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Step 2 – Event code extraction

The file myeegfile.eeg now contains the digitized signals and the event codes for stimuli and responses. For further processing, these event codes and positions in time have to be extracted by the following command line:

eegpos myeegfile.eeg myeegfile.pos

This will create the text file myeegfile.pos containing the following information:

(time sample when the event occurred) (event code) (rejection status: see Step 4)

6745    10   0
7785    10   0
8765    10   0
9745    20   0
9995   100   0
10755   10   0
11775   10   0
.......

In this example (odd-ball paradigm), sampling rate was 1000 Hz, and stimuli were presented with an onset-to-onset inter-stimulus interval of about 1000 ms (randomized). Standard stimuli had trigger code 10 and target had code 20. Subjects had to press a button after target, generating an event code 100. The reaction time in this particular example for the target was 250 samples (9995-9745), i.e., 250 ms.

The event position file myeegfile.pos is a text file that can be easily processed for possible event recoding (see poschseq) or reaction time computation (see posrt). It can also be transformed by user’s specific software (Matlab or C program). More details in Event Processing Tools.