One possibility is that the LME model might not be able to account for some potential confounding effects in your data. As a result, the "residuals" could contain some strongly correlated structure across space, leading to extremely high (and unrealistic) ACF parameters. One solution is to use some empirical ACF parameters from a similar dataset for 3dClustSim. Alternatively, try your best guess of FWHM and feed them into 3dClustSim -fwhm ...
Gang
Edited 1 time(s). Last edit at 04/30/2021 03:52PM by Gang.