Hi Mario, thanks a lot for your response.
When I ran this line
VBLSyncTest(1000, 0, 0.6, 0, 0, 1, 0)
the output is
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PTB-WARNING: Flip for window 10 didn't use pageflipping for flip. Visual presentation timing and timestamps are likely unreliable!
PTB-WARNING: Something is misconfigured on your system, otherwise pageflipping would have been used by the graphics driver for reliable timing.
PTB-WARNING: Read the Linux specific section of 'help SyncTrouble' for some common causes and fixes for this problem.
INFO: PTB's Screen('Flip', 10) command seems to have missed the requested stimulus presentation deadline
INFO: a total of 10 times out of a total of 1005 flips during this session.
INFO: This number is fairly accurate (and indicative of real timing problems in your own code or your system)
INFO: if you provided requested stimulus onset times with the 'when' argument of Screen('Flip', window [, when]);
INFO: If you called Screen('Flip', window); without the 'when' argument, this count is more of a ''mild'' indicator
INFO: of timing behaviour than a hard reliable measurement. Large numbers may indicate problems and should at least
INFO: deserve your closer attention. Cfe. 'help SyncTrouble', the FAQ section at
www.psychtoolbox.org and the
INFO: examples in the PDF presentation in PsychDocumentation/ Psychtoolbox3-Slides.pdf for more info and timing tips.
PTB missed 73 out of 1000 stimulus presentation deadlines.
One missed deadline is ok and an artifact of the measurement.
PTB completed 0 stimulus presentations before the requested target time.
Have a look at the plots for more details...
****
but after running Screen('Preference', 'SuppressAllWarnings', 1) or Screen('Preference', 'Verbosity', 0)