Fig. 5. Voxel-wise negative covariation analysis (remifentanil dosage as covariate of interest) of H215O–positron emission tomography data during administration of saline, low-dose remifentanil (0.05 μg · kg−1· min−1), and moderate-dose remifentanil (0.15 μg · kg−1· min−1). Decreases of regional cerebral blood flow with increasing remifentanil dosage are shown during painful stimulation. The statistical parametric maps (thresholded at P < 0.001) are overlaid on normalized structural magnetic resonance images in axial planes (average of 27 T1-weighted scans of the same individual from the Montreal Neurological Institute) as provided by SPM99. 1 = Thalamus; 2 = insula/secondary somatosensory cortex; 3 = parahippocampal cortex; 4 = prefrontal cortex; 5 = basal ganglia; 6 = temporal cortex.