[Freesurfer] Sign option of Monte Carlo simulation in QDEC

Dear experts:

In examining QDEC results of a contrast of interest, I noticed the results are mainly in cold color (blue), with only two small clusters in warm color (red). When I proceeded with Monte Carlo simulation, if I chose the abs as the sign option in simulation, then no clusters survived. In contrast, some clusters would remain, if I chose the neg sign. Because the uncorrected results were mainly in cold color, it seems reasonable to me to use negative only in the simulation. But I am wondering how the option of abs/neg/pos leads to the disparities and will you please elaborate how the simulation works based on abs/neg/pos.

Thank you in advance.

Karl