[Eeglablist] ERSP calculation

Michael Boyle mrboyle at live.unc.edu
Tue Sep 22 15:53:22 PDT 2015

Hey everyone,

I had a quick question about ERSP calculations. Lets assume my EEG time
series are simple trials with a baseline period and a period of interest,
and have been time-frequency transformed through some method of choice and
the result is a complex spectrogram, from which we extract the amplitude
spectrogram by simply taking the magnitude. For a single trial, is it
correct that one averages the spectra during the baseline window (across
time) to get the baseline spectrum that is in the ratio denominator (or
used as the baseline subtraction, same thing) for ERSP calculation?

If so, then is it preferred to work with amplitude spectrograms/spectra and
get dB power by taking 10*log10(amplitude^2/avgBaselineAmplitude^2) or to
work with power spectrograms/spectra and get dB by taking
10*log10(power/avgBaselinePower). These two expressions aren't equivalent
because mean(baselineAmplitudes)^2 is typically not equal to
mean(baselineAmplitudes^2) (sometimes very different even), nor is
mean(baselineAmplitudes)^2/mean(baselineAmplitudes^2) constant, so you
don't just get a difference of a constant when considering the two results.

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