ERPs associated with decisions based on slowly increasing warm and painful stimuli

The event-related potential (ERP), produced by subjects who were experiencing a gradually increasing heat stimulus, was recorded 3.5 sec before and 1 sec after a motor response. Subjects responded when the stimulus was 'just warm,' 'just at prickling pain' or 'above the temperature of just prickling pain.' Two control procedures mimicked the readiness potential (RP) and motor potential (MP) experiments. The results showed that the steadily rising negative MP was modified to a positive wave before the decision of 'warm.' During the 'pain' and 'above pain' conditions, the pre-decision slow wave was less negative than the MP but less positive than that of the 'warm' condition. A temperature control condition confirmed that rising temperature alone did not produce these results. Principal components analysis confirmed these differences by producing factors which were interpreted as representing the ERPs of the conditions of 'warm,' 'pain,' 'above pain,' RP and MP. It was evident from pilot studies that uncontrolled electrical activity from the skin potential response (SPR) was contaminating the results. SPR forced all ERP curves in the positive direction and several attempts to eliminate the SPR from the ERP data failed. While the SPR was produced differentially across conditions and across the experimental session, the smaller the variability in the electro-oculogram (EOG) recorded from above and below the orbits, the smaller was the SPR contamination that we recorded in the ERP. Therefore, we selected subjects for this experiment who showed a stable orbital EOG.