instigation of volitional acts, Libet suggested that it may still have a part to play in suppressing or withholding certain acts instigated by the unconscious."

From here it devolved into a debate between dualists philosophers, scientists, and others. At this point, I lost interest in the subject. I thought it was all but done. That is where this quote came from:

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.

The veto act had dualist connotations I didn't really feel comfortable with.

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

http://www.ncbi.nlm.nih.gov/pubmed/?term=PMC3733500
Canceling actions involves a race between basal ganglia pathways.
Schmidt R, Leventhal DK, Mallet N, Chen F, Berke JD.

As far as the Schmidt et al. study, I have no reason to doubt their findings, the problem is that it is questionable what relevance they have to the Libet controversy. The Schmidt study dealt exclusively with subcortical processes related mostly to basal ganglia structures and as far as I could tell did not even take any readings from the neocortex at all:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3733500/
"...rats in Experiment 1 received implants containing 21 individually drivable tetrodes targeting basal ganglia structures (STR, GP, STN, and SNr)"

In contrast, the Libet experiment focused solely on the neocortex. Not only that, it was recorded with scalp EEG electrodes. So it's very difficult to extrapolate the Schmidt data to the Libet data. The Libet data was designed to test the unconscious "cerebral" initiative in a voluntary act, which has all sorts of connotations to cognition, the neocortex, and especially executive function in the prefrontal cortex. Rats are too removed from us phylogenetically to use in a comparison here. For a long time it was debated whether or not rats even had a prefrontal cortex at all. I think the consensus now is that they have a very tiny one. But in any case, the problem is that, although older phylogenetic regions of the brain are mostly conserved, the growing outer mantle of the cortex likely reorganized subcortical circuits to some degree in order to accommodate newer cognitive capacities found in primates. That's why we can't be sure the Schmidt circuits are reliable homologues to circuits in humans, and why I'd be suspect to read much into them.

So, is it reasonable to consider whether the Libet "delay" in expression of intended action (to whatever degree that actually is) is related to this "go/stop" neurological competition?

See above.

As far as the OP's study:http://www.pnas.org/content/109/42/E2904.full

I don't see how this challenges or changes the Libet debate much. I only browsed it, but it seems to me as if the authors are only stating that we can't be sure that the readiness potential (RP) is the only factor "pre-cluing" in the decision to initiate a movement, there may be some stochastic processes or spontaneous neural fluctuations that compound with the RP that mask the pure role of the RP in the initiation of that movement:

http://www.pnas.org/content/109/42/E2904.full
"One can extend this idea to choice behaviors such as choosing between reaching for a coffee cup or reaching for a muffin. If you are somewhat hungry and also somewhat thirsty, then spontaneous fluctuations might play a role in tipping the scales in favor of one or the other, as is the case with cued perceptual decisions."

and

"We assume that spontaneous fluctuations are ongoing even when the subject is not preparing to move and that these can often approach the threshold without crossing it. Therefore, false alarms will be a limiting factor in the overall sensitivity of an interface that uses slow fluctuations to infer movement intention."

 

[Graeme M]

I found this review ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4064703/ ) to be of some interest in terms of my original question. It is relatively recent (2014) and is a general review of research aimed at more objective analyses of the Libet type experiments. That is, they attempt to identify more objective methods for measurement than relying on subjective observation.

By identifying neural correlates to the subjective perception of the urge to act and act itself (Libet's W and M moments), they can quantify when the W and M moments arise in a more objective quantitative sense.

Although the paper is more concerned with the sense of agency rather than whether spontaneous events arise according to a conscious volition, it does for me at least offer greater insight (although I found the paper hard going given my lack of background in the current science).

I found the notion of binding particularly interesting (when an action results in a sensory feedback, the perception of these events temporally shift closer together relative to either separately - the action is perceived later while the feedback is perceived earlier).

The discussion around both "comparator model within optimal motor control theory", and "active inference theory" was a bit out of my league but nonetheless I gathered that prediction models are compared internally to perceptual responses (sensory feedback) to generate (or contribute to) the sense of agency. That is, the closer the predictive model to the resulting perceptual model the greater the sense of agency (I think though that this really just says the more something looks like I did it, the more I think I did it). The active inference theory outlines how this unfolds in terms of the hierarchy of cortical processes.

Again, while mostly this discussion was above my level of understanding, I felt that it broadly described how the conscious sense of directing an act is largely retrospective. That sort of accords with my intuitive sense about how conscious direction might operate.

The 'binding' discussion to me is especially relevant by illustrating how subjective perception is adjusted internally and my take - and here I'd welcome any comment by those who are far more familiar with the subject matter - is that the W and M perceptions arise from internal temporal adjustments to correlate motor actions with external feedbacks (that is, it's better to have an organism sense its behaviours as aligned with environmental interactions in a cause/effect relationship than otherwise, which would seem to my naïve view a clear evolutionary likelihood).

For example, while the Libet data indicates the W moment precedes the actual moment of action by around 200ms and the M moment around 85ms, research shows that the later the W moment (ie closer to M), the less the sense of agency.

By the way, going back to an earlier comment, I still have no idea how anyone can identify a W moment in the first place...

 

[Graeme M]

And just to show I am happy to comment on things I know nothing about, here's an interesting study regarding unconsciousness that I think has some relevance in terms of earlier comments regarding defining consciousness.

http://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1003271

This study notes (and I suppose it's stating the bleeding obvious) that Propofol induced unconsciousness is characterised by a loss of wide scale integration of information processing (increased clustering of connectivity) and reduced efficiency in information distribution. They also note that changes to both long and short connections were generally similar and that long range connections were not more affected, contrary to other reports.

Given I am not familiar with current scientific thinking around consciousness and the general position of neuroscience I am probably out of order with this observation, but is this not somewhat suggestive of the reverse idea that consciousness arises from wide scale integration of neural information processing? For example, the sense of agency discussed in the paper in my earlier comment above must depend upon the integration of a wide range of internal information from a variety of brain regions. If I consider that for the conscious mind to generate a sense of agency, actions, their outcomes AND the modelling of those relationships must occur quickly enough to provide some gross temporal synchronicity then it makes sense to me that it is the effectiveness and the speed of the integrated network that makes consciousness possible.

 

[Graeme M]

Apropos my earlier comment regarding consciousness simply being what it is to be a brain processing information, I found this today. I offer it for anyone who is interested and to ask if anyone who has read the book "Consciousness and the Social Brain" or knows more about this fellow's ideas would care to comment.

http://aeon.co/magazine/philosophy/how-consciousness-works/

Michael Graziano is a professor of neuroscience at Princeton, and unless I misunderstand his ideas from this relatively simple overview, this is exactly what I was getting at.