In 8 healthy subjects we have recorded cerebral evoked potentials and reaction time (RT) to CO2 laser stimulation of the hairy and glabrous skin at low and high stimulus intensities, corresponding to subjective reporting of detection and pain, respectively. At each intensity we were able to identify an evoked potential; the latencies of the major vertex positive (VP) components fell into 2 distinct populations 320 +/- 30 (VP300) and 778 +/- 80 (VP800) which did not differ between stimulation sites. The frequency of the VP300 responses was greatest in the high stimulus conditions and lower in the low stimulus conditions whilst the opposite was true for the VP800 responses. BImodal distributions of RT were seen at both stimulus intensities. In a further group of of 10 subjects we recorded the latency shift of the vertex negativity following proximal and distal stimulation of hairy skin of the left upper limb and derived conduction velocities for the VP300 (13.21 +/- 2.8 m/sec) and VP800 (1.26 +/- 0.29 m/sec) responses. These results suggest that, following CO2 laser stimulation of both hairy and glabrous skin, two different fibre populations are activated. The VP300 responses appear to be related to A delta activation, while the characteristics of the VP800 responses are consistent with activation of thermoreceptors mediated by C fibres.

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