I have a question regarding the outputs of fixelcfestas. I apologize that this issue might be discussed several times, but I’m still confused about what I got so far.
I performed a longitudinal FBA and would like to know whether there is any change across time. I subtracted each time-point 1 image from the time-point 2, and smoothed the outcome images. My design matrix has 2 columns: intercept (1s) and a demeaned covariate. My contrast is 1 0 and -1 0, which I assume reflect global change for the whole sample. I also added a mask to exclude <50 fixels.
I did not find any significant results, however the fwe1-p values are the minimum (1/n), null_contributions are more than 3000, which seem wired. Please see the outputs of the contrast 1 0 as below (similar for -1 0).
uncorrected1-p: 0 ~ 1.17e-38
fwe1-p: 0 ~ 0.0002
null_contributions: 0 ~ 3069
My question is that even if the global changes of my sample are not significant, they couldn’t be zero (the zstats are not trivial), then why 1-p values are so small? I encountered this type of problem every time when I looked at the significance of the global intercepts with demeaned covariates in the models. Which step that I could have done wrong? I would appreciate any help you could provide!