<p></p><p><a name="_Hlk530944452">Here is a video on Morphology and Montages. If you want to skip straight to montages, go to minute 24.</a></p><p><a name="_Hlk530944452"><a href="https://www.youtube.com/watch?v=dpY2F_2nrAo&amp;t=1580s">Morphology and Montages Video</a><br></a></p><p><a name="_Hlk530944452">An EEG montage is the placement of
electrodes. All EEG signals are measured by subtracting the signal measured at
2 sites: the active electrode and the reference electrode.&nbsp; This is called
common mode rejection.&nbsp; The reason for this is that we need to subtract
out all the environmental electrical noise (which is much larger than the
brainwave signal) in order to see the brain's activity.&nbsp; Any scalp
electrode site measurement must be referenced to some other measured
site.&nbsp; In the past, the most common site for reference was linked ears.&nbsp;</a> But you can use the nose, or any other scalp location or, as explained
above, any average of multiple sites.&nbsp; So, there are many montages
available and they all have strengths and weaknesses.&nbsp; There is no best
montage for all purposes.</p><h3><a name="_Toc29929020">Referential
Montages</a></h3>

<p>Many will refer to the “active”
electrode and the “reference“&nbsp; electrode.
Commonly used “referential montages” include the ear references (linked ear,
ipsilateral or contralateral ear), the Cz location or a “vertex”
reference.&nbsp; Local function in the EEG
does not shown well with linked ears (that is, if you had a focal slowing,
Linked Ears would not be a good montage to view it). Linked ears montages show
global firings well.</p>

<p>The linked ear montage can spread
temporal lobe activity into reference electrodes. The activity seen at T5 and
T6 are most likely to be seen in the electrodes on the ears but activity at T3
and T4 may influence the ear electrodes, as well as F7 and F8 in rarer cases.</p><h2><a name="_Toc29929021">Linked Ears Montage</a></h2><p></p><p style="text-align: center;"><a name="_Toc29929021" style="background-color: rgb(255, 255, 255); color: rgb(82, 96, 105); font-size: 1rem;">&nbsp;&nbsp;</a></p><a name="_Toc29929021"></a><p style="text-align: center;"><a name="_Toc29929021"></a><a name="_Toc29929021" style="background-color: rgb(255, 255, 255); color: rgb(82, 96, 105); font-size: 1rem;">&nbsp; &nbsp; &nbsp;<img src="@@PLUGINFILE@@/LE%20Montage.png" alt="Linked Ears" width="519" height="512" style="text-align: center; " class="img-responsive atto_image_button_middle"></a></p><p style="text-align: center;"><br></p><p></p><br><p></p><p></p><h2><a name="_Toc29929022" style="color: rgb(55, 71, 79); font-size: 2.143rem; background-color: rgb(255, 255, 255);">Cz Reference Montage</a><br></h2><p style="text-align: center; "><a name="_Toc29929022" style="color: rgb(55, 71, 79); font-size: 2.143rem; background-color: rgb(255, 255, 255);"><img src="@@PLUGINFILE@@/Cz%20Montage.png" alt="Cz Montage" width="800" height="522" class="atto_image_button_middle"><br></a></p><br><p></p><p><br></p><p><a name="_Toc29929023" style="color: rgb(55, 71, 79); font-family: &quot;Open Sans&quot;; font-size: 1.715rem; background-color: rgb(255, 255, 255);">Sequential
Montages</a><br></p>

<p>Sequential
references (commonly termed “bipolar”) are those that we see in the transverse
bi-polar and longitudinal transverse bi-polar montage. Sequential montages
(such as transverse or longitudinal transverse, are often used in neurology
practices to determine the location of any focal findings in the EEG. This is
seen with “phase reversals’ in the EEG, where two electrodes seem to “point” to
one another (i.e. one line moves eletro-negative and the one next to it moves
electro-positive.&nbsp; This is a marker of a
physical abnormality and can point to the seizure focus, a lesion,
hypoperfusion, localized infarct (a small localized area of dead tissue), or
white matter ischemia. This is an “X marks the spot” type of finding. You can
determine the site of the finding based upon the <u>shared electrode between
the two pairs</u>. Phase reversals are relevant in slow frequencies (typically
8Hz and under).</p><h3><a name="_Toc29929024">Phase Reversals located at T5 in a
double banana montage</a></h3><p><a name="_Toc29929024"><img src="@@PLUGINFILE@@/Phase%20Reversals.png" alt="Phase Reversals" width="600" height="1036" class="img-responsive atto_image_button_left"></a></p><p><a name="_Toc29929024">Phase reversals are a
marker of a physical abnormality and can point to the seizure focus, a lesion,
hypoperfusion, localized infarct (a small localized area of dead tissue) or
white matter ischemia. This is an “X marks the spot” type of finding. When
there is a lack of blood flow and oxygen to the area, there is often a
consequential slowing of alpha at the area. As the area degrades, sometimes the
alpha slows considerably below the band. But this depends on how severe the damage
is (Classen and Foreman, 2012). </a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p><a name="_Toc29929024"><br></a></p><p></p><h3><a name="_Toc29929025"><br></a></h3><h3><a name="_Toc29929025"><br></a></h3><h3><a name="_Toc29929025"><br></a></h3><h3><a name="_Toc29929025"><br></a></h3><h3><a name="_Toc29929025">Video of localization of ischemia via
identifying phase reversals in a bipolar montage</a></h3>

<p><a href="@@PLUGINFILE@@/Ischemias.mp4">Ischemias.mp4</a><br></p><p><br></p><p><br></p><p><br></p><p></p><p></p><h2><a name="_Toc29929026">Transverse Bi-Polar Montage</a></h2><p style="text-align: center;"><img src="@@PLUGINFILE@@/transverse%20bi-polar%20montage.png" alt="Transverse bi-polar montage" width="600" height="551" class="atto_image_button_middle" style="font-size: 1rem;"></p><p></p><p><br></p><p><br></p><p><a name="_Toc29929027" style="color: rgb(55, 71, 79); font-family: &quot;Open Sans&quot;; font-size: 2.143rem; background-color: rgb(255, 255, 255);">Longitudinal Bi-Polar Montage (aka “double banana montage”)</a><br></p><br><p></p><p style="text-align: center; "><img src="@@PLUGINFILE@@/Double%20Banana%20Montage.png" alt="Double Banana" width="683" height="610" class="img-responsive atto_image_button_text-bottom"><br></p><p></p><h3><a name="_Toc29929028">Double Banana Montage</a></h3><br><p></p><p></p><p style="text-align: center;"><span style="font-size: 1rem;">&nbsp;</span><img src="@@PLUGINFILE@@/Double%20Banana%20Montage%20%281%29.png" alt="Double Banana Montage" width="548" height="383" class="img-responsive atto_image_button_middle" style="font-size: 1rem; text-align: center;"></p><p style="text-align: center;"><br></p><p></p><p></p><h2><a name="_Toc29929029" style="color: rgb(55, 71, 79); font-size: 2.143rem; background-color: rgb(255, 255, 255);">Average Montages</a><br></h2>

<p>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.</p><h3><a name="_Toc29929030">An EEG with an Ear Reference contaminated
by a heartbeat (which is thus seen in every channel)</a></h3><p style="text-align: center;"><span style="font-size: 1rem;">&nbsp; &nbsp;</span></p><p style="text-align: center;"><span style="font-size: 1rem;">&nbsp;</span><img src="@@PLUGINFILE@@/Contaminated%20Ear%20Reference.png" alt="Contaminated Ear Reference" width="775" height="541" class="atto_image_button_middle" style="font-size: 1rem; text-align: center;"></p><p></p><p style="text-align: center; "><br></p><p style="text-align: center; "><br></p><p></p><p style="text-align: left;">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). </p>

<p style="text-align: left;">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). </p><p style="text-align: left;"><span style="font-size: 1rem;">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,&nbsp;</span></p><a name="_Toc29929024"><p style="text-align: left;">this “weighted average” reference is very popular when using dense arrays in topographic mapping</p></a><p></p><span style="font-size: 1rem;"><p style="text-align: left;"><span style="font-size: 1rem;">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.”</span></p></span><p></p><p></p><h2 style="text-align: left;"><a name="_Toc29929031">Switching from Linked Ears to an Average
Reference to identify focal findings</a></h2><p style="text-align: center;"><span style="font-size: 1rem;">&nbsp;&nbsp;</span><img src="@@PLUGINFILE@@/Switching%20from%20Linked%20Ears.png" alt="Switching" width="1000" height="563" class="img-responsive atto_image_button_middle" style="font-size: 1rem; text-align: center;"></p><p style="text-align: center; "><br></p><p style="text-align: center; "><br></p><p></p><h2 style="text-align: left;"><a name="_Toc29929032">Appearance of Diffuse Alpha in Lined Ears
Reference, due to a contaminated reference electrode, properly localized to T3
when montage is shifted</a> </h2><p style="text-align: left;"><a name="_Toc29929032"></a><a href="@@PLUGINFILE@@/Shared%20%20Variance.mp4">Shared  Variance.mp4</a><br></p>

<p style="text-align: left;">&nbsp;In the above video, you will see that every channel looks similar in phase and waveform. I confirm this by examining small portions of the record, seeing that there is shared variance (meaning that the ear electrode is picking up a real brainwave and casting it across all the channels, as the reference. When switching the montage to a Laplacian montage, we are able to confirm that the alpha signal is coming from one of the temporal leads (affected the ear in a linked ears reference). This kind of EEG must be read from an average montage for accuracy.</p>

<p style="text-align: left;">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. </p><h3 style="text-align: left;"><a name="_Toc29929033">QEEG Map with False Hypercoherence, due
to contaminated reference</a></h3><p style="text-align: center;"><span style="font-size: 1rem;">&nbsp;</span><img src="@@PLUGINFILE@@/Contaminated%20Reference.png" alt="Contaminated Reference" width="717" height="953" class="img-responsive atto_image_button_middle" style="font-size: 1rem; text-align: center;"></p><p style="text-align: center;"><br></p><p style="text-align: center;"><br></p><p></p><p style="text-align: left;">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;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </p>

<p style="text-align: left;">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><h3 style="text-align: left;"><a name="_Toc3535249">Power displays</a></h3><p><a name="_Toc29929034"></a></p>

<p style="text-align: left;">There are a number of ways power is
seen in EEGs. Different power metrics are: 1) raw power measured in amplitude
(microvolts squared) or magnitude (microvolts); 2) absolute power (seen in
norm-referenced quantitative EEGs); and 3) relative power (seen in
norm-referenced EEGs).&nbsp; Furthermore,
power can be displayed in spectrometers, head maps, and other ways. </p><h3 style="text-align: left;">Depiction of Power Display in IsyncBrain</h3><p style="text-align: center;"><span style="font-size: 1rem;">&nbsp;&nbsp;</span><img src="@@PLUGINFILE@@/isync%20map.png" alt="isync map" width="703" height="681" class="img-responsive atto_image_button_middle" style="font-size: 1rem; text-align: center;"></p><p></p><p style="text-align: center; "><br></p><p style="text-align: left;"><br></p><p></p><h3 style="text-align: left;">Absolute Power</h3>

<p style="text-align: left;">Absolute Power is power
measured in discreet Hertz bins, by amplitude or magnitude. This can be
displayed in a norm-referenced fashion or in raw individual hertz bins. </p><h3 style="text-align: left;">Absolute Power Displayed in Raw EEG (not norm-referenced)</h3><p style="text-align: center;"><span style="font-size: 1rem;">&nbsp;</span><img src="@@PLUGINFILE@@/Raw%20Data%20in%20Neuroguide.png" alt="Raw Data in Neuroguide" width="717" height="874" class="img-responsive atto_image_button_middle" style="font-size: 1rem; text-align: center;"></p><p></p><p style="text-align: center; "><br></p><p></p><h3 style="text-align: left;">Absolute Power Displayed in Z-Score EEG
(norm-referenced)</h3><p style="text-align: center;"><span style="font-size: 1rem;">&nbsp;&nbsp;</span><img src="@@PLUGINFILE@@/Absolute%20Power.png" alt="Absolute Power" width="716" height="825" class="img-responsive atto_image_button_middle" style="font-size: 1rem; text-align: center;"></p><p></p><p style="text-align: center; "><br></p><p></p><h2 style="text-align: left;"><a name="_Toc29929035"></a><a name="_Toc3535250"></a><a name="_Toc525053815">Relative
Power</a></h2>

<p style="text-align: left;">The
differences in absolute and relative power are differences between the
total voltage, or energy, that the brain is emitting, measured in disparate
frequencies and typically portrayed after processed via a “normative database”
(absolute power) and the measure of clients’ EEGs, viewed in relation to their
absolute power profile, used to determine what may be deviant, in relative
terms (relative power).&nbsp; Relative maps
are subject-specific, in the sense that we are witnessing the deviances (and
excesses) in and of the client, taking into account the particulars of the
subject’s overall amplitudes.</p>

<p style="text-align: left;">Relative power is especially telling when a
client is low amplitude overall, or high amplitude overall. In the next
figures, we see an individual who has a high amplitude EEGs—meaning that the
overall voltage that their brain emits is larger than their same aged peers, in
every bandwidth. In this case, we can see in what frequency band they spend the
most time.</p><p style="text-align: left;">Because the absolute power row is high voltage
(2+ Z-scores above the norm in every frequency bin) we have to use the relative
power rows to determine where this person has the most energy allocated. In the
image we can see that the client spends the most time (relatively) in the alpha
band and the least amount of time in the delta range.</p><h3 style="text-align: left;">High Amplitude Absolute Power EEG</h3><p style="text-align: center;"><span style="font-size: 1rem;">&nbsp;</span><img src="@@PLUGINFILE@@/map.png" alt="Map of High Amplitude Absolute Power" width="560" height="721" class="img-responsive atto_image_button_middle" style="font-size: 1rem; text-align: center;"></p><p></p><p style="text-align: center; "><br></p><p style="text-align: left;"><br></p><p></p><p style="text-align: left;">In the next figure, we see an EEG that is low
voltage, overall. We can see that the client is 1-2+ standard deviations low in
every bandwidth. When looking at relative power, we learn that the balance of
his EEG is quite good—he is spending similar amounts of time in all five
bandwidths measured. </p><h3 style="text-align: left;">Low Voltage EEG</h3><p><br></p><p style="text-align: center;"><img src="@@PLUGINFILE@@/Low%20Voltage%20EEG.png" alt="Low Voltage EEG" width="560" height="739" class="img-responsive atto_image_button_text-bottom" style="font-size: 1rem;"></p><p></p><p></p>