[From Bjorn Simonsen (2004.06.14,11:36
EuST)]
[From Bill Powers (2004.06.11.1018
MDT)]
Positive feedback can occur at higher frequencies while the feedback at low
frequencies (or steady-state) is still negative. The result is not runaway,
but
oscillation (tremor, vacillation). You can see an example of this by
pushing
your hands together as hard as you can. After a while, the muscles fatigue
and the hands begin to shake. This is caused by the fact that muscles are
exponential springs, with a spring constant that increases with tension.
The
tension that is pushing the hands together raises the spring constant, and
that raises the loop gain at high frequencies more than low. Fatigue in the
muscles increases their lag, and the combination of increased lag and
increased loop gain produces positive feedback at a frequency of around
3 to 5 cycles per second. The average push remains the same, but the
amount of push varies up and down very rapidly.
I read your first sentence as “Positive feedback can occur at higher frequencies ….
and I know you have rendered a
possible positive feedback loop on figure 3.10 in B:CP. Later in the book I
have not seen positive feedback
loops mentioned. Have I overlooked anything?
I have
seen positive feedback loops discussed mostly in psychological literature, but
here positive feedback is juxtaposed with affects/emotions. They give
expression to positive feedback loops as if they recruit more neural
components to an
emerging
constellation and they say that negative feedback couples them in a stabilizing
regime. (Just as fig. 3.10 ??). Do they/you know if any investigation really
has pointed out positive feedback loops in the nervous system?
I have tried your example and also pushed my hands
together with clenched fists. With clenched fists, my hands don’t begin to
shake. Maybe I don’t push them together strong enough? Or is it still negative
feedback loops functioning, but with to little gain when the push is strong
enough? If this is correct I also prefer to substantiate the shaking from an
ineffective gain instead of using the word fatigue. This I think is a concept
describing an emotion.
My argument for not
finding positive feedback loops in the nervous system is the presence of
Renshaw cells. They terminate the incoming first level neural current and these
signals have an inhibitory effect. The positive feedback loop is prevented from
the Renshaw cell. Am I wrong, please tell me.
bjorn