Magnetoencephalographic (MEG) responses to selective laser activation of A or C fiber thermonociceptors

Abstract

Since activation of myelinated (A) vs. unmyelinated (C) nociceptors feels very different, understanding cortical responses to their selective activation may be critical in attempting their amelioration. Different diode laser (DL) stimulation protocols produce selective activation of these two fiber types in cells, animals and human volunteers. Evoked potential recordings in humans demonstrated clear differences in the temporal pattern of cortical responses to these stimuli. Similarly, application of DL stimuli during fMRI BOLD testing produced clear spatial distinctions in cortical response. However, fMRI provides only limited temporal resolution (approximately 4s), disallowing observations of early spatiotemporal pattern distinctions. Because MEG allows for both spatial (∼ 5mm) and temporal (∼2 ms) resolution we examined MEG spatiotemporal patterns for DL stimulus activation of A or C fiber nociceptors. A 275 channel whole-head MEG system was used while 100 each of supra-pain threshold 60 ms (A fiber activating) or 1.5 s DL (C fiber activating) pulses were applied to non-repeating sites on the dorsal surface of the hand of subjects while event-related field responses were recorded. While overall, spatial patterns observed for MEG were consistent with fMRI patterns, distinct MEG spatiotemporal patterns were observed for A vs C fiber mediated pain. For short (A fiber) pulses, activation began at 100 ms in the contralateral primary and secondary somatosensory cortex, then spread to the insula within 300 ms, eventually including deactivation of ipsilateral primary somatosensory cortex around 1000 ms. In contrast, responses to long (C fiber) pulses began at 650 ms after the stimulus with bilateral activation in primary somatosensory cortex and subsequent deactivation in the contralateral sensory association area. When compared to evoked potential/EEG (surface electrical activity) and fMRI blood oxygenation signals recorded with similar stimulation protocol, MEG is uniquely suited to directly elaborate deep-brain spatiotemporal distinctions in cortical activation patterns for different pain types.