Dear Mark, 

Thank you for your insights. The Y shape example is absolutely clarifying and now I see why reversing the seed and target may give different results depending on the anatomy even if the diffusion information at each point is the same. 

All the best,
Noelia

2017-08-05 12:09 GMT+02:00 Mark Jenkinson <[log in to unmask]>:
Dear Noelia,

Although the diffusion information at each point is the same in either direction, tracing a tract through the brain differs.  For instance, if you consider a tract that splits into two parts (like a Y shape) then starting at the common end (the base of the Y) and tracing in one direction will give you both parts of the split, whereas starting at the end of one of the splits (e.g. the top left part of the Y) and tracing in the opposite direction will ignore the other split, since the angle at the junction is too great for the tractography algorithm (which is a deliberately imposed constraint).

So in general it is useful to see what you get when reversing the seed and target, since this can show you different aspects of the anatomy.

All the best,
        Mark


> On 3 Aug 2017, at 16:05, Noelia Martinez Molina <[log in to unmask]COM> wrote:
>
> Dear experts,
>
> I've performed a probabilistic tractography study using BEDPOSTX and PROBTRACKX tools in FSL. My question is: can the roles of seed and target be exchanged since probabilistic tractography does not contain directionality information?
>
> Thank you in advanced for your assistance.
>
> Kind regards,
> Noelia



--

Noelia Martínez-Molina
Cognition and Brain Plasticity Group
Department of Basic Psychology
Universitat de Barcelona
Campus Mundet, Edifici de Ponent
Passeig de la Vall d'Hebron 171
08035 Barcelona, Spain

Office: (34) 93 403 47 68
twitter:@NoeliaMrtnez

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