Sure - the easy thing is to use a more robust summary statistic that
is not so affected by outliers - for example, just use the median
values from the Featquery output instead of the mean.
Steve.
On 13 May 2007, at 06:45, Xiaoyun Liang wrote:
> Hi Steve,
> Thank you very much! Yes, the average psc might be being driven by
> negative outliers caused for example at brain/CSF boundaries.But I
> selected ROI from group level activated map, the voxels of no interest
> should be excluded. And I wonder if any methods is available for
> solving
> such problem.
> Many thanks!
>
>
>
> Xiaoyun
>
>
>
>
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>
>
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>
> On Sat, 12 May 2007 07:01:36 +0100, Steve Smith <[log in to unmask]>
> wrote:
>
>> 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
>>
>>
>> ---------------------------------------------------------------------
>> ---
>> ---
>> 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
>> ---------------------------------------------------------------------
>> ---
>> ---
>> =====================================================================
>> ====
------------------------------------------------------------------------
---
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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