http://www.pnas.org/content/109/39/15959.abstract
Abstract
The dominant theories of human placebo
effects rely on a notion that consciously perceptible cues, such as
verbal information
or distinct stimuli in classical conditioning,
provide signals that activate placebo effects. However, growing evidence
suggest
that behavior can be triggered by stimuli presented
outside of conscious awareness. Here, we performed two experiments in
which the responses to thermal pain stimuli were
assessed. The first experiment assessed whether a conditioning paradigm,
using clearly visible cues for high and low pain,
could induce placebo and nocebo responses. The second experiment, in a
separate
group of subjects, assessed whether conditioned
placebo and nocebo responses could be triggered in response to
nonconscious
(masked) exposures to the same cues. A total of 40
healthy volunteers (24 female, mean age 23 y) were investigated in a
laboratory
setting. Participants rated each pain stimulus on a
numeric response scale, ranging from 0 = no pain to 100 = worst
imaginable
pain. Significant placebo and nocebo effects were
found in both experiment 1 (using clearly visible stimuli) and
experiment
2 (using nonconscious stimuli), indicating that the
mechanisms responsible for placebo and nocebo effects can operate
without
conscious awareness of the triggering cues. This is
a unique experimental verification of the influence of nonconscious
conditioned
stimuli on placebo/nocebo effects and the results
challenge the exclusive role of awareness and conscious cognitions in
placebo
responses.
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