<p></p><p>Montages are logical and orderly
arrangements of channels (electrode pairs, with waveforms representing the
potential difference between the two electrodes).</p><p>One never records from a single
electrode, but the difference of voltage between&nbsp; (at least) two electrodes (the active and the
reference electrode). Each EEG channel records variations in double ended
voltage differences.&nbsp;<b>Common Mode Rejection</b>:&nbsp;The ability of the amplifier to suppress voltages&nbsp;<b>common</b>&nbsp;to both electrodes, i.e. varying together over time, is called the&nbsp;<b>common mode rejection</b>. An example is the noise from a 50 or 60 Hz line current.</p>There are two classifications of montages: monopolar (aka referential) and bipolar. <u>(</u>The term monopolar is a misnomer, since all recording must come from the difference of two channels). The use of two channels is what makes a "differential amplifier" differential. It is measuring the difference between two electrodes, and by way of that removing all common noise or signals between the two to highlight their differences.<p></p><p><img src="@@PLUGINFILE@@/Montages.jpg" alt="Montages" width="600" height="344" class="img-responsive atto_image_button_middle"></p><p><b>Average montages</b> (such as laplacian, average, hjorth, current source density, and weighted average). These are good for identifying focal issues.</p><p><b>Bi-polar montages</b> are from one scalp site to another scalp site. They can be widely spaced or closely spaced. More widely spaced electrodes should give greater amplitude because there is less data in common and thus, less common mode rejection. These are also good for focal findings.</p><p><b>Referential montages</b> use sites like singular (right or left) ear, ipsilateral ear, contralateral ears linked ears, Cz (the exception to the rule, as it is a scalp reference), mastoids, or the nose as a point of reference. With linked ears, for instance, the activity at the T3 and T4 electrodes will be of lowest voltage, because of the greatest common mode rejection with the reference site. These are good for global findings.</p><p>Every montage has its benefits and its shortcomings.&nbsp; In a general sense, referential montages will have the least amount of cancellation of activity and will then show a purer signal, but bi-polar montages can localize more subtle deficits.</p><p>A Cz montage can dull and diminish activity at the sites surrounding that central electrode (in as such, C3, C4, F3, Fz, F4, P3, P4, Pz will all have lower amplitudes, however the activity at the temporal lobes will be magnified, since the ears are removed from the common mode rejection equation).</p><h3>Shared Variance in Linked Ears Montages</h3><p>One of the oft-overlooked and most insidious artifacts found in EEG comes from "contaminated ear references" in any referential montage using the ears. This phenomenon is best explained with an anology. Imagine that there is very high amplitude, slow activity, stemming from the temporal lobe. If the activity is of sufficient size, the ear lobe will also reflect the same EEG finding (i.e. it will cease to be a "neutral" reference). Then, two things will happen: 1) the actual focus of the slow wave at T3 will be <u>removed</u>&nbsp;from the T# electrode via common mode rejection, and the slow wave will be distributed in all of the other channels not proximal to the actual source of the slow wave.&nbsp;</p><p>The image on the left is a screen shot of the artifact of shared variance. The image on the left is the shared activity between the left ear and the T3 electrode.</p><p><img src="@@PLUGINFILE@@/Shared%20Variance.png" alt="Shared Variance" width="1000" height="565" class="img-responsive atto_image_button_middle"><br></p><p><span style="font-size: 1rem;">The image below is an easier one to grok, in term of shared variance. In the below image, there is a cardiac artifact that is obvious in the EEG record. We know it to be cardiac, for sure, by its rhythmicity (once per second, just like a heartbeat). It is appearing in every channel because the origin of the heartbeat is the ear lobe, itself. Thus, this is different from every channel on the scalp and showing up in every channel on the scalp, due to common mode rejection.&nbsp;</span><br></p><p><br></p><p><img src="@@PLUGINFILE@@/heartbeat%20shared%20varnace.png" alt="heartbeat shared variance" width="1104" height="668" class="img-responsive atto_image_button_middle"><br></p><p><span style="font-size: 1rem;">Here is a video showing one of the most common impostors in shared variance, erroneous diffuse alpha. In this video, the montage is switched from Linked Ears (which shows false global alpha) and current source density (which localizes the alpha waveform).</span><br></p><p><a href="@@PLUGINFILE@@/Shared%20Variance.mp4">Shared Variance.mp4</a><br></p><p>When the ear electrodes are contaminated, the quantitative EEG can produce erroneous coherences, usually showing as hypercoherences, such as those seen in the below image.</p><p><br></p><p><img src="@@PLUGINFILE@@/Hypercoherences.jpg" alt="Erroneous Hypercoherences" width="517" height="518" class="img-responsive atto_image_button_left"><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><h3>Bi-Polar Montages</h3><p>Many neurologists use bi-polar montages to look for findings in EEG becuase they are commonly looking for epileptiform discharges in the EEG. The location of these abnormal findings is found in the electrode shared between two pairs.&nbsp;</p><p><img src="@@PLUGINFILE@@/Phase%20Reversals.png" alt="Phase Reversals" width="750" height="445" class="img-responsive atto_image_button_left"><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p><br></p><p>The video below is of a woman dealing with age-related cognitive decline, as seen in phase reversals localizing slowing at her temporal lobes (T5 and T6), once we switch the montages to bipolar montages.</p><p><a href="@@PLUGINFILE@@/Phase%20Reversals.mp4">Phase Reversals.mp4</a><br></p>