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

Abstract

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 data from primary afferent electrophysiology a stimulus level (40°C) was chosen, which is above C-fiber heat threshold, but clearly below Aδ-nociceptor heat threshold in order to excite selectively C-fibers without concomitant excitation of Aδ-fibers. Feedback-controlled stepped heat stimuli to 40°C elicited ultralate laser evoked potentials (LEPs) at the vertex in a high proportion of experiments (90%). Estimates of conduction velocity calculated from latency shifts between the hand and forearm sites 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 resolution of contingent negative variation (CNV), an endogenous potential related to expectation and response preparation, by its scalp topography. Strong heat stimuli of 48°C, which is suprathreshold for most Aδ- and C-fiber nociceptors, elicited the well-known late LEPs mediated by nociceptive Aδ-fibers confirming previous studies. The LEP waveform to strong heat stimuli also contained an ultralate component reminiscent of an ultralate 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 Aδ- and C-fiber-mediated afferent pathways and the related cortical processing without the complication of dissociating A-fiber nerve blocks.