Cortical mechanisms underlying point localization of pain spot as studied by event-related potentials following CO2 laser stimulation in man

To elucidate cortical mechanisms underlying point localization of a pain spot, we investigated event-related potentials (ERPs) while using a CO2 laser beam to apply a pain stimulus to the hand dorsum in 16 healthy men. The stimulus spot (pain spot) was shifted for each stimulus, while the subject was requested to identify the stimulated spot as accurately as possible and to use a pointer in the non-stimulated hand to indicate the corresponding spot on a figure of a hand that was projected onto a screen (localization condition). For the control condition, the subject pointed to a single predetermined spot, regardless of the location of the stimulation (control motor task condition). Electroencephalograms were recorded from 21 electrodes, referenced to the linked earlobes, and were averaged time-locked to the stimulus onset for each task separately. Under the control rest condition (neither point localization nor motor task), only two early components (N2 and P2) were recorded. During the control motor task condition (no point localization), in addition to N2 and P2, a steep negative-going slope was recorded at the fronto-central region. Exclusively during the localization condition, a positive peak (647 ms, 5.6 mu V for the left and 634 ms, 5.7 mu V for the right hand stimulation) was identified; this was maximal at the midline centro-parietal area and distributed symmetrically over the scalp. It is suggested that the late positive component detected exclusively during the localization task is related to the somatotopic point localization of the pain spot. From the distribution of this ERP, the task most likely involves bilateral activation of the superior parietal cortices.