bci2000.org BBS

I wonder if there is any way to change the delay time between the auditory stimuli and visual stimuli in the application P3Speller?

When I see the stimuli output on an oscilloscope, the auditory stimulus appears about 50ms after the visual stimulus.

We will like to get a zero delay or the posibility to change it.

Best regards,

Ariel.

Ariel,

Technically, it is very difficult to achieve perfect synchronization between auditory and visual stimuli and EEG data acquisition (e.g., there are latencies in the audio driver, or on the monitor). If EEG are to be processed in real time as well in relation to the stimuli, it becomes even more difficult.

Thus, while perfect synchronization can be achieved, it is quite difficult, and most likely results in a system that is extremely specific to the one particular experiment you are trying to implement.

With BCI2000, we do not try to achieve perfect synchronization in favor of a platform that can be much more general than a specific implementation that wants to achieve perfect synchronization. However, we just submitted a paper on a timing evaluation system (that is now part of the BCI2000 distribution). Using that system, and properly chosen components, it is possible to get audio and video latencies that are both small and not much different from each other (i.e., around 10 and 15 ms, respectively).

As a general rule, at least at present, stay away from Vista and stick with XP SP3. You also may want to use a CRT monitor instead of an LCD monitor (set to 1024 x 768 @ >= 100 Hz). We achieved the best audio latencies with a Dell Optiplex system.

I hope this helps.

Gerv

Dear Grev:

Thank you very much for your reply.
In view of the comment on the hardware, our concern would be if we could introduce few lines of code into the source to cause a delay of the visual stimulus. If this can be done, we would appreciate if you could point out in what place do it.

Ariel