Fig. 4. Voxel-wise SPM99 analysis of H²¹⁵O–positron emission tomography data. The statistical parametric maps (threshold at  P < 0.005 for descriptive purposes) are overlaid on normalized structural magnetic resonance images in sagittal planes (average of 27 T1-weighted scans of the same individual from the Montreal Neurological Institute) as provided by SPM99. (  A ) Remifentanil-induced regional cerebral blood flow increases during painful heat stimulation are shown [(0.15 μg · kg⁻¹· min⁻¹remifentanil and pain) minus (0.05 μg · kg⁻¹· min⁻¹remifentanil and pain)]. (  B ) Remifentanil-induced regional cerebral blood flow increases during nonpainful heat stimulation are shown [(0.15 μg · kg⁻¹· min⁻¹remifentanil and no pain) minus (0.05 μg · kg⁻¹· min⁻¹remifentanil and no pain)]. (  C ) This modified analysis of the data already shown in  A was intended to separate pain-specific from -nonspecific (nonpainful heat-related) effects of remifentanil. Thereby, the data from  B were used as an exclusive mask of the analysis shown in  A , i.e. , voxels that were significant in  B , were excluded from this analysis. ACC = anterior cingulate cortex.