Response plasticity of pain evoked reactions in man

Pain estimation (E), evoked cerebral potential (EP), electrooculogram (EOG), electromyogram of withdrawal reflex activity (EMG) and skin conductance reaction (SCR) were measured in response to electrical skin stimuli in 11 male volunteers (age 21-31 years). Constant current stimuli (20 msec; 2, 3, 6, 10 mA) were applied to a finger tip (averaged pain threshold at 5.1 mA). Interstimulus intervals (20-40 sec) and stimulus intensities were delivered in quasi-randomized order, each intensity appearing 40 times per session. Four sessions were repeated with intervals of two days between sessions. With randomized stimulus intensities, power spectral density functions of the prestimulus EEG indicated a stable non-specific arousal level between and within sessions. Under these conditions no significant response plasticity was found for E and EP, not in the prepain or the pain range, and also not between or within sessions. In contrast, the amplitudes of EOG, EMG and SCR decreased drastically with time, especially between the first and second session, and between the first 10 and second 10 stimuli of equal intensity in each session. EP and E remained highly correlated in repeated sessions. A EOG-EP contamination could be ruled out because of their different time course to repeated stimuli.