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

[Iversen, John]

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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