[Eeglablist] epoch length varied during computation

Bradley Voytek bradley.voytek at gmail.com
Sat Dec 11 17:52:01 PST 2010


Zach:

Because EEGLAB uses sliding-window wavelets on epoched data to
decompose the ERSP, your parameters will have a large effect on your
decomposition window. This line is key:

The window size used is 285 samples (1113.28 ms) wide.

Because your window size if 285 samples (determined by the number of
cycles used, the lowest frequency of interest, etc.), and your
sampling rate is probably 265 samples/second, then your decomposition
has no resolution at any time points before 56.6 ms or after 439.5 ms.

The math is simple:

* 1113.28/2 = 556.64.
* -500 + 556.64 = 56.64
* 996 - 556.64 = 439.36

You need to increase your epoch length or--if I remember newtimef
correctly--you might be able to increase your padratio to change the
size of these windows.

::brad

On Thu, Dec 9, 2010 at 09:45, Saim Rasheed <saim_rasheed at hotmail.com> wrote:
> Hi,
>
> I am using EEGLAB v9.0.3.4b
> Created a STUDY with 9 different conditions and 5 subjects. Sampling rate is
> 256 Hz.
> preComputed channel measures with default parameters using GUI.
> epoch length is -500 to 1000 ms.
> I have a problem while precomputing channel measures as per following
> message displayed.
>
> "Each trial contains samples from -500 ms before to
>   996 ms after the timelocking event.
>   Image frequency direction: normal
> Using 3 cycles at lowest frequency to 64 at highest.
> Value of 'timesout' must be <= frame-winsize, 'timesout' adjusted to 99
> Generating 99 time points (56.6 to 439.5 ms)
> Distribution of data point for time/freq decomposition is perfectly uniform
> The window size used is 285 samples (1113.28 ms) wide.
> Estimating 100 log-spaced frequencies from 3.0 Hz to 128.0 Hz.
> Processing time point (of 99): 10 20 30 40 50 60 70 80 90
> Computing the mean baseline spectrum"
>
> ERSP plots are not displayed according to my epoch lenght. It only displayes
> ERSP for the time points computed above (red text in bold). Indeed it should
> compute for the range -500 to 1000 ms. Where can I fix this parameter?
> Moreover, my data is already bandpass filtered from 1 to 30 Hz but it
> computes frequencies from 3 Hz to 128 Hz. How to confine computation from 1
> to 30 Hz. Although I can display from 1 to 30 Hz.
>
> Probably I am missing some parameter fixing.
>
> Any one please help.
> thanking you in advance.
>
>
> Saim
>
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