Hi,
This is perfectly reasonable for a couple of reasons:
1 - the volume change is not very spatially specific (or consistent)
and hence you can have a strong overall volume change
without a local vertex change (that is above threshold).
2 - the FDR thresholding is not the most sensitive technique for
surface inference (we are working on coding up better ones)
and so it is possible that it is just not quite sensitive enough
although you might see interesting results in the uncorrected
analyses. Unfortunately, the FDR results are very dependent
on the small p-values which can be significantly influenced by
slight outliers. It is worth looking at the uncorrected results to
see if there is some consistent change which is just failing to
meet the FDR threshold.
I hope this helps.
All the best,
Mark
On 22 Nov 2010, at 04:58, Bhavani shankara wrote:
> Dear FSL experts,
>
> I am using FSL-FIRST, on extracting the Volumes and statistics were done on R software and found the subcortical structures and also both left and right hippocampus were significantly differing between the groups (AD and controls (all are right handed) ) (p<0.01).
>
> However on vertex analysis only on left side hippocampus is not differing between the groups standing out at FDR correction (on using surface_fdr input.vtk). However other structures including the sub-cortical deep gray matter regions were differing at FDR threshold output (p value) below 0.05.
>
> Is it possible ?? is it due to technical/analysis issue or due to biological reason??
>
> Thanks in advance for your response
>
> --
> Dr. BHAVANI SHANKARA BAGEPALLY
> MBBS, (PhD in Clinical Neuroscience) NIMHANS,
> Bangalore,
> INDIA-560029
> email: [log in to unmask]
>
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