I am interested in only the positive sulc and would like to make group analysis of the postive sulc values. Hence I would like to mask off the negative values (as the lh.sulc file has both positive and negative value). Before making group analysis, I have to resample and smooth the sulc value. How do I smooth the positive sulc in a way that the negative sulc values will not have an effect on the smoothing process? Example if I smooth the positive sulc directly with the negative values, it will have some effect on the positive values since gyri and sulci are alternating.

If I set all the negative value to be 0 and leave the positive value unchanged, it will also have an effect on the positive value, but to a less degree. Would the latter method be a feasible and valid way to smooth the positive sulc or is there better way for me to smooth the positive sulc and make a group analysis of just the positive sulc values?