Bilateral brainstem activation by thermal stimulation of the face in healthy volunteers

Contralateral sensory deficits in Wallenberg's lateral medullary syndrome suggest bilateral processing of trigeminal afferent input in the human brainstem. On the basis of experiments in rodents and clinical data, the present study addresses the hypothesis of bilateral projection of facial nociceptive input onto the spinal trigeminal nucleus (STN) in healthy humans. Nociceptive processing in the brainstem was investigated by functional magnetic resonance imaging (fMRI) in 18 healthy volunteers. Heat stimuli (39, 43, 46 degrees C) were applied by a Peltier type thermode to the left forehead (V1) and the left mental region (V3). Analyses of fMRI data were performed with SPM2 and BrainVoyager software. A region-of-interest approach analyzed local activation in the STN. Heat evoked significant bilateral activation in the STN (P < 0.01, T > 2.8). Contralateral activation was more frequent during stimulation of the V1 than of the V3 region. Whereas activation by V1 stimulation was located in caudal STN, V3 stimulation induced activity in more rostral parts of the STN. Functional MRI data in humans suggest bilateral brainstem activation when heat is applied to the face. Contralateral brainstem activity is more pronounced by stimulation of V1 as compared to V3. The results indicate similar nociceptive processing in humans and rodents and may explain clinical findings.