Stimulus duration in error?

[andi]

Hi there,


the following question:
in my experiment I present in the follwing order:

fixation cross (500ms) - stimulus (1000ms) - blank screen (random time
between 1000 - 1200 ms).


I am using event mode as the duration of the stimulus needs to be
constant (EEG recording - ERP experiment) - always 1000 ms.

Times are adapted to vertical refresh (i.e. fixation cross duration
483 ms instead of 500 ms). The stimulus preload is 100 ms.


Problem: in the edat-file (constructed during the presentation) I am
told the stimulus duration is 1000 ms (as wished). The time difference
between the onset and offset of stimulus equals 900 ms (for example
onset: 191503, stimulus offset: 192403).

Why is that? Does that mean the stimulus is presented only for 900 ms?
If yes, what can be done about it?



Thanks for your help!


Andrea
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