freqdescriptives

Robert Oostenveld roberto at SMI.AUC.DK
Thu Nov 4 09:21:31 CET 2004


Hi Masao,

On 4 Nov 2004, at 4:06, Matsuhashi, Masao (NIH/NINDS) wrote:

> Additional question. What is the unit/dimension of the source activity
> calculated by
> sourceanalysis(cfg.method='coh_refchan')->sourceinterpolate-
> >sliceinterp?
> It may be [0,1] as in Gross(2001) if it is coherence, but the colorbar
> showed something like 0 to 20 and 0 to 140. It seems like absolute or
> power
> of cross-spectra....

The unit depends on what estimated source parameter you are looking at:

source.avg.pow is dipole power, which is expressible in (nAm)^2. But
since Fieldtrip is unaware of the physical units of all the
"dataobjects" (volume conductor, data, sensor geometry) that go into
the computation, the result should be interpreted as arbitrary units.

source.avg.coh is the coherence, which is equal to the squared
coherency. Both coherence and coherency are between 0 and 1.

source.avg.noise is the estimated projected noise, where the noise is
assumed to be uncorrelated and of equal magnitude between channels and
where the amplitude of the noise is scaled with the smallest singular
value of the cross-spectral density matrix. The projected noise has the
same units as the power, i.e. arbitrary units in principle.

After sourcedecriptives, source.avg.nai equals the neural activity
index, which is defined as pow/noise. It is dimensionless and always
 >=0.

best regards,
Robert


----------------------------------------------------------------------
Robert Oostenveld, PhD
Center for Sensory-Motor Interaction (SMI)
Aalborg University, Denmark

and

F.C. Donders Centre for Cognitive Neuroimaging
University Nijmegen
P.O. Box 9101
NL-6500 AH Nijmegen
The Netherlands

Tel: +31 (0)24 3619695
Fax: +31 (0)24 3610989
----------------------------------------------------------------------
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