Martin - it would seem that your experiment requires the timing accuracy and precision that DirectRT harnesses. MediaLab is not powerful enough to specify the display of an image for a period of time as small as 40 ms. DirectRT on the other hand, has been designed to make exactly this type of experiment very easy to create and manage.

The advantages to using DirectRT would include the ability to directly render to the user's screen (meaning fast and precise display), time inputs to their lowest and most precise unit of measurement, and the ability to manage both the data and input files through Excel.

See also:

http://www.empirisoft.com/Support/showthread.php?t=15
http://www.empirisoft.com/Support/showthread.php?t=17