Archive for December, 2007

Picture: brains. The New Scientist had a rather rambling piece on recent ideas about the subconscious the other day. One thing I thought was interesting was the four-part model of the mind it described, attributed to Dayan, Daw and Niv. I haven’t been able to find a paper which actually sets out the four-part system: the one referred to by the New Scientist is really about how control is passed between two of the four systems – but the gist is fairly clear. One of the appealing things about this line of thought is that each system has a fairly robust basis in neurology, with evidence of functions being localised in particular brain areas; but also a rationale in terms of what the system does for us and why this particular combination of systems might work well.

To begin with, we have the ‘Pavlovian’ system. Pavlov, of course, is famous for his work on conditioned reflexes: training dogs to salivate at the sond of a bell, and so on. Here the term is used rather loosely to cover instincts and a range of ‘automatic’ behaviour. It may in fact seem doubtful whether all such behaviour stems from a single system, and indeed in this case a single clear neurological location doesn’t exist, though various places have a role. The chief advantage of behaviour this hard-wired is clearly speed, and the main limitation is the stereotyped nature of the behaviour: well-suited to cases where the required action is clear and urgent, and not so good elsewhere.

Then we have the habitual controller, which I think we could call the ‘autopilot’: responsible for more complex learned behaviour. This system can take control of behaviour for an extended period, and respond appropriately to inputs so long as they fall within the expected range; so that when driving, for example, we may take account of the progress of the car in front without having to think about it consciously. This second system can cope with much more complex problems than the Pavlovian one, but it is still largely stereotyped and when something unexpected comes up it hands over abruptly to another system.

That might be the episodic controller, which produces behaviour which makes decisions in a conscious but unreflective manner, drawing on memory of what happened before. It works well in circumstances where events are as it were, following a script and the broad outlines of what is going on are known. Although it takes a little more time and thought than either of the preceding systems, it is still fast and it has the advantage of working relatively well when detailed knowledge is not available: so long as the circumstances are broadly familiar, we can go on producing generic behaviour which is likely to be appropriate.

The fourth system is the goal-directed controller; here for the first time the brain attempts to look forward and decide which behaviour is going to achieve its goals. It is slower and consumes more resources than the other systems, and it can only really work where enough information is available, but these disadvantages are outweighed by its sophistication and flexibility. Dayan et al describe it as searching through a tree of possibilities, which I think may under-rate or misdescribe it – the brain seems to be able to devise goal-oriented strategies by some other means than working through alternatives – but that doesn’t invalidate the overall model.

So if the model has a good basis in both neurology and function, how does it fare against introspection – does the mind really feel like a four-part operation from the inside? I think in many respects the model is recognisable, though I’d quibble over some details. The main weakness, as I see it, is that the perspective which the whole thing is based on is very much one in which the function of the mind is to produce good output behaviour from a range of different environmental inputs. That’s fine as far as it goes, but consciousness is generally taken to be about awareness and experience as much as decision-making. When I’m sitting still and admiring the view across the bay, which system is working? None, it would appear, but in old-fashioned two-part terminology, I should have said that both conscious and subconscious were hard at work. Perhaps all the systems are working in harmony to produce the appropriate output behaviour of not doing anything?