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Abstract
We compared brain activations in response to acute noxious thermal stimuli in controls
and chronic back pain (CBP) patients. Pain perception and related cortical activation
patterns were similar in the two groups. However, nucleus accumbens (NAc) activity
differentiated the groups at a very high accuracy, exhibiting phasic and tonic responses
with distinct properties. Positive phasic NAc activations at stimulus onset and offset
tracked stimulus salience and, in normal subjects, predicted reward (pain relief)
magnitude at stimulus offset. In CBP, NAc activity correlated with different cortical
circuitry from that of normals and phasic activity at stimulus offset was negative
in polarity, suggesting that the acute pain relieves the ongoing back pain. The relieving
effect was confirmed in a separate psychophysical study in CBP. Therefore, in contrast
to somatosensory pathways, which reflect sensory properties of acute noxious stimuli,
NAc activity in humans encodes its predicted value and anticipates its analgesic potential
on chronic pain.
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