The reference electrode used in recording EEG data is
usually termed the 'common' reference for the data -- if all the
channels use this same reference.
Typical recording references in EEG recording are one mastoid (for
example, TP10 in the 10-20 System, the electrode colored red in the picture below),
linked mastoids (usually, digitally-linked mastoids, computed post hoc,
the vertex electrode (Cz), single or linked earlobes, or the nose tip.
Systems with active electrodes (e.g. Biosemi Active Two), may record data
"reference-free." In this case, a reference be must be chosen
post hoc during data import. Failing to do so will leave
40 dB of unncessary noise in the data!
There is no 'best' common reference site.
Some researchers claim that non-scalp
references (earlobes, nose) introduce more noise than a scalp channel reference
though this has not been proven to our knowledge. If the data have been recorded
with a given reference, they can usually be re-referenced (inside or outside EEGLAB)
to any other reference channel or channel combination.
Converting data, before analysis,
from fixed or common reference (for example, from a common earlobe or other channel reference)
to 'average reference' is advocated by some researchers,
particularly when the electrode montage covers nearly the whole head
(as for some high-density recording systems).
The advantage of average reference rests on the fact that
outward positive and negative currents,
summed across an entire (electrically isolated) sphere,
will sum to 0 (by Ohm's law).
For example, in the figure below
a tangentially-oriented electrical source is associated with a positive
inward current to the left (here, blue) and an opposing outward negative current
to the right (red).
If the current passing through the base of the skull to the neck and body is
assumed to be negligible (for instance, because of low conductance of the skull at
the base of the brain), one may assume that the sum of the electric field values recorded at all
(sufficiently dense and evenly distributed) scalp electrodes is always 0
(the average reference assumption).
The problem with this assumption is that "true" average reference data
would require the distribution of electrodes to be even over the head.
This is not usually the case, as researchers typically place more electrodes
over certain scalp areas, and fewer (if any) on the lower half of the head surface.
As a consequence, an average reference result using one montage
may not be directly comparable to an average reference result obtained using
another montage.
Below, we detail the process of transforming data to 'average reference'.
Note that in this process, the implied activity time course at the former reference electrode
may be calculated from the rest of the data (so the data acquires an additional
channel - though not an additional degree of freedom!).
Also note that if the data were recorded using nose tip or ear lobe
electrodes, you should not include these reference electrodes
when computing the average reference in (1) (below),
Thus, in the example below the dividing factor (in (3)) would be 64 instead of 65.
Note that in localizing sources using the EEGLAB DIPFIT plug-in,
'average reference' will be used internally (without user input).
The choice of data reference does color (literally) the plotted results of the data analysis.
For example, plots of mean alpha power over the scalp must have a
minimum at the reference channel, even if there are in fact
alpha sources just below and oriented toward the reference channel!
However, no (valid) reference can said to be wrong - rather, each reference
can be said to give another view of the data. Howeever, the nature of the
reference needs to be taken into account when evaluating (or, particularly, comparing) EEG results.
For ICA decomposition (covered later in the tutorial),
the selection of reference is not so important. This is because changing
the reference only amounts to making a linear transformation of the data
(in mathematical terms, multiplying it by a fixed re-referencing matrix),
a transformation to which ICA should be insensitive.
In practice, we have obtained results of similar quality from data recorded
and analyzed with mastoid, vertex, or nose tip reference.
We advise recording eye channels (conventionally four channels, two for
vertical eye movement detection and two for horizontal eye movement detection)
using the same reference as other channels, instead of using bipolar montages.
One can always recover the bipolar montage activity by subtracting the activities
of the electrode pairs. We term these channels 'peri-ocular EEG' channels
since what they record is not exclusively electrooculographic (EOG) signals,
but also includes e.g. prefrontal EEG activities.
ICA can be used to decompose data from either average reference, common reference,
or bipolar reference channels -- or from more than one of these types at once.
However, plotting single scalp maps requires that all channels use either
the same common reference or the same average reference.
Robert Oostenveld advises that peri-ocular channel values, even
in ICA component maps, may best be omitted from inverse source modeling using
simple head models, since these are apt to poorly model the conductance geometry
at the front of the head.
We will now describe how to specify the reference electrode(s) in EEGLAB
and to (optionally) re-reference the data
Exploratory Step: Re-reference the Data.
Select 
