Dear List,
I was wondering if anyone could comment on the meaning of the results of
a ‘distance-corrected’ probtrackx calculation as a metric of connectivity
strength, relative to a non-‘distance-corrected’ probtrackx calculation. I
understand that the latter will represent the total number of tracks which
go from a seed to a target mask (units = tracks), while the former will
represent the total distance these tracks traveled to reach the target
(units = voxels). The non-distance-corrected output will always have a
standard proportional value based on the number of tracks used, i.e. 1000
out of 5000, say, tracks made it from a seed to a target. This means I
will always have an absolute measure of connectivity relative to the
maximum possible strength (in this case 5000 out of 5000). With the
distance-corrected output, however, there is no single upper limit to the
value it could output for tracks going from a seed to a target, and that
limit will change depending on where my target is. The distance correction
does enhance the results for targets far away from a seed which would be
washed out in a non-distance-corrected analysis, but how can I
meaningfully compare these values across subjects (or even within a
subject)? Is a 100 generated by one track traveling a long distance
equivalent to a 100 generated by 10 tracks traveling a short distance, in
terms of strength of connectivity?
Further, if, for example, we compare fronto-occipital connections in two
groups, one with long brains and the other with short brains, there will
be a difference between the two in non-distance corrected data (assuming
distance does diminish the likelihood of a track successively reaching its
target), while there may not be one in the ‘distance-corrected’ data.
However, the distance corrected data are now expressed in units of length
of connectivity, which does not sound like a very meaningful biological
measure, while the probability of connections (non distance-corrected) is
more appealing.
Another related question: what determines the reduction in probability of
connection with increasing distance from the seed? Is it only the
probabilistic nature of the algorithm (with increasing likelihood of
deviating from the main path as the procedure is repeated on successive
voxels) or also the architecture of crossing paths along the way?
Any thoughts or comments would be greatly appreciated.
Thanks for the help!
Sincerely,
Timothy Laumann
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