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Hi Emma The curvature threshold used by default in probtrackx is only there to avoid paths tracking back on themselves and artificially increasing the probability along regions that were already visited. The angle of 80 degrees is high enough, however, to allow the sampling to account for local uncertainty. We see the spread in the path spatial distribution not as increased false positives, but as increased uncertainty :) That said [or written], I have tried the thalamus segmentation using decreasing threshold angles (increasing curvature thresholds) - see attachment. The picture shows that the general pattern is conserved for 60 degrees, which is a reasonable threshold, but voxels "inside" the thalamus tend to have higher directional uncertainty, which leads to the pattern you get for the conservative threshold of 45 degrees. Note that even at this level, the general pattern is conserved, especially on the external shell of the thalamus. Cheers, Saad.