User`s guide

E-Prime User’s Guide
Appendix B: Considerations in Research
Page A-27
At the end of the foreperiod, the stimulus is presented. In many experiments there is only a
single event making up the overall stimulus. In others, there may be distracter elements
displayed on the screen, or stimuli that serve as primes. In either event, timing of the reaction
begins when the critical stimulus is displayed. The critical stimulus refers to the element in the
display that determines the appropriate reaction (i.e., which key to press). This is sometimes
called the “imperative” stimulus. The stimulus duration (how long it remains in view) will largely
be controlled by the nature of the stimulus display. For example, if eye movements during the
stimulus presentation could affect the experiment, a very brief (say, 100msec) presentation is
often used, since it takes about 200msec after the stimulus appears for an eye movement to
begin. If the stimulus duration is so short that the subject gets only a glance at the stimulus, the
display is described as a data-limited display. Other situations involving data-limited displays are
discussed below.
Another issue for defining a trial is that of how long to give the subject to respond. Typically, the
subject must respond with a key-press within some limited time. The choice of that time depends
on the sorts of RT's expected, with the time allowed being set so as to encompass any legitimate
trials. If the task is an easy one, with RT on most trials being less than 500msec, the time
allowed for a response may be relatively brief (e.g., two seconds or so). If no response occurs in
that time period, the trial is counted as an omission. Many harder tasks, however, have typical
RT's of 1-2 seconds. In this case, the time allowed for a response should be increased
accordingly.
Feedback about accuracy and/or RT is usually given following a response. Feedback about
accuracy is usually provided, telling subjects whether they were right or wrong in their choice of a
response. It should be noted, though, that subjects are generally aware of having made an
incorrect response. The accuracy feedback emphasizes the importance of correct responding.
Because the usual RT instructions emphasize speed of reactions, RT feedback is important,
since it lets subjects monitor their own performance. Many researchers prefer not to report RT on
error trials, to avoid encouraging subjects to respond so quickly that accuracy is reduced.
Other terminology.
The inter-trial interval (ITI) is the time from the end of one trial to the beginning of the next. If the
subject controls initiation of the next trial, the subject also controls the ITI. When it is important to
control ITI, trial initiation must be controlled by the computer or experimenter.
In some experiments, there may be more than just a single stimulus presented on each trial, or
there may be a prime and then a stimulus that calls for a response (sometimes called the
imperative stimulus). For example, if the subject must judge whether two letters they see are the
same or different, they might see one letter and then see the second some short time later. That
delay before the second stimulus is the inter-stimulus interval (ISI). The ISI is time from the onset
of the first stimulus to the onset of the second. Another term for this is stimulus onset asynchrony
(SOA)
In the example experiment on visual acuity, a central fixation mark would be required so that
measures of stimulus location would be accurate. Because the locations must be specified and
the proper size letters chosen to compensate for distance from fixation, it would be necessary to
control the distance from the subject to the screen, using a viewing hood or chin-rest. The
distance to the screen, and the resulting display sizes (in degrees of visual angle—see below)
should be included in the Methods section of the final report. To be sure that subjects do not turn
their eyes and re-fixate the letter in central vision, a data-limited display would be needed. A 150-
msec display would control for this. Subjects might employ a strategy of guessing the location,