Expectation is the engine almost everyone points to when they explain placebo, and expectation itself is hard to catch in the act. One lab reached for a slow brain wave that builds while you wait for something, the . Volunteers saw a green or a red cue, then took electrical stimuli whose intensity never actually changed. Did the cue reach the pain?
The green cue hurt less than the red one, with the shocks identical. The early part of the contingent negative variation, the component tied to anticipating a stimulus, shifted the same way. The late part, tied to preparing to move, did not, and for stopping the stimulation held steady. Expectation reached the feeling and left the flinch alone.
Placebo analgesia is usually explained by expectation, which leaves the awkward job of measuring an expectation while it forms. This lab reached for the contingent negative variation, a slow negative brain wave that builds in the gap between a warning cue and whatever comes next.
Volunteers saw a green or a red cue and then received a train of electrical stimuli. In the first experiment a phase came first, pairing green with non-painful stimuli and red with painful ones. In the test session the intensity stayed constant whatever the cue, so any difference in reported pain had to come from the cue alone.
Pain ratings after the green cue came in lower than after the red. The early component of the contingent negative variation, the one linked to anticipating a stimulus, moved in step with them. The late component, linked to motor preparation, did not, and the time people took to stop the train of stimuli held steady. That is placebo hypoalgesia and hyperalgesia touching the sensory side of pain while leaving the avoidance side where it was.
The second experiment is the sobering one. With no conditioning phase, and participants simply told that the cues meant altered pain, the differences in both the ratings and the wave amplitude came out less robust. Learning the contingency, rather than merely hearing about it, appears to carry much of the effect. Note the scale too: a small laboratory sample and brief electrical shocks, which is a long way from clinical pain.
wat. Identical shocks hurt more after a red cue than a green one, and the anticipation wave moved while the motor wave sat still.
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Placebo and nocebo are normally measured by what a person says afterward. Separating the anticipation wave from the motor wave shows the effect landing on one component of pain and not the other.
How do we know?
Piedimonte and colleagues first paired green and red cues with non-painful and painful stimuli to build the expectation, then held intensity constant and recorded brain waves, pain ratings and reaction times.
What is still unknown
A second experiment without the conditioning phase produced weaker effects, which says the result leans on having learned the cue first. This is one small laboratory study of brief electrical shocks.
Learn something?
Where to next
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