User`s guide

E-Prime User’s Guide
Appendix B: Considerations in Research
Page A-34
as the number of responses becomes larger. This effect is not usually of much concern, but must
be kept in mind when comparing the results of several experiments (i.e., if they used different
numbers of response alternatives, the RT's cannot be directly compared).
Intensity and contrast
At least for low levels of illumination, the more intense the stimulus, the faster the RT. Once the
stimulus reaches an intensity where it is clearly visible, however, further increases will have little
effect. Similarly, increasing contrast (the difference in intensity between the stimulus and the
background) will decrease RT, up to a point where the stimulus is clearly visible. Either low
intensity or low contrast would produce a data-limited display. A very brief stimulus is another
example of a data-limited display.
One common problem in controlling intensity and contrast is ambient light (the light present in the
room). A display that may seem very weak under ordinary room lighting may seem quite bright
when room lights are off and windows covered. In experiments employing brief, data-limited
stimulus displays, it is important that ambient light be carefully controlled.
In addition to lowering apparent intensity and contrast, ambient light may result in glare or
reflections on the display screen of the computer. In this case, lights must be shielded or the
computer moved to prevent such interference.
Stimulus location
The location of the stimulus can have a powerful effect on both RT and error rates. Visual acuity
drops quickly as stimuli are moved away from the fovea—the narrow area of vision straight ahead
that is about 2° wide. A person with 20/20 vision in the fovea will typically have about 20/80
vision 2.5° from straight-ahead. At 10° from straight ahead most people have worse than 20/300
vision. To put this in perspective, at a viewing distance of 57 cm (22.5”), each centimeter is about
1° of visual angle, so a letter displayed 2.5 cm (about 1”) from fixation will be seen quite poorly.
For these reasons, retinal locus (where on the retina the image of the stimulus falls) must be
controlled by randomization or counterbalancing if the stimuli are not all presented in the same
location. If one type of stimulus is presented in the fovea, and another in the periphery,
differences in RT might occur (or fail to occur). However they could be due to differences in the
location of the stimuli, rather than to differences in the stimuli themselves.
Note that the relative size of the stimuli is a function of distance from the eye. If the relative size
of stimuli is a concern, then the location of the subject's head relative to the screen must also be
controlled. This is often done by use of a chin rest or viewing hood to keep the subject's head
relatively stable. In this case, the viewing distance should be specified in the Method section of
the final report. Sizes of stimuli are also reported, in degrees of visual angle, rather than
millimeters or inches.
Calculation of stimulus sizes in degrees of visual angle can be done using the law of cosines. A
good approximation is obtained by the formula
Size in degrees of visual angle = 57.3W/D
…where W = width of display and D = viewing distance, with W and D in the same units of
measurement.