This is counter-intuitive but reasonable and expected. The distribution of cluster sizes changes as you change the CFT. When you raise the CFT, clusters get smaller, but the clusters under the null also get smaller  too. It may be that the smaller cluster size is actually less likely under the null when using the higher CFT

On 10/30/2024 8:52 PM, 이종훈(보건과학대학 의공학부) wrote:

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Dear Freesurfer expert,

I hope this email finds you well. I am currently working on a project involving FreeSurfer's cluster-wise correction methods, and I encountered an unexpected result when adjusting the Cluster-Forming Threshold (CFT) and Cluster-Wise Probability (CWP) values. I would greatly appreciate your expertise on this matter.

In my analysis, I observed that some regions were not detected when using a CFT of 1.3 and a CWP of 0.05. However, these regions emerged as significant when I increased the CFT to 2.0 while keeping the CWP constant at 0.05. This finding appears counterintuitive, as a higher CFT typically results in fewer clusters being detected due to the increased threshold for significance.

Could you please help clarify why this phenomenon might occur? Specifically:

1. How might the higher CFT (2.0) interact with the CWP (0.05) to reveal regions that did not appear with a lower CFT (1.3)?
2. Is it possible that the increase in CFT reduces noise or increases signal clarity in such a way that previously undetected clusters reach significance?

Thank you very much for your time and guidance. I look forward to any insights you can provide on this topic.

Jonghun Lee Master's Student, Advanced Image Processing Technology Lab Yonsei University

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