difference between Cedrus response pad and exp.time()
I'm designing a priming study using OpenSesame and I have a difference from around 11ms to around 20ms between the responses collected using a button box and the recording of exp.time(). Below more infos.
The task is to decide if the 2nd word appearing on the screen is word or not pressing a botton (if it's not a word, the participant has to wait for 3000ms for the new stimulus). I'm using the Cedrus RB-530 response pad to collect the reaction times. As discussed in this forum, I've downloaded the pyxid library and I'm using it to get the time measurements of the button box.
the design of the experiment is the following:
1stWord(prime) --> 50ms blank --> 2ndWord(target) --> next trial
If no button is pressed the target word will stay on the screen for 3000ms, otherwise, it will move to the next stimulus when a button is pressed.
In order to control for this, I set up a coroutine around the target sketchpad. In the function associated with this coroutine, I set the duration of the corutine (by default set to 3000ms) to 0 when a button is pressed. In the same corutine function, I'm doing assigning t1=exp.time() in the 2nd line of the function (immediately after def) and t2=exp.time() when a button clic is detected. At that point, I compute tt=t2-t1 as the time between the visualization of the stimulus on the screen and the button press. Unfortunately, there is always a difference of btw 11 to 20ms from the button box results and tt (tt is always higher than the buttonbox response). Does anyone experienced the same problem? Which measurement should I consider more accurate?
thanks a lot for your help,