Dear SPM list
I would like to ask your advice on the best way to analyse my data. I have two groups and I am trying to find differences between them on a working memory task. I have three seperate runs of the working memory task plus a baseline task for each subject, however, due to movement (the subjects are children) some participants have only two or even one usable run. To be specific, I have 15 subjects in group A and 13 in B. In Group A 10 have 3 usable runs and 5 have 2. In Group B 5 subjects have 3 usable runs, 3 have 2 and 5 have only 1. Each individual run contains three blocks of the WM condition and three baseline, each block containing 10 stimuli (resulting in 30 for each run). Therefore, I think, there are enough stimuli for one run alone to be usable, but obviously we would like to use all the data that we have to increase the power of the analysis.
I have done the first level analysis on everyone, on all runs seperately (WM minus baseline) and produced contrast maps. For the second level analysis, I wished to look at activation in group A minus Group B and vice-versa. I ran a full monty analysis on the two groups, defining each subject and the number of contrast images for each subject (as mentioned some subjects had three runs and others had two or one), for each group. However, when I tried to define the contrast, SPM would not accept it. I defined the contrast as 1 -1 (Group A being 1 and B -1) and also tried defining each subject within the group (i.e. 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1). The design matrix looks correct to me, but as I mentioned SPM would not accept these contrasts.
Does anyone have any advice on the best way to analyze this data set or on what I may be doing wrong? I would very much appreciate step-by-step directions, as I am quite unfamiliar with SPM.
Thanking you in advance for your help,
Catherine
Catherine Fassbender PhD
Dept of Child Psychiatry, Research Division
University of Maryland, Baltimore
Room 436, 737 W Lombard St
Ph: 410 706 4723
Fax: 410 706 6220
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