An average reference that is often used in EEG is based on Laplacian mathematics; it is variously called the Laplacian reference, Hjorth reference, and local average. Other computerized references (meaning no individual reference electrode is required)
include the common average or global average and the weighted average reference (Scherg, et al., 2002). A Laplacian montage is useful for understanding what is happening on a focal level (i.e. Laplacian montages are good at seeing focal findings). An
average reference is a good way to understand what is happening without referencing the ear electrodes, which are often contaminated with real (EEG) or fake (artifactual) findings.<br><br>
<p style="text-align: center;"><img src="@@PLUGINFILE@@/ear-reference-contaminated.png" alt="An EEG with an Ear Reference contaminated by a heartbeat (which is thus seen in every channel)" width="800" height="558" class="img-responsive atto_image_button_text-bottom" style="font-size: 1rem;"></p>
<p style="text-align: center;"><span style="font-size: 1rem;">An EEG with an Ear Reference contaminated by a heartbeat (which is thus seen in every channel)</span></p>
<p style="text-align: center;"><span style="font-size: 1rem;"><br></span></p>
<p>The Laplacian technique (aka “local average” or “Hjorth” montage) is quite good at showing localized findings. The Laplacian montages are good at eliminating cardioballistic artifacts (as seen in the previous image). However, Laplacian montages are thought
    to have an “unforgiving” nature of display, due to minor distortions at the edge electrodes, commonly a small percentage error (not a real problem for clinical utility).&nbsp;</p>
<p>The Laplacian montage is not good at displaying global or regional EEG findings, due to false localization to the perimeter or edge of the finding (Scherg et al., 2002).&nbsp;</p>
<p>An “average” montage uses the average of all electrodes as the reference. This technique is not good for displaying generalized changes, especially generalized paroxysmal activity. However, this “weighted average” reference is very popular when using
    dense arrays in topographic mapping.</p>
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<p>No montage is the “right one” or the “best” montage, they each have their own strengths and weaknesses. It is important to switch references from a linked ears (or other referential montage) to an average or a Laplacian or Hjorth, to understand the differences
    between montages. One example where linked ears may be deceiving is when strong temporal alpha contaminates the ear lobe. In this case, the ear references will create the appearance that alpha is global, when it is in fact located in the temporal
    and picked up (and thus transmitted) by the reference electrode site—the ear. Thus, the ear references when contaminated create “false alpha.”&nbsp;</p>
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<p style="text-align: center;"><img src="@@PLUGINFILE@@/identify-focal-findings.png" alt="Switching from Linked Ears to an Average Reference to identify focal findings" width="920" height="517" class="img-responsive atto_image_button_text-bottom"><br></p>
<p style="text-align: center;">Switching from Linked Ears to an Average Reference to identify focal findings</p><p style="text-align: center;"><br></p>
<p style="text-align: center;"><iframe src="https://player.vimeo.com/video/408145826" width="640" height="399" frameborder="0" allow="autoplay; fullscreen" allowfullscreen=""></iframe><span style="font-size: 1rem;">Appearance of Diffuse Alpha in Lined Ears Reference, due to a contaminated reference electrode, properly localized to T3 when montage is shifted</span></p><p style="text-align: center;"><span style="font-size: 1rem;"><br></span></p><p style="text-align: left;"><span style="">The failure to shift montages creates problematic coherences, which are impossible for the human brain to create. When we failed to shift montages, in cases of shared variance, we will often end up with global hypercoherence, which is artifactual and should never be considered real.&nbsp;<br></span></p><p style="text-align: left;"><span style=""><br></span></p><p style="text-align: center;"><span style=""><img src="@@PLUGINFILE@@/qeeg-hypercoherence.png" alt="QEEG Map with False Hypercoherence, due to contaminated reference" width="717" height="953" class="img-responsive atto_image_button_text-bottom"><br></span></p><p style="text-align: center;"><span style="">QEEG Map with False Hypercoherence, due to contaminated reference<br></span></p><p style="text-align: center;"><span style=""><br></span></p><p>Using a variety of montages is a part of the minimum guidelines for EEG and a commonly adopted guideline developed by the American Society of Electroneurodiagnostic Technologists (ASET). This practice of switching perspectives avoids the reader being fooled by a false localization when a reference is contaminated with voltages as described above. The proper selection of montages will allow the reader to get a good view of the EEG phenomenon and its distribution across the cortex. Understanding of the topographic distribution of brain function is required to understand what the neuropsychological impacts of the EEG changes might be. If you switch to global and you see that it is very different, you must determine which EEG is accurate and which is not.&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;</p><p><span></span></p><p>The montage is like a perspective, it is used to present information from a particular point of view. Each montage has strengths as well as weaknesses, including the problem of failed neutrality of the reference. A single ear reference will avoid any problem associated with the other ear’s contamination though it does not cancel the EKG as well as the linked ear reference, as well as creating an apparent asymmetry due to the systematically different inter-electrode distances between the two hemispheres. The Cz reference will give good resolution for the temporal areas but not the central area. The Cz montage is also a poor choice where there is drowsiness, with the associated vertex sharp waves and spindles which are seen maximally along the midline anteriorly and at the vertex which will contaminate the vertex electrode.</p>