Dr. Gutman, thanks for your (and your cat’s) response. The point that the probability of connection should not be equated with ‘strength’ of connection is well taken, as crossing fibers will change the probability but not necessarily the ‘strength’ of a region to region connection. What I’m still trying to understand, however, is how the distance weighting adds a further complication to the interpretation of the probtrackx results. With distance weighting you no longer simply receive the probability of connection, but a value that reflects both the probability of connection (in the number of tracks out of the total possible which get from region A to region B) and the total length of each of the tracks which made it. When I then compare the distance-corrected output values for (region A to region B) and (region A to region C), I can no longer say that one had 0.5 probability of connection and the other has 0.8; I instead have two values which don’t tell me the relative probabilities of these two connections (a 100 value could mean 1 out of 1000 tracks went 100 voxels, or 100 out of 1000 tracks went 1 voxel), nor do I even know the relative distances tracks had to travel in each case (unless you perform probtrackx again without distance correction and divide this number (total tracks) into the distance-corrected value  – which may not work exactly as the outputs may not perfectly overlap due to the probabilistic nature of the calculation). I’m just not sure how to interpret a comparison of these values across subjects when the underlying sources of the values could have such variant meanings.

Thanks again for your thoughts!

Tim

Nuts-- cat decided to send my message before I was done.

Anyway-- be careful about interpreting the raw numbers as strength of connection-- they are actually probability's of connection, and the distance correction attempts to somewhat control for the decrease in probability as you get farther away from the seed region.

For example, assuming your using more than one seed voxel, to have values at a given voxel > 5000, for example if all of the threads from neighboring voxels pass through a given point, that value will be >> 5000.

More relevant, the probability of connection is not directly equatable to a strength of connection.   Imagine a tract that is going straight to the frontal pole-- say with 1000 threads running down the highway to the frontal pole.   Now in subject 1 the tract goes straight to the frontal lobe;  in this analogy imagine this as a 5 line highway.  

In subject 2, the 5 lane highway also has an exit/offramp.  Of the 1000 threads that were going to go down to the frontal pole, 200 of them take the "offramp" and the other 800 continue straight.  You've done nothing really to the underlying "strength" of connection (i.e. both subjects have a 5 lane highway), but the probability of getting to the frontal lobe has now decreased because you have more options, 20% of the time you get off the exit, 80% of the time you've gone straight.  Thus when framing questions, you have to be careful how you interpret the numbers.  While the probability of getting from point A to point B might have changed (and this is interesting perhaps), the actual "strength" of the connections has not necessarily changed.

Tim and Saad can likely provide oodles more detail about this


dg

-- 
David A Gutman, M.D. Ph.D.
Department of Psychiatry & Behavioral Sciences
Emory University School of Medicine