[Eeglablist] average reference and connectivity

[Makoto Miyakoshi]

Dear Roy,

We are actually studying Dezhong Yao's zero referencing approach. There is
a possibility that we switch to his method completely.

Makoto

On Fri, Jun 26, 2015 at 12:22 PM, Roy Cox <roycox.roycox at gmail.com> wrote:

> hi all,
>
> 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, http://www.sciencedirect.com/science/article/pii/S0167876015001907)
>
> 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.
>
> Best,
>
> Roy
>
>
> On Mon, Jun 22, 2015 at 5:23 PM, Roy Cox <roycox.roycox at gmail.com> wrote:
>
>> 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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>>
>>
>
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-- 
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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