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If you want to address a direct comparison, individual group results cannot be generalized to a between group comparison. That is to say, if you find a significant effect in one group and not in the other, you still cannot say that that shows one group is greater than the other, you need to do a direct comparison.

Something else to consider, if there are anatomical differences associated with your patient population it may not be valid to do a whole-brain comparison using standard MNI normalization. to get around this you can either create a common template to normalize to, or do a functional ROI based analysis.

To compare groups, you can use an unpaired (2-sample) t-test. However, that is assuming that data from each group is normal. Otherwise, you can use something like a permutation test or other non-parametric test.

hope this helps...

Cheers,
Michael

On Sun, Mar 13, 2011 at 2:37 PM, Toxopeus, CM <[log in to unmask]> wrote:
Dear all,

We performed an fMRI study with a patient and a control group, using a specific movement task. We performed simultaneous recording of a behavioural measure. This parameter was significantly different between groups.

Now, we would like to know whether brain activation specifically related to this parameter is different between the patients and healthy subjects, thus on second level.

We first tried a two-sample t-test and using the parameter as 2 covariates, one for each group (padded with zeroes), which is the conventional method for investigating the effect the possible role of a covariate when comparing activation of two groups.

However, because our parameter is not just a covariate as body mass or bloodpressure as in normal statistics, but measures actual motor output, which should be organized by the brain, we figured we also could use a single t-test. This way, we do not model group-differences, since we actually are only interested in differences brain activity related to this covariate in the two groups.

I hope someone can give me some advice whether this is a correct thought????

Thank you very much!!

Best regards,
Carolien Toxopeus, MD, PhD student



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Research Associate
Gazzaley Lab
Department of Neurology
University of California, San Francisco