User`s guide

E-Prime User’s Guide
Chapter 3: Critical Timing
Page 116
able to maintain synchronization within a millisecond after 10 minutes. Note that you can rerun
the calibration test with a scaling factor applied in order to verify the results of rescaling.
Once a scaling factor is computed, the actual rate of drift is minimal as long as both machines are
operating at a stable temperature. However, if the hardware or operating environment of either
the E-Prime computer or the biological recording system is changed significantly, the recalibration
will need to be performed and a new clock scale factor must be computed.
Timing Paradigm 5. Continuous sequences of events at high rate with short
stimulus times
The fifth timing paradigm involves presenting stimuli at a high rate in some form of continuous
loop. What makes this difficult is that the display time of the stimulus approaches the duration of
the preparation time for the stimulus, and it is not possible to display at a rate faster than the time
needed to setup the stimulus. Figure 22 illustrates this effect. The upper row shows the timing
under the Event timing mode conditions in order to display stimuli at a rate of 50ms, aligned to be
every third refresh. The 10ms rule (see Timing Paradigm 2, section 3.6.3.2) was used to set the
Duration to 40ms. After the 40ms duration, the next object waited for the next refresh event to
begin the display (at 50ms). The second row shows what occurs when we attempt a 33ms
presentation rate. In this case, the specified Duration is less than the preparation time necessary.
At the end of the preparation time (38ms), the program looks for the next vertical blanking signal.
However, since the display did not look for the vertical blanking signal until after the 33ms signal
had already passed, the display must wait until the next refresh at 50ms.
Figure 22. Inability to display stimuli faster than the preparation time of the stimuli in Event mode
timing.
The maximal rate of stimulus presentation is limited by the preparation time required by
each stimulus. The preparation time is dependent on the processor speed and the complexity of
the preparation task or stimulus itself. For example, presenting ImageDisplays typically involves
reading bitmap images from the disk, a process which takes approximately 30ms on a 266MHz
computer. The following table shows the maximum presentation rates for 3 different computers
attempting to present full screen pictures at progressively faster presentation rates.