Estimating the power in EEG frequency bands

Thomas Thesen thomas at UCSD.EDU
Thu Feb 22 15:39:43 CET 2007

Hi FieldTrippers,

Following up on this conversation. It seems that the 'induced activity'
contains both phase-locked and non-phase-locked activity, whereby the
'evoked' activity contains only phase-locked activity. Is it then kosher to
separate these components by linear subtraction? For example, if we first
compute the 'induced' activity by averaging power over individual trials,
and from that subtract the 'evoked activity' (calculated based on average
response) to get the induced activity without any phase-locked activity?

So if

Induced = Phase + Non-Phase


Evoked = Phase


Non-Phase = Induced - Evoked

Or does the fact that this is a linear operations on data that have been
constructed through a non-linear process render this somehow invalid? It has
certainly been done before. Your comments would be much appreciated.




From: FieldTrip discussion list [mailto:FIELDTRIP at NIC.SURFNET.NL] On Behalf
Of Christian Hesse
Sent: Thursday, February 22, 2007 3:41 AM
Subject: Re: [FIELDTRIP] Estimating the power in EEG frequency bands

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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