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Hi Jonathan, Thank you, I will check this toolbox. The question was exactly about you explain (my e-mail was poorly explained), in spite of applying the isotropic Gaussian Kernel (and the logit transform) the data is not quite normally distributed. I have read, it is possible improving non-stationarity by modelling the total amount of grey matter as a confounding effect. Your are right about my first question, it was about I have read something about a Gaussian in the k direction, but the smooth is done by an isotropic Gaussian Kernel, so I was confused. (I have already found where my confusion was! ;-) Thanks and Best regards, Gemma 2009/3/28, Jonathan Peelle <[log in to unmask]>: > > Hi Gemma > > Are you perhaps referring to the issue of nonstationarity in VBM data > (mentioned in Ashburner & Friston, 2000)? The issue isn't one of the > smoothing kernel (which is typically isotropic), but rather that the > data themselves are not equally smooth over space. The Gaussian > smoothing is not enough to overcome the inherent nonstationarity in > the data. > > One consequence of this is that by chance, larger clusters tend to > occur in regions where the image is more smooth, and smaller clusters > where the image is less smooth. This invalidates the typical random > field-based cluster-level correction in SPM. However, the > nonstationarity toolbox will correct for this: > > http://fmri.wfubmc.edu/cms/NS-General > > Hope this helps, > > Jonathan > > > > On Fri, Mar 27, 2009 at 10:16 PM, Gemma Monte <[log in to unmask]> wrote: > > > > There is a concept I am confused about: (in pre-processing of VBM) I > thought > > that the convolution with the Gaussian over the data is describing the > > smoothness in direction k. This leads to the data aren't non-uniformly > > (non-isotropic) smoothed. But when we apply the smooth to the data, > > we introduce the value in the x, y and z directions, why is > non-isotropic? > > > > And if the data are non-uniformly smoothed, it has an influence over the > > results, which are these effects over the results? How it is possible to > > correct for them? > > > > Best regards, > > Gemma > > > > > > >