<div dir="ltr">Dear Kyongje,<div><br></div>> When the baseline is not removed, time-frequency analysis function gives me the power calculated in different unit, which is 10*log_10 (uV^2/Hz). This unit is also displayed when I just want to check out a specific channel property (pop_prop function). To me, this is quite confusing since this unit is exactly the dB unit with respect to the identify unit of the measurement (i.e., 1uV). The term “uV^2/Hz” within the parenthesis is nothing different from “(uV^2/Hz) / (1uV^2/Hz)”.<div><br></div><div>Well, this is one of those ever confusing things for me. One time I tested it. </div><div>+/- 1 microV sine wave (peak to peak), the PSD peak level was 0dB.</div><div>+/- 10 microV, 20dB</div><div>+/-0.1 microV, -20dB</div><div><br>> Also, with limited knowledge on signal processing, I’m not aware of any power calculation with Hz dividing the voltage measure (uV^2 /Hz). The division by Hz would not matter if the power is calculated in this way: “(uV^2/Hz) / (1uV^2/Hz)”, not just “(uV^2 /Hz)”. But, as it is now, the term “(uV^2 /Hz)” is very confusing. I don’t think EEGlab is actually dividing the voltage measure with the corresponding frequency. To me, the “/Hz” term is semantics, not mathematics. I think the better way of displaying the unit is something like this: “10*log_10 (uV^2) per Hz” or “10*log_10 (uV^2/ 1uV^2) per Hz” to be precise. But, again, these can simply be written as “dB”.<br><br>I thought /Hz represents 'per second' (don't ask me why), if I understood correctly.<br><br>> Am I misunderstanding anything here?<div class="gmail_extra"><br></div><div class="gmail_extra">No, I think your understanding is good, unless I'm wrong.</div><div class="gmail_extra"><br></div><div class="gmail_extra">Makoto</div><div class="gmail_extra"><br></div><div class="gmail_extra"><br></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, May 18, 2016 at 8:39 AM, Kay Sung <span dir="ltr"><<a href="mailto:ksung3@jhmi.edu" target="_blank">ksung3@jhmi.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext">Dear Eeglablisters,
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext">This is my thought about the measurement unit of power spectrum analysis in EEGlab and I want to make sure if I’m not misunderstanding something here.
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext">Typically, I use EEGlab (time-frequency analysis) to obtain two different power calculations. One is to baseline removed ERSP and the other without the baseline
removal. <u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext">When the baseline is removed, ERSP plot shows power in dB unit. This is clear since the power of cortical oscillation after, say, stimulus onset is calculated
with respect to the baseline power, the reference point. <u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext">When the baseline is not removed, time-frequency analysis function gives me the power calculated in different unit, which is 10*log_10 (uV^2/Hz). This unit
is also displayed when I just want to check out a specific channel property (pop_prop function). To me, this is quite confusing since this unit is exactly the dB unit with respect to the identify unit of the measurement (i.e., 1uV). The term “uV^2/Hz” within
the parenthesis is nothing different from “(uV^2/Hz) / (1uV^2/Hz)”. <u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext">So, the power calculated in this way is always in dB unit with different referencing point even if we don’t remove the baseline.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext">Also, with limited knowledge on signal processing, I’m not aware of any power calculation with Hz dividing the voltage measure (uV^2 /Hz). The division by
Hz would not matter if the power is calculated in this way: “(uV^2/Hz) / (1uV^2/Hz)”, not just “(uV^2 /Hz)”. But, as it is now, the term “(uV^2 /Hz)” is very confusing. I don’t think EEGlab is actually dividing the voltage measure with the corresponding frequency.
To me, the “/Hz” term is semantics, not mathematics. I think the better way of displaying the unit is something like this: “10*log_10 (uV^2) per Hz” or “10*log_10 (uV^2/ 1uV^2) per Hz” to be precise. But, again, these can simply be written as “dB”.
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext">Am I misunderstanding anything here?
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext">Thanks.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:13pt;font-family:Calibri,sans-serif;color:windowtext"><u></u> <u></u></span></p>
<p class="MsoNormal" style="margin-bottom:12pt"><b><span style="font-size:10pt;font-family:Arial,sans-serif;color:rgb(0,51,127)">Kyongje (Kay) Sung, Ph.D</span></b><span style="font-size:11pt;font-family:Calibri,sans-serif"><u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:10pt;font-family:Arial,sans-serif;color:rgb(0,51,127)">Research Associate<br>
<b>Johns Hopkins University School of Medicine</b><br>
Cognitive Neurology & Neuropsychology – Department of Neurology</span><span style="font-size:11pt;font-family:Calibri,sans-serif"><u></u><u></u></span></p>
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