Figure 4.

(A) The false‐positive error rate per SPM volumetric image for the beamformer weights designed for the 0. 2 nAm source at 3 cm depth. Ideal false positive rates, where the achieved rate should match the required rate are shown as a dotted line. The use of a global thresholding strategy (circles), where uniform smoothness is assumed, tends to produce over inflated error rates. The use of local thresholding (solid), where the threshold for each voxel is determined from its extent in resel space, is very close to ideal. (B) The distribution of per voxel thresholds (histogram) as compared to the global threshold (point) for corrected significance level P < 0.05. It is clear that there is a wide spread of per voxel thresholds, being lowest around the simulated source, where the images are least smooth (Fig. 2).