Sensory modulation in cingulate motor area by tonic muscle pain: A pilot study

We investigated the central modulatory effects of tonic muscle pain on phasic nonpainful stimuli delivered within the same muscle. Somatosensory evoked potentials (SEPs) were recorded during nonpainful intramuscular electrostimulation with and without simultaneous infusion of hypertonic saline. Source modeling was used to localize the modulatory effect within the brain. In most of the subjects, the phasic nonpainful muscle stimuli were perceived as less intense during tonic muscle pain. The only consistent change in SEPs was localized to contralateral caudal cingulate motor area (CMA) and was manifested as a reduction in source strength for the corresponding dipole. Our results indicate that sensory integration in caudal CMA may be based on contextual relevance relative to other input including affective, cognitive and sensory/pain dimensions. Such a mechanism may be related to motor defensive strategies and shaping of an orienting response towards behavioral relevant stimuli.