Discrepancy between perceived pain and cortical processing: A voxel-based morphometry and contact heat evoked potential study.

Objectives: The purpose of this study was to determine if local gray and white matter volume variations between subjects could account for variability in responses to CHEP stimulation.

Methods: Structural magnetic resonance imaging was used to perform voxel-based morphometry (VBM) of gray and white matter in 30 neurologically healthy subjects. Contact heat stimulation was performed on the dorsum of the right hand at the base of the thumb. Evoked potentials were acquired from a vertex-recording electrode referenced to linked ears.

Results: Controlling for age, total intracranial volume, and skull/scalp thickness, CHEP amplitude and pain rating were not significantly correlated between subjects. A VBM region of interest approach demonstrated a significant interaction between pain rating and N2 amplitude in the right insular cortex (p<0.05, family-wise error corrected, FWE). In white matter, a significant interaction was localized in the right inferior frontal occipital fasciculus (IFOF, p<0.05 FWE).

Conclusions: Accounting for gray matter volume in the right insular cortex, resulted in a significant relationship between CHEP amplitude and pain rating.

Significance: This finding suggests that the discrepancy between pain ratings and the amplitude of evoked potentials is not solely related to measurement artifact, but rather attributable, in part, to anatomical differences between subjects.