On Fri, 10 Apr 2009 13:58:42 -0400, Julie McEntee <[log in to unmask]>
wrote:
>Hi all,
>
>I have searched the archives but could not locate anything on this topic.
>
>I am using SPM5 (latest update) on Windows XP 2002 and Matlab 7.0.
>
>I would like to test the difference between 3 groups (fMRI). I chose full
>factorial at second-level analysis and defined 1 factor (Group) with 3
>levels. I selected the .con files for each group/level of factor. After
>specifying the test and estimation, the following contrasts were generated:
>
>001{F}: Average effect of condition
>002{F}: Main effect of Group
>003{T}: Positive effect of condition_1
>004{T}: Positive effect of Group_1
>005{T}: Positive effect of Group_2
>
>When I look at the results for the 2nd contrast (I¹m assuming this is the
>test of difference between groups), there are no p values at the cluster
>level (only at the voxel level, although cluster size is indicated). Can
>anyone explain this or identify what I have done incorrectly?
I think it's because there's no cluster-level correction implemented for F-
contrasts.
See this thread for details:
https://www.jiscmail.ac.uk/cgi-bin/webadmin?
A2=SPM;mjpolQ;20080211132411%2B0100
>Also, the T tests that were generated look like the difference between
>Group_1 minus Group_2 (contrast 4) and Group_2 minus Group_3 (contrast 5).
>Does this mean that the difference between the groups is accounted for by
>the difference between Group_1 and Group_2 and Group_2 and Group_3, so
the
>difference between Group_1 and Group_3 is meaningless? One last question:
>the T tests indicate ³Positive effect²; does this mean increase in BOLD
>response, such that if I wanted to see if there was a difference in decrease
>in BOLD response between groups, I would have to define that contrast and
>use the flexible factorial model?
>
>Thank-you for your help,
>
>Julie
>
>Julie E. McEntee, MA, CCRP
>Senior Research Program Coordinator
>Department of Psychiatry- Neuroimaging
>Johns Hopkins University School of Medicine
>600 N. Wolfe St./Phipps 300
>Baltimore, MD 21287
>Phone: 410-502-0468
>Fax: 410-614-3676
>
>
