Effects of motor response expectancy on cortical processing of noxious laser stimuli

Previous studies have shown pain reductions during motor cortex stimulation or voluntary movements. To shed more light on cortical changes associated with decreases in pain during heightened level of motor preparedness in absence of movement, we decided to analyse the effects of motor readiness on EEG laser-evoked potentials (LEPs) by manipulating the expectancy of motor responses. Noxious laser stimuli were administered to the right hand in absence of any movements during periods associated with either high or no expectancy of motor response (HMRE or NMRE, respectively). Subjects reported greater pain intensity during NMRE than HMRE trials. The N1 component of LEPs, peaking at 141 ms and generated in the contralateral operculo-insular cortex, was larger during HMRE than NMRE periods. The amplitude of the N1 component during NMRE correlated with pain intensity. The P2 component peaked earlier during HMRE (336 ± 30ms) than NMRE (356 ± 29 ms, P<0.05) condition and its amplitude showed statistically significant positive correlation with subjective pain intensity. Results suggest that pain reduction during high motor expectancy may be related to summation of effects of motor readiness and nociceptive processing in operculo-insular cortex. Subjective pain intensity appears to be formed at an early, sensory stage of processing of laser stimulus in the absence of motor task and only later, during the period in which multiple behavioural challenges are evaluated, if motor readiness is heightened.