User`s guide

E-Prime User’s Guide
Chapter 3: Critical Timing
Page 100
Stimulus Duration to Specify = (16.63ms/cycle * 3 cycles) –10ms
= 49.89ms – 10ms
= 39.89ms rounded 40ms
Since E-Prime only accepts durations as integer numbers of milliseconds we round 39.89ms up
to 40ms. Continuing with the example, if the stimulus duration is set to 40ms, the fourth vertical
blanking (hardware signal indicating the beginning of the next refresh cycle) would be expected to
fall at 49.89ms. So, by subtracting the extra 10ms, the experiment has ample time to
continuously monitor the hardware for the vertical blanking signal so that the next display may be
presented at the beginning of the fourth refresh cycle. Figure 12 illustrates this sequence of
events.
-10 ms-
STIM1
Specified Duration 40 ms
STIM2
0 16.63 33.26 49.89
STIM1 visible,
STIM2 monitoring
for next Vertical
Blank (Onset Delay)
Onset to Onset
Vertical
Blank Events
0
Time 39.89 ms
Figure 12. Correct specification of duration to ensure 3 refreshes of Stimulus 1.
When a display object uses Event timing mode with its Onset Sync property set to “vertical blank”
(the default setting), the actual stimulus onset will be delayed until the next vertical blanking event
is detected. That is, once the vertical blanking is detected, the base for the duration is effectively
set to 0ms, and the timing of the duration begins (i.e., wait for the vertical blank event to occur,
present the stimulus, then wait the specified duration of 40ms). If the next event (STIM2) is using
Event timing mode with its Onset Sync property set to “vertical blank”, its actual onset will be
delayed until the next vertical blank signal at 49.89ms. Note, the second stimulus is not starting
at an integer multiple of milliseconds but rather as soon as the vertical blanking event occurs.
When the synching signal (i.e., vertical blanking) is detected, the next event considers this signal
as the actual onset time of the stimulus, and processes the duration from this start time rather
than from the end time of the previous stimulus. This is necessary in order to stay synchronized
with the vertical blanking events. If timing does not re-synchronize with the actual vertical
blanking, the display timing and display updating will drift. The result is that some displays will be
more than, and others will be less than, the intended display times (e.g., refer to section 3.2.2).
The combination of specifying a reduced display duration and synchronizing with the vertical
blanking allows the experiment displays to be accurate even with minor timing variations in the
refresh rate. It is important to note that even using the same model display card on two identical
computers may result in slightly different refresh rates. Hence, a 50ms duration on one monitor
might result in a period of 49.9 to 50.1ms across cards. By setting a duration such that E-Prime
begins looking for the next refresh at 10ms before it is expected, minor variations across cards
and delays in an E-Prime program (typically 0.5ms standard deviation due to rounding to the next