Right-hemisphere preponderance of responses to painful CO2 stimulation of the human nasal mucosa

We recorded whole-scalp cerebral magnetic fields from healthy adults to painful CO2 pulses (duration 200 ms, concentration 65-90%), led to the left or right nostril once every 20 or 30 s. The stimuli were embedded in a continuous airflow (140 ml/s, 36.5 degrees C, relative humidity 80%) to prevent alterations in the mechanical and thermal conditions of the nasal mucosa. The recording passband was 0.03-90 Hz and 16 single responses were averaged per run. Five out of the 9 subjects showed replicable and artifact-free responses 280-400 ms after stimulus onset. The main responses originated close to the second somatosensory cortex (SII), most frequently in the right hemisphere, and also in the rolandic areas, mostly on the left. The signals were considerably stronger over the right than the left frontotemporal region, with a right-to-left ratio of 2.3 for areal mean signal amplitudes calculated across 16 channels, for both left and right nostril stimuli. Air puffs delivered to the nasal mucosa resulted in a trend for right-hemisphere dominant responses, but responses to air puff stimulation of the lip and the forehead were symmetric. The right-hemisphere dominance of the SII responses may be associated with the painful, and thus unpleasant, nature of the CO2 stimulus, thereby suggesting involvement of the right hemisphere in emotional/motivational aspects of trigeminal pain, in agreement with the role of the trigeminal pathways as a general warning system.