Andrew,

If you are wanting to say what the activity is in a subject, the best thing to do is to average across runs based on the number of trials. If its a block design, you probably have equal number of blocks in each run, and it can be simplified to averaging across runs.

The idea of using 1s and 0s was simple that it was easier than entering fractions and as long as all subjects had the same number of runs, then it wasn't a problem. Additionally, the spmT or spmF maps are the same for the 1s and 0s or if you use 1/N and 0s where N is the numebr of runs.

Since you want to compare across subjects with varying numbers of runs/sessions, you need to make the sum of the contrast (assuming its A versus baseline) equal to 1.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School
Office: (773) 406-2464
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On Tue, May 10, 2011 at 2:51 PM, Andrew Westphal <[log in to unmask]> wrote:

Hello fellow SPM users,

With some investigation into con images, my lab found that con images appear to be sums of the beta images that one specifies depending on the weight that is assigned (i.e. 1 0 0 0 0 1 0 0 0 0 - you would add the two beta images together). This appears to be the way it is calculated instead of a mean value. So does SPM account for the fact that subjects will have varied block counts, such as one subject having 5 sessions and another having 2 sessions? Or is the subject with 5 sessions going to contribute 2.5 times as much at the group level as the subject with two sessions? For it to be valid, does one have to put in a contrast vector like 1/5 0 0 0 0 1/5 0 0 for the 5 session subject and 1/2 0 0 0 0 1/2 0 0 for the 2 session subject?

Andrew