Cerebral processing and temporal summation of human skin and muscle pain

Background. The cerebral correlate of temporal summation of painful stimuli has not yet been described. The aim of this study was to determine the brain responses evoked by repetitive painful electrical stimulation of skin and muscle and relate the EEG findings to psychophysical measures of temporal summation. Methods. Twenty subjects were stimulated with trains of five electrical pulses applied to the skin and muscle. Latencies and peak-to-peak amplitudes of the major negative and positive deflections at Cz, C4, C3 and Fpz evoked by the first and fifth stimulus were analyzed and topographic maps and electrical source localization were determined. Results. The psychophysical data demonstrated temporal summation with significantly higher ratings after the fifth stimulus compared to the first stimulus. The major negative peak at Ct was later for painful muscle stimulation than for skin stimulation and the first negative peaks were significantly later than the fifth negative peaks. The amplitude caused by painful skin stimulation was greater compared to muscle stimulation and the first stimulus in the train caused greater amplitudes than the fifth stimulus. The topographic maps of paired t tests (skin vs. muscle) suggested differences after the first stimulus over the contralateral somatosensory area followed by a spread to the ipsilateral somatosensory area. The electrical sources were located in the contralateral somatosensory area and deep midline vertex area with muscle stimulation and in nearly identical dipoles with skin stimulation. The fifth stimulus was associated with significant shifts of the vertex and somatosensory dipoles. Conclusions. The study demonstrated a clear temporal summation of psychophysical ratings; its dissociation from simple amplitude measures, and a possible correlation with shifts in dipoles.