<div dir="ltr">hi all,<div><br></div><div>I've been recently made aware of this simulation study where different dipoles with different phase relations are modeled and power and phase estimates are then obtained using different montages. ( Tenke and Kayser, 2015, <a href="http://www.sciencedirect.com/science/article/pii/S0167876015001907">http://www.sciencedirect.com/science/article/pii/S0167876015001907</a>)</div><div><br></div><div>it appears that CSD is the overall winner, and the average reference gives some spurious results quite in line with my worries as I first expressed them.</div><div><br></div><div>Best,</div><div><br></div><div>Roy</div><div><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Jun 22, 2015 at 5:23 PM, Roy Cox <span dir="ltr"><<a href="mailto:roycox.roycox@gmail.com" target="_blank">roycox.roycox@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Thanks all:<div><br></div><div>Dezhong: definitely a relevant paper, thanks!</div><div><br></div><div>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.</div><div><br></div><div>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..</div><div><br></div><div>Best,</div><div><br></div><div>Roy</div><div><br></div><div><br></div></div><div class="HOEnZb"><div class="h5"><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Jun 19, 2015 at 5:20 AM, Mathis Kaiser <span dir="ltr"><<a href="mailto:mathis.kaiser@charite.de" target="_blank">mathis.kaiser@charite.de</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
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<div><div><br>
<br>
<div>On 17.06.2015 19:47, Roy Cox wrote:<br>
</div>
</div></div><blockquote type="cite"><div><div>
<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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