Using these design
features, the pain model system just described can accommodate
for trait differences between subjects, such as individual differences
in sensitivity to and the suppression of pain, and can ensure
a state of tonic pain of predefined intensity that is experientially
adjusted and maintained for the duration of the experiment.
After modeling the initial response characteristics, the adaptive
element functions as a second input. It includes strategies
to bring pain intensity back in case the upper or lower preset
target pain intensity bounds are exceeded and it recalculates
any previously established model parameters. Pain intensity
ratings, obtained every 15 sec., are filtered for greater model
stability. Because bolus volumes for some sensitive muscles
may be very small and/or special experimental constraints may
require a long polyethylene tube to deliver the stimulus, the
system design accommodates for variations in compliance of the
delivery tube (Zhang,
Ashton-Miller et al., 1993).
| Figure
8.1: Diagram of System to Induce and Maitain Pain
Intensity
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Block
diagram of the system designed to induce and maintain
pain intensity between
preset VAS scores by infusion of an algesic substance
into deep tissue. The main feature
of the adaptive control delivery system is a proportional-integrative-derivative
(PID)
controller. It accepts the error signal that is based
on the difference between actual and
estimated pain intensities. Once the subject’s
response characteristics are modeled, the
adaptive element functions as a second input. The
adaptive element includes strategies
to bring pain intensity back in case the upper or
lower preset bounds are exceeded and
recalculates any previously established PID parameters.
Since bolus volumes for some
muscles are small, the mechanical properties of the
tubing affect the system performance,
requiring system characteristics to be modified to
accommodate the tube compliance
properties and mechanical resistance to infusion flow.
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