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

OBJECTIVE: By co-activating A partial partial differential- 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 partial partial differential-fibre related late laser evoked potential (LEP). However, when avoiding concomitant activation of A partial partial differential-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 partial partial differential- 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 partial partial differential-fibres occurring at similar latencies as the late LEP; and (2) a widespread post-stimulus event-related desynchronization (ERD) induced by both A partial partial differential- and C-fibres.
CONCLUSIONS: A partial partial differential- and C-fibre related LEPs could be electrophysiological correlates of similar brain processes, which, when already engaged by A partial partial differential-fibres, cannot or do not need to be reactivated by the later arriving C-fibre afferent volley. A partial partial differential-fibre related ERD could reflect a transient change of state of brain structures generating these responses.