[Mne_analysis] sLORETA

Elena Orekhova Elena.Orekhova at neuro.gu.se
Thu Sep 1 11:32:32 EDT 2011
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Dear Hari,

The scaling you suggest will make the data look ‘nice’, with the same mean and SD in the baseline. However, if will not resolve the problem.

According to MNE manual sLORETA scales signal to noise in some way. It seems that it proportionally decreases amplitude of the signall-of-interest if it contains more noise.  Is there a way to correct for this down-scaling of the signal-of-interest by the sLORETA or would it be more correct to use MNE?

Thanks, 
Elena



________________________________________
From: Hari Bharadwaj [hari at nmr.mgh.harvard.edu]
Sent: Thursday, September 01, 2011 3:02 PM
To: Elena Orekhova
Cc: mne_analysis at nmr.mgh.harvard.edu
Subject: Re: [Mne_analysis] sLORETA

Hi Elena,
   Just my 2 cents (below) before Matti gives his expert opinion..
On Thu, September 1, 2011 4:07 am, Elena Orekhova wrote:
> Dear Matti,
>
> I would like to compare amplitudes of activation in response to auditory
> stimuli in the right and left temporal labels.  I used sLORETA and noticed
> significant differences in the timecourses during pre-stimulus baseline.
> As the sLORETA uses noise normalization, I guess that this effect is due
> to different level of noise in the two labels.  Would it be correct to
> scale the sLORETA timecourses by e.g. dividing them by the mean
> pre-stimulus value before statistical comparison of the left and right
> labels?
>

Dividing by the mean is slightly less principled since mean is not a
'scale' parameter but rather a 'location' parameter. The scaling used in
EEGLAB (especially for time-frequency maps) in this situation is a z-score
derived as follows:

If x(t) is the original timecourse in a label:

z(t) = (x(t) - mean(x(baseline)))/std(x(baseline)) ;

This would give use the same time series whether you use MNE or dSPM or
sLORETA since they are just scaled versions of each other with the scale
factor being constant in time.


> I have also a related question.  The MNE software uses the same inverse
> operator for MNE and sLORETA.  Is it at all identical to the sLORETA by
> Pascual-Marqui (Standardized low-resolution brain electromagnetic
> tomography (sLORETA): Technical details. METHODS AND FINDINGS IN
> EXPERIMENTAL AND CLINICAL PHARMACOLOGY, 2002, 24: 5-12) ?
>


It is identical to Pascual-Marqui's solution. 'sLORETA' is a misnomer in
the sense that it is not a noise-normalized version of LORETA but rather a
normalization of MNE using a slightly different denominator (related to
the resolution matrix).

Rey Ramirez's article on scholarpedia is a nice condensation of many of
the source localization methods in use:
http://www.scholarpedia.org/article/Source_localization


Regards,
Hari

> With regards,
> Elena
>
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>
>


--
Hari Bharadwaj


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