Non-phase locked electroencephalogram (EEG) responses to CO2 laser skin stimulations may reflect central interactions between A∂- and C-fibre afferent volleys

Objective: By co-activating A∂- and C-fibre nociceptors, intense CO2 laser heat stimuli produce a dual sensation, composed of first and second pain, but induce only a single A∂-fibre related late laser evoked potential (LEP). However, when avoiding concomitant activation of A∂-fibres, C-fibre related ultra-late LEPs are recorded. This poorly understood phenomenon was re-investigated using a method which, unlike time-domain averaging, reveals electroencephalogram (EEG) changes whether or not phase-locked to stimulus onset. Methods: CO2 laser stimuli were applied to the dorsum of the hand. Reaction-time was used to discriminate between A∂- and C-fibre mediated detections. Analyses were performed using a method based on the time-frequency wavelet transform of EEG epochs. Results: This study revealed: (1) a novel non-phase-locked component related to the activation of A∂-fibres occurring at similar latencies as the late LEP; and (2) a widespread post-stimulus event-related desynchronization (ERD) induced by both A∂- and C-fibres. Conclusions: A∂- and C-fibre related LEPs could be electrophysiological correlates of similar brain processes, which, when already engaged by A∂-fibres, cannot or do not need to be reactivated by the later arriving C-fibre afferent volley. A∂-fibre related ERD could reflect a transient change of state of brain structures generating these responses.