mailto: j.mccrone@btinternet.com
web site: http://www.btinternet.com/~neuronaut/
There has been some discussion in your group on an article I wrote for the
New Scientist magazine on a dynamical view of the neural coding issue
(check my site for a copy of the article). Also some of you have emailed to
ask if I am aware of PCT theory. The short answer is no - but I have done a
quick skim job as a result.
You seem quick to dismiss the dynamics story - and you are right in that
most of those who call themselves dynamic don't really get it. Only a very
few combine both a grasp of dynamics and the necessary in-depth knowledge
of how the brain is organised and so can talk about the very particular
dynamics of the brain (its hierarchical organisation, its subcortical
"bottlenecking" organs, its multi-dimensional plasticity, etc). But there
is something brewing there.
Personally, I am sympathetic to some of the PCT themes. The book I have
coming out in October is really all about how consciousness is not a state
of contemplation but a reaction. Brains are designed to formulate
intentions and actions. So in some sense, the processing is controlled by a
goal of "best action". But it strikes me that PCT is a very
one-dimensional, mechanical, way of describing what takes place. Sure you
will tell me that it is a proper theoretical construct whereas I just talk
in easy analogies. But to me, that desire for something rigid is just part
of the whole reductionist science problem. Dynamism is about learning to
live with a way of doing science that is more organic, more flexible -
looser so that it can ultimately wrap tighter around its subject.
Anyway, a couple of things that may not be getting their proper emphasis
within PCT. Where do you stand on anticipation? The brain takes time to
evolve a reaction to whatever is just happening - the psychophysics says
this is at least a tenth of a second and probably half a second (to go
through all the stages). This creates a gap. The brain gets over the gap by
making predictions (or forming plans, expectations, reafference messages,
priming, mental images, inklings and intuitions, or whatever you want to
call it).
The second big thing is that the brain is heavy with habits. The first time
the brain has to deal with a situation, it does so in a "conscious" (that
is globally exploratory, highly unstable, third of a second to stabilise)
sort of way. Then very quickly, with experience, it "shrinks down" to a
more habitual solution (the population of neurons involved is whittled down
to a skeleton crew that can do the job swiftly and reliably, the habit
pattern also sinks down to "inhabit" lower levels of the hierarchy such as
the basal ganglia/insula cortex). And these habits are sensory as well as
motor (no real hard distinction anyway if you are a true dynamicist). So a
new-born kid is born pretty bare of circuitry and then learns how to
respond to the world. It experiments with the perceptual categorising and
thought and motor responses needed to make itself more adapted to the world
it finds itself in. Within a few years, this fat layer of habitual reaction
can deal with most of what life throws at it, leaving "consciousness" (the
most globally coherent shift in brain state during a particular moment) to
tackle the very small residue of novelty or significance left over.
This sets up a general cycle of processing. The brain generates a state of
anticipation (what it wants or expects to happen during the coming moment),
the moment then starts to happen and as much of possible gets dealt with at
a preconscious, habitual, level, then the bit left over is escalated
(prefrontal lobes, cingulate cortex, basal ganglia loops) so that it
receives special attention. It becomes the nub of the moment and creates a
wash of activation back down the brain hierarchy that stimulates all the
associations, thoughts and motor planning that we loosely call anticipation
- the wheel has turned a full circle.
At first glance, PCT does not seem to accommodate anticipation (which is
all about open destinations rather than controlled end states), nor the
habit vs focal consciousness trade-off in the processing loop. Thirdly, you
seem to be pushing a systems level view of brain function without actually
talking about any brain structures themselves - is there any neuroscience
in all of this?
Judging by the rather evangelical tone of some of the postings I have read,
I'm certain I will shortly be getting a rather blunt reply.
Cheers all!
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from: John McCrone -- science writer