[Eeglablist] measurement unit of EEGlab power spectrum analysis

[Kay Sung]

Dear Eeglablisters,

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.

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.
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.

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)".
So, the power calculated in this way is always in dB unit with different referencing point even if we don't remove the baseline.

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".

Am I misunderstanding anything here?

Thanks.



Kyongje (Kay) Sung, Ph.D
Research Associate
Johns Hopkins University School of Medicine
Cognitive Neurology & Neuropsychology - Department of Neurology

1629 Thames Street, Suite 350
Baltimore, MD 21231
P 443-287-8019 | F 410-955-0188
ksung3 at jhmi.edu<mailto:ksung3 at jhmi.edu>
web.jhu.edu/cognitiveneurology/index.html<http://web.jhu.edu/cognitiveneurology/index.html>
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