Hi,
I copy the answer of chris roden on the MriCroN forum. May be it helps.

"As regards to positive and negative correlations, I assume you are using NiiStat not NPM or Stephen Wilson's VLSM. With NiiStat, a positive statistic means that brighter images are correlated with higher behavioral scores and a negative correlation means that brighter images are associated with lower behavioral scores. The convention is to draw our lesions as bright regions (lesioned voxels = 1) on a dark background (unlesioned voxels = 0). The inference depends on your behavior: on many standardized tests like WAB-AQ or MoCO lower scores mean poorer performance. On the other hand, for reaction time tasks, higher response times means poorer performance."