Dear Urs,

The values are normalized by the mean power across vertices and
conditions within subject. So if you then subtract evening from
morning it does make sense to compare those numbers across subjects
just you need to keep in mind that you are talking about changes in
relation to mean activity over the whole brain for that subject.

Best,

Vladimir

On Fri, Nov 25, 2011 at 1:30 PM, Urs Bachofner <[log in to unmask]> wrote:
> Dear Vladimir,
>
> thank you for your inputs. I'm checking out Marsbar now and it seems this is
> the tool that will work for me.
>
> Creating those masks I had a very big concern about my Nifti images created
> by the Source Reconstruction: Are those activation patterns absolute values?
> If not then it's not even possible to compare them with other subjects,
> right?
>
> Thank you for an answer,
>
> Urs
>
>
> Am 24.11.2011 13:28, schrieb Vladimir Litvak:
>>
>> Dear Urs,
>>
>> On Tue, Nov 22, 2011 at 3:03 PM, Urs Bachofner<[log in to unmask]>
>>  wrote:
>>>
>>> Dear SPM Experts,
>>>
>>> I need help building a statistical analysis in SPM8. I know exactly what
>>> I
>>> want to do, but there seem to be a lot of different ways of doing it.
>>>
>>> Here's the plot: Of 40 subjects of different age groups (Age 8 to 18) I
>>> have
>>> activation images (.nii) of an oddball task, generated by 3D source
>>> reconstruction of EEG files. They all did the same oddball task in the
>>> evening and in the morning, after sleep.
>>>
>>> In earlier studies my research group found that the GMFP in such a task
>>> decreases over night, so now I wanted to test if the involved areas also
>>> decrease over night.
>>>
>>> My first idea was to use imCalc to subtract the morning-images from the
>>> evening-images, but I encountered the problem, that the areas involved in
>>> solving the task differ in differerent age groups, so of course those
>>> differential images also show different voxels involved.
>>> So even though I find a greater neuronal response in the evening in all
>>> subjects, it makes no sense to t-test the differential images, since the
>>> remaining activation is in different voxels.
>>>
>>> The next idea was to define ROIs of the same size in all differential
>>> images
>>> (evening minus morning) and average the activation therein. Does this
>>> make
>>> sense? What functions of the fMRI part of SPM8 is able to do this?
>>>
>> It's not completely clear to me whether you are talking about very
>> close areas that just don't overlap precisely or about completely
>> different areas. In the former case you could use group inversion to
>> try to make sure that the sources are the same or even use fMRI priors
>> option (with a mask for your areas of interest). You can also increase
>> the smoothing parameter (available in the batch tool 'M/EEG source
>> reconstruction results'). You could also average over an ROI covering
>> the voxels where you see an effect in all the subjects as long as you
>> could justify the choice of that ROI in advance and not based on the
>> data (which should be OK in your case as there is a lot of literature
>> about the sources of MMN). I think the tools to work with ROIs are not
>> in the main SPM but in toolboxes like MarsBar but I'm not familiar
>> with these toolboxes myself.
>>
>> If you are talking about completely different and non-adjacent areas I
>> don't see any physiological sense in putting them in the same test. I
>> would then just do separate tests for the different age groups which
>> do share the same sources and report the results of these tests
>> separately (with Bonferroni correction across age groups).
>>
>>> Apart from this, as I am primarily interested in activation of frontal
>>> regions, is it necessary to cut away the rest of the activation, for
>>> exmaple
>>> by using MarsBaR? Or is there a tool in SPM8 wherein I can set all areas
>>> I'm
>>> not interested in to zero?
>>
>> You don't need to set them to zero, but there is an option to use
>> explicit mask when you set up a statistical test or you can do small
>> volume correction with a mask image to define your volume.
>>>
>>> Another idea was to simply use a two-paired dependant t-test using both
>>> morning and evening images, but I think this won't get me any information
>>> about the decrease of activation that happened over night.
>>>
>> Paired t-test is equivalent to single-sample t-test on difference images.
>>
>>> Also, I'd be interested if the decrease of activation (or of involved
>>> clusters, respectively) changes with age. How should this be done? Do I
>>> have
>>> to use the differential images or can I work with the images of evening
>>> and
>>> morning?
>>
>> I think you should correlate the differential images with age but
>> it'll only make sense if you are looking at the same area/ROI across
>> subjects.
>>
>>
>> Best,
>>
>> Vladimir
>>
>>>
>>> Any hints, solutions and clues are greatly appreciated.
>>>
>>> Thank you,
>>>
>>> Urs Bachofner
>>>
>>>
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>
>