Habituation effects on Somatosensory Event-related Potentials (SEPs) elicited by painful electrical stimuli

Background and Aims: Pain perception is typically measured by questionnaires and behavioural responses. Somatosensory Evoked Potentials (SEPs), however, provide a direct measure of stimulus processing in the brain. We studied how stimulus repetition influences the SEPs.

Methods: A multi-channel EEG was recorded (band pass 0.1−100Hz, sample frequency 1000Hz) from 11 volunteers. Electrical stimuli were applied to the middle phalange of the left ring finger. Each trial consisted of 30 stimuli (1 s duration); inter-stimulus interval of 4 seconds. Three trials were recorded, corresponding with subjective intensity levels 5 (pain detection), 7 (moderate pain) and 9 (pain tolerance). Electric stimuli thus ranged from 1 to 25 mA. Mean amplitudes were extracted from single EEG epochs at Cz: N1: 115–125 ms, P2: 185–195 ms, N2: 195–205 ms and P3: 250–270 ms.

Results and Discussion: The N1 component of the SEP decreased between the first and second stimulus. The P3 wave decreased slower; over 4 to 5 stimuli. For the amplitude of the N1 no differences were observed between the three VAS scores. For those of the P3, the amplitudes of the VAS 5 were lower than those of the two higher VAS scores. The amplitude of the N1 reaching an asymptotic level after the second stimulus in the trains, is consistent with those of many earlier studies [ref: J. Kekoni]. The slow habituation of the P3 component contrasts the sensitization in subjective VAS scores found in a parallel experiment. It will be interesting to study whether this SEP habituation differs between chronic pain patients and healthy volunteers.