fMRI catch trial response messing up timing of presentation
I'm working on an fMRI experiment with a 2000 ms TR in which the first 1000 ms is a scan, and next 1000 ms is a silent gap in which we present auditory stimuli.
At first, I tried using the "absolute timing" script that Sebastiaan provided to make the trials be presented every 2000 ms. This worked for the first block, but because we are using a multiband sequence that takes the scanner 40s to warm up, the "time_to_pad" variable would inevitably get messed up between blocks, and the second block's timing sequence would be off. Instead, it presented consistently presented stimuli every 1700 ms.
I abandoned the absolute timing in favor of having every trial triggered by the "5" keyboard response sent by our scanner to the testing computer. At the beginning of a TR, the scanner sends an emulated "5" keypress to the computer. As such, I put a keyboard response that required a "5" at the beginning of the block sequence to initiate the trial. It is then followed by a 1000 ms advanced delay, so that the audio doesn't start playing during the noisy scan sequence. Following the advanced delay is a sketchpad with a fixation cross that has a duration of 0, then the sampler that plays the auditory sequence, and then a keyboard response. On some trials, the stimulus is played more quietly and we use that as a catch trial to ensure participants are paying attention. Here is what this looks like
I currently have the timeout for the catch_response set at 1000 ms. However, when I look at the timing in my log file for the sampler called "stimulus", I see that trials are lasting 4000 ms instead of 2000 ms.
When I remove the "catch_response" item, the stimuli go back to being presented every 2000 ms and the timing is perfect. However, this breaks our ability to have catch trials.
I'm really struggling with this but I believe it has something to do with the timeout for catch_response. Any help would be much appreciated with this issue, as it will allow our lab to use OpenSesame for all of our future fMRI experiments.