Hi Yaser,
>Hi SPM Listserv members
>
>I ran VBM analysis for Grey matter for some subjects but in the MSU print
>out of the activated clusters it shows a lot of white matter activation.
>Now I only ran grey matter segments so I dont understand why the MSU is
>showing a lot of white matter. Could anyone help me on that.
>Thanks
>Yaser
>
I'm also running a VBM study ... but for my first time! so I'm not an
expert on that. However, I guess that it could be related to your
threshold (given that segmented images are ok and well normalized).
For my study I use Christian Glaser's script. On his website, he gives
this piece of advice:
"You should use an absolute threshold for masking around 0.1..0.15 (and
finally up to 0.2). This prevents that you get (opposite) results in
white matter although you only made an analysis for gray matter. This
effect is due to the negative correlation of gray and white matter
intensity values around the border between gray and white matter. If you
choose an absolute threshold masking you will only compute your
statistic in those areas that are above this threshold. Areas below this
threshold will be excluded. Be careful if you get any result in white
matter although you made an analysis of gray matter. These results are
suspected to show an opposite effect".
Maybe you haven't used this absolute threshold?
Hope it helps,
Best,
Cyril
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