EEG Signal (re)Reference

EEG signal is measured as a potential difference between two electrodes placed on a person’s scalp. Typically there are more than two electrodes used in EEG signal measurements and they can be combined in different ways in EEG acquisition systems. There are two major montages typically used in EEG recordings: bipolar and referential. 

Bipolar montage. In a bipolar setup, the potential difference is typically measured between two adjacent electrodes. The whole acquisition system is formed by having many electrode pairs measuring EEG activity over the cortex region of interest. For example, one EEG signal can be acquired from the F3-F7 electrode pair, while another one from the F4-F8 electrode pair.
Referential montage. In a referential setup, there is a single reference electrode and the potential difference is measured between any electrode placed on the scalp and this reference electrode. This setup results in as many EEG recordings as in bipolar montage with almost half the number of electrodes. It also allows acquiring a more global picture of EEG activity than in a bipolar setup, however, it comes at the expense of potentially having more noise as the distances between the reference and other electrodes are typically larger.

Reference electrode positions. There is no standard for the position of reference electrode placement, but the most common options include: left mastoid, left ear (with clip electrode), both mastoids or ears. However, ear and mastoid locations are quite susceptible to EEG artifacts from muscle movements that may be undesirable in some cases. In BrainAccess Kits, for example, Fp1 is typically used as a reference electrode. It is important to ensure that the reference electrode has good skin contact as it will affect the measurements for all the electrodes.

As there is no standard position for the reference electrode, comparing EEG signals obtained by different measurement setups is not a straightforward task. Therefore, post-processing techniques are used to transform EEG signals obtained by different setups so that they are reference-independent. This then allows researchers to compare their results regardless of the referential setup involved. The most notable ones are average reference and REST (Reference Electrode Standardization Technique).

Average reference. As the name suggests in this re-referencing technique an average signal is calculated from all the recorded EEG signals at different locations and this is then used as a reference signal, i.e. this average signal is then subtracted from each EEG signal recorded at different positions. 

REST reference. REST technique transforms each EEG signal recorded at different locations as if it were referenced with a point located at infinity. Hence, it acts as an ideal neutral reference location. It was introduced in 2001 and is becoming more widely adopted.

Both re-referencing methods would typically require a relatively large number of electrodes distributed over the whole cortex region for them to work effectively. Lastly, it is worth noting that while not explicitly stated here, all the setups also require a ground electrode positioned somewhere on the scalp for the valid voltages to be observed on reference and other electrodes.