User`s guide

E-Prime User’s Guide
Chapter 3: Critical Timing
Page 85
or synchronizing timing with external equipment (e.g., brain wave monitoring), it is important to
understand timing concepts described here to accurately specify the experiment.
In this section, we describe five techniques for achieving accurate and precise timing in
computerized research. Each of these techniques is grounded on a concept about timing and
computers that may be novel to the reader. It is important to understand both the concepts of
what makes precise timing difficult and the techniques E-Prime offers to deal with these concepts.
Once these underlying concepts are understood, you will find that you can use a variety of
methods to achieve the timing accuracy and precision required for your particular paradigm. The
next subsections describe concepts and methods. The following section (3.4 - Implementing
Time Critical Experiments in E-Prime) presents specific E-Prime models for implementing
common experimental paradigms.
3.3.1.1 Technique 1: Using PreRelease
Computers can require significant time periods to prepare stimuli for presentation, and it is critical
to conceptualize and account for this time. Preparation time for the next stimulus must occur
concurrently with the present stimulus display events in order to avoid delaying the time between
stimulus events. The most serious cause of timing inaccuracy arises from the false assumption
that preparing stimuli for presentation takes negligible time (Refer to Appendix E for a description
of the timing of events involved in the execution of an object). A specification might read “Present
a series of words in 18 point font for 100ms each.” The actions of selecting the stimuli,
generating the stimuli in a high-resolution font, and displaying the stimuli could take
approximately 30ms (on a 120MHz computer). So the stimuli are presented after a time delay
that was not originally considered in the specifications.
The table below illustrates the effect of the preparation time of the stimulus on the timing
specification. The term onset time is used to define the time at which the stimulus would be first
written to the display (or audio) hardware (all response times in E-Prime are also time-stamped
relative to this onset time when the object enables response collection). The table below
illustrates the onset times for the events that are expected to occur at 100ms intervals. The first
line assumes a 0ms setup time. The second line shows the times with a more realistic 30ms
setup time.
Stimulus Onset Time
Sequential Stimulus
1 2 3 4
• Expected time assuming 0 preparation
0 100 200 300
• Time with 30ms preparation time
30 160 290 420
• Time with preparation of next stimulus being
concurrent with display of current stimulus
(use of PreRelease in E-Prime)
30 130 230 330
With a setup time of 30ms, the stimuli onset would occur at 30, 160, 290, and 420ms. Notice that
the onset times keep drifting from the intended time of 100ms intervals. The last line shows what
the onset times would be if you could somehow prepare the next stimulus, while the current
stimulus was still being presented. Since a 0ms preparation time is not possible, only the last line
meets the timing specification outlined.
Now that you understand this underlying problem conceptually, you are ready to ask how E-Prime
addresses the issue. E-Prime does this by providing a PreRelease property on each stimulus
presentation object, which allows an event to be prepared prior to the effective termination of the
previous event. Specifying the PreRelease time allows one stimulus to absorb the setup time for
the next stimulus, while it waits for the targeted end-time of the first event.