Hi Cyril and Mike,
yes and no (to Cyrils question), as you can see from the figure in my previous email, some data already seem to converge to an appropriate correction already at a liberal voxel-wise thresholds (p<.01 and even p<.05 for VBM for HC for example and the same for functional connectivity in PD patients in our data set) and some do not. In contrast, you can also see that for VBM in our data p<.001 resulted in a significantly over-conservative correction (which is not as problematic but also indicates that the correction in general is not very accurate being only correct for a specific p-value range). Correspondingly, these general statements on what is okay and not okay are kind of overgeneralized and need to be empirically determined for the specific data. What seems definitely correct, is that the cluster threshold behaves quite different and inaccurate depending on the data and it seems to be highly dependent on smoothness (below also a figure on this). Basically, the only way to have general validity would be really to go to a non-parametric approach as recommended by Eklund or to determine the validity of a specific parametric cluster and voxel-wise combination empirically as for example for the data in my table (which becomes a kind of mix between both - non-parametric cluster threshold on parametric voxel-wise data. Considering that an approach like the one used to create the table or a fully non-parametric one as by Eklund et al. provide a valid empiric null distribution for any voxel-wise threshold, not sure why one would want to go forward with a cluster threshold which one never knows if it is currently correct, over- or under-conservative).Regarding Figure 1 in Friston et al., yes, it says that low CDT is a bad idea because it OVERESTIMATES the expected cluster size (which should lead to an over conservative correction). In contrast, Eklund et al. show that it UNDERESTIMATES it. So, yes, low CDT is a bad idea but for the opposite reason than in this figure therefore an argumentation with reference to this figures appears problematic. Actually, the caption of the figure 1 also says that smoothness does not matter (in contrast it seems to matter very strongly).
