[Freesurfer] denominator of the t-test in fsfast?

freesurfers,

i have a question about how t-tests are computed in fsfast.

so t = ces/sqrt(cesvar), but how exactly is cesvar calculated?

i thought it would be: cesvar = variance_of_residual * c*(X' * X)^-1*c' where X is the stim matrix and c is the contrast vector and ' denotes transpose

two questions:

1. i tried computing cesvar myself, but it is not the equation above (for the one subject i was playing with, cesvar was ~1.5x the above equation). what is the equation that is used?

2. intuitively, the second half of that [ c*(X' * X)^-1*c ] penalizes for correlations in your stimulus matrix, particularly for those relevant to your contrast. is that a good way to think about it?

thanks in advance!

alex