[FieldTrip] spectral resolution & interpretation of power estimates
Helene Gudi
helene.gudi at uni-hamburg.de
Tue Nov 25 13:15:46 CET 2014
Dear FieldTrip List,
Since a while I am struggeling with a question regarding the spectral
resolution and its consequences for interpreting the power vaules. I
would appreciate any comments or hints which help me understand the issue.
As described in the tutorial on TFR analysis when using the Morlet
wavelets the spectral bandwidth at a given frequency is determined by
the formula: F/width*2, meaning that when I define cfg.width=7 and look
into the estimated power values of e.g. 8Hz, what I get is the power
values not for 8 Hz but for a spectral band of 8Hz+/- (8/7*2)/2.
Am I correct?
If yes, is this also true when estimating the power values using a
Hanning window? As described in the tutorial when using the Hanning
taper the frequency resolution is defined by: 1/length of the sliding
window. Let's say my sliding window = .5s, resulting in a frequency
resolution of 2 Hz. Now, if i look into the power estimate for 10 Hz, do
I get the spectral band of 2Hz at a given frequency eg. 9-11Hz, for 10Hz
etc.?
My next question refers to the 'cfg.pad' parameter, in the help it says:
"the padding determines the spectral resolution". I unfortunately could
not find any further explanations. What exactly does it mean? How do I
compute the exact frequency resolution once cfg.pad has been used? Is it
1/cfg.pad?
Does the padding allow to estimate power values for frequencies other
than given by '1/length of time window'-resolution? As in the code below
power estimates are calculated in 1Hz steps. Do the power estimates at
3Hz,4Hz etc. make sense at all, given the frequency resolution of 2.5Hz
(as defined by 1/cfg.t_ftimwin)? What spectral band is actually included
in the single 1Hz bins? Is it a problem if the frequency resolution ist
not an integer?
cfg = [];
cfg.output = 'pow';
cfg.channel = 'all';
cfg.keeptapers = 'no';
cfg.pad = 7;
cfg.method = 'mtmconvol';
cfg.toi = -1.75:0.05:1.75;
cfg.taper = 'hanning';
cfg.keeptrials = 'yes';
cfg.foi = 3:35;
cfg.t_ftimwin = ones(length(cfg.foi),1) .* 0.4;
ft_freqanalysis(cfg, data);
I would be very thankful for any help!
Lena
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