Hi Steve,
Thank you for your response.I wonder whether the computed psc with
high value is meaningless.And how can I obtain the reasonable psc?
Thanks
Xiaoyun
On Fri, 11 May 2007 08:09:47 +0100, Steve Smith <[log in to unmask]>
wrote:
>Hi,
>
>
>On 10 May 2007, at 22:25, Xiaoyun Liang wrote:
>
>> Hi,
>> I have a problem now. When I computed the percent signal change
>> using
>> featquery for event-related data, I found some results are strange.
>> For
>> example, in one ROI, the max psc is 9.32, and the min psc is -17.
>> So I tried
>> to find what is the problem with it, and I found the value of the
>> correponding voxels is very low, about 5000. But, actually, values
>> of most
>> voxels should be around 10000. What's the problem?
>
>The 4D dataset is scaled to be 10000 on average, but there is
>variability around that (otherwise the timeseries analysis would be
>slightly pointless.....)
>
>So for example voxels at the edge of the brain can have much lower
>values (unless you make the thresholding in the preprocessing a lot
>more aggressive) and hence you can get very high % signal change -
>this is an obvious limitation and danger of quantifying effects via %
>signal change.
>
>Cheers.
>
>
>
>> In my research, I defined ROIs, such as FFA, Amygdala, OFC
>> etc., by
>> using random-effects group activation with all faces vs. baseline.
>> And I
>> created spheres using coordinate of peak activation in the group
>> activation
>> as center. Then I intersected the sphere with group level
>> activated map and
>> obtained the functional mask. Then I applied the same mask to
>> individual
>> subject for specific ROI, and computed percent siganl change(PSC) with
>> several contrasts.
>> Thanks a lot!
>>
>>
>> Xiaoyun
>
>
>------------------------------------------------------------------------
>---
>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
>------------------------------------------------------------------------
>---
>=========================================================================
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