Hello!

I'm trying to make sure I'm understand how the different parts of how 3dclustsim works.

I noticed if I don't specify a grey matter mask (i.e., no mask flag) in my 3dclustsim analysis, my k values are much lower. Intuitively, I would have expected the opposite. My goal is to include the whole brain in my calculation.

Could you help tease apart why my values vary so dramatically between using a grey matter mask (more stringent p/k value ratios) and no mask? I'm assuming the k values provided with the grey matter mask are correct because they are more stringent, but I just want to make sure I'm understanding why.

Thanks again!