Hi Martin,
This probably isn't a great approach as it relies on exact spatial
left-right symmetry in order to be well conditioned. The more normal
approach is to specify L and R ROIs and compare results within them -
this is simpler and doesn't rely on exact spatial symmetry.
You could for example, generate the ROIs from the standard space
atlases, or maybe from the thresholded activation masks - after L-R
flipping and averaging those masks, etc. etc.
The measure of activation to compare is something that is not
generally agreed upon - e.g., peak Z, supra-threshold voxel count,
etc. I would recommend reading the Mitsis NeuroImage paper for
investigations of options here.
Cheers, Steve.
On 2 Apr 2008, at 20:02, Martin Kavec wrote:
> Hi,
>
> I am trying to test for statistical difference of activation in
> bilateral
> task. I have simple boxcar paradigm, which gives nice activations in
> the both
> L and R hemispheres and I would like to know which one is
> statistically
> higher. I do the following:
>
> 1. flip slice-wise the whole time series so the AP orientation stays
> the same,
> just L->R
>
> 2. merge in time the original and the flipped time series
>
> 3.setup the FEAT with EV1 and EV2, where the EV1 stops after 180s
> (the actual
> length of the the original time series) and the EV2 skips 180s.
>
> 3. set C1 [1, -1] and C2 [-1, 1], though the second one is probably
> not
> necessary
>
> The problem is that I don't get any difference even in the data,
> where the
> analysis of the original time series is evidently asymmetric. I would
> appreciate if anybody could give me a hint what could be wrong with
> this
> approach.
>
> Thanks a lot in advance,
>
> Martin
> --
> **********************************
> Senior Clinical Research Associate
> MRI Unit of the Department of Radiology
> Erasme Hospital
> Lennik Street 808
> B-1070 Brussels
> BELGIUM
>
> tel: +32-2-555-4325
> fax: +32-3-555-3994
> email: [log in to unmask]
> email: [log in to unmask]
> **********************************
>
> -----------------------------------------------
> Find a way, or make one!
>
---------------------------------------------------------------------------
Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
---------------------------------------------------------------------------
|