C- and A delta-fiber components of heat-evoked cerebral potentials in healthy human subjects

Feedback-controlled laser heat was used to stimulate the hairy skin of the hand dorsum and forearm, and heat-evoked cerebral potentials were recorded at midline (Fz, Cz, Pz) and temporal (T3, T4) scalp positions. Based on dat a from primary afferent electrophysiology a stimulus level (40 degrees C) w as chosen, which is above C-fiber hear threshold, but clearly below A delta -nociceptor heat threshold in order to excite selectively C-fibers without concomitant excitation of A delta-fibers. Feedback-controlled stepped heat stimuli to 40 degrees C elicited ultralate laser evoked potentials (LEPs) a t the vertex in a high proportion of experiments (90%). Estimates of conduc tion velocity calculated from latency shifts between the hand and forearm s ites of ultralate LEPs (2.4 m/s) and of reaction times (2.8 m/s) confirmed mediation of ultralate potentials by unmyelinated nerve fibers (nociceptors and/or warm fibers). The ultralate LEP could be differentiated from resolu tion of contingent negative variation (CNV), an endogenous potential relate d to expectation and response preparation, by its scalp topography. Strong heat stimuli of 48 degrees C, which is suprathreshold for most A delta- and C-fiber nociceptors, elicited the well-known late LEPs mediated by nocicep tive A delta-fibers confirming previous studies. The LEP waveform to strong heat stimuli also contained an ultralate component reminiscent of an ultra late LEP following the late LEP. Ultralate and late LEP had identical scalp topography. In conclusion, the method of temperature-controlled laser heat stimuli allows the selective and reliable examination of A delta- and C-fi ber-mediated afferent pathways and the related cortical processing without the complication of dissociating A-fiber nerve blocks. (C) 1999 Internation al Association for the Study of Pain. Published by Elsevier Science B.V.