User`s guide

E-Prime User’s Guide
Chapter 3: Critical Timing
Page 98
timing precision is unnecessary for most experiments. Generally, experimental paradigms have a
specific set of critically timed segments that must be carefully checked and verified, while the rest
of the events of the experiment do not require the same level of precision (e.g., whether
instruction screens had 14 or 52ms of preparation time is typically irrelevant in most experiments,
while the length of time to prepare and display a probe stimulus is typically critical).
Recall that there are two distinct timing modes within E-Prime; Event mode and Cumulative
mode. Choosing the appropriate timing mode is contingent upon the timing requirements of the
individual paradigm at hand. Specifically, paradigms vary in terms of when they use Event and
Cumulative mode timing, and when they use the PreRelease processing feature.
Precise timing is influenced by a number of things, such as the refresh rate of the display
hardware, the speed of the machine, and the actual E-Prime experiment. The refresh rate of the
monitor and video card specifically determine the shortest duration possible for any display.
Furthermore, since some displays may require more setup or preparation time than others (e.g., a
full screen, full color image will require more setup time than a single text string), the computer
must be capable of quick processing.
3.4.2.1 Refresh rates and display duration calculation in E-
Prime
Recall that all visual stimuli will be presented for an integer multiple of the refresh rate, and the
refresh rate varies with the video card, screen resolution, color depth, and monitor settings of the
computer. The refresh rate is effectively set at the beginning of the experiment and cannot be
reset during the experiment without significant, and possibly serious, visual disruptions. Stimulus
presentations must be both synchronized to and multiples of the refresh rate.
Remember that when moving an experiment from one computer to another, or changing
other display characteristics (e.g., color depth or screen resolution), the refresh rates are
likely to change and settings will not be optimal, potentially causing a different stimulus
presentation than is specified in the original method.
For example, the table below illustrates the variation in refresh rates for display parameters
constant across three computers, all running with a display resolution of 1024x768 and 8-bit color
depth (256 colors). In general, the refresh rates of the video card cannot be directly controlled
reliably
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. The table illustrates results from timing tests conducted while attempting to present a
stimulus for 20, 60, and 100ms.
Observed durations across different computers and video hardware
Specified Duration (ms)
20 60 100
Computer
Refresh Rate
(Hz)
Refresh
Duration (ms) Actual Duration Observed (ms)
1 60.15 16.63 33.26 66.52 116.41
2 75.07 13.32 26.64 66.60 106.56
3 85.00 11.76 23.52 70.56 105.93
Max Error 66% 18% 16%
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Most video cards ignore the requested refresh rate parameter. Video device drivers look up an internal
table for resolution and color depth and then set, but do not report, the refresh rate. In E-Prime, the refresh
rate is measured and logged at the beginning of each run of the experiment.