Trigger shifting when recording ASSR

Post Reply
GerardEncina
Posts: 1
Joined: Tue Feb 05, 2013 1:43 pm

Trigger shifting when recording ASSR

Post by GerardEncina »

Hi all,

We are trying to record Auditory Steady-State Responses (ASSR) with Biosemi ActiveTwo and we are getting quite noisy data due to trigger shifting problems. The stimuli is generated in Matlab though the soundcard, and the trigger is also generated in Matlab thought the parallel port. We first set the trigger on (1), wait some time and we unset the trigger (0). Then, we send the stimuli. A trigger point the beginning of a sweep, which contains an integer number of epoch which will be averaged off-line when the recording is done. The complete instructions we implemented to Matlab are shown at the end.

Depending on the stimuli length and the pause between setting on and off the trigger, we get trigger time errors diverging 10 to 40 milliseconds from the expected time. Obviously, this time shift is a huge amount when dealing with ABRs or ASSRs.

Does anyone has any solution or idea to fix this issue? I would really appreciate any comment or suggestion.
Many thanks in advance.

Gerard Encina
Technical University of Denmark

PD: Matlab code:

%Set parallel port
ioObj = io32;
status = io32(ioObj);
address = hex2dec('D900');

%Set trigger (on, wait, off)
io32(ioObj, address, 1);
pause(0.001);
io32(ioObj, address, 0);

%Send stimuli
wavplay(sign_out, fs,'sync');
disp('...DONE...')

clear io32; %Clean up

Post Reply