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Okay, I understand what you did. Are you tracking a pathway along a non-primary fiber orientation? That could cause a negative correlation between waytotal and FA (i.e. the stronger the pathway, the lower the FA because of crossing effects). It would help to know the pathway as well, but if you don't want to publically say, you could tell me off list. Peace, Matt. _____ From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf Of hd x Sent: Thursday, May 12, 2011 1:49 AM To: [log in to unmask] Subject: Re: [FSL] Highly negatively correlated waytotal and average FA Hi Matt., I made an error in describing how I normalized. I didn't divide the waytotal, but divided the number of streamlines per voxel along the pathway. But in case you got what I meant, are you suggesting to take the average FA on a not-thresholded pathway? Best, HD On 11 May 2011 18:33, Matt Glasser <[log in to unmask]> wrote: What happens if you don't do this normalization. Peace, Matt. _____ From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf Of hd x Sent: Wednesday, May 11, 2011 3:53 AM To: [log in to unmask] Subject: [FSL] Highly negatively correlated waytotal and average FA Dear list, I'm working on a study examining the relationship between a behavioral measure and a particular white matter pathway defined using probtrackx (part of the arcuate fasciculus), and I found: 1. The behavioral measure is negatively correlated with the waytotal for the pathway but positively correlated with average FA along the pathway. 2. There is a strong Negative correlation between waytotal and average FA along the pathway. The above two points seem to suggest something circular. Would this be due to a problem of low thresholding or something related to the probabilistic algorithm? Ps, I divided the waytotal by the total sample sent out to normalize the pathway and took a threshold of 5X10-6 . Any thoughts or experience are highly appreciated. Best regards, hd