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

[Eelke Spaak]

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
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>>
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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/
>
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