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

Eelke Spaak eelke.spaak at donders.ru.nl
Fri Feb 7 12:56:13 CET 2014

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.)


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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