[Mne_analysis] group dSPM?

[Alexandre Gramfort]

hi,

MNE assembles orientation with sqrt(x^2 + y^2 + z^2)
The problem that can happen is that since you're likely to use loose
orientation the variance of x, y and z component will be different
but also the number of trials for each subject will vary causing
amplitude differences in dSPM values.

@hari : do you agree? From your experience does your chi2->T transformation
somehow reduces the problem?

Alex

On Fri, Jun 8, 2012 at 5:30 PM, Hari Bharadwaj <hari at nmr.mgh.harvard.edu> wrote:
> Hi Elena,
>  If you are using a fixed orientation solution, then the dSPMs are z
> scores and can be combined in group studies using t-stats etc..
>
> If you are using a free orientation solution, then depending on how you
> combine the compnents you end up with an F or a chi2.. Atleast as a first
> pass, you could compute group stats using a transform such as that goes
> from chi2 (with 3 dof) to Z such as:
>   y = (x/3)^(1/3)
> y is then approximately distributed normally with mean (1 - 2/27)and
> variance (2/27)
>
> See: Wilson, E.B.; Hilferty, M.M. (1931) "The distribution of
> chi-squared". Proceedings of the National Academy of Sciences, Washington,
> 17, 684–688.
>
>
> We have usually dSPM scores non-parametrically (i.e using permutation type
> tests) at the group level which might be the cleanest way of doing it but
> very slow.
> See: http://www.ncbi.nlm.nih.gov/pubmed/11747097
>
>
> Regards,
> Hari
>
>
>
>
> On Fri, June 8, 2012 10:22 am, Alexandre Gramfort wrote:
>> Hello Elena,
>>
>> there is one way that would consist is keeping track on the noise
>> normalization
>> used for each subject dSPM, then morph the noise normalization values too
>> and then do the maths to average MNE estimates and divide by the mean
>> noise normalization.
>>
>> Unfortunately there is build in way to do this currently...
>>
>> Alex
>>
>> On Fri, Jun 8, 2012 at 11:55 AM, Elena Orekhova
>> <Elena.Orekhova at neuro.gu.se> wrote:
>>> Dear MNE community,
>>>
>>>
>>>
>>> I would like to estimate reliability of the MNE activations on a group
>>> level.  dSPM, that give us F values, can be performed in a single
>>> subjects,
>>> but what is about GROUP dSPM?
>>>
>>>
>>>
>>> I have seen the presentation by Daniel Goldenholz, where he speculates
>>> on
>>> possibility of the ‘group dSPM’ and I have seen the post by Marina
>>> Papoutsi
>>> (from February 18, 2009), where she tried to implement this analysis.
>>> However, there was no news on this topic since then.  Does anybody use
>>> the
>>> ‘group dSPM’ and/or has any practical hints on that?
>>>
>>>
>>>
>>> Elena
>>>
>>>
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>
>
> --
> Hari Bharadwaj
> PhD Candidate, Biomedical Engineering,
> Boston University
> 677 Beacon St.,
> Boston, MA 02215
>
> Martinos Center for Biomedical Imaging,
> Massachusetts General Hospital
> 149 Thirteenth Street,
> Charlestown, MA 02129
>
> hari at nmr.mgh.harvard.edu
> Ph: 734-883-5954
>
>