[FieldTrip] time-frequency analysis: temporal alignment of output

[Aaron Schurger]

Hi, Eelke,
Sure, here it is:

cfg=[];
cfg.method = 'wavelet'; % or 'tfr'
cfg.toi = [];
cfg.foi = [1:12,14:2:30,35:5:100];
cfg.width = 7;
cfg.gwidth = 5;
cfg.trials = btn;

I would be keen on using the mtmconvol method, but then I have to
decide how to choose all of the parameters. Any guidelines?

Thanks!
Aaron


On Fri, Feb 7, 2014 at 5:57 PM, Eelke Spaak <eelke.spaak at donders.ru.nl> wrote:
> Hi Aaron,
>
> I had been assuming you were using cfg.method = 'mtmconvol', which
> does an implicit wavelet convolution, but lets you specify the
> parameters in terms of a window length, taper to use, and smoothing
> parameter (if using slepian taper sequence). Mtmconvol is the method I
> would almost always use, as it provides a greater degree of control
> over and flexibility in the parameters. However, now I understand you
> might be using cfg.method = 'wavelet'. Could you post the entire cfg
> you are using for ft_freqanalysis?
>
> Best,
> Eelke
>
> On 7 February 2014 17:39, Aaron Schurger <aaron.schurger at gmail.com> wrote:
>> Hi, Eelke,
>> Thank you for your reply - that is very helpful indeed. One question
>> remains then: how do I know what W is for each frequency band? I think
>> W will of course depend on the frequency (in the case of morlet
>> wavelets), being shorter for higher frequencies and longer for lower
>> frequencies. I know that wavelets are defined in terms of the standard
>> deviation of the envelope, but the window itself of course extends out
>> beyond 1 std of the envelope, to some point beyond where the wavelet
>> tapers to zero (I guess). So, if you know how I can compute W for any
>> arbitrary frequency band, in the case of morlet wavelets, I would be
>> very grateful.
>> Thanks again!
>> Aaron
>>
>> On Fri, Feb 7, 2014 at 12:56 PM, Eelke Spaak <eelke.spaak at donders.ru.nl> wrote:
>>> Hi Aaron,
>>>
>>> Indeed, the value you get at t = 0.3 will correspond to the wavelet
>>> (or, equivalently, the time window that was tapered and
>>> Fourier-transformed) centered at that time point. So if your time
>>> window (cfg.t_ftimwin) is 400ms long, the wavelet will have stretched
>>> from t = 0.1 to t = 0.5 s.
>>>
>>> There is no default option to align the time points of interest to an
>>> edge of the wavelet. However, there is a very straightforward solution
>>> to what you are asking: if you want to make sure that the wavelet
>>> 'knows nothing' of the future after e.g. t = 0.5s, then only consider
>>> the time points up to and including t = 0.3s for any subsequent
>>> analyses. (Or more general up to t = tA - W/2, where tA is the point
>>> of interest beyond which no info can bleed into the wavelet, and W is
>>> the window length.)
>>>
>>> Best,
>>> Eelke
>>>
>>> On 7 February 2014 12:43, Aaron Schurger <aaron.schurger at gmail.com> wrote:
>>>> Hi,
>>>> I am running a time-frequency analysis using ft_freqanalysis with the
>>>> wavelet method. I assume that the output in each frequency band is
>>>> temporally aligned to the midpoint of the wavelet (thinking in terms
>>>> of convolution): i.e. the power value that I get at time 0.300, for a
>>>> given frequency f, is precisely the dot product of f's wavelet
>>>> CENTERED at 0.300 sec in the data. Is that correct? If so, how can I
>>>> ask for the output of freqanalysis to be time aligned to the leading
>>>> edge of the wavelet rather than the center? I.e. I want to make sure
>>>> that each point in the output only has information about the past and
>>>> knows nothing about the future, with respect to that time. Is there a
>>>> simple way to do this?
>>>> Thanks!
>>>> Aaron
>>>>
>>>> --
>>>> Aaron Schurger, PhD
>>>> Senior researcher
>>>> Laboratory of Cognitive Neuroscience
>>>> Brain-Mind Institute, Department of Life Sciences
>>>> École Polytechnique Fédérale de Lausanne
>>>> Station 19, AI 2101
>>>> 1015 Lausanne, Switzerland
>>>> +41 21 693 1771
>>>> aaron.schurger at epfl.ch
>>>> http://lnco.epfl.ch/
>>>>
>>>> _______________________________________________
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>>>> fieldtrip at donders.ru.nl
>>>> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
>>>
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>>
>>
>>
>> --
>> Aaron Schurger, PhD
>> Senior researcher
>> Laboratory of Cognitive Neuroscience
>> Brain-Mind Institute, Department of Life Sciences
>> École Polytechnique Fédérale de Lausanne
>> Station 19, AI 2101
>> 1015 Lausanne, Switzerland
>> +41 21 693 1771
>> aaron.schurger at epfl.ch
>> http://lnco.epfl.ch/
>>
>> _______________________________________________
>> fieldtrip mailing list
>> fieldtrip at donders.ru.nl
>> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
>
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-- 
Aaron Schurger, PhD
Senior researcher
Laboratory of Cognitive Neuroscience
Brain-Mind Institute, Department of Life Sciences
École Polytechnique Fédérale de Lausanne
Station 19, AI 2101
1015 Lausanne, Switzerland
+41 21 693 1771
aaron.schurger at epfl.ch
http://lnco.epfl.ch/