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).

In a unipolar setup, there will be one reference electrode, and the potential difference between this electrode (connected to Vin-) and every other electrode on the head (connected to Vin+ on separate amplifiers) will be measured.

For bipolar recordings, each active (Vin+) electrode will have its own reference (Vin-) connection.

BIOPAC Systems use a common ground connection for all amplifiers that are electrically connected to each other (i.e., part of the same MP system and not passing through a wireless stage as the BioNomadix modules do). In these cases, there should not be more than one ground connection to the subject unless all but one of the ground connections are through AC coupled leads. For instance, recordings made through a single MP36 system should have only one ground connection to the subject. Similarly, recordings made through EEG100C or EEG100C-MRI amplifiers all connected to a single MP150 system should have only one ground connection (total!) to the subject. BioNomadix wireless physiology data systems necessarily have a separate ground for each transmitter (note that each transmitter has two amplifiers on it). In this case, each transmitter should have one ground connection to the subject even if all transmitters communicate with the same MP150 system.