<p>Spectral power is an often used measure in qEEG studies. The amount of power for each frequency or frequency band for a given electrode is computed and compared to the database mean value. The results are often represented as z-scores, that is the difference between the mean score of a population and the patient’s individual score divided by the standard deviation of the population. Z-scores detect and characterize potential abnormalities from the tested individual, relative to the normative population within his/her own age range. It is not strictly the number of deviations, rather the pattern of deviations that is most relevant. The use of parametric statistics assumes a normal (Gaussian) distribution of the variable(s) in question.&nbsp;</p><p></p><p>The Gaussian distribution is a fundamental distribution that is used throughout science, for example, the Schrodinger wave equation in Quantum mechanics uses the Gaussian distribution as basis functions (Robinett, 1997).&nbsp; The application of the EEG to the concept of the Gaussian distribution requires the use of standard mathematical transforms by which all statistical distributions can be transformed to a Gaussian distribution (Box and Cox, 1964). Quantitative EEG uses transforms that are applied to the power spectrum of the digital time series in order to approximate a normal distribution.&nbsp;</p><p><br></p><p style="text-align: center;"><img src="@@PLUGINFILE@@/gaussian-distribution.png" alt="Gaussian distribution" width="763" height="555" class="img-responsive atto_image_button_text-bottom"><br></p><p style="text-align: center;">Gaussian distribution<br></p><p style="text-align: center;"><br></p><p style="text-align: left;">The next two figures are representations of raw, un-normed data, and Z-score data, respectively, in spectral graphs. The top image shows the raw absolute power of the EEG in microvolts squared. We see that 10Hz shows up at approximately 16mV2 . The second image shows the data in Z-scores, or how deviant from the norm these frequencies are. In the second image, we can see that the purple line (representing one electrode) is 3+ standard deviations above the norm between 14 – 16 Hz, and 4+ standard deviations above the norm at 28 Hz. Both of these findings are potentially artifactual, but this cannot be determined without an analysis of the raw EEG.<br></p><p style="text-align: left;"><br></p><p style="text-align: center;"><img src="@@PLUGINFILE@@/microvolts-spectral.png" alt="Spectral Power in Microvolts squared (y-axis) and hertz (x-axis)" width="599" height="553" class="img-responsive atto_image_button_text-bottom"><br></p><p style="text-align: center;">Spectral Power in Microvolts squared (y-axis) and hertz (x-axis)<br></p><p style="text-align: center;"><br></p><p style="text-align: center;"><img src="@@PLUGINFILE@@/spectral-power-z-score.png" alt="Spectral Power in Microvolts squared (y-axis) and hertz (x-axis)" width="595" height="541" class="img-responsive atto_image_button_text-bottom"><br></p><p style="text-align: center;">Spectral Power in Z-Scores (y-axis) and hertz (x-axis)<br></p><p style="text-align: center;"><br></p><p style="text-align: left;">The next figure we see is showing, one channel per graph, the raw data in the EEG on a spectral display. This imagery is from WinEEG. Each peak represents a rhythm in the data, showing kurtosis, or the expected shape of a peak with sides that indicates it is a frequency range.<br></p><p style="text-align: left;"><br></p><p style="text-align: center;"><img src="@@PLUGINFILE@@/spectra-raw-eeg.png" alt="Spectra of Raw EEG (not Norm referenced) as seen at each electrode in WinEEG" width="900" height="572" class="img-responsive atto_image_button_text-bottom"><br></p><p style="text-align: center;">Figure #1 - Spectra of Raw EEG (not Norm referenced) as seen at each electrode in WinEEG<br></p><p style="text-align: center;"><br></p><p style="text-align: left;">The figure below illustrates a common modern quantitative EEG analysis where EEG traces are viewed and examined at the same time that quantitative analyses are displayed so as to facilitate and extend analytical power.<br></p><p style="text-align: center;"><img src="@@PLUGINFILE@@/raw-EEG-quantitative-EEG.png" alt="Raw EEG and Quantitative EEG" width="1920" height="1160" class="img-responsive atto_image_button_text-bottom"><br></p><p style="text-align: center;">Raw EEG and Quantitative EEG<br></p><p style="text-align: center;"><br></p>