[Freesurfer] sig.nii.gz

Hi,

I am trying to compare the quality of fMRI data with different artifact inducing objects in the magnet. I ran expanding and contracting rings and followed the retinotopy fsfast analysis in FreeSurfer 5.1.0. I am trying to compare the data by looking at the sig.nii.gz (which from the website should correspond to the voxel-wise significances) with the different artifacts. I am not sure though how to interpret the negative values for a retinotopy analysis? Can someone clarify what the negative values represent. I would have expected the significance to only be positive, as there isn't a traditional contrast in the smoothly varying stimulus.

Thanks! Dan