[Eeglablist] FW: What is meant by 'epoch' and 'frames' in EEG Lab

[Alonso Valerdi, Luz M]

Hello Rajesh:

Firstly, the data is recorded in CONTINUOUS mode, and when you import it into EEGLab the data is always in continuous mode.

Secondly, when we record EEG signals, we often make use of triggers (markers that help you to know when specific events happened). Those triggers are recovered by EEGLab and it's very frequently to study our EEG signals according to them. For instance, in the paradigm illustrated below, we have four events (4 different kind of sounds) that were marked by four triggers (1,2,3,4). The recording is continuous, but we have marked the moments that we are interested in. When we "epoch" the data, we indicate to EEGLab what are the segments of the continuous we are interested in. Then the software will extract only the EEG segment that are framed by the triggers (according to the time points you previously indicate) along the whole continuous data. After this procedure, you stop having continuous data.

[cid:image003.jpg at 01CCFB22.7A571910]

Finally, as far as I understand, the frames correspond to the number of samples that your equipment record per second. This sample frequency depends on the characteristics of your equipment, it may be 128 samples per second, 256 samples per second, or 512 samples per second (these are just examples).

Hope this helps!
Luz

From: eeglablist-bounces at sccn.ucsd.edu [mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of D.rajesh
Sent: 03 March 2012 01:45
To: eeglablist at sccn.ucsd.edu
Subject: [Eeglablist] What is meant by 'epoch' and 'frames' in EEG Lab


 Hi ,

           I am new to EEG lab and to the subject. I'd like to know what is meant
by 'epoch' in EEG. Isn't the EEG signal is collected continuously in time ? Then where
does the concept of 'epoch' and 'frames' come from and what do they mean ?


Thanks and regards

Rajesh.D

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