Hi
--------------------
Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington,
Oxford. OX3 9 DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask]
http://www.fmrib.ox.ac.uk/~steve
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On 20 Feb 2009, at 07:49, Sarah Master <[log in to unmask]> wrote:
> Dear FSL users,
> We have run a study with 17 participants, each with 3 runs. After
> running
> our models on each run (lets say 3 EV’s per run), we combined the 3
> runs per
> subject under a fixed effects framework. We then combined the
> resulting
> COPE files using FLAME 1&2 and threshold at Z=2 and cluster p<.05.
>
> In reviewing the results, we find very few, if any, significant
> voxels in
> the thresh_zstat images. However, when reviewing the zstat images,
> there
> are quite a few voxels (and only in a priori, hypothesized regions)
> with z’s
> above 2.0. We assume that the cluster correction is the sole or
> primary
> source of the lack of significance. Is this correct?
Yes unless there are problems in the analysis such as data artefacts,
misregistrations or misspecification of the model.
>
>
> We have tried running the analyses using FLAME 1 with the same
> results. We
> also tried running the data under a Fixed effects framework for
> exploration
> purposes. This obviously led to far more identified voxels, but
> running
> Fixed effects analyses are not appropriate at this stage.
>
> We have several questions.
> Is it correct to interpret this pattern as resulting from just sub-
> threshold
> activation/effects?
> Might there be a problem we are missing?
See above.
>
> Would it be appropriate to report any statistics or findings from the
> uncorrected zstat images in a write-up of results? (for example,
> voxels with
> z-values above a certain number?)
Not in general, but if you have a priori ROIs then you can use these
in prethreshold masking to reduce the effects of multiple comparison
correction.
Cheers
>
> Short of collecting more data (which is not an option), do you have
> any
> suggestions?
>
> We have been working on analyzing the data using ROI analyses to
> work around
> the relatively stringent correction in whole brain analyses but
> suggestions
> will be appreciated.
>
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