[FieldTrip] negative connectivity strength from mvgc

[Tyler Grummett]

Hey Jorn,

Thanks for the prompt reply. Would it matter that the segments of data going into the multivariate analysis are non-time locked? They are just segments of data from a free running task (so to speak).

I'm worried because the adjacency matrices (viewed using imagesc) don't look right.

Tyler



> On 9 Sep 2015, at 9:30 pm, Jörn M. Horschig <jorn at artinis.com> wrote:
> 
> Hi Tyler,
> 
> as far as I know only the instantaneous causality term might become
> negative, not the granger values x->y or y->x themselves. Reasons for this
> are a bit dubious as far as I understood (within my limits). In [1], Ding et
> al. explained that that the instantaneous causality term can become negative
> for "certain situations" and that it thus might not have a "readily
> interpretable physical meaning". It could have something to do with SNR or
> with real, third region influences. But this is speculative and in fact I
> have no idea. I hope this helps, or at least provides a reference to read up
> on this ;)
> 
> Best,
> Jörn
> 
> 
> [1] Ding, M., Chen, Y., and Bressler, S. L. (2006). Granger Causality: Basic
> Theory and Application to Neuroscience. In Handbook of Time Series Analysis,
> B. Schelter, M. Winterhalder, and J. Timmer, eds. (Wiley-VCH Verlag GmbH &
> Co. KGaA), pp. 437-460. Available at:
> http://onlinelibrary.wiley.com/doi/10.1002/9783527609970.ch17/summary
> [Accessed April 8, 2014].
> 
> 
> --
> 
> Jörn M. Horschig, PhD, Software Engineer
> Artinis Medical Systems  |  +31 481 350 980 
> 
>> -----Original Message-----
>> From: fieldtrip-bounces at science.ru.nl [mailto:fieldtrip-
>> bounces at science.ru.nl] On Behalf Of Tyler Grummett
>> Sent: Wednesday, September 9, 2015 10:52 AM
>> To: FieldTrip discussion list <fieldtrip at science.ru.nl>
>> Subject: [FieldTrip] negative connectivity strength from mvgc
>> 
>> Hello fieldtrip experts,
>> 
>> I decided to try multivariate granger causality today, using the steps in
> the
>> tutorial 'analysis of sensor- and source-level connectivity'. I noticed
> that I am
>> getting negative connectivity measures in the granger.grangerspctrm. Is
> this
>> normal? If so, why does this happen?
>> 
>> I apologise if this has been asked before!
>> 
>> Tyler
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>> fieldtrip at donders.ru.nl
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> 
> 
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