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Subjects vary widely
in non-specific effects of pain (e.g. anticipation of pain) as
well as the previously-mentioned sensitivity to and suppression
of pain, which are complexly controlled traits. Experimenters
should therefore consider, whenever feasible, using a randomized
and counterbalanced crossover design in which both treatments
(hypertonic and isotonic saline) are applied to each individual,
so that each subject forms his or her own control. In other words,
except for the order and time, the "pain group" and
the "control-group" are identical (See
Chapter 6).
Subjects should know
that one of the two infusions causes pain. However, they should
be blinded with respect to what solution is infused. Infusion
rates for isotonic saline should be based on the average infusion
profile required to produce pain of target intensity at the given
application site to produce similar volumetric conditions for
both pain and the saline-control trial.
With pain being induced
and sustained by the continuous infusion of hypertonic saline,
the investigator studies the alternatives to the null hypotheses
(H0) that the experientially-adjusted
rate of infusion of hypertonic saline and the resulting graded
experience of pain cause the hypothesized outcome. Grading of
pain is required in order to permit comparison of mean phenotypic
values for a given intensity of pain. The experiential titration
of the stimulus, such as keeping subjects at similar pain intensities,
is useful for the comparative study of response behaviors of subjects
with particular genotypes or significantly different phenotypes.
| Figure
11.1: Subject in Pain
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First
Frame of Quicktime movie (not linked).
Movie
shows subject in pain induced by the intramuscular
infusion of the algesic substance into the jaw
muscles.
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