Somatotopic organization of brainstem analgesic circuitry

Rationale: By conducting a placebo analgesia conditioning paradigm using a
falsely labelled and described cream, 93 pain-free participants believed
this cream could reduce the intensity of painful heat applied to either
their face, arm, or leg. In reality, this “lidocaine” cream was a placebo,
and the temperature of a thermode surreptitiously lowered whenever placed
onto this cream relative to a control Vaseline cream site. During
collection of images of brainstem activity using a 7-Tesla Magnetic
Resonance Imaging scanner, both the placebo and control cream sites
received identical temperatures, with a decrease in perceived pain
intensity during the placebo cream site stimulation being a placebo
analgesia response. We analysed activity changes in the PAG and an area
that the PAG contacts to inhibit pain, the rostral ventromedial medulla
(RVM), during analgesia responses on the face, arm, and leg. Further,
using the hidden application of a third cream to a separate body site, we
explored whether analgesia was restricted to the area of stimulation or
whether it was more widespread. Results: Our findings show that analgesia
evoked on the face, arm, and leg changes activity in a somatotopically
organized fashion, i.e. face analgesia is towards the rostral (top) of the
PAG and RVM, arm and leg analgesia represented more caudally (lower).
These somatotopically organized analgesic responses occurred on opposite
sides of the PAG but at the same locations as pain activity changes, and
the analgesia was restricted to the site where conditioning was applied.