Hi Chris,
On Fri, Nov 20, 2009 at 9:44 AM, Christophe Phillips
<[log in to unmask]> wrote:
> I see the point about using the leadfield for data reduction BUT this
> hampers one key (at least to me) feature of DCM-EEG: the possibility to
> compare models with different source configurations (number/location)!
That's a very good point that I wanted to mention in my reply to Zoi,
but didn't have time for it yesterday. With the way things are
presently implemented to compare models with different sources one
needs to include all those sources in all the models so that the
models will only differ in the connectivity part. If you don't connect
some of the sources to any inputs, these sources will be silent and
will not be able to fit any data, but they will still count for
reduction to modes. So for instance if you want to compare between two
models which differ in location of a particular source, put sources in
both alternative locations and then connect either one or the other to
the rest of the network.
When in doubt about the validity of your model comparison, you can use
the 'Verify data identity' option in the BMS GUI. This option is now
off by default but if you turn it on, it'll only allow valid model
comparisons to be performed.
Best,
Vladimir
> Vladimir Litvak a écrit :
>
> Dear Zoe,
>
> I briefly looked at the code and it seems that what it does now is not
> exactly the same as what was done when those papers were being
> prepared. Now the spatial modes are based on the leadfields of the
> sources included in DCM models rather than on the data. So there is no
> place in the code where the percentage of data variance explained by
> these modes is computed. This could possibly be done with some custom
> code but writing such code would require some expertise.
>
> Best,
>
> Vladimir
>
> On Thu, Nov 19, 2009 at 3:50 PM, Zoi Da <[log in to unmask]> wrote:
>
>
> Dear Vladimir,
>
> My mistake for not explaining.
>
> In various articles of Garrido et al. (2007;2008;2009), they state that
> 'the use of three principal eigenvariates (spatial modes) preserved more
> than 73% of the variance in all subjects.' I was wondering how someone can
> test that? I have used 8 spatial modes, how can i test how much variance
> they explain in al subjects? Where are these values stored?
>
> Hope that it is clearer now!
>
> Best
>
> Zoi
>
> P.S. Thanks for taking the time
>
> ________________________________
> Von: Vladimir Litvak <[log in to unmask]>
> An: Zoi Da <[log in to unmask]>
> CC: [log in to unmask]
> Gesendet: Donnerstag, den 19. November 2009, 15:23:17 Uhr
> Betreff: Re: [SPM] WG: DCM_EEG
>
> Dear Zoi,
>
> It's not quite clear to me what you want to do. Perhaps you could
> describe the aim of your analysis and then it's be easier to help you.
>
> Best,
>
> Vladimir
>
> 2009/11/19 Zoi Da <[log in to unmask]>:
>
>
> ----- Weitergeleitete Mail ----
> Von: Zoi Da <[log in to unmask]>
> An: [log in to unmask]
> Gesendet: Donnerstag, den 19. November 2009, 14:56:12 Uhr
> Betreff: DCM_EEG
>
> Hi everyone
>
> My name is Zoi and I have been analyzing EEG data with DCM. I have
> computed
> the DCMs to eight spatial modes. I would like know how I can compute the
> variance of the data?
>
> Thanks a lot
>
>
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