263 EFFECTS OF EXPECTATION ON SOMATOSENSORY EVENT-RELATED POTENTIALS (SEPS) ELICITED BY PAINFUL ELECTRIC STIMULI

Abstract

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. To investigate whether expectancy influences SEPs we applied a cross‐modal oddball paradigm. The P3 EP component is modulated by expectancy, anticipation and novelty. We hypothesize that the number of background stimuli preceding the painful target determines the expectancy of the target and consequently the amplitude of the P3. Methods: A multi‐channel EEG was recorded (band pass 0.1–100 Hz, sample frequency 1000Hz) from 11 volunteers. Trials commenced with 3, to 11 auditory stimuli, followed by a painful electric target stimulus (ring finger, 1ms duration, individually set to a subjective pain intensity level of 9; 0 = no pain; 10 = maximum pain). Inter‐stimulus intervals were 800 ms. After target presentation the participants rated the stimulus painfulness (0–9). Results and Discussion: An increase in the number of auditory background stimuli preceding the painful stimulus was accompanied with a larger lateralised N130, possibly due to an increase in attention. The centrally localized P350 decreased, as hypothesized, as did the late N450. SEP changes were observed up to 7 preceding background stimuli, after which the effect levelled off. No effects of number of backgrounds was found with respect to the perceived intensity. We propose that SEP P350 components elicited by oddball paradigm behave similarly to P3 EP components observed in other modalities. It will be interesting to study whether cognitive SEP modulation differs between chronic pain patients and healthy volunteers.