[Eeglablist] ersp times: what are these values and how are they formed?

Makoto Miyakoshi mmiyakoshi at ucsd.edu
Fri Aug 20 14:50:25 PDT 2021


Dear Sean,

This may help.
https://sccn.ucsd.edu/wiki/Makoto's_preprocessing_pipeline#A_tip_to_compute_time-frequency_transform_.28i.e._ERSP_.26_ITC.29_.2801.2F11.2F2017_updated.29

Makoto

On Fri, Aug 20, 2021 at 11:02 AM Iversen, John via eeglablist <
eeglablist at sccn.ucsd.edu> wrote:

> Hi Sean,
>
> When you precompute ERSPs there are a number of parameters that determine
> the times at which the ERSP is calculated. By default it computes 200
> evenly spaced points, but it will never include the full epoch because of
> edge effects in the wavelet decomposition--it will chop off time equal to
> half of the lowest frequency wavelet's length on either end of the epoch.
> That wavelet's length is determined by the lowest frequency you ask for as
> well as the wavelet parameters (# cycles). Raising the lowest frequency or
> reducing the cycles will reduce the amount of data lost. Check out the
> cycles, freqs and timesout parameters in newtimef.m
>
> So, if you wanted to see the full 0-28 seconds in your ERSP, your original
> epochs would have to be padded to be longer. The amount of padding depends
> on the lowest wavelet.
>
> I can't speak to the specific time values you got back, which don't make
> sense to me since they don't represent a symmetric loss of data on either
> end of a 0 to 28 second epoch: 0.559 to 14.443 seconds. Curiously, it
> represents about exactly half of your original epoch length. Without
> knowing the parameters that went into computing ERSPs it's hard to
> interpret.
>
> Best,
>
> John
>
> On Aug 16, 2021, at 12:13 PM, Sean Gilmore via eeglablist <
> eeglablist at sccn.ucsd.edu<mailto:eeglablist at sccn.ucsd.edu>> wrote:
>
> Dear EEGlab List,
>
> I am using the function 'std_erspplot' to calculate a time frequency
> decompositions for a specified cluster of dipoles. Data from each
> participant contains 60 events, with 30 unique triggers (each stimuli was
> played twice). Each event is epoched into 0sec  - 27.99sec segments.
>
> I have run the std_erspplot function on a specific cluster using this
> syntax.
>
> [STUDY erspdata ersptimes erspfreqs pgroup pcond pinter] =
> std_erspplot(STUDY,ALLEEG,'clusters', cluster)
>
> The output variable ersptimes contains a 1x 200 array of latencies from
> 0.0559 - 1.4443 * 1.0e+04. This array is peculiar because it does not
> correspond to the time points that are defined in the epochs (0 - 28sec).
>
> Does anyone know how ersptimes is calculated? What determines the size of
> this array, and further, why is it is producing this array which doesn’t
> correspond to the times in the epoch.
>
>
> Thanks for your time. Cheers
>
> -
> Sean Gilmore, M.A. (PhD candidate)
> Department of Psychology
> SMART lab
> Ryerson University
>
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