I am programming a simple experiment, showing letter strings with a fixed duration of 1000ms and interstimulus fixation crosses with variable durations (using a trial variable ISI durations). However, each stimulus takes 9 to 11 ms longer than the specified time, which wouldn’t be so bad if the error were not cumulative. In this case, it leads to 2 seconds extra on 100 trials.
I know this may have to do with the monitor refresh rate, but the error occurs whether I set this to 60 or 75 Hz. I have also tried expanding the trials (the presented words use stimulus lists) and setting the trial time manually (because it is different for each trial - a trial being word presentation and fixation cross), making sure the time of the individual events is shorter than the trial time. This, however, still yields the same error, although aproximately cut in half, of course. Even if this were to solve the problem though (which it doesn’t), this is of course very time consuming and labor intensive, so it’s definitely not what I want.
So, here’s my questions:
- Is there no way around the strict dependence on the monitor refresh rate (which I assume is the cause of the error)?
- Is there a way to at least not make the error cumulative?
- Why is it not possible to use presentation time lists for trials, just like for events?
If no other solution works, I guess I have to recalculate my presentation times into intervals that fit the monitor refresh rate, but this certainly does not seem to be the ideal solution, if only because I may wish to present the experiment on different machines …
Thanks for your help!