Dear Anders,
Why don't you calculate the contrasts on the 1st level? You will then
get an idea of what the activation looks like. I hope you didn't smooth
your EPIs prior to the 1st level Bayesian analysis.
After you have your 1st level contrasts you will have to smooth them in
order to increase the chance of overlap between subjects. The smoothing
factor depends on what you want to look at. Here's a paper about smoothing:
Ball, T., Breckel, T. P. K., Mutschler, I., Aertsen, A.,
Schulze-Bonhage, A., Hennig, J., & Speck, O. (2011). Variability of
fMRI-response patterns at different spatial observation scales. Human
Brain Mapping, 1171(January 2011), 1155–1171. doi:10.1002/hbm.21274
At the 2nd level you will take the smoothed con images, do a classical
estimation and then do a Bayesian estimation. I usually use the batch
editor and estimate on the prior dependency for the classical way, then
create another dependency from the classical to do the Bayesian
estimation. The standard deviation from each subject is not used in the
2nd level analysis, since you only plug in the contrasts.
I hope this helps.
Regards,
Glad
On 08.03.2015 10:54, Anders Eklund wrote:
> Dear SPM users,
>
> I'm trying to do a Bayesian group analysis in SPM8. I have looked at the
> SPM8 manual but it is still a bit unclear to me.
>
> I have gone through each subject with a Bayesian first level analysis,
> so I have Cbeta_* and SD_beta* for all regressors. I would like to
> obtain a group level PPM of the first beta weight being larger than 0. I
> used "Specify 2nd-level" and selected the Cbeta_0001.img for each
> subject. I then first do a classical estimation (which seems to be
> required prior to a 2nd level Bayesian estimation) and then do the
> Bayesian 2nd-level estimation. Does this procedure seem correct?
>
> If the procedure is correct, is the standard deviation from each
> subject, i.e. SDbeta_0001.img, used in the group analysis, or is only
> the posterior mean of each subject used?
>
> Regards,
> Anders
>
>
--
P. Glad MIHAI, M.Sc. Biomedical Physics
Functional Imaging | University Clinic Greifswald
Walther-Rathenau-Straße 46 | 17475 Greifswald | Germany
Tel: +49 3834 86 69 44 | Fax: +49 3834 86 68 98
www.baltic-imaging-center.de
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