Psychophysical and EEG responses to repeated experimental muscle pain in humans: pain intensity encodes EEG activity

Clinical pain is often characterized by repetitive and persistent occurrence in deep structures, but few studies investigated repetitive tonic pain in humans. To determine cerebral responses to repetitive tonic pain, psychophysical responses, and electroencephalographic (EEG) activation to five trials of repeated tonic muscle pain induced by hypertonic saline were examined and analyzed in 13 male subjects. The study was composed of two experimental sessions performed in separate days. Five sequential injections of hypertonic saline (5.8%) were used to induce repeated muscle pain in the left forearm, and five sequential injections of isotonic saline (0.9%) acted as control. Visual analogue scales (VAS) for pain intensity and 32-channels EEG activities were recorded simultaneously. Five trials of relatively stable muscle pain were induced by intramuscular injections of hypertonic saline, but no evident pain was induced by the injections of isotonic saline. Significant decreases in alpha-1 and -2 activities in posterior part of the head were found during repeated muscle pain in comparison with non-pain. In comparison with baseline, alpha-1 and -2 activities reduced significantly during the first two trials, and gradually resumed in the following three trials of muscle pain. However, beta-2 activity increased consistently throughout the five trials of muscle pain compared to baseline. Alpha-1 activity was negatively, but beta-2 activity was positively correlated to the pain intensity and pain area on the skin. Throughout five injections, the reduction of alpha-1 activity was contrary to the changes of pain intensity. These results indicates that pain-related EEG activities were encoded by the pain intensity. The thalamo-cortical system and descending inhibitory neuronal networks may be involved in the regulation of pain intensity.