<p></p><h3></h3><h4>Windowing</h4><p><span style="font-size: 1rem;">You will have to match the ends up when you cut the EEG and
make epochs. If you splice ends together and there’s discontinuity you get
Gibb’s artifact or “leakage.” A square wave (a square wave is all of the
frequencies combined)</span><br></p>

<p>Windowing- gently turns on the epoch at the beginning and
turns it off at the end, this is called smearing—this is preferential to Gibb’s
artifact.</p>

<p>An ideal window starts at 0 . The Hamming Window does not
start at 0—it does not get down to the 0 point. Bartlett (triangular window)
has a narrow peak at the top. There are various ways of fixing the
EEG—(Bartlett, Hamming, Hanning, Blackman)—the QEEG is a extension of the
analysis of the EEG.</p><p><br></p><div><div><p></p><h4>Voltage vs. Amplitude</h4>Voltage and amplitude are terms that are closely linked but are not synonymous.&nbsp;<p></p><p>The voltage of an EEG waveform is measured in microvolts. The voltage can vary depending on the recording technique. The voltage seen on the EEG recording is the difference in electrical potential between the 2 inputs. It may appear higher or lower depending on the type of montage used to record it. Remember, the montage selection, bipolar or referential, will alter the display or deflection due to cancellation or summation of the EEG signal.&nbsp;<b>This makes it difficult to know the true or absolute voltage of a waveform</b>.&nbsp;</p><p><img src="@@PLUGINFILE@@/bipolar%20montages.png" alt="Bipolar Montages" width="600" height="310" class="img-responsive atto_image_button_left"><br></p><p>Bipolar&nbsp;<a title="Montages" href="https://schoolofneurotherapy.com/classroom/mod/lesson/view.php?id=183">montages</a>&nbsp;with closely spaced electrode derivations produce lower voltage than widely spaced referential electrode derivations.&nbsp;</p><p>The amplitude of the EEG waveform is dependent on the sensitivity setting of the EEG instrument, and the voltage of the EEG signal being recorded. The amplitude which is the peak to-peak measurement (millimeters) is multiplied by the sensitivity, in microvolts per millimeter to calculate the voltage of the EEG wave or in some digital instruments the sensitivity may be "full scale" such as 50uV or 100 uV, in other words, the maximum voltage that can be displayed full scale. A waveform with a peak-to-peak measurement of 9 millimeters recorded at a sensitivity of 10 microvolts per millimeter would have a voltage of 90 microvolts or a peak to peak amplitude measurement of 13 millimeters recorded at a sensitivity of 10 microvolts per millimeter has a voltage of 130 microvolts.&nbsp;</p><p><br></p><h4>Symmetry vs. Asymmetry</h4><p>Symmetry is used to describe EEG amplitude and frequency in homologous, or similar, regions&nbsp;of the head. In general, normal EEG activity that occurs over both hemispheres in expected to be&nbsp;symmetric. It may be difficult to assess the symmetry of a sinusoidal pattern because of&nbsp;amplitude changes that are characteristic of the pattern. The range of amplitude changes,&nbsp;however, should be similar.</p><p><b>Symmetric - EEG waveforms that are equal in frequency and voltage&nbsp;in homologous (corresponding) regions of the head.&nbsp;</b></p><p><b>Asymmetric - EEG waveforms that are NOT equal in frequency and voltage&nbsp;in homologous (corresponding) regions of the head.&nbsp;</b><br></p><p>Asymmetry describes EEG activity that is different (present/absent or increased/decreased) in one hemisphere when compared to the same region of the opposite hemisphere. Asymmetry can be caused by damage to an area of the brain or even by skull defects, which are changes in the skull bone caused by injuries or medical procedures. An increase in the amplitude is seen over the skull defect. A minimal degree of asymmetry is acceptable in normal EEG recordings. Alpha rhythm, for example, is usually of higher amplitude over the right hemisphere.&nbsp;<br></p><p>In right-handed individuals, alpha should be found to be somewhat higher in amplitude in the right hemisphere.&nbsp; However, when alpha frequency is found to be slower by 1 Hz or more in one hemisphere, this should be considered as potentially indicative of a focal abnormality (Markand, 1990) warranting a neurological consult, and should particularly be considered abnormal if it is greater than 1.5 Hz (Duffy, et al., 1989).&nbsp; Likewise, an amplitude difference in alpha between the two sides of more than 50% is considered significant (Hughes, 1994), and because in right handed individuals, alpha rhythm is usually of higher amplitude on the right side, a decrease of even 35% on the right may be significant (Duffy et al., 1989).&nbsp; Focal lesions may cause alpha to be disorganized ipsilaterally, to be of lower or higher amplitude, and may cause alpha to fail partially or totally to block following eye opening on the affected side.&nbsp; But, “in the absence of other findings, one should use caution in interpreting isolated&nbsp;<i>amplitude</i>&nbsp;asymmetry of the alpha rhythm, because amplitude asymmetries of the order as high as 2:3 are not uncommon in normal subjects” (Markand, 1990, p. 181, emphasis added).&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br></p><p><b>Both muscle contamination and poor electrode application can lead to asymmetry.</b></p><p><img src="@@PLUGINFILE@@/symmetry%20vs.%20asymmetry.jpg" alt="Symmetry vs. Asymmetry" width="720" height="540" class="img-responsive atto_image_button_left"><br></p><h4>Synchrony</h4><p>Synchrony describes waveforms that occur simultaneously. Patterns that occur at the same time in both hemispheres may be called bilaterally synchronous. Eye blinks seen on an EEG are an example of activity that is bilaterally synchronous. Asynchronous - waveforms with timing that is not the same. The eye blinks in the above image are synchronous.</p></div></div><br><p></p>