[Eeglablist] Bipolar HEOG and VEOG channel locations

[Dezhong Yao]

Dear Nike,
       In general, if you want to check the EOG independently, you may use bipolar HEOG or VEOG to ge the message.
Otherwise, you should use monoploar montage sharing the same reference with other EEG electrodes. For this situation, EOG channels just like  EEG channels ( If there is no EOG at a moment, it contains just EEG, otherwise, it contains both EOG and EEG simultaneously), then you can check the spatial distribution over the scalp (including all eeg chennels and EOG channels), and the EOG artifacts would be clearly revealed. 
      In EEG recordings, we only may get the difference potential between an active electrode and a reference electrode, the best  reference should be zero or constant potential, then we may get the true potental variation at the active electrode. Otherwise, what we get is the difference between the active and the reference. As there is no point on the scalp surface where the potential is zero  or constant, the difference is certainly not the true potential at the active electrode position. 
    When a bipolar recording is adopted, we  only get the difference between the two positions, it does not provide the true activities at any one position of the two electrodes.  
     When EOG is recorded by bipolar, and EEG is recorded by monopolar, it means that they are recorded with different references, then they can not be shown together physically (spatial distribution), and difficult to interpretate together.
    In summary, for actual recordings, the usual choice is monpolar recordings,  as
    1) if one does need to check the bipolar potential, it is very easy to transform off-line the monopolar data to bipolar one-- a  direct subtraction between the two monopolar data.
    2) the relative spatial relations of all channels' potential are maintained, thus for any a moment, a spatial tomporaphic map is meaningful to see the relative activities.
    3) The most importantly, after a monoplar data with more than 20 channels is obtained, it is easy to transform it to the approximate zero reference, the desired reference for the field of EEG, by a recent developed technique -- REST (reference electrode standardization technique),  the free  software can be found at www.neuro.uestc.edu.cn/rest.  The merits of REST have been known by more and more recent applications in facing the various reference problems.
   Best wishes
 
On Sun, Nov 16, 2014 at 11:12 PM, Nike gnanateja <nikegnanateja at gmail.com> wrote:
Dear list,
We are using a Neuroscan 64 channel Synamps2 equipment at our lab. We use bipolar channels for the EOG recordings. When I import the recordings into eeglab and try to load the electrode locations from the existing sample locations, it assigns a single location to the bipolar channel. But actually, they are from two differential electrodes.
Here are myquestions
1. how do I add a separate reference channel to the EOG  Bipolar channel?
2. Does addition of a separate reference channel to the EOG electrodes actually matter? What I have visually observed is that the scalp topography of the eye movements are falsely lateralized, and the actual dipolar nature of the horizontal movements not clearly visualized in the EOG channels 
3. Is using monopolar channels for EOG better than using bipolar channels.

regards
Nike

--
G Nike Gnanateja, MSc (Audiology)
Junior Research Fellow,
Department of Audiology,
All India Institute of Speech
and Hearing Mysore-06
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--
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego----------------Dezhong Yao  PhD
Cheung Kong Professor of Neuroengineering, Neuroimaging
E-mail: dyao at uestc.edu.cn
Fax: 86-28-83208238; Tel: 86-28-83201018
Director, Key Laboratory for NeuroInformation,  Ministry of Education, China
              International Joint Research Center for NeuroInformation, Ministry of Science and Technology, China 
Dean,      School of Life Science and Technology, University of Electronic Science and Technology of China, Sichuan,Chengdu 610054, China
Director,  Center for Information in BioMedicine, University of Electronic Science and Technology of China, China
http://www.neuro.uestc.edu.cn/bci/member/yao/yao.html
尧德中 电子科技大学 生命科学与技术学院/信息医学研究中心/神经信息教育部重点实验室/神经信息国际联合研究中心
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