Hello

i havve problem with the results of my roi analysis.
if i have a roi with all limb. structures including the amygdala, the results show me no amygdala activation
but if i have a roi analysis just with the amgydala i have a activation in this region
how can we explain this?? can somone explain me the logic of the Roi analysis

thnx

Hi Yeliz - depending on how exactly you are implementing these ROI analyses, the likeliest explanation is that the ROI analysis adjusts the correction for multiple comparisons when thresholding. Therefore, when running the analysis with all limbic structures, there are more voxels, i.e. more statistical tests, which results in a more stringent threshold (relative to amygdala alone), which no voxels in amygdala surpass; when including only amygdala, there are fewer voxels, a lower threshold, and therefore some above-threshold voxels. Hope this helps.
-Ben Turner

Hi Yellz,

just what to clarify something first:
1. The two ROIs, are they in different size?
2. The activated regions related to amygdala ROI, are they mostly in limbic regions?

If the answer to the first one is yes, then it might be a SNR issue. including time series from more voxels in the bigger ROI might diffuse the signal in amyg. thus you cannot see the same/similar activation pattern which were shown when  you use smaller ROI. this is just my guess though...

The answer to question 2 might confirm or reject my guessing.


Yang

Hi Ben
thnx for your help

Yang thnx for your input.
one question is still exist. is there any role or guideline to create the ROI Analysis
according to my hypothesis i use in the roi the limbic regions.
as i saw there is no amygdala activation than i just look just for fun for the amygdala alone and found a activation but the question is how
signifikant is this result? should i compare the T value or p ?

ps sorry for my bad english

Yeliz - again, depending on how you are implementing this ROI analysis, neither the p- nor t-values should change. Instead, what changes is the threshold. You should certainly interpret this significance cautiously; according to standard statistical practice, if you intended a priori to test this hypothesis in this way (that is, to do an ROI analysis in amygdala), then you are "safe", but if not, then you are dredging. However, there is a logical limit to this reasoning, since in some sense, you could say you were interested in the activity in every voxel in a whole-brain analysis (but few would find that a compelling defense of failing to correct for multiple comparisons). As long as you are clear about what steps you took, there is nothing "wrong" with what you did; however, based on your description, you should certainly describe this analysis as an exploratory one, and the significance level will therefore be understood (at least, by savvy readers) to be more liberal.