That does sound plausible, and in fact, a portion of the cluster is
outside the amygdalar mask as you predicted. Thanks, this helps me a
great deal.
On Dec 6, 2004, at 11:29 PM, Stephen Smith wrote:
> Hi - I suspect that the masking is interacting with the cluster sizes
> to
> give what you're seeing.
>
> I suspect that when you use the large mask the "amygdalae cluster",
> whilst
> existing (before p-thresholding) within this mask, has had it's full
> extent curtailed by the masking. In other words, the original cluster
> was
> partially in and partially out of the large mask, so that after masking
> it's estimated size was decreased, so that it failed the
> p-threshold-test.
>
> Then, when you reduced the mask still further, the reduced stringency
> of
> the multiple-comparison-correction (due to small masking) allowed this
> cluster to then pass the test.
>
> Does this sounds plausible? Cheers, Steve.
>
>
> On Mon, 6 Dec 2004, Richard Albistegui-DuBois wrote:
>
>> This may reflect my abysmal ignorance of statistics, but I am stuck
>> and
>> confused.
>> When I run a group analysis on a certain dataset with no prethreshold
>> masking, and cluster correction at p<0.05, I observe widespread
>> clusters of positive Z's, including in the amygdalae. If I use a
>> prethreshold mask on several cortical areas and the amydalae (and I
>> have confirmed the the amygdalar portion of the mask includes the +Z
>> clusters in the unmasked results), the amygdalar clusters do not
>> appear. If, however, I use a mask which ONLY includes the amygdalar
>> portions of the previous mask, the clusters do appear.
>>
>> I thought that using a mask would make the cluster correction less
>> stringent, and so I am confused as to why the amygdalar clusters would
>> show up in the unmasked image, not in the large masked image, but
>> would
>> show up in the amydalar masked image. Have I completely misunderstood
>> cluster correction?
>>
>> I have checked, and all other aspects of the analysis were identical.
>> The mask images are 0 outside the mask and 255 within the mask.
>>
>> Thanks in advance.
>>
>> Richard Albistegui-DuBois, Ph.D.
>> UCLA Brain Mapping/Neuropsychiatric Institute
>> phone: (310) 825-3814
>> email: [log in to unmask]
>> AIM: dubistegui
>>
>
> 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
>
>
Richard Albistegui-DuBois, Ph.D.
UCLA Brain Mapping
660 Charles Young Drive South
(310) 774-1305 phone
(310) 794-7406 fax
[log in to unmask]
AIM: dubistegui
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