Hi Tommy,

well, you should combine several ROIs for correction of multiple comparisons, when you are going to report activation in any of the ROIs. To put it another way, when you have a chance of a false positive per voxel of 0.05, and you report results (a priori) from regions altogether spanning 1000 voxels, you will see 50 voxels activated purely by chance when no effect is present. I do not know a reference of this other than statistical common sense ( p(false positive)=Number_of_comparisons*p(false_positive_per_comparison) ) ...

Therefore, the reviewer is probably right (it wasn't me).

So you do the same as before, but as your search space you now use the volume defined by the combination of all your ROIs. And you report the surviving activation peaks therein.

Perhaps things are different when you have 1 a-priori question (i.e. contrast) per ROI, and only report results from that particular ROI for that particular a-priori contrast, but such cases are often the exception, not the rule.

Or you scan a few additional subjects ;-)

Cheers,

Bas


Op 08-02-12 03:52, Tommy Ng schreef:
[log in to unmask]" type="cite"> Dear SPM experts

I seek your help concerning a question raised by a reviewer.

I did SVC separately for 4 noncontiguous local maxima and reported activations that survived FWER correction.

The question raised by the reviewer was whether I correct for multiple tests across my SVCs?

Is this required? If yes, how do I do this? If no, can someone point me to some references?

Thank you in advance.

Best,

Tommy





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