[Eeglablist] Epoch length in EEGLab

[Clemens DICKHUT]

Dear Joseph,

generally your trial length and your research question/post-processing. If you want to calculate event-related potentials you usually need 1000-500ms pre-stimulus baseline activity to extract the mean amplitude (which is then subtracted from post-stimulus time points). Depending on the components you want to investigate, i.e. N200/P300, consequentially you will need at least 500ms post-stimulus, better 1000. A good start probably is -500 1000.

For time-frequency transforms you will need longer epochs, because, depending on the wavelet starting frequency, a substantial amount of time points will be cut away both pre and post stimulus (this is due to the wavelets that take only the middle time point). I.e. epochs from -1000 3000ms time-frequency transformed with a wavelet starting from 1Hz will leave you with data values from ca. -500 2500. So, be aware of that and go for longer epochs of you want to observe time-frequency activity. Some additional thing: for time-frequency analyses you may (this is a later step in the analysis) indicate the baseline (if) to be subtracted from -800 to -200 or something like that, to avoid subtracting post-stimulus activity “smearing” into the baseline (due to the reason mentioned above, especially relevant for low frequencies).

For both approaches, if you have non-random stimulus overlap or responses leaking into the pre-stimulus period, its sometimes better to go for shorter pre-stimulus times.

Hope this helps.

Best,
Clemens




On 25 Jun 2016, at 22:06, Joseph Nuamah <jknuamah at aggies.ncat.edu<mailto:jknuamah at aggies.ncat.edu>> wrote:

Dear All,

I am new to EEGLAB.

My question is: what determines the choice of epoch length in EEGLAB.

Thanks.

Joseph.
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