[From Peter Cariani (970220.1130)]
[From Rupert Young (970220.1130]
(From Rick Marken (970219.1750))
> ..... research that has led
> people to the (foregone, it turns out) conclusions that feedback
> can be "too slow", that behavior is not "modulated" by sensory
> feedback and that movement "commands" are centrally specified.
> I suppose it's worth it to go over these things again with new
> people on the net, though, even if it is of little benefit to Hans.Yes please. At the recent workshop on autonomous behaviour I went to I heard
people (biologists, I think) giving an example of model-based response. I
can't remember the exact explanation but it went something like this. The body
responds to stimuli/disturbances on the hand in under 300ms, but it takes
700ms for the info from the stimulus to reach the brain so "therefore" there
must be some model involved 'cos the feedback's too slow. What's the PCT
explanation ?
Feedback can be quite fast. There are neural loops in the auditory
system that
provide efferent control over the cochlea within tens of milliseconds.
There
is also the example of the ability to repeat speech that one has heard
with
(if I remember correctly) only a few hundred millisecond delays. There
is the
example from Reichardt's work on fly vision-navigation feedback loops
that
suggests that flies make flight course corrections within 30 msec, and I
wouldn't
be surprised if bats achieve similar kinds of feats as they close in on
flying
insects or fly through trees. I've been told that someone was able to
train a
bat to fly through a FAN. (These kinds of fast mechanisms arguably take
advantage of the precision of spike times to process
information with only a handful of precisely timed spikes.)
In many systems there are "reflex" loops that are much shorter than the
longer
loops that go all the way up to "higher" processing centers and then
down again
to motor systems. A big advantage of having a layered, hierarchical
control structure is
that the shorter loops give you faster response times, and this provides
relative
stability for the "higher" rungs of the control ladder to cope with the
longer-term
perturbations.
There are, of course, some systems that could be open-loop, like the
release of
the frog's tongue, but the aiming of the tongue could conceivably be the
result of
a feedback fire-control system with some delay offsets.
I think the message is: don't underestimate the potential for fast
feedback in the
natural world. There are plenty of ways to get it, and if there are ways
to get it,
nature will eventually find them...........(whether by accident or 'on
purpose').
Maybe a second message is: don't trust any of the dogmas about time in
the nervous system.
Peter Cariani