Pain measurement with evoked potentials: combination of subjective ratings, randomized intensities, and long interstimulus intervals produces a P300-like confound

Abstract

Evoked potentials in response to painful stimuli have been studied as objective measures of pain. Bromm has advocated experimental conditions in which, (1) stimulus intensities are randomized, and (2) subjects rate each stimulus. However, a cognitive, i.e. information processing, 'late positive component' (LPC), e.g. the P300, may be elicited by these same conditions, whether or not the stimuli are painful. The LPC may overlap, and interfere with the measurement of, responses that are only seen with painful stimuli. We compared the LPC in two experimental protocols using ten subjects and electrical stimuli. In the 'Rating Protocol', shocks of different intensity levels were randomly presented and subjects rated the intensity of each stimulus. In the 'Oddball Standards Protocol', the same levels were used, but each was presented in a separate block of a single level. Stimuli were presented more rapidly and subjects had to push a button in response to occasional double shocks (oddball targets), but not to single shocks (oddball standards). The oddball targets served to direct subjects' attention to the stimuli, but only the evoked potential responses to the oddball standards were used for data analysis. To look at the difference between protocols, we computed a difference condition (Rating protocol responses minus Oddball Standards protocol responses) which we called Incremental activity. The Incremental LPC (average amplitudes from 350 to 650 ms) had a more parietal topography (amplitude at electrode Pz greater than at Cz) than the Oddball Standards LPC (Cz > Pz; protocol x electrode interaction P<0.001). This implies that the Rating Protocol LPC included P300-like activity. The parietal Incremental activity began as early as 250-350 ms after the stimulus in the responses to the most painful stimuli and therefore can confound the measurement of pain activity in the evoked potential.

Attention to Painful Stimulation Enhances γ-Band Activity and Synchronization in Human Sensorimotor Cortex

“…These studies substantiated the role of attention in pain processing and allowed the identification of separate and overlapping components sensitive to pain and task factors including EEG P3-like phenomena (Becker et al, 2000;Legrain et al, 2002;Dowman, 2004) (for review, see Lorenz and Garcia-Larrea, 2003). A promising approach to analyzing the internally driven dynamics of cortical pain processing is to evaluate oscillations that are induced by painful stimuli.…”

Pain perception, obstructive imagery and phase-ordered gamma oscillations

“…For further off-line analyses, only EEG sweeps corresponding to standard stimuli were analyzed. This choice was suggested by Becker's et al (2000) findings showing that classic experimental protocols, which require to evoke ERPs to target stimuli (while subjects are engaged to rate pain sensation elicited by painful targets) produced a parietal late P300-like peak (peaking between 350 and 650 ms), which was mainly dependent on cognitive activity for detection and classification of the target stimulus rather than on pain sensation. In contrast, standard stimuli elicited a more anterior P3 component (peaking at about 300 ms), which was related to pain sensation.…”

“…Becker et al (1993Becker et al ( , 2000 have provided experimental evidence that standard stimuli elicit a P300 component of the ERPs which is dpain specificT and it is not influenced by the cognitive information processing of stimulus evaluation (as it happens for target stimuli). For this reason, gamma responses to the oddball standard stimuli were used in the present study.…”

Attentional set effects on spinal and supraspinal responses to pain

“…For example, it has been shown that under some~but not all! circumstances, the pain-related P2 overlaps with the P3b event-related potential~Becker, Haley, Urena, & Yingling, 2000;Becker, Yingling, & Fein, 1993;Siedenberg & Treede, 1996!. P3b amplitude is dependent, among other things, on the task relevance of the stimulus~Donchin & Coles, 1988!.…”