Hi, the most likely difference is in the estimation of the data
spatial smoothness.
When you run FEAT, it has access to the GLM residuals (res4d), which
is the most accurate thing to estimate the spatial smoothness from.
When you run easythresh, it doesn't have access to this (it could if
you knew where your residuals are and amended the script), so it
estimates the smoothness from the unthresholded input statistic image;
this is slightly less accurate than the former approach and will give
slightly different smoothness estimation, resulting in slightly
different p-values.
Cheers, Steve.
On 26 Jan 2008, at 01:32, Dasa Zeithamova wrote:
> Hi,
>
> I have noticed that cluster p-values provided by easythresh command
> and those that come from feat GUI when using cluster threshold in
> post-stats are different. This happens both when using BET-extracted
> whole brain ("mask.nii.gz") and when using a small volume mask (such
> as occipital cortex mask). The cluster locations and sizes are
> identical, but the associated p-values differ (are smaller, "more
> significant" from feat GUI). I was wondering what algorithms are
> used to estimate the cluster-associated p-values and what may
> account for this discrepancy.
>
> Thank you,
> Dasa
>
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Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
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