[Eeglablist] Is power calculated in STUDY and in dataset is abs or relative?

[Aleksandra Vuckovic]

Dear Arno,
that is what I've expected but how can one then compare powers between two group ( looking at a peak frequency of e.g. two groups of patients) being sure that the difference does not come from individual difference in EEG amplitude?
Or other way around, how can we replace a value of the absolute power in a STUDY with the relative power and then visualise it using the STUDY tools?
Many thanks,
Aleksandra

________________________________
From: Arnaud Delorme [arno at ucsd.edu]
Sent: 12 October 2012 02:28
To: Aleksandra Vuckovic
Cc: tarikbelbahar at gmail.com; M Abul Hassan; eeglablist at sccn.ucsd.edu
Subject: Re: [Eeglablist] Is power calculated in STUDY and in dataset is abs or relative?

Dear Aleksandra,

power is absolute when plotting the spectrum and relative to baseline when computing ERSP as Tarik mentioned.
Best,

Arno

On 9 Oct 2012, at 01:48, Aleksandra Vuckovic wrote:

Hi,
what Abul Hasan is asking is whether the Power calculated in EEGLab in certain frequency band is absolute or relative (e.g. for example is it Power (8-12 Hz) or Power (8-12 Hz)/Power(1-40 Hz) )
This is a relevant question if one wants to compare mean Power values between two groups in a STUDY. We need to be sure that the difference is not coming from the fact that people in one group have a higher amplitude EEG  but that their relative power in certain frequency band is higher as compare  to the other group.
Can anybody give us a clue how to get a relative power using STUDY so we can use all nice graphical presentation and statistics?

Many thanks,
Aleksandra

________________________________
From: eeglablist-bounces at sccn.ucsd.edu<mailto:eeglablist-bounces at sccn.ucsd.edu> [eeglablist-bounces at sccn.ucsd.edu<mailto:eeglablist-bounces at sccn.ucsd.edu>] On Behalf Of Tarik S Bel-Bahar [tarikbelbahar at gmail.com<mailto:tarikbelbahar at gmail.com>]
Sent: 08 October 2012 22:41
To: M Abul Hassan
Cc: eeglablist at sccn.ucsd.edu<mailto:eeglablist at sccn.ucsd.edu>
Subject: Re: [Eeglablist] Is power calculated in STUDY and in dataset is abs or relative?

Greetings Abdul,

I am not an expert on the mechanics here,
but I believe if you are referring to ERSP computation using the newtimef function,
then the baseline of values before the 0 time point in your epochs is automatically subtracted.
Please also see  multiple messages on this topic in recent months in the eeglablist archives.

Respectfully,
Tarik





On Fri, Oct 5, 2012 at 2:37 AM, M Abul Hassan <abulhassan85 at yahoo.com<mailto:abulhassan85 at yahoo.com>> wrote:
Dear all

I want to know the power spectrum generated by EEGLAB both in a STUDY and in each dataset seperately is using absolute values or relative values.

If it based on relative values than how a frequency range can be known?

If it is based on absolute values than how can I calculate the relative power in EEGLAB?

Thanks
Best Regards
Muhammad Abul Hasan


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