# coordinate structures and fig-leaves

**URL:** <http://discourse.iapct.org/t/coordinate-structures-and-fig-leaves/13476>\
**Category:** CSG1993\
**Created:** [October 3, 1993, 4:27pm UTC](http://discourse.iapct.org/t/coordinate-structures-and-fig-leaves/13476 "1993-10-03T16:27:57Z")\
**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:** [October 3, 1993, 4:27pm UTC](http://discourse.iapct.org/t/coordinate-structures-and-fig-leaves/13476/1 "1993-10-03T16:27:57Z")

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[Avery Andrews 931003.1030]

First, a bit on the theme that 2nd order feedback systems are implementations  
of coordinative structures. Consider the task of holding a coffee cup  
vertically with a three jointed arm (shoulder, elbow, wrist), all movement  
in the vertical plane. If you measure the angles appropriately, and  
assume the right kind of grip on the cup, keeping the cup vertical will  
follow from maintaining the following relationship between the joint  
angles:

&nbsp;&nbsp;&nbsp;0 = shoulder+elbow+wrist

But this is just the format of a coordinative structure. Furthermore,  
if we want to tilt the cup at an angle T, we can do so by `tuning' the  
structure by using some value of T instead of 0:

&nbsp;&nbsp;T = shoulder+elbow+wrist.

Similarly we can specify a shoulder-cup elevation angle by the relationship:

&nbsp;&nbsp;E = shoulder+1/2\*elbow

and a shoulder to cup distance:

&nbsp;&nbsp;D = (pi-elbow)

&nbsp;&nbsp;(nonlinear distance scale, in the range [pi, pi-elbow\_flexion\_limit])

What a coordinative structure is supposed to be is some kind of dynamic  
system that tends to enforce these relationships, but the neo-Gibsonians  
seem to be rather vague on the details of what kind of system might  
actually do the job. (Though I note in passing the Fowler and Turvey  
1980 (Butterworth vol.) state explicitly that any kind of `index  
over muscle function', including perceived joint angles, might be  
linked by a c.s..

2nd order feedback systems, as illustrated by Rick in the spreadsheet  
model, will clearly be able to implement these kinds of relationships,  
but I can think of at least two considerations that might serve as  
fig-leaves for F&T.

First, the original description of 2nd order feedback systems in BCP  
doesn't correspond to the working systems presented later by Rick,  
because it doesn't have the integrators that Rick has in the  
first-order reference functions. If I'm reading the spreadsheet  
correctly, a first order reference signal should be thought of  
as produced by a circuit element that tends to output a constant  
signal, with higher order output signals tending to raise or  
depress the signal (I'd think that a capacitor-type element would  
be more realistic here, in fact). These thing aren't in the BCP model,  
as far as I can see, so it's not clear to me that the original model  
would actually work. This makes it less reprehensible that F&T  
didn't like or understand it.

A second consideration is this: contemporary 2nd order control systems  
do implement coordinative structures, but can we be sure that there  
aren't other kinds of systems that would as well? If we can't be,  
then the notion of coordinate structure shouldn't be immediately  
discarded, because it is a concise description of a problem for which  
we have a proposed solution (or rather, family of solutions, since  
there are various different 2nd order control systems that could be used  
to implement a single coordinative structure), but perhaps not the  
only solution.

Avery.Andrews@anu.edu.au