Group: Forum Members
Hi there Inquisit wizards,
I have a timing issue that has been bugging me for some time. I have attached a very simple example script which illustrates the problem. The issue is with the / trialduration command in the trial element.
I need my trials to be a specified duration, with as minimal an error as possible. The attached program presents a single word on the screen for two seconds, and this is repeated for 80 trials. This should give a total running time for the program of 140 seconds (80 trials x 2 secs). Trial duration is controlled by /trialduration = 2000.
The problem is, there is a consistent error such that the program always runs approximately 3.5 seconds long (timed 'by hand' with a stopwatch). It appears that Inquisit is adding approximately 35-40 ms to each trial's duration. The logged data simply say that each trial's duration is 2000 ms. It doesn't seem to matter how long the trial is i.e. if I specify a trial length of 1 second or 10 seconds, there always seems to be a bit extra added to every trial.
35-40 ms doesn't seem like much, but over a large number of trials it adds up. Previously I've been coping with this by simply specifying trial length to be 1960 ms, and this appears to give trials of approximately 2000 ms, however this has always made me somewhat nervous as I'm not really sure where the error is coming from and whether it really is consistent across trials. I know that specifying exact timings is difficult because of refresh rates etc. etc. but I think the error due to Inquisit waiting for the next screen refresh should be less than this, yes? I've tried this on several different computers, running both Vista and XP, and changed all the parameters I can think of in the program, and it seems to be consistent and reproducible. I've now run into a situation where I really do need absolutely precise timing information (for synchronisation with a MEG machine, which can sample at very high temporal resolutions) so would appreciate anybody's thoughts on this. I know Inquisit is very good at, for instance, presenting items for a single screen refresh etc. so I'm surprised that this error seems so pervasive.
Apologies for the length of this post, and many thanks for your help with this.
Matt Wall (Royal Holloway, UK).