Estimating the power in EEG frequency bands

Christian Hesse c.hesse at FCDONDERS.RU.NL
Thu Feb 22 09:40:42 CET 2007

Hi Shantanu,

> When I want to analyze the power in different eeg frequency bands
> (alpha,
> beta, etc), do I use a grand average for all subjects, or do i take
> only
> data trial by trial for all electrodes, estimate the powers, and then
> average them for all trials?

You use the the latter approach, averaging the power over trials
(subject average) and then over subjects (grand average) if you wish
to look at INDUCED activity, i.e. task or event related modulation in
the amplitude time-course of ongoing oscillatory activity (this
analysis is not sensitive to the phase differences of the
oscillations on different trials).

You can also average the signals first (in the time domain) and then
compute the power spectrum, in which case you will see the spectral
properties of EVOKED activity, i.e., activity that is phase or time
locked to the event / stimulus / response.

Both of these time frequency visualizations give information that is
complementary to looking a the time course of your event-related or
evoked potentials (for EEG) or fields (for MEG) which you get by
doing a time-locked average of the raw signals.

Hope this helps to clarify,

Christian Hesse, PhD, MIEEE

F.C. Donders Centre for Cognitive Neuroimaging
P.O. Box 9101
NL-6500 HB Nijmegen
The Netherlands

Tel.: +31 (0)24 36 68293
Fax: +31 (0)24 36 10989

Email: c.hesse at

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