# [Eeglablist] Power spectrum

Makoto Miyakoshi mmiyakoshi at ucsd.edu
Tue Feb 7 18:14:50 PST 2017

Dear Leonardo,

> 1- The code that you recommended computes power spectrum in "10*log10(uV^2/Hz)".
In this case, can we consider this unit without Hz (i.e., only 10*log10
(uV^2))?

It is called power spectrum density (PSD). It means whatever power *per
second*. Hence you have Hz in the unit. If it is not per second, 20 s long
data would have twice as large power than 10 s long data. So you can't
remove Hz from the unit to show what you exactly mean.

> 2- In fact, my major aim is computing power spectrum in "uV^2" unit, and
the code you recommended does different thing--it, actually, computes power
spectrum in power spectrum in unit "10*log10(uV^2/Hz)" (please see the
attached file). Kindly, could you guide me to way of computing the
power in "uV^2"
unit ?

Again, if you remove Hz, then you multiply the power spectrum by data
length in second. You don't want to do it. If you want to convert it from
log scale to linear scale, use 10^(x/10)

> 3- Is there any formula that converts power spectrum from "10*log10(uV^2/Hz)"
unit into "uV^2" unit?

Again, 10^(x/10) but you still want to do uV^2/Hz, otherwise you can't
compare data with different lengths.

Makoto

On Thu, Feb 2, 2017 at 5:46 PM, Leonardo Lion <llionname at gmail.com> wrote:

> Dear Makoto,
>
> Thank you so much for your help.
>
> I have two main issues:
>
> 1- The code that you recommended computes power spectrum in "10*log10(uV^2/Hz)".
> In this case, can we consider this unit without Hz (i.e., only 10*log10
> (uV^2))?
>
> 2- In fact, my major aim is computing power spectrum in "uV^2" unit, and
> the code you recommended does different thing--it, actually, computes power
> spectrum in power spectrum in unit "10*log10(uV^2/Hz)" (please see the
> attached file). Kindly, could you guide me to way of computing the power in "uV^2"
> unit ?
>
> 3- Is there any formula that converts power spectrum from "10*log10(uV^2/Hz)"
> unit into "uV^2" unit?
>
> Kind regards,
> Leonardo
>
>
>>  power.png
>>
> On 3 February 2017 at 02:08, Makoto Miyakoshi <mmiyakoshi at ucsd.edu> wrote:
>
>> Dear Leonardo,
>>
>> See my wiki page for the example code. Unfortunately, you can't do this
>> from GUI.
>> https://sccn.ucsd.edu/wiki/Makoto%27s_useful_EEGLAB_code#How
>> _to_extract_EEG_power_of_frequency_bands
>>
>> Makoto
>>
>> On Tue, Jan 31, 2017 at 9:25 AM, Leonardo Lion <llionname at gmail.com>
>> wrote:
>>
>>> Hi all,
>>>
>>> How to convert the EEGLAB power spectrum "10*log10(uV^2/Hz)" into "uV^2"
>>> only?
>>>
>>> Thanks.
>>> Leonardo
>>>
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>>
>>
>> --
>> Makoto Miyakoshi
>> Swartz Center for Computational Neuroscience
>> Institute for Neural Computation, University of California San Diego
>>
>
>

--
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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