Hi Saad,

I am curious about how the --rseed option works.  Do you give it a number to
seed a random generator, or does it use a random number to seed the random
generator?  I am wondering if I could use this to parallelize probtrackx on
a number of samples basis.  For example, if I wanted to run 25000 samples
for each seed, could I break it up into 5 unique --rseed results of 5000
each and then sum them?  I remember that this did not used to be possible.



-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
Of Saad Jbabdi
Sent: Tuesday, August 11, 2009 12:08 PM
To: [log in to unmask]
Subject: Re: [FSL] waytotal results

Hi Rachel,

Is this consistent across trials? (i.e. if you re-run probtrackx for  
different --rseed options, do you always get the same pattern?).

Note: it should be the opposite on average! The drop in waytotal  
represents the following probabilities:

76 = probability of passing through an exclusion mask
30474 = probability of NOT passing through an inclusion mask

40118 = probability of either passing through an exclusion mask or NOT  
passing through an inclusion mask

So the sum of the first two should be larger than the third on average  
because you are counting twice the events where you either pass  
through an exclusion mask or do not pass through an inclusion mask...


On 11 Aug 2009, at 17:29, Rachel Hawe wrote:

> Thanks for your response.  What I am more confused about is that  
> adding only
> an exclusion mask caused a drop of 76 in the waytotal; adding only a
> waypoint mask caused a drop of 30474; but adding both caused a drop of
> 40118.  Why is the drop in waytotal greater when using both an  
> exclusion and
> waypoint mask compared to the sum of the drops when adding each  
> separately
> (i.e. 76 + 30474 < 40118)?
> Thanks,
> Rachel

Saad Jbabdi
University of Oxford, FMRIB Centre

JR Hospital, Headington, OX3 9DU, UK
+44 (0) 1865 222523  (fax 717)