[Eeglablist] How to perform Morlet wavelet freq-time transforms (Tallon Baudry)?

[Edward Rooke]

Dear all,

I am struggling to apply a morlet wavelet frequency-time transform some
data. I would like to apply exactly the same data analysis as used my
Tallon-Baudry in the 1999 paper; Oscillatory gamma activity in humans and
its role in object representation.

I notice the function timefreq() has a 'dftfilt3; option which is labelled
exact Tallon-Baudry. But also in the new_timef() information it says it is
possible to select 'dftfilt3' for Morlet wavelets. I was keen to use
new_timef() as it is the function that is called through the GUI for
channel time-frequency plots. Are the  'dftfilt3' Morlet wavelets in
new_timef() the same as the ones used in timefreq() and in Tallon-Baudry`s
paper? Which function would be best to use?

I have just been reading through the inputs for the new_timef() function
attempting to select the values that would produce the same wavelet used
in Tallon-Baudry`s paper but I am finding this extremely difficult. This
is due to an incomplete knowledge of both the EEGlab function and the
maths behind the analysis in the paper. Does anyone know which input
values I should select for this type of analysis? (I have tried to give
more of an outline of the analysis below) Any ideas which inputs I
definitely need to change or not change? (I think there are some which are
nothing to do with the wavelet that are confusing me).

The paper says "The signal to be analyzed s(t) is convoluted by complex
Morlet’s
wavelets w(t,f) having a Gaussian shape both in the time domain (standard
deviation U), and in the frequency domain (U=1/(2pi * U)) around a
frequency
f : w ( t , f ) = A . e x p ( – t^2 / (2*U^ 2) * exp(2i pi ft) with
A being a normalization factor: A=1/(U (pi^0.5))^0.5." In previous papers
they mention the wavelet is normalized so the total energy = 1, f/U = 7
and they use a 1 Hz step. (NB:-  "U" = sigma).

I also have another, more simple question; My understanding is that the
time-frequency transform in EEGlab is built up from data averaged accross
all trials producing a plot which represents evoked responses. I would
also like to build up time-frequency plots for individual trials and then
average these to produce a plot which shows induced responses. How would I
go about doing this?

Does anyone has a script for the Tallon-Baudry analysis they would be will
to share?

I apologies if my queries are a bit trivial I am very new to EEGlab.

Best regards

Ed Rooke


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