Hi Mahinda and Stam,


>> A belated thank you for your advice regarding this thread. 

You're welcome!

>> So if my interpretation of your comments is correct, changes in the 
>> mode (ie an increase from negative to more positive values) might 
>> represent a shift from a "disc" to more "cigar" shaped anisotropy 
>> especially if this increase is in areas where one expects significant
>> amounts of crossing fibres.  In areas where there is less likely to be >> crossing fibres, a similar increase in the mode may alternatively 
>> represent a shift to a more linear anisotropy, which may for example be >> due to an increase in the 1st eigen value in the tensor model or a 
>> decrease in the 2nd/3rd eigenvalues.  

Absolutely.

>> Would use of tbss_x aid interpretation further.  I understand that it
>> ensures that f1 and f2 belong to the same fibre bundles across 
>> subjects, so that it is then possible to compare measures across 
>> subjects that are related to the same fibres.  If therefore an increase >> in mode of anistrophy/FA picked up by TBSS is due to selective 
>> degeneration of a fibre bundle, would one then expect to see decreases >> in F2 in that same area ?  That is, would one expect to see a decreased >> proportion of the diffusion signal being modelled by f2 in those 
>> regions ?  

Yes, in theory you are right, you should be able to see that f1 remains the same (the unaffected tract) and that f2 decreases (the affected, secondary tract crossing in this region). However in practice, it is actually not necessarily what you will be able to see. To make a (very) long story short, you would need very high b-values (maybe 2000 or 3000) to see this exact effect on f1 and f2. What I had for instance in my study was a decrease of f2 with an increase of f1...

>> Conversely if one sees an increase in f1 in those regions would that 
>> again suggest 
>> that the modal changes seen represent a more linear anisotropy, which >> as Gwenaelle points out you would maybe see in healthy subjects with, 
>> say, motor learning for instance, just like an increase of FA would 
>> then also be seen.

> There is no reason for only f2 being affected in the first case. If a 
> disk-shaped profile becomes more cigar-shaped, this might indicate a 
> degeneration of any of the two crossing fibres. Thus, you might see that > f2 is not affected and f1 is decreased or vice versa.

What Stam wrote above is true in the case of two crossing fibres with the exact same "strength". 
In practice, most of the time, you have one slightly dominating fibre, and the increase of mode (and FA) can then only be due to the degeneration of the secondary tract crossing in this region of increase (if it was a degeneration of the primary one, you would then see a decrease of mode and FA). Hence, again theoretically, you should see a decrease of f2 and a stable f1... Your knowledge of anatomy and the impact of the disease (and a possible decrease of mode and/or FA in the region where the secondary fibre becomes the largely predominating one) should point towards the right answer.

Hope this makes sense!
Gwenaelle