Tonic pain abolishes cortical habituation of visual evoked potentials in healthy subjects

We investigated changes in visual cortex excitability by analyzing visual evoked potential (VEP) habituation in healthy subjects during tonic pain evoked by the cold-pressor test (CPT). We tested VEP amplitude habituation (slope of the linear regression line for N1-P1 amplitude from the 1st to 6th block of 100 sweeps) in 19 healthy volunteers during 4 experimental conditions: baseline; no-pain (hand held in warm water, 25 degrees C); pain (hand held in cold water, 2-4 degrees C); and the after-effects of tonic pain. During baseline and no-pain sessions, VEPs habituated normally across the 6 consecutive blocks (mean slope -.28 and -.18%), whereas during pain and its after-effects they failed to decrease (0%, and -.11%). Tonic pain induced by the CPT abolishes normal VEP habituation and the lack of habituation persists after the CPT is stopped. Tonic pain probably abolishes VEP habituation by acting on brainstem neural structures which modulate thalamo-cortical activation thereby changing visual cortex excitability.

This study shows that tonic pain alters visual cortex excitability, a brain region unrelated to pain processing. These changes probably reflect defensive strategies against pain. Extending the study from healthy volunteers to patients with migraine between attacks would offer the opportunity to investigate visual cortical excitability under conditions when baseline habituation is absent.