Brain activations observed following the intranasal administration of 500 ms pulses of gaseous CO₂ at 75% v/V, which was clearly above pain threshold.

<p><b>Left column:</b> Pain-stimulus associated activations. <b>Middle and right column:</b> Pain-stimulus associated brain activations where the contrast pain > smell was significant (conjunction of second-level t-contrasts 1 −1 0 and 1 0 −1 denoting CO₂, H₂S and vanillin stimuli associated responses respectively). The left Rolandic operculum and insular cortex contralateral to the stimulation displayed these pain predominant activations although activations were observed bilaterally. Statistically significantly activated voxels (p<0.05 FWE-corrected; t>5.14) are presented overlaid (red) on 3D surface renderings of a standard MNI brain (Panel A) and as coloured overlay on the horizontal and sagittal plane of a structural standard T1-weighted MRI template (left). In the right parts of the figure, the colour depth of the displayed voxels reflects the respective t value of the voxel. Furthermore, stimulus related brain activations corresponding to the different stimuli are reported as mean percent signal change in a 5 mm spherical search volume around a selected FWE-corrected peak coordinate (MNI 42 −10 22; bottom). Single subject activations are depicted as dots and the 95% confidence interval as white bars. Results reflect a 12-subject group analysis.