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Hi Roy,<br>
<br>
not claiming to be an expert on matters of averaging, but it might
be useful to think about the connectivity measure you're applying:<br>
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).<br>
<br>
Cheers,<br>
Mathis<br>
<br>
<div class="moz-cite-prefix">On 17.06.2015 19:47, Roy Cox wrote:<br>
</div>
<blockquote
cite="mid:CABafTZfiDXCzxwufEBMTzWfeYg=y8_8ZPcVzA0tzJh7ELNQYKQ@mail.gmail.com"
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<div dir="ltr">Hi all,
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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).</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>I imagine amplitude-envelope correlations could also suffer
from spurious, average reference-induced, oscillations.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>Roy</div>
</div>
<br>
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