User`s guide

E-Prime User’s Guide
Appendix B: Considerations in Research
Page A-26
While we have defined RT as the time from stimulus onset to a response, it is sometimes defined
in other ways. In much research in kinesiology, for example, RT is defined in relation to the onset
of a muscle potential (electromyographic signal), while the time from that first electrical activity in
the muscle to when the response movement itself is completed is called Motor Time. Because
RT is sometimes defined differently, and because it can depend on the nature of the response
apparatus, it is important in RT research that the definition of RT and the nature of the response
be made explicit, and reported in the Procedures section of the research report.
RT is also sometimes classified as simple RT or choice RT. In simple RT, a subject makes one
response to a single stimulus. This requires only a judgement about the presence of a stimulus,
and does not involve a decision about the nature of the stimulus. Choice RT is measured when
more than one type of stimulus can occur, and the subject must indicate the stimulus type by his
or her choice of responses. Because research on simple RT is rare, "RT" means choice RT
unless noted otherwise.
General Considerations
In developing the general considerations for RT research, we examine issues concerning the
events that take place on each trial, how blocks of trials may differ, and finally how these combine
to form the experiment as a whole.
An example experiment
To permit concrete examples of the issues discussed, we begin by outlining an experiment that
could be fairly easily implemented in E-Prime. The intent here is clarity, rather than scientific
importance. Suppose that you wish to examine how RT to a stimulus is affected by changes in
the location of the stimulus. Visual acuity is best for objects in foveal vision—the small, central
part of the visual field, and drops rapidly for objects further away in peripheral vision. But does
that affect RT? The following experiment would help answer that question.
The principal dependent variable is RT, with accuracy (percent correct) as a secondary
dependent variable. The independent variables are the location of the stimulus and whether it is
adjusted in size to compensate for poorer peripheral acuity. The stimulus is a letter, presented in
a random location on the screen. Stimulus letters are centered on locations 0, 2, 4, 8, and 16, left
and right of straight-ahead. Letter sizes are chosen to compensate for the distance from central
vision (reference). The letters to be presented are C, G, O, and Q, with one response for C and
O and another for G and Q. These letters were chosen because C and G share a lot of feature
overlap, as do O and Q, so the discrimination is fairly difficult. Four different letters are used so
that subjects cannot rely on a single feature, such as the tail of the Q, for the discrimination.
What happens on each trial?
Typically, RT experiments consist of one or more series (blocks) of trials. While the specific
stimulus may vary from trial to trial, certain aspects of the experiment are usually the same on
each trial. There is often a fixation mark of some kind, to let the subject know where he or she
should be looking when the trial starts. Initiation of a trial may be under the subject's control,
allowing the subject to begin a trial whenever he or she is ready. Alternatively, initiation of a trial
may be automatic, controlled by the experimenter or computer. In this case, a warning signal is
typically given, to allow the subject to get ready for the trial. Sometimes the appearance of the
fixation mark acts as the warning, and sometimes a tone or other signal is used. After a trial is
initiated (by the subject or automatically), there is usually a brief delay before the stimulus
appears. This delay is called the foreperiod, and may vary from trial to trial or be fixed
(unvarying). The foreperiod is usually fixed for choice RT tasks.