Timing Question

[Emz]

Hello,

I need some help with a timing problem in a reasonably complex set-up
(the complexity is in in-line scripts that output to another system,
which requires the use of the IsPending() function, the task itself is
just an N-back variant). I have tried and tested many minor changes in
the set up and have managed to eliminate all of the timing problems
except one: the actual duration of stimulus presentations are always
either one of two values, each different than expected (as based on
multiples of screen refresh rate).

The specifics are as follows:

The screen refresh rate is 75.031Hz, giving a duration of 13.3. I wish
to have presentation times of about 500 ms and 2000 ms, so as per
manual I have selected about 10 ms less than the nearest multiples
(505.4 and 2008.3), resulting in the set presentation duration times
set to 495ms and 2000ms.

Errors are consistently as follows:

527.3438 - 505.4000 =  21.9438 (many more of this one)

558.5938 - 505.4000 =  53.1938 (only about 15% of the ~500 ms display
items)

2027.3483 - 2008.3000 = 19.0483 (equal proportions with below for the
~2000ms items)

 1996.1 – 2008.3 =  -12.2

I am currently not using the pre-loading option as I read somewhere
this was not necessary for this sort of task. I also have some in-line
scripts between display items, and the before mentioned IsPending loop
in some places, for which I don’t know if the pre-loading would even
make sense. The aforementioned IsPending loop doesn’t seem to be the
problem as trials with and without it have exactly the same errors.
Deleting or moving the in-line scripts (which are used to output data)
appear to produce a bigger variety of errors.

Ideas ?



E.

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