Tag Archives: DMN

The will to persist

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salienceI was interested to see reports here and there the other day that scientists had discovered the seat of the will in the anterior midcingulate cortex.

That’s not precisely the case, of course; there’s an article here which describes the research. The scientists in question had an unusual opportunity to use electrodes in the brains of two patients; although they did indeed operate in the anterior midcingulate cortex they believe they were stimulating the brain’s salience network, which is quite widely distributed. The effect was apparently to create feelings of needing to persist against challenging circumstances; the researchers themselves call it “the will to persevere”. The patients were fully conscious and able to describe their feelings, but alas no tests were carried out to see whether they were in fact more persistent when stimulated.

The correct interpretation of the results seems difficult to me. As I understand it, the theory of the salience network holds that brain activity is controlled by neural networks which stretch across several regions of the brain. The default mode network, or DMN, is the one that operates when we’re not focused on anything in particular, perhaps daydreaming. It has been suggested that loss of this function is what distinguishes people who have “locked-in” syndrome from those who are in a “persistent vegetative state” – if you lose your DMN you’re not really there any more, in other words.

When we concentrate on a task, another network takes over – the central executive network, or CEN. The role of the salience network, if I’ve got this right, is primarily to act as arbitrator between the two. It spots something that deserves attention – something salient, indeed – and switches control from DMN to CEN. That’s fine, but it doesn’t seem to have much to do with persistence; it’s actually about changing the object of attention, not sticking with it. But perhaps strong or continuing stimulation of the salience network has that kind of effect. The salience network says “you need to look at this”, so perhaps when it operates emphatically we think “yes, I’m going to look the hell out of that alright; I’m going to look at that intensively; when I’ve finished looking at that, by golly it’s going to stay looked at”.

More plausibly it might all be to do with physiological effects; besides directing attention the salience network has a role in gearing up our “fight or flight” state, and it might just be that in that state we feel ready for a challenge ( in which case a readiness to persist comes into it, but surely isn’t the whole point); that would be a William James style emotion, originally a matter of the gut more than the mind.

Anyway, this really has nothing to do with an organ of the will. That is an interesting notion, though, isn’t it? My own assumption is that the will emerges from the operation of general cognition and that there couldn’t be a separate will module. If such a module determined the actions to be willed, it would surely have to encompass almost the whole of cognition, and so be far more than just a module; if it merely willed the actions selected for it by the rest of the brain it wouldn’t amount to much at all.

People do, of course, often hypothesise that there might be a special function for assigning value, or flagging up those things we ought to pursue as desirable. To me, though, that seems a bit different; the will, properly understood, is not a matter of basic motivation, but a faculty which might over-ride that motivation, either by operating at a meta level or simply by acting as a restraining and countervailing force.

Would that even be a distinct faculty of its own? Some would probably question whether talking about the will is a useful approach at all, rather than a relic from outmoded ideas about the soul controlling the body through acts of will. I must admit I find it hard to think of any subject that can’t be adequately discussed without mentioning  the will. Even free will doesn’t really lose anything if we talk about free action.  So is the will even worth persisting with? I can feel my DMN kicking in…

Zoned-out rat consciousness

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Zoned-out ratThe New Scientist suggests that zoned-out rats may give us a clue to consciousness.

It’s all to do with the Default Mode Network, or DMN. You might think that when we stop concentrating on a particular task and sit back for a few quiet minutes the level of activity in our brains would fall, but it turns out this isn’t really so: instead, more or less the same level of activity appears to continue, but it switches to a different set of areas – in particular, a linked set of areas in the cortex and elsewhere. This is the DMN, but what is it doing?

A completely honest answer, I think, would be that we don’t exactly know except that it’s something other than concentrating on a task.  In human subjects the DMN seems to be associated with daydreaming, but also with other detached modes of thought.  Why would this help explain consciousness? It seems that in patients with locked-in syndrome, where consciousness is fully retained but the patient is unable to move, the DMN is functioning normally, whereas in persistent vegetative syndrome, where consciousness is absent, it is disrupted.

I can think of a further reason to think that this might shed light on consciousness. It’s not much of a stretch to see DMN activity as being the kind of thinking that isn’t directly related to inputs and outputs. When we’re working on a task those are crucial, but one plausible account of the role of consciousness is exactly that it lets us escape from giving instant responses to our surroundings and lets us develop longer-term plans, deeper understanding, and more complex behaviour. If the DMN represents useful mental activity detached from inputs and outputs it is exactly the thing whose existence the behaviourists denied, which is pretty much the same as one conception of consciousness.

The New Scientist and others speak of the DMN as associated with introspection, but I can’t see the evidence for that. To be daydreaming or thinking in general terms about stuff that is or might be going on is not introspection. I think there’s some confusion going on here between thinking internally and thinking about what’s going on internally: and perhaps a further suggestion that introspection= self-awareness = consciousness: those are tenable but debatable equations which don’t seem to be vindicated or disproved by the mere existence of the DMN. So perhaps the excitement is premature.

The rats are not that reassuring either. The New Scientist reports that analogues of the human DMN have been found in monkeys, and now even in rats. That’s interesting, but unless we rate the consciousness of rats unusually highly it seems to show that the DMN  cannot explain any uniquely human level of consciousness. Fair enough: I don’t disdain rat consciousness altogether: but it’s worse than that because, as I understand it, the evidence currently suggests that younger human children don’t have an identifiable DMN. It would be somewhat weird to attribute to rats a level of reflective consciousness which is absent in human infants – wouldn’t it?  If more were needed to put us off, it is not quite 100% agreed that the DMN is in fact a functional entity in itself; it could yet turn out to be more like the mere absence of the TPN, the Task Positive Network which is its opposite (or complement) – the similar set of areas which appear to work together when we’re engaged in a specific task. Perhaps the level of neuronal activity in the brain stays high, not because the DMN is really processing anything, but because the brain just uses a lot of energy to tick over?

Still, if the DMN doesn’t explain what consciousness is, it’s hard to resist the view that it’s telling us something about how it works. Problems with the DMN have been put forward as possible causes of Alzheimer’s, autism, and schizophrenia (I think everything has been put forward as a possible cause of schizophrenia). The range of problems is perhaps an indication of the vagueness of the theories. There is some good evidence of a correlation between Alzheimer’s and disrupted DMN: but then the DMN includes quite a siginficant sampling of some important areas of the brain, so that may not mean all that much. It could be that when consciousness is disrupted the DMN tends naturally to get disrupted too, without that implying that the DMN actually runs or constitutes even the less-focused forms of consciousness.

At the end of the day what we’re left with is that our brains – and even rat brains – don’t use the same circuits for task-related and non-task related activity, but go through a fairly large-scale switch of resources.  Even if we’re idly daydreaming about driving into town already, it seems we bring in a different set of neurons to do it with. There has to be some good reason for this, but what…?