# slowness, central patterns, deafferentation

**URL:** http://discourse.iapct.org/t/slowness-central-patterns-deafferentation/14996
**Category:** CSG1993
**Created:** [January 23, 1993, 9:10pm UTC](http://discourse.iapct.org/t/slowness-central-patterns-deafferentation/14996 "1993-01-23T21:10:14Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![\_Avery\_Andrews](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/_/848f3c/32.png) [@\_Avery\_Andrews](http://discourse.iapct.org/u/_Avery_Andrews)
#### Post date: [January 23, 1993, 9:10pm UTC](http://discourse.iapct.org/t/slowness-central-patterns-deafferentation/14996/1 "1993-01-23T21:10:14Z")

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[Avery.Andrews 920123.1601]

Some thoughts on various issues:

First, the `feedback too slow' issue. While many think that  
feedback is too slow to be involved in many activities, just about  
everybody agrees that many fast reflexes are modulated by sensory  
information of various kinds. But feedback is just modulation  
of the intensity of the response w.r.t. the extent to which the  
needed result already exists. So these two popular positions are  
contractory, as far as the \*initiation\* of responses is concerned.  
And, of course, feedback inhibition can be done with a small  
number of synapses, so the time it adds would typically be minimal.

For termination of responses, things might be different. What is required  
is that the force delivered by the actuators be approximately zero  
when the required result is achieved. It seems plausible to me that  
simple linear feedback schemes might indeed be too slow for certain  
kinds of behaviors, but then I'm not an engineer, & am perfectly happy  
to be shown wrong on this by people who are. Supposing it is, one  
can certainly envision that nonlinear feedback schemes might overcome  
the problem. E.g. the commands sent to the actuators specify zero  
force before the desired result is achieved. Much of the effort in  
learning highly skilled activities might plausibly go into tuning  
the nonlinear widgets so that they will work right.

At any rate, initiation & termination are different issues, & maybe  
nonlinearity is more important than PCT writings tend to suggest.

And, thinking about this issues makes me vividly aware of how little  
the typical cognitivist (e.g., myself) knows about actuators, and  
what it takes to drive fingers, fins, legs, lips, etc. I wonder  
how many psychologists are better off than I am in this respect.

Second: Central Pattern Generators and Planning.

There appears to be widespread confusion between CPGs and Planning, at  
least at the level of terminology. A plan is something that you write  
out in advance, and then follow. E.g. generate a constituent structure,  
and then speak it (how Penni says that speech production doesn't work).  
But all a CPG has to do is spit out a waveform. In the general case,  
I guess, a CPG is just a nonlinear transducer, perhaps emitting a  
sinewave of variable amplituded & frequency, as determined by two  
input wires. So the slowing circuits in Arm are simple trajectory  
generators, converting a step change in their inputs into a ramp  
in their outputs. Evidence for CPGs is thus not evidence against  
PCT.

This feeds into the deafferentation issue. In a post of Bill Powers'  
some time ago (csg-l (921025.0800)), he pointed out that if the feedback  
return is cut, the reference signals will convert directly to error  
signals, creating exaggerated movements (since the feedback return is  
inhibitory), which is apparently what you get right after  
deafferentation (it would be  
good to have the references for this). But the nonlinear widgets  
will still basically be doing the right sort of thing, even if they  
have to be extensively retuned to deliver effective behavior. So the  
centrally generated patterns will basically be within the `competence'  
of the organism, even if it takes a bit of time for it to be able to  
implement them adequately in `performance'.

So it may well be that PCT actually does a \*better\* job of explaining  
the consequences of deafferentation, tho I think there's a fair amount  
of substantiatory scholarship to be done on this issue.

Avery.Andrews@anu.edu.auti