[FieldTrip] spectral resolution & interpretation of power estimates
Jörn M. Horschig
jorn at artinis.com
Wed Nov 26 08:47:39 CET 2014
please allow me to correct my first sentence:
> as an addition, maybe keep in mind that using *tapers* you in fact widen
your
--
Jörn M. Horschig, Software Engineer
Artinis Medical Systems | +31 481 350 980
> -----Original Message-----
> From: fieldtrip-bounces at science.ru.nl [mailto:fieldtrip-
> bounces at science.ru.nl] On Behalf Of Jörn M. Horschig
> Sent: Wednesday, November 26, 2014 8:44 AM
> To: 'FieldTrip discussion list'
> Subject: Re: [FieldTrip] spectral resolution & interpretation of power
> estimates
>
> Hey,
>
> as an addition, maybe keep in mind that using filters you in fact widen
your
> main lobe (i.e. decrease the effective frequency resolution, e.g. for a
> Hanning taper the main lobe drops to zero at twice the Raleigh frequency),
> and you do this to decrease magnitude of the side lobes. Most people tend
> to forget such things when interpreting their data (incl. me).
>
> Best,
> Jörn
>
> --
>
> Jörn M. Horschig, Software Engineer
> Artinis Medical Systems | +31 481 350 980
>
> > -----Original Message-----
> > From: fieldtrip-bounces at science.ru.nl [mailto:fieldtrip-
> > bounces at science.ru.nl] On Behalf Of Lam, N.H.L. (Nietzsche)
> > Sent: Tuesday, November 25, 2014 3:34 PM
> > To: FieldTrip discussion list
> > Subject: Re: [FieldTrip] spectral resolution & interpretation of power
> > estimates
> >
> > Hi Helene,
> >
> > The idea in general is to design one's experiment in a way that allows
> > one
> to
> > observe effects at the frequency of interest. Otherwise, what you
> > might end up with is an experiment that doesn't allow you to estimate
> > power at the frequencies of interest.
> >
> > For wavelets, given your specifications, you would get a spectral
> bandwidth
> > of 8/7*2 = 2.28 Hz.
> > So 8 +/- 2.28 Hz ( 5.72 - 10.28) i.e. the bandwidth calculated
> > refers
> to the
> > bandwidth on one end, not both. Therefore, you shouldn't divide by two.
> > Similarly with multitapers, if you have a smoothing for 8 Hz, that's
> > -8Hz
> and
> > +8Hz.
> >
> > For the Hanning taper, 1/0.5s = 2Hz, means you get estimates of
> > power
> that
> > are multiples of 2 Hz, so 10, 12, 14 Hz ..etc. With regards to what
the
> spectral
> > bandwidth is, the short answer is that it is plus and minus half the
> frequency
> > resolution. So, for 10 Hz, it is 9 - 11Hz. The reasons behind this
> answer are to
> > do with the spectral profile of the taper (e.g., a Hanning window)
> > that
> you
> > apply to your data. I would suggest that you take a look at one of
> > our
> lecture
> > video's here by Robert:
> >
> https://www.youtube.com/watch?feature=player_detailpage&v=QLvsa1r1V
> > oc#t=741 where he provides a much more detailed explanation.
> >
> > cfg.pad parameter in ft_freqanalysis: This parameter is helpful if
> > your
> trials
> > are of different length. If you set cfg.pad = 7, that means any trial
> shorter
> > than 7s long will be padded to become 7s worth of data. The type of
data
> > that you use to pad is specified in cfg.padtype. The frequency
> resolution of
> > your data is indeed 1/cfg.pad. There is a suggested limit for how
much
> you
> > should pad given your data, but I'm not sure what the limit is, maybe
> > someone else knows.
> >
> > Now with your code,
> > cfg.toi = -1.75:0.05:1.75;
> > cfg.taper = 'hanning';
> > cfg.foi = 3:35;
> > cfg.t_ftimwin = ones(length(cfg.foi),1) .* 0.4;
> >
> > You have a 0.4 second window. So 1/0.4 = 2.5Hz frequency resolution,
> > as
> you
> > figured out.
> > This means you can only estimate in steps of 2.5Hz so doing power
> estimates
> > at 1 Hz steps does *not* make sense. To get any sensible estimate,
> > you would need something like this cfg.foi = 2.5:2.5:35;
> >
> > Finally, no, the frequency resolution does not have to be an integer.
> > We don't know what frequency(ies) the brain oscillates, but integers
> > allow
> for us
> > to deal with the data more easily.
> >
> > I hope this helps, and I hope that if I've not been entirely accurate
> > with
> my
> > explanations that someone will jump in to correct me.
> >
> > Best,
> > Nietzsche
> >
> > ________________________________________
> > From: fieldtrip-bounces at science.ru.nl
> > [fieldtrip-bounces at science.ru.nl] on behalf of Helene Gudi
> > [helene.gudi at uni-hamburg.de]
> > Sent: 25 November 2014 13:15
> > To: fieldtrip at science.ru.nl
> > Subject: [FieldTrip] spectral resolution & interpretation of power
> estimates
> >
> > 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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> >
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>
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