From Mervyn van Kuyen (970928 12:30 CET)
[Bill Powers (970926.0419 MDT)]
If you have just one error signal output, indicating "mismatch" without any
indication of the sign, you have at best a hill-climbing system, not a
control system. A control system needs an indication of the sign as well as
the amount of an error, so its action can always be aimed toward reducing
the error. We have discussed this on CSGnet, years ago.
You forget to leave some elements of PCT at home that are not part of my
neural servo model. It's very easy to 'prove' that *bicycles* will never
work by removing two wheels from your *car*. The point is, if you want to
explore a fundamentally different design, you have forget about the 'car'.
In PCT the reference is hidden from the network that corrects the
perceived errors. But in my model the network produces the
actions *and* the references. Therefore it has perfect implicit
knowledge about the 'sign' of the disturbance. And you can be sure
it *will* exploit this knowledge because the sign is, as you mention,
a very important property. It will simply react differently to error
signals, depending on the nature of the references it is creating.
Maybe you should have *tested* it, years ago 
Regards,
Mervyn