Ground vs. reference for EEG recording

The terminology for electrical connections to human subjects is not universally agreed upon. “Ground” and “reference” are often used interchangeably. This is not a good practice as there are different purposes for different connections.

Ground and reference are better understood as two different types from the perspective of both physiology and amplifier design. BIOPAC uses a relatively pervasive terminology here.

• The ground is used for common mode rejection. The primary purpose of the ground is to prevent power line noise from interfering with the small biopotential signals of interest. By design, amplifiers should not be affected by large changes in potential at both the active and reference sites. A ground electrode for EEG recordings is often placed on the forehead (but could be placed anywhere else on the body; the location of the ground on the subject is generally irrelevant).
• The reference lead is the lead that connects the reference electrode; in EEG recordings, this electrode is usually placed at the ear or, in the case of “summed ears,” to a pair of electrodes, one at each ear. The measured electrical potential differences are ideally the voltage drops from the active electrode (connected to Vin+ on the amplifier) to the reference electrode (connected to Vin- on the amplifier).

Good, Better, Best:
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The best ways to set up participants to record good data for life science research and teaching labs.
– Which electrode?
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– Do I need Ground (GND)?
– Best placement?
– Best recording system?

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