[Eeglablist] average reference and connectivity

[Roy Cox]

Thanks all:

Dezhong: definitely a relevant paper, thanks!

Pal: I like the CSD approach and I've used it in the past. I never
considered that differences in impedance between left/right mastoids leads
to a virtual reference that "floats" in between somewhere. Very good to
know.

Mathis: I'm aware of methods that get rid of zero-phase lag connectivity,
and I've used PLI in the past. But as I see it, average reference would
result in opposite phases for distant areas (frontal peaks matched by
posterior troughs). I'm not sure how these methods deal with anti-phase
connectivity. Plus I would imagine phase differences are never going to be
exactly pi so you'd still pick up connectivity there..

Best,

Roy



On Fri, Jun 19, 2015 at 5:20 AM, Mathis Kaiser <mathis.kaiser at charite.de>
wrote:

>  Hi Roy,
>
> not claiming to be an expert on matters of averaging, but it might be
> useful to think about the connectivity measure you're applying:
> while spurious connectivity is an issue when using the PLV, it should not
> be present for measures that are based on the imaginary part of coherency
> (and therefore don't take into account zero-phase lag synchronization, see
> Nolte et al. 2004). Proposed improvements on the ImC include the PLI (Stam
> et al. 2007) and wPLI (Vinck et al. 2011).
>
> Cheers,
> Mathis
>
>
> On 17.06.2015 19:47, Roy Cox wrote:
>
> Hi all,
>
>  I would like to get some expert opinions on the use of the average
> reference when investigating phase-based connectivity (e.g., PLV, PLI,
> etc), and a potential problem when using this approach.
>
>  While no referencing scheme is optimal, it is often argued that the
> average reference offers "the best" solution given a sufficient amount of
> electrodes (we use 60). The reference can be interpreted as the "height"
> from which the topographical landscape of voltage amplitudes is viewed.
> While any perspective is valid, average referencing places the viewpoint at
> the average of all electrodes, which is declared zero. Importantly, this is
> done on a sample-by-sample basis, meaning that the average is always zero.
>
>  Enter widespread synchronous oscillations. I've often noticed that when
> strong in-phase alpha activity is present over posterior cortex, the
> average reference results in equally strong and anti-phase alpha
> oscillations over anterior regions (with a small region in between where
> alpha activity is relatively absent). Similarly, during deep sleep there
> are very strong frontal slow oscillations that are inverted in polarity
> over posterior regions.
>
>  Now, any phase-based metric will return beautiful long-range
> (anti-phase) connectivity, which is entirely (or at least largely) an
> artefact of the referencing.
>
>  When using average mastoids as a reference (also not perfect - I know),
> it is evident that there is no phase reversal from anterior to posterior
> areas: alpha activity is (visually) absent from frontal regions, and sleep
> oscillations are synchronous (in-phase) across most of cortex (but, by
> necessity, relatively small close to the sites used for referencing).
>
>  In sum, while I'm sure the average reference is valid on a
> sample-by-sample basis, problems seem to arise when time enters the
> equation and widespread large negativities have to be matched by widespread
> positivities to keep the average zero.
>
>  I imagine amplitude-envelope correlations could also suffer from
> spurious, average reference-induced, oscillations.
>
>  This is all terribly hand-wavy and non-mathematical, so it would be
> great if someone could comment on this to support or disprove my reasoning.
>
>  Roy
>
>
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