  Pain is a multidimensional process, which can be modulated by emotions,
however, the mechanisms underlying this modulation are unknown. We used
pictures with different emotional valence (negative, positive, neutral) as
primes and applied electrical painful stimuli as targets to healthy
participants. We assessed pain intensity and unpleasantness ratings and
recorded electroencephalograms (EEG). We found that pain unpleasantness, and
not pain intensity ratings were modulated by emotion, with increased ratings
for negative and decreased for positive pictures. We also found two consecutive
gamma band oscillations (GBOs) related to pain processing from time frequency
analyses of the EEG signals. An early GBO had a cortical distribution
contralateral to the painful stimulus, and its amplitude was positively
correlated with intensity and unpleasantness ratings, but not with prime
valence. The late GBO had a centroparietal distribution and its amplitude was
larger for negative compared to neutral and positive pictures. The emotional
modulation effect (negative versus positive) of the late GBO amplitude was
positively correlated with pain unpleasantness. The early GBO might reflect the
overall pain perception, possibly involving the thalamocortical circuit, while
the late GBO might be related to the affective dimension of pain and top-down
related processes.
