E Prime and Black Box

[Shivani R]

Hi,
I am using E Prime to develop the experiments. In this experiment,
the
subject sees a fixation mark followed by a string of letters. He/she
also hears a tone after 50 or 100 or 200 or 400 or 800ms, the string
has been displayed on the screen. The subject's task is to determine
if the string of letters was a word or non-word. I am using an
external device (Coulbourn) to produce the tone after 50 or 100 or
200
or 400 or 800ms duration, the stimulus has been displayed on the
screen. Also I am using black box tool kit to measure the duration of
the fixation point, stimulus, and also to measure the time after
which
the tone is played. The duration of fixation mark is 1 s, the
duration
of stimulus is 2 s and the time after which tone should be played is
50 or 100 or 200 or 400 or 800 ms.

After using black box to measure the timing precision, I found out
that there is no issue with the duration of fixation point and
stimulus, but the tone seems to be played before the stimulus is on
the screen for 50 ms tone lag. I don't know why it is so, because the
script in the experiment clearly says that tone is played after the
stimulus has been displayed on the screen.


Time lag                                   Actual time lag
in script(ms)                             I am getting in
                                                 real time(ms)

800                                               600
400                                               250 or 262
200                                               172 or 82.5 or 95
100                                                20
50                                                 -80

 (-) means I am getting the tone earlier than it should be. This
shows
that for 50ms, tone is getting produced before the picture comes up
on
the screen. 50ms is such a small time unit and it is not discernible
for me to figure out this. This is really important for me to figure
out why this difference is appearing. I mean in my code I have set
tone lags as 50, 100, 200, 400 or 800 but I am getting all of them as
something less in real time. This experiment would be used for
scientific studies and so timing is really really important for this.


Any help is really appreciated.


Regards
Shivani

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