Expectation of pain enhances responses to nonpainful somatosensory stimulation in the anterior cingulate cortex and parietal operculum/posterior insula: an event-related functional magnetic resonance imaging study

Although behavioral studies suggest that pain distress may alter the percep tion of somatic stimulation, neural correlates underlying such alteration r emain to be clarified. The present study was aimed to test the hypothesis t hat expectation of pain might amplify brain responses to somatosensory stim ulation in the anterior cingulate cortex (ACC) and the region including par ietal operculum and posterior insula (PO/PI), both of which may play roles in regulating pain-dependent behavior. We compared brain responses with and subjective evaluation of physically identical nonpainful warm stimuli betw een two psychologically different contexts: one linked with pain expectatio n by presenting the nonpainful stimuli randomly intermixed with painful sti muli and the other without. By applying the event-related functional magnet ic resonance imaging technique, brain responses to the stimuli were assesse d with respect to signal changes and activated volume, setting regions of i nterest on activated clusters in ACC and bilateral PO/PI defined by painful stimuli. As a result, the uncertain expectation of painful stimulus enhanc ed transient brain responses to nonpainful stimulus in ACC and PO/PI. The e nhanced responses were revealed as a higher intensity of signal change in A CC and larger volume of activated voxels in PO/PI. Behavioral measurements demonstrated that expectation of painful stimulus amplified perceived unple asantness of innocuous stimulus. From these findings, it is suggested that ACC and PO/PI are involved in modulation of affective aspect of sensory per ception by the uncertain expectation of painful stimulus.