[Eeglablist] epoch length varied during computation

Arnaud Delorme arno at ucsd.edu
Sat Dec 11 16:07:35 PST 2010


Dear Sain,

> 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?


You cannot. To compute time-frequency decompositions, it is necessary to extract time windows. The latency of each time-frequency estimate corresponds to the center of each of these windows. Therefore it is not possible to obtain a time-frequency estimate at -500 ms (it would mean that your window size is 1 sample and it does not make sense to perform FFT or wavelet decomposition on a single sample). For instance, assuming data limits of -500 ms to +1000 millisecond, a sampling frequency of 1000 Hz (1000 samples per second), and a window size of 500 samples, the earliest time-frequency estimate may be obtained at -250 ms  latency (-500 ms is -500 samples compared to the time-locking event and the first window spam from latency -500 ms to 0 ms and has its center at -250 ms). Hope it makes more sense.


> 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.


Use the 'freqs' option of the newtimef function (or enter [1 30] for the frequency limit in the graphic interface).

Best regards,

Arno

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