Different EEG topographic effects of painful and non-painful intramuscular stimulation in man

To clarify the specific effects of muscle pain on electroencephalogram (EEG) activation in man, painful and non-painful sensations were produced by intramuscular injections of capsaicin and vehicle solution in the left brachioradialis muscle, with identical procedures in 15 male volunteers. Thirty-one channel EEG data acquired before, during and after the two injections were analysed and compared in respect of topography and power spectrum. Although the painful and non-painful muscular stimulations evoked similar EEG topographic patterns, statistics demonstrated that distinct EEG activation over different areas of the head were induced by the painful and non-painful stimulation compared with the baselines. The decreases in theta and alpha-1 (8-10.5 Hz) activity in central and posterior parietal parts were evoked by non-painful stimulation, but the decreases in alpha-1 and alpha-2 (11-13.5 Hz) activities in the posterior part of the head were induced by painful stimulation. The alpha-2 activity augmented during the waning pain following a decrease in the overt pain. Comparing the EEG changes between baseline, non-painful and painful stimulations as well as waning pain, we found that the increase in beta-2 activity during muscle pain was significant over the extensive areas of the head, whereas a significant increase in alpha-2 activity took place at the posterior part of the head during waning pain following a marked decrease in overt pain. These results may imply that the painful and non-painful muscular stimulations evoke distinct EEG activation in different neural networks of the human brain and the intensity of nociceptive input from muscle may encode the variety of topographic EEG changes.