On 30/4/96 Chris Kirtley <[log in to unmask]> wrote:
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An Honours student and I just finished a research project with the
Australian Neuromuscular Research Institute, and funded by the PD
Association of Western Australia. We looked at dynamic control of the
centre of gravity in PD with a Neurocom Balance Master system. Basically we
found that people with PD have the same timing but smaller amplitudes of
movement when tracking a moving target. This was a bit of a surprise to us
at first, but we now understand it by modelling the motor control system as
a typical engineering control system. It looks like people with PD reduce
the loop gain and increase damping to prevent instability (tremor), which
results in the lower amplitude movements.
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I asked Chris to explain this more simply and this is his reply:
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Hello - nice to hear from you. I guess my terminology is a bit obscure, but
the engineering theory is also a little tricky!
Basically, all control systems have both gain and damping. The gain of the
system sets its sensitivity to discrepancies between what's required and
what is achieved. For example, in a central heating control system, the
thermostat turns on when the temperature falls between a certain threshold
- the heater then warms the room up, and when the temperature rises above
the threshold again, the thermostat turns off again.
Now actually, when the room warms up it takes a little bit of time before
the thermostat turns off, so there's a bit of an overshoot in temperature.
Instead of being constant, the temperature snakes up and down above the
required value. The amount of this overshoot is related to the damping of
the control system.
Control systems like the central heating controller (called "closed-loop"
controllers, because they monitor their outputs) are very useful, but they
have some drawbacks. The main problem is that if the gain (sensitivity) is
too high and/or damping is too low (too much overshoot), the system becomes
unstable and the output oscillates out of control.
I believe our work has shown that this is just the sort of problem we have
in PD, although, in a sense the other way about: because the system is
unstable (which manifests itself in the tremor that you see in your
brother), he subconsciously reduces gain and increases damping to maintain
control. The side effect of this is to make his movements slow and stiff.
When he wants to do something (what's called the "step response of the
control system), it takes longer than usual to get moving. We had expected
that we'd see this slowness in our testing, but we didn't. We now
understand because we were using a oscillating target as our input - people
had to move their bodies rhythmically back and forth to follow it. It turns
out that control systems respond to such an oscillating input in a
different way to a step response: they follow it at the same speed but with
smaller amplitude (less big) movements, and a little out of step with the
target movement. The fact that we found this confirmed to me that the
slowness and stiffness symptoms in PD should be understood as being
secondary compensations, rather than primary problems.
We have yet to publish, and obviously we have to do more work, but I think
it has helped to use the engineering approach to understand what is going
on in PD.
Hope this helps,
Chris
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Dr. Chris Kirtley MB ChB, PhD [log in to unmask]
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Lecturer, Bio-engineering --_ / \
/ \
School of Physiotherapy, Perth #_.---._/
Curtin University of Technology, V
GPO Box U1987,
Perth 6001, Tel +61 9 351 3649
Western Australia. Fax +61 9 351 3636
WWW: http://www.curtin.edu.au/curtin/dept/physio/pt/staff/kirtley/
Internet Relay Chat: irc.curtin.edu.au (port 6667) "bio-engineering"
NetPhone: IP address 34311.180.105
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