Hello,
We are currently writing up a program in Eprime and were going to send a trigger immediately preceding the presentation of a picture stimulus.. we were then going to keep the stimulus up for 800ms and then turn the trigger off (after the picture had disappeared)... our question is does the biosemis system (activeView2) use the falling edge or the rising edge of the trigger pulse to determine where the trigger fell? If it's the rising edge then our scheme should work... otherwise I assume we have to turn the trigger on, then off, and then put up our picture...
Thanks in advance....
EEG trigger- rising edge or falling edge?
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wesleyclapp
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Coen
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ActiView displays both the rising and falling edges of the triggers. It is instructive to compare the "analog" and "decimal" display formats while the system is running.
The recording of triggers in the BDF file is level sensitive, not edge sensitive. For example, bit 0 of the trigger/status channel (last channel) is High for every sample where trigger input 1 on the USB receiver is held at 5V, and Low for every sample where the input is held at 0V.
Please note that several review software packages only display the (rising) edges found in the trigger channel, and ignore the information of the falling edges.
The downsampling options in ActiView are based on the assumption that positive trigger logic is used (High = stimulus, Low = no stimulus). Using negative trigger logic may result is missed triggers, because the downsampling routine uses an OR function between the triggers of a group of samples.
Best regards, Coen (BioSemi)
The recording of triggers in the BDF file is level sensitive, not edge sensitive. For example, bit 0 of the trigger/status channel (last channel) is High for every sample where trigger input 1 on the USB receiver is held at 5V, and Low for every sample where the input is held at 0V.
Please note that several review software packages only display the (rising) edges found in the trigger channel, and ignore the information of the falling edges.
The downsampling options in ActiView are based on the assumption that positive trigger logic is used (High = stimulus, Low = no stimulus). Using negative trigger logic may result is missed triggers, because the downsampling routine uses an OR function between the triggers of a group of samples.
Best regards, Coen (BioSemi)