Hi.
> We have been running blocked design (30sec on and off)language paradigms on
> a number of subjects and we want to test laterality. Having already looked
> at voxel count measures we want to look at the data from the point of view
> of HRF amplitude.
>
> What we want to do is compare the HRF in the left and right hemisphere and
> derive laterality measures from this and compare these to our voxel measures.
>
> Firstly is it possible to derive HRF amplitudes using FSL? If so how do I do
> that?
In your case, this is easy, yes. The values in the pe or cope images are
proportional to the strength of response, assuming that the regressors are
always the same height in all analyses (which they will be, of height 1 by
default for block designs).
> Secondly, I assume that I should use the first level analysis as a mask for
> the HRF measurements.
Yes, this makes sense. Note that Featquery should easily be able to do
what you want for getting mean PE values inside a mask.
> However, on some subjects there were no active voxels
> in one hemisphere with z set to > 3.5. I assume for this part of the study I
> could reduce z so that all the subjects had active voxels in both
> hemispheres, however, would I be correct in saying that this would reduce
> the overall quality of the HRF's measured?
That's correct, yes, if by quality you mean statistical significance of
the voxels used. An alternative (but there doesn't seem to be general
agreement on these issues) is to reduce the Z threshold slightly and then
instead of using the mean PE in the mask, use the max. There's pros and
cons to each approach.
Cheers, Steve.
Stephen M. Smith DPhil
Associate Director, FMRIB and Analysis Research Coordinator
Oxford University Centre for Functional MRI of the Brain
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
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