Sustained thermal pain modulates spontaneous sensorimotor rhythms

Resting human brain could generate spontaneous rhythmic activity in areas of visual, somatosensory and motor areas. Previous studies have shown that excitation in these areas in response to external stimuli will counter-modulate the spontaneous rhythmic brain activity. In the current study, we investigated the effects of sustained thermal pain on the cortical activity by measuring 64-channel scalp EEG on healthy subjects. During the experiment, each subject experienced a sustained painful stimulus using a thermode placed on their dorsal side of the left wrist. The temperature of the thermode was kept at 46°C for 30 seconds during the stimulus-on condition. The temperature was then reduced to 32°C for 60 seconds during the stimulus-off condition. Each trial was repeated 6 times. Independent component analysis (ICA) was performed. One independent component with sensorimotor alpha rhythm was found to be negatively correlated with the stimulation state with correlation coefficient equals to -0.6. Source localization based on this component yielded cortical activation at the sensorimotor area which corresponds to the site of stimulation i.e. left wrist. Frequency analysis was subsequently performed. In the temporal aspect, we found that the alpha activity at the contralateral sensorimotor electrodes was suppressed during the painful stimulation in comparison to no stimulation condition (p< 0.05). The alpha power at the contralateral area was also found to be statistically smaller than the alpha power of ipsilateral brain region during the painful thermal stimulation (p< 0.05). In summary, we found that the spontaneous sensorimotor alpha rhythm was suppressed on the contralateral side of the sensorimotor region during sustained painful thermal stimulation. Although the current finding is based on the study of healthy subjects with sustained pain from external stimuli, it may also help to broaden our understanding of cortical response in patient population who suffer from chronic pain.