Millisecond Forums

Hello all!

I am writing to ask about a discrepancy we're noticing in our Inquisit data files, specifically between the "elapsedtime" field and the "trialduration" fields. These data were collected with Inquisit Lab 6.5.1, in a script that presents 

Our trials look like this (full script attached):

<trial POSO_posBG_shrt_ITI_6pt4>
/ validresponse = ("1", "4")
/ correctresponse = ("4")
/ stimulustimes = [0 = fixation; 800=noreplace (POSO);
1000 = blanks;
1100 = poso_goodtarget;
1600 = blanks]
/ trialdata = [POSO poso_goodtarget]
/ beginresponsetime = 1100
/ trialduration = 8000
/ responseinterrupt=frames
</trial>

The trialduration field in our data files matches exactly what we specified in the trial element itself in the script. For our final few subjects, we also had Inquisit save an audit/log file, which perfectly matches our expected trialduration fields, except in rare situations where a participant didn't respond (in which case there was a 26-27 ms delay in the onset of the next trial).

However, the elapsedtime field in our data files sometimes seem to diverge by tens or hundreds of milliseconds from the trial duration.

The reason we're double-checking this carefully is because these data were collected in an fMRI experiment, and we're using the Inquisit data to create event-timing files for analyzing the fMRI data.

What exactly is the elapsedtime field measuring? We can provide more example data to illustrate this point if it's helpful. Thank you!