310 Action Selection
CHANGES IN ACTION SELECTION
WITH PRACTICE
Choice RT decreases with practice at a task, with equivalent
amounts of practice producing larger changes earlier in prac-
tice than later. Teichner and Krebs (1974) reviewed numer-
ous studies of visual choice reactions and concluded that the
stage of processing that benefits most from practice is re-
sponse selection. Newell and Rosenbloom (1981) proposed
that the changes in RT with practice follow a power function:
RTBN,whereNis the number of practice trials, Bis RT on the first
trial, and is the learning rate. The power function has come
to be regarded as a law to which any model that is intended to
explain practice effects must conform. Although the power
law provides a good description of changes in RT with
practice averaged across subjects, Heathcote, Brown, and
Mewhort (2000) contend that it does not fit the functions for
individual performers adequately. They demonstrated that
exponential functions provided better fits than power func-
tions to the data for individuals in 40 data sets, and proposed
a new exponential law of practice.
Beginning with Merkel (1885), several investigators have
shown that the slope of the Hick-Hyman function decreases
with practice (e.g., Hyman, 1953; Mowbray & Rhoades,
1959). Seibel (1963) used all combinations of 10 lights as-
signed directly to 10 keys. After more than 75,000 trials had
been performed, the RT for the 1,023-alternative task was
only approximately 25 ms slower than that for a 31-alternative
task. Practice also is typically more beneficial for incompati-
ble than compatible mappings. However, SRC effects do not
disappear even with considerable practice (Dutta & Proctor,
1992; Fitts & Seeger, 1953).
Proctor and Dutta (1993) had subjects perform two-choice
tasks for 10 blocks of 42 trials each. In the critical conditions,
they performed with the hands uncrossed and crossed in
alternate blocks. Whether compatible or incompatible, when
the spatial mapping of left-right stimulus locations to left-
right response locations remained constant, there was no cost
associated with alternating the hand placements: Overall RT
and changes with practice with the alternating placements
were comparable to those of subjects who practiced with the
same hand placement for all blocks. In contrast, when the
mapping of stimulus to response locations was switched be-
tween blocks so that the same hand was used to respond to a
stimulus when the hands were crossed or uncrossed, there
was a substantial cost for participants who alternated hand
placements compared to those who did not. These results
imply that the S-R associations that are strengthened through
practice involve spatial response codes.
Practice with an incompatible spatial mapping alters the
influence of stimulus location on performance when location
becomes irrelevant to the task. Proctor and Lu (1999) had
subjects perform a two-choice task for 3 days using an in-
compatible spatial mapping. On the 4th day, they performed
a task for which stimulus location was irrelevant. For this
task, the Simon effect was reversed, with RT faster for non-
corresponding responses. Tagliabue, Zorzi, Umiltà, and
Bassignani (2000) found a similar effect of prior practice
with an incompatible mapping and showed it to be present
even when subjects were tested a week later. Thus, a limited
amount of practice produces new spatial S-R associations
that persist at a sufficient strength to override the preexisting
associations between corresponding locations.
Nissen and Bullemer (1987) demonstrated that when the
trials in a compatibly mapped four-choice spatial reaction task
follow a sequence that repeats regularly (every 10 trials in their
study), performance improves more with practice than when
the trial order is random. Considerable effort has been devoted
subsequently to determining whether this sequence learning is
implicit or explicit, and to examining the nature of what
is learned. Because this research is summarized in the chapter
by Johnson in this volume, we restrict mention here to a study
by Koch and Hoffmann (2000). Across four experiments, they
varied whether the stimuli were spatial or symbolic and
whether the responses were spatial or symbolic. Their results
showed that the effect of sequence repetition and structure on
performance was much stronger for spatial sequences than
for symbolic sequences, regardless of whether the stimulus or
response set was involved. Koch and Hoffmann also specu-
lated that learning of the response sequence is greater for in-
compatible S-R mappings (e.g., random mappings of digits to
response locations) than for compatible S-R mappings. Re-
gardless, they emphasized, “the selective impact of S-R com-
patibility on learning stimulus and response sequences in SRT
[serial reaction tasks] seems to us an important issue...that
has not received much attention” (p. 879).APPLICATIONSContemporary research on action selection has its roots in dis-
play-control design issues. Paul Fitts, who formalized the con-
cept of SRC and conducted much of the groundbreaking
research on action selection, was the founder of what is now
the Fitts Human Engineering Division of the Armstrong Labo-
ratory of the U.S. Air Force and made many contributions to
human factors. Although most of the research on action