User`s guide

E-Prime User’s Guide
Chapter 3: Critical Timing
Page 105
In the trial procedure, the Probe stimulus is removed from the screen by the Feedback object
(i.e., the Feedback is displayed over top of the Probe, thus effectively removing it from the
display). The variable Probe.RT stores the reaction time data (in milliseconds) for the response
collected by the Probe object. Note that the timing is based on a stimulus that would appear at
the top of the screen. The lower the stimulus is placed on the screen, the later the expected time
would be relative to the beginning of the refresh cycle (i.e., when the redraw of the screen
begins). For example, with a 16ms refresh duration and time 0ms being the time at which the first
pixel is redrawn, the times for stimulus placement at the 25%, 50% and 75% locations of the
vertical screen would be at about 4, 8 and 12ms respectively (e.g., 16ms * 0.25 = 4ms, 16ms *
0.50 = 8ms, 16ms * 0.75 = 12ms). There is also some keyboard response delay in the range of
5ms, although it varies with keyboard manufacturers (each input device has its own degree of
response delay refer to Appendix A – Timing Test Results for a review of delays for common
input devices). If all of the conditions present stimuli in the same location of the screen, the
screen position delay effect is often not reported (since it is small compared to other motor
effects, and equally represented in all conditions). The PST Refresh Detector System may be
used to get the actual delays of various positions on the screen if desired (e.g., using a photo-
diode the device triggers a response via the PST Serial Response Box every time the pixels
under the photo-diode are refreshed).
Since all of the timing occurs within a single object within this paradigm, no special timing
analysis is required. The duration of the Fixation object will be that specified in the Duration field
for the object (500ms), plus the generation time for the Probe object, plus the synchronization
with the vertical blank time, for a total fixation time of approximately 515ms (depending on the
refresh rate).
Timing Paradigm 2: Critical sequence of events
This masking paradigm illustrates a critical sequence of events. In this paradigm, the
experimental task presents the fixation for a given period of time, then the probe for a critical
period, and then the mask. Both the stimulus durations and the inter-stimulus intervals are time
critical. The displays of the Fixation and Probe must be integer multiples of the refresh rate. To
eliminate any delays due to generation time, the critical stimuli should include PreRelease time to
prepare for the next event. The Structure view of this task is shown below.
Figure 16. Paradigm 2 with ellipse over critical
sequence of Fixation, Probe, and Mask events.