The placement of a reference electrode is one of the most critical aspects of electroencephalography (EEG) because it helps ensure the accuracy and reliability of recorded brain electrical activity. Typically, EEG references are placed on the mastoids, ears, or nasion, as opposed to being placed on an inert area of the scalp. Below is a step-by-step explanation of why these specific locations are chosen, supported by medical literature and research:

1. Importance of Reference Electrode in EEG

In EEG, the reference electrode serves as a common ground to which the electrical activity measured at various sites on the scalp can be compared. Without a proper reference, it would be difficult to interpret the brain's electrical activity accurately. The reference electrode is critical because it defines the baseline (or "zero") against which the differential voltages between the active electrode and the reference are measured.

2. Why Not Use an Inert Site?

Using an inert site for the reference (i.e., a location where no electrical activity is expected) may seem ideal at first, but in practice, it presents several challenges:

  • Noise and Interference: Inert areas often exhibit electrical noise due to environmental factors, including muscle activity, eye movements, and electrical equipment interference. These noise sources could compromise the signal quality and make it difficult to interpret the brain's activity accurately.
  • Electrode Impedance: The electrical impedance of an inert site may vary, leading to poor signal quality. Differences in skin properties or other physiological factors can introduce inconsistencies that may affect the measurements.
  • Lack of a Clear Reference Point: An inert site might not have a consistent and predictable electrical potential, leading to unreliable and varying reference points across different recordings and sessions.

3. Mastoid and Ear Placement

The mastoids (located just behind the ears) and the ears themselves are commonly used as reference sites because:

  • Minimal Muscle Activity: These areas are distant from the main areas of brain electrical activity and are less likely to be influenced by muscle movements (such as those from facial muscles) or other peripheral electrical noise.
  • Stable Electrical Activity: The mastoids and ears provide stable, predictable electrical potentials that can be reliably used for reference. Their proximity to the scalp and their relatively inert electrical characteristics make them ideal for creating a baseline for EEG recordings.
  • Practical and Easy Access: These sites are anatomically easy to access and are relatively simple to attach electrodes to, ensuring ease of use in both clinical and research settings.

4. Nasion Placement

Another common reference site is the nasion, the region between the eyes at the bridge of the nose. While less frequently used than mastoid or ear placements, it may be selected under specific circumstances, such as when the mastoid or ear electrodes are not feasible. Reasons for using the nasion include:

  • Good Anatomical Location: The nasion provides a central point on the head, offering a balanced and consistent reference when other locations are unsuitable.
  • Minimizing Noise: The nasion is located in an area where muscle activity from the face and jaw is less likely to interfere, similar to the mastoid regions.

5. Considerations and Challenges

Although these reference sites (mastoid, ear, and nasion) are commonly used, there are some considerations to keep in mind:

  • Reference Effects on the Data: The choice of reference site can affect the EEG signal itself, particularly in terms of spatial distribution. This is important in studies where specific patterns of brain activity are being examined.
  • Common-Mode Rejection: Modern EEG systems often employ "common-mode rejection" techniques, which help eliminate any common noise that might affect both the reference and active electrodes, improving signal fidelity.

6. Conclusion

The placement of the EEG reference on mastoids, ears, or nasion is done to ensure that the recorded signals represent brain activity as accurately as possible while minimizing interference from other sources of electrical activity. These sites are chosen because they are stable, easy to access, and provide reliable baselines for EEG measurements. Additionally, they help reduce noise, muscle activity, and other confounding factors, ensuring that the EEG recordings are as clear and interpretable as possible.