Re-referencing and EOG channels preparation

If you record EEG with easycap64, you probably use electrode 64 to register vertical eye movements. In this case you attach it below the left eye. (Pic.1) In the raw data the signal from this electrode, as well as from others, is monopolar and referenced to the reference channel. To detect vertical eye movements you should re-reference it to the electrode that placed directly above the eye. Use number 50. The code is following:

%Eogv channel
cfg = [];
cfg.channel = {'50' '64'};
cfg.reref = 'yes';
cfg.implicitref = [];
cfg.refchannel = {'50'};
eogv = ft_preprocessing(cfg, data);
eogv.label = {'dummy2', 'eogv'};
cfg = [];
cfg.channel = 'eogv';
eogv = ft_preprocessing(cfg, eogv); % only keep one channel, discard the  dummy

To detect horizontal eyes movements, again, you should compute bipolar signal between electrodes that placed on temples closely to eyes. Use number 51 and 61 or 60 (depends on how you place electrodes). The code is following:

%Eogh channel
cfg = [];
cfg.channel = {'51' '61'};
cfg.reref = 'yes';
cfg.implicitref = [];
cfg.refchannel = {'51'};
eogh = ft_preprocessing(cfg, data);
eogh.label = {'dummy1', 'eogh'};
cfg = [];
cfg.channel = 'eogh';
eogh = ft_preprocessing(cfg, eogh); % only keep one channel, discard the  dummy

Finally, append eogv and eogh channels to your data:

% Finally, add eogv and eogh channels to your data 
cfg = [];
data_with_eog = ft_appenddata(cfg, data, eogh, eogv);

Pic.1

If you record EEG with easycap64, you probably use the following electrodes arrangement (Pic.2). The REF electrode is attached to the left mastoid. If you want the signal to be referenced to linked mastoids, you may place electrode 53 on the right mastoid. In this case you need to re-reference data. First, add implicit channel 'REF' to your channels. Then assign 2 reference channels:

cfg.reref = 'yes';
cfg.implicitref = 'REF';
cfg.refchannel = {'53' 'REF'};

Pic.2