Signal valence in the nucleus accumbens to pain onset and offset.

Pain and relief are at opposite ends of the reward-aversion continuum. Studying them provides an opportunity to evaluate dynamic changes in brain activity in reward-aversion pathways as measured by functional magnetic resonance imaging (fMRI). Of particular interest is the nucleus accumbens (NAc), a brain substrate known to be involved in reward-aversion processing, whose activation valence has been observed to be opposite in response to reward or aversive stimuli. Here we have used pain onset (aversive) and pain offset (rewarding) involving a prolonged stimulus applied to the dorsum of the hand in 10 male subjects over 120s to study the NAc fMRI response. The results show a negative signal change with pain onset and a positive signal change with pain offset in the NAc contralateral to the stimulus. The study supports the idea that the NAc fMRI signal may provide a useful marker for the effects of pain and analgesia in healthy volunteers.