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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.

Positron emission tomographic scanning has no demonstrable benefit in the diagnosis, staging, or prognosis of patients with tumors of 2 cm or less in size.

Whether attending to stimuli affects the orienting response and its habituation is not yet clear. EEG alpha suppression responses, electrodermal responses, and EEG evoked potential responses of two groups of subjects were compared. The Attend group was given instructions to pay attention to and count the 59 moderate intensity clicks (interstimulus interval = 15 sec). The Ignore group was instructed to “try not to let the clicks disturb your relaxed state.” Separate ANOVAs with repeated measures were used to evaluate all measures, and all showed significant effects for click repetition (habituation). Alpha suppression and change in log conductance showed no differences related to group. The P300 component of the average evoked potentials was significantly larger for the Attend group. We concluded that attending to, versus ignoring, stimuli does not have an appreciable effect on these two traditional measures of the orienting response, but it does affect the evoked potentials. The relationship between attention and “significance” is discussed.

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