[Eeglablist] Re-referencing before or after creating bi-polar HEOG and VEOG channel

[Marco Simões]

Melissa,

Regarding the eyes, if I understand you correctly, you acquired VEEG with 2
electrodes referenced to the left ear, and want to know if you should
re-reference before create the bi-polar channel (high electrode - low
electrode).

The answer is that it doesn't matter, because each electrode is the
difference to the ref, and then you are creating a difference between them
which will discard the refference:

high_electrode_measure = high_electrode - REF;
low_electrode_measure = low_electrode - REF;

VEEG_bipolar = high_electrode_measure - low_electrode_measure <=>
 VEEG_bipolar = (high_electrode - ref) - (low_electrode - ref) <=>
VEEG_bipolar = high_electrode - low_electrode + ref - ref <=> VEEG_bipolar
= high_electrode - low_electrode

Hope it helps!
-- Marco Simões

2015-07-28 19:20 GMT+01:00 Stephen Politzer-Ahles <spa268 at nyu.edu>:

> Hi Melissa,
>
> What was your electrode layout during acquisition? If you already had a
> true bipolar montage (e.g., if you had had a pair of electrodes above and
> below an eye but they only come out to one channel of data in the
> acquisition) then those are already referenced to each other, so I believe
> there is no need to re-reference those to something else (in fact I'm not
> sure what it would mean); in my experience using such a setup, we
> specifically excluded the bipolar EOG montages from re-referencing.
>
> This may also depend on what method you intend to use for blink detection
> and correction.
>
> Best,
> Steve
>
>
>
> Stephen Politzer-Ahles
> New York University, Abu Dhabi
> Neuroscience of Language Lab
> http://www.nyu.edu/projects/politzer-ahles/
>
> On Tue, Jul 28, 2015 at 6:47 AM, melissa scott <melissascott6 at gmail.com>
> wrote:
>
>> Dear Colleagues,
>>
>> My advisor's lab recently started collecting ERP data. We decided to
>> reference the electrodes to the left earlobe during data acquisition and
>> plan to re-reference to the average of the left and right earlobes offline.
>> We think we should also re-reference the electrodes above and below the
>> eyes before we create our bipolar channel (VEOG) that we will subsequently
>> use for blink detection and correction. Does this seem appropriate? We
>> appreciate your thoughts and suggestions.
>>
>> thanks, Melissa
>>
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