Hi,
You're partly right, in melodic the temporal mean is first removed
and re-introduced after the decomposition as outlined in the
technical report (see first few lines in section 'Maximum Likelihood
estimation' of the technical report available at our website for the
technical treatment).
The mean time course in effect corresponds to a spatial map where all
voxels contribute equally, i.e. it is a signal which does not contain
any spatial specificity - as such, the mean time course itself
carries no useful information wrt the final (spatially specific, i.e.
not everywhere the same) maps, particularly not during the estimation
stage. The part which lies in the space of the modelled time courses
is only useful in that it re-sahpes the corresponding time courses
while leaving the spatial maps in tact.
hope this helps
best
christian
On 6 Jun 2007, at 15:35, Pierre Lafaye de Micheaux wrote:
> Dear members,
>
> When i do a PCA in melodic, i get the following message:
>
>> Excluding voxels with constant value
>> Data size : 120 x 158227
>> Removing mean image ... done
>> Estimating data covariance ... done
>> Removing mean time course ... done
>> Starting PCA ... done
>
> So, it seems that you center the data BOTH for the lines and the
> columns.
> I thought that if you make a spatial PCA, you should center the
> data only for the variables (time) and not for the statistical
> units (space): removing mean tim course only.
> In a temporal PCA i thought you should also center only for the
> variables (space in this case): removing mean image only.
>
> Am i right? Do you have a theoretical justification for the
> procedure used in Melodic?
>
> Best regards,
>
> Pierre Lafaye de Micheaux, Ph.D.
>
> --
> Pierre Lafaye de Micheaux
> Bureau 210, bâtiment BSHM
> 1251 avenue centrale BP 47
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> FRANCE
>
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____
Christian F. Beckmann
University Research Lecturer
Oxford University Centre for Functional MRI of the Brain (FMRIB)
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK.
[log in to unmask] http://www.fmrib.ox.ac.uk/~beckmann
tel: +44 1865 222551 fax: +44 1865 222717
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