e-prime task runs for variable durations

[Jim]

Forum,

I'm developing a basic CPT task to use in conjunction with fMRI. The
task displays a letter for 250ms, a blank screen for 1 second, then
scores responses using inline code. Rinse and repeat.

One thing that I've noticed with e-prime is that the duration of a
task changes across trials, even when all objects are set to
cumulative (and objects are set to run for a specified length of time
as opposed to terminating with user input). For example, I ran a task
this morning 4 times, 2 with 100ms prerelease and 2 without
prerelease. The difference between the longest and shortest trials
accross task versions was 5 seconds; the difference between the
average duration for the 2 task versions was 2 seconds. This is
problematic for me since most of the time I am using e-prime in
conjunction with fMRI or physio data collection. I can't get a handle
on how long I should set my scanner program for if the e-prime program
has a variable duration.

Is this variable duration to be expected or something problematic with
my task? If it is to be expected, how do folks synch scanner/physio
programs with e-prime?

Jim

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