Tonic pain attenuates phasic pain

Pain perception is closely related to the attentional level and the vigilance of the subject [4]. The intensity of a pain stimulus and its sensation do often not coincide. A common experimental example is the so-called pain-inhibiting-pain effect: A stressful stimulus, e.g. persistent or chronic pain, often elevates the threshold for brief pain stimuli applied during the same time. Stressful stimuli, which deflects attention from a painful event, may induce the so-called Stress Induced Analgesia [6],[8]. Phasic pain stimuli in the fingertip cause a pin-prick like sensation and lead to a typical activation sequence of the cortex. An activation of the SII region at 100–120 ms which is related to the rational, discriminating component of pain sensation is followed by an activity at 250–350 ms, originating from the cingulate gyrus (CG) which is related to the evaluative perception of pain [9] (for a review, see [1]). This study investigates the change in brain activation, if short pain stimuli were superimposed by a persistent, tonic pain stimulus. Due to the high resolution needed in time, MEG and EEG are most suited to record such activities. The alteration of brain activity patterns in MEG/EEG during the application of additional tonic pain is compared against an additionally recorded, psychophysical parameter. Thereby it is possible to identify the cortical level where the pain inhibition takes effect.