Relevant Stimulus Information 303locations or effectors and that response selection benefits
from repetition of the stimulus category when it maps onto a
salient feature of the response sets.
Soetens (1998) examined sequential effects for tasks in
which subjects responded to four stimuli located at the cor-
ners of an imaginary square by pressing the left key if the
stimulus was to one side and the right key if it was to the
other. When left-right stimulus locations were mapped com-
patibly to left-right responses, the repetition benefit at the
short RSI (50 ms) was primarily associated with the response
(i.e., the benefit was evident when the stimulus side was
the same as on the previous trial, but the location was differ-
ent). At the long RSI (1,000 ms), a small alternation benefit
was evident. With an incompatible S-R mapping (i.e., left
side to right response), the results were similar, but with an
increased benefit for repeating the same stimulus, particu-
larly at the short RSI. When up-down responses were made to
the left-right stimulus locations, response and stimulus repe-
tition benefits of similar magnitudes were found at the short
RSI. At the long RSI, the only effect was a repetition benefit
for the same stimulus. Soetens concluded that automatic fa-
cilitation shifted toward stimulus-related processes as the
mapping became less compatible. Together, the studies of
Pashler and Baylis (1991), Campbell and Proctor (1993), and
Soetens indicate that response repetition, without stimulus
repetition, is beneficial when there is a structural relation be-
tween the stimulus and response sets and that repetition of the
stimulus is more important when the mapping is arbitrary.
Although first-order sequential effects have been most
widely studied, second- and third-order repetition effects, in-
volving the sequence of the preceding two or three stimuli,
respectively, are larger and more consistent (Soetens, 1998).
For two-choice tasks, at short RSIs, RT benefits from multi-
ple repetitions, regardless of whether the present trial is a
repetition or an alternation. For example, responses on the
current trial tend to be faster if the three preceding trials were
repetitions than if they were alternations. At long RSIs, how-
ever, a prior string of repetition trials is beneficial if the
current trial is also a repetition, but a prior string of alterna-
tion trials is beneficial if the current trial is an alternation.
These two patterns of results can be attributed to automatic
activation and subjective expectancies, respectively. The
higher order effects in Soeten’s study also showed the pat-
terns indicative of automatic facilitation at the short RSI and
subjective expectancy at the long RSI.
Is the Hick-Hyman Law an Artifact of Repetition Effects?
Kornblum (1967, 1968) noted that, unless explicitly con-
trolled, the proportion of repetition trials decreases as set size
increases. Therefore, he proposed that the Hick-Hyman law
is an artifact of repetition effects. Kornblum (1968) used a
four-choice task in which four lights were mapped to four re-
sponse keys and information was varied by manipulating
stimulus probabilities. For three levels of information, condi-
tions were constructed in which the probability of repetition
was high or low. RT was shorter for the high-repetition con-
ditions than for the corresponding low-repetition conditions,
and these latter conditions showed only a nonsignificant ef-
fect of information on RT. Kornblum (1967) conducted a
similar experiment in which the number of alternatives was
two, four, or eight. For four- and eight-choice tasks, RT was
shorter on repetition than on nonrepetition trials, with the
slope being less for repetition trials. Within these tasks, RT
for repetition trials increased as the amount of stimulus infor-
mation increased, but RT for nonrepetitions did not.
Hyman and Umiltà (1969) noted that the RSI in
Kornblum’s (1967, 1968) experiments was approximately
140 ms, a short interval that would maximize repetition ef-
fects and minimize preparation for the subsequent trial. They
replicated three of Kornblum’s (1968) conditions, but used an
average RSI of 7.5 s. Although RT was faster for repetition
than nonrepetition trials, the slopes of the two functions were
approximately equal. Hyman and Umiltà concluded, “There
seems little doubt that the information hypothesis is much
more compatible with our results than those of Kornblum’s”
(p. 47). In other words, the Hick-Hyman function is not an
artifact of the proportion of repetition trials when there is
adequate preparation time.Advance InformationWarning EffectsPreparation is usually studied by presenting a neutral warning
signal at various intervals prior to the onset of the imperative
stimulus. Bertelson (1967) had subjects press a right key to a
right light and a left key to a left light. The warning signal
was an auditory click that, in different blocks, occurred 0, 20,
50, 100, 150, 200, or 300 ms prior to the visual stimulus. At
the 0-ms warning interval, RT was approximately 265 ms. It
decreased to a minimum of 245 ms at the 150-ms interval and
then increased slightly to 250 ms at the two longest intervals.
However, the error rate increased from about 7% at the
shorter intervals to 12% at the 100- and 150-ms intervals, and
decreased slightly to 9% at the longer intervals. Thus, the
effect of the warning signal was to increase readiness to
respond quickly, but at the expense of accuracy.
Posner, Klein, Summers, and Buggie (1973) obtained sim-
ilar results for a two-choice task in which the compatibility of