Hi - if psc in a single voxel is negative, it is because the effect
(contrast of parameter estimates) is negative - for example, you are
getting less BOLD response at rest than during task.
However, if psc is negative when averaged over a region of interest,
it does not necessarily mean that the effect is also negative on
average: the average psc might be being driven by negative outliers
caused for example at brain/CSF boundaries.
Cheers.
On 12 May 2007, at 05:32, Xiaoyun Liang wrote:
> Hi,
> When I computed the psc, I found that the mean psc is negative
> in some
> ROIs, such as FFA, with the stimulus of face. However, I think it
> should
> be positive. Can anyone give me a reason? Thanks a lot!
>
>
>
>
> Xiaoyun
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Stephen M. Smith, Professor of Biomedical Engineering
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FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
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