Extraction Method And Optimal Parameter Settings Of Human C-LEPs

Pain is related to the encoding and processing of the nociceptive afferent information in the brain, which is important to protect the human body by avoiding any potential or actual tissue injury. Pain includes four components:nociceptive component, sensory discrimination component, motivational-affective component and cognitive-behavioral component. Pain involves in the physical, social and psychological factors, which not only shows the subjectivity and complexity of pain, but also shows the very strong practical significance for the study of pain.A8-and C-fiber endings are skin nociceptors, which locate in the superficial skin layers. Being conducted by Aδ fibers and C fibers, nociceptive information is processed by multiple brain structures, and normally generated a typical double sensation:an initial pricking pain (first pain) and a prolonged burning sensation (second pain), which is mainly due to the different conduction velocity of Aδ (about 15 m/s) and C (about 1 m/s) afferents.Intense laser heat pulses can selectively excite nociceptive free nerve endings in the superficial skin layers. Such stimuli can elicit changes in brain responses (laser-evoked potentials, LEPs) in the ongoing electroencephalography (EEG). Some of these clear components at latencies compatible with the conduction velocity of A8-fibers are called A8-LEPs and some compatible with the conduction velocity of C-fibers are called C-LEPs. LEPs can not only be used to investigate the peripheral and central processing of nociceptive sensory input, which is important in various clinical applications, but also be adopted in some basic researches (e.g., aging problem and social cognition) as a novel neurophysiologic technique.Till now, six reliable components were detected in A8-LEPs:A8-N1, A8-N2, A8-P2, and A8-P4 waves in the time domain and alpha-band event-related desynchronization (α-ERD) and gamma-band event-related synchronization (y-ERS) in the time-frequency domain. Aδ-Nlis recorded in the center of the stimulating contralateral area. As the earliest composition of Aδ-LEPs, the latency of A8-N1 is 160 ms after the stimulus. The early lateralization response probably originated from the electrophysiological activity of primary somatosensory cortex (SI) and operculoinsular cortex. Aδ-N2/P2 are followed A8-N1 and its latency are 200-350 ms after the stimulus. The A8-N2 extends bilaterally toward temporal regions, whereas the A8-P2 is morecentrally distributed. The amplitude of Aδ-N2/P2 have significant correlation with intensity of laser stimulation and pain perception and Aδ-N2/P2 may be reliable electrophysiological indicator for pain perception. Aδ-P4 is a novel positive component followed Aδ-N1and Aδ-N2/P2. The peak latency of Aδ-P4 is about 390-410 ms when stimulating the hands. As Aδ-N1, scalp and roots results show that the AA-P4 is mainly from electrophysiological activity in the contralateral SI.In the past three decades, researchers spent great efforts to design experiments for exploring human C-LEPs. These techniques include using laser beams of extremely small diameters (because of the higher density of C-fibers than A8-fibers); performing a nerve compression block (because of the higher resistance of C-fibers to ischemia); adjusting the stimulus intensity (because of the lower heat threshold of C-fibers than AA-fibers). Importantly, all these approaches are complex and difficult to implement, and yield a weak C-fiber input (because of, e.g., stimulation of few afferent at low-energies), resulting in C-LEPs of low SNR.The present study investigates the neural electrophysiological features of LEPs and their cognitive significance, and most importantly, by optimizing the experiment paradigm and parameter, two components of C-LEPs could be reliably detected:C-N2 and C-P2 waves in the time domain. Here we applied advanced signal processing techniques (peak alignment and time-frequency decomposition) to electroencephalographic data collected in two experiments conducted in 34 and 96 healthy participants. We show that, when using optimal stimulus parameters (delivering> 80 stimuli within a small skin territory), C-LEPs can be reliably detected in most participants. Importantly, C-LEPs are observed even when preceded by Aδ-LEPs, both in average waveforms and single trials. By providing quantitative information about several response properties of C-LEPs (latency jitter, stimulus-response and perception-response functions, dependency on stimulus repetitions and stimulated area), these results define optimal parameters to record C-LEPs simply and reliably.