[Eeglablist] average reference and connectivity

[Mathis Kaiser]

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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