Psychophysiological and psychophysical responses to experimental pain induced by two types of cutaneous thermal stimuli

Evoked potentials (EPs) to noxious thermal stimulation of skin may provide information about integrity of the nociceptive afferent system and thus small afferent fiber integrity. Here we describe subjective report, EPs and response times to noxious contact thermal and laser stimuli in the same subjects. Pain quality to both forms of stimulation was consistently reported as a brief pricking or stinging sensation (first pain), occasionally followed, after a silent period, by a diffuse burning sensation (second pain). EPs to laser generated heat pulses consisted of bi- or triphasic waveforms with a large positive wave at approximately 300 ms following arm stimulation and 360 ms following stimulation of the leg. EPs to contact heat pulses consisted of a single, scalp positive wave occurring approximately 830 ms following arm stimulation and 890 ms following leg stimulation. Both forms of noxious stimulation activated afferents with a conduction velocity consistent with that of A-delta fibers (similar to 10 m/s) and with the psychophysical attributes of first pain.