Hi - the effect of the burnin is more important in bedpostX than it  
was in bedpost, as it is not easy to get a good initialisation of the  
model when there us more than one fibre population, so you are doing  
a good thing by testing this.   A good thing to look at is the  
difference in how often you see crossing fibres (e.g. where is  
mean_f2samples greater than 0.05).   The amount of burnin you need  
may well depend on your data, but more should be better! It is  
obviously a trade off with how long you want to wait.

Cheers

T


On 27 Nov 2007, at 20:18, Ted Yanagihara wrote:

> After searching the archives I found that Mahinda Yogarajah asked a  
> question similar to the one I have just come up with, but there  
> doesn't seem to be any reply. So, I'll try again.
>
>
> I have been playing with the default settings for bedpostX to see  
> if there are any major changes to the data. To do this quickly I  
> have been running single voxel analyses on individual subjects.  
> When I seed a voxel in the rostral ACC at a burnin of 1000 I get  
> fibers that go from the frontal pole, cross the midline, and go to  
> the contralateral frontal cortex. If I then use a burnin of 5000  
> these fibers start going through the genu, but then stop and do not  
> reach the contralateral frontal cortex.
>
> Does anyone have suggestions on the right way to decide the  
> appropriate burnin? I only vaguely understand what the purpose of  
> the burnin is, so it's hard for me to make any educated guesses.  
> Also, increases in processing time are not much of an issue, at  
> least for the time being.
>
> Thank you!
>
> ted