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The reference influences the amplitude at each EEG channel and time point: it defines the level of zero voltage and all other channels are expressed in relation to it. How does the choice of the reference affect the data? This can be useful if the online reference channel (for example Cz or FCz) is of relevance for your data analysis.Ģ.2. This way, the signal of the online reference, which has been implicitly contained in all other channels, can be regained with respect to a new reference. The same process can in fact also be applied to the online reference, which – after the subtraction of the new reference – is simply the equivalent of the negated new reference ( – = online reference – new reference). In the same way, the new reference is subtracted from itself during re-referencing and thus reflects the new level of zero voltage. Afterwards, the voltage at these channels reflects the difference to the new reference and no longer to the online reference ( – = channel – new reference ). During re-referencing, the signal of the new reference is subtracted from each EEG channel.

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However, note that the choice of the online reference is not too decisive: you still have the option to choose a different reference during offline referencing (see 2.1.).īefore re-referencing, the new reference itself reflects the difference in electrical potential to the online reference (new reference – online reference).

The online reference is hence not available as a channel with meaningful information. Recording the reference against itself (online reference – online reference) would mean amplifying and recording zero Volt differences. You will find that the reference itself is not displayed as a channel in your data. Otherwise neither the contributions nor the signal quality (impedances) of the different channels forming the online reference can be disentangled and controlled. In general, it is recommended to use a single channel as online reference during recording (instead of e.g. In other cases, dedicated electrodes can be attached to the nose, earlobes or mastoids (the bones behind the ears) to serve as online reference. Many EEG caps have a pre-defined reference channel, for example at the position of FCz or Cz. Therefore, when looking at your EEG data, keep in mind that the amplitude at any channel constitutes the difference in electrical potential to the chosen reference.ġ.2. Differential amplifiers amplify this (less noisy) voltage difference between EEG channels and reference. Relating the EEG channels to the reference cancels out the noise stemming from the ground circuit ( – = channel – online reference). This means the signal at the other EEG channels is expressed as the difference in electrical potential to this reference instead of to the ground. To eliminate this ground-related noise, an online reference is chosen out of the recorded EEG channels, which is used as “baseline” for all remaining EEG channels. Consequently, the voltage difference between ground and EEG scalp electrodes is also affected by this noise. As the name indicates, this electrode is connected to the ground circuit of the amplifier, and it thereby picks up electrical noise that does not reach the other scalp electrodes.

Primarily, this other recording site is the ground electrode.

This means the signal displayed at any channel is in fact the difference in electrical potential to some other recording site. The relationship between ground, reference and scalp channelsĮEG recordings measure differences in electrical potentials between two points (Voltages) and are usually expressed in units of microvolts (µV).

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