308 Action Selection
Proctor (2001) used stimulus color to designate the mapping
and obtained similar results with left-right physical-location
stimuli, as Shaffer used, as well as with left-right pointing ar-
rows. These findings are consistent with the fact that, in a
variety of situations, performance of the easier of two tasks
is harmed more by mixing (Los, 1996). However, Vu and
Proctor found that when the stimuli were the words leftand
right,the advantage for the compatible mapping was en-
hanced compared to pure blocks of one trial type. These re-
sults, along with many others, suggest that words are
processed differently than physical locations and arrows.
Proctor and Vu (2002) also showed that mixing location-
relevant and location-irrelevant trials within a trial block
alters the stimulus-response compatibility (SRC) effects ob-
tained for each task. When physical location stimuli were
used to convey the location information, the standard SRC
effect was eliminated for location-relevant trials. However,
the SRC effect was not affected with arrow stimuli and was
enhanced with location word stimuli. Mixing the two trial
types also affects the Simon effect obtained for the location-
irrelevant trials. For all stimulus types, when the location-
relevant mapping was compatible, the Simon effect was
enhanced compared to pure blocks of Simon trials; when
the location-relevant mapping was incompatible, a reverse
Simon effect was obtained. With arrows and words, the re-
verse effect was smaller than the positive effect. However,
with physical locations, the reverse Simon effect was at
least as large as the positive effect obtained with the compat-
ible location-relevant mapping. This outcome implies that
there was no automatic activation of the corresponding re-
sponse. The reversal for physical location stimuli obtained
when the location-relevant mapping was incompatible was
evident even when the trial type was precued by up to 2.4 s
before presentation of the stimulus. This outcome indicates
that the reversal does not reflect only a strategy of preparing
the noncorresponding response in anticipation that location
may be relevant to the trial.
Psychological Refractory Period
In a common dual-task procedure, subjects perform two
different choice-reaction tasks, Task 1 (T1) and Task 2 (T2),
on a single trial. The stimulus onset asynchrony (SOA) between
the stimuli for T1 (S1) and T2 (S2) is varied. The typical finding
is that RT for the second task (RT2) is slowed as the SOA de-
creases. Telford (1931) called this phenomenon the psycholog-
ical refractory period (PRP) effect. Extensive research on
the PRP effect has been conducted over the past 50 years,
and explanations have been proposed in terms of information-
processing bottlenecks, demands on limited capacity resources,
and strategies adopted to satisfy task constraints (Meyer &
Kieras, 1997; Pashler, 1998). The most widely accepted ac-
count in recent years is a response-selection bottleneck model
advocated by Pashler and colleagues (see Figure 11.5). Accord-
ing to this model, stimulus identification and response execu-
tion occur in parallel for the two tasks. However, response
selection operates serially because it requires a single-channel
mechanism.
The evidence for the response-selection bottleneck model
comes primarily from using locus of slack logic (Schweickert,
1983) to interpret the patterns of additive and interactive ef-
fects produced by variables presumed to selectively affect
stimulus identification, response selection, and response exe-
cution. According to the model, identification of S2 com-
mences immediately upon its presentation, regardless of the
SOA. At long SOAs response selection can begin as soon as
stimulus identification is completed, but at short SOAs it can-
not begin until response selection for T1 is finished. Conse-
quently, there is slack in the processing sequence for T2
between the completion of stimulus identification and initia-
tion of response selection. At short SOAs, the slack can ab-
sorb, at least in part, an increase in time to identify S2. This
leads to the effect of the stimulus-difficulty manipulation
being smaller at the short SOAs than at the long SOAs. In con-
trast, for variables that affect response selection or response
execution, which have their influence after the bottleneck,
the extra time cannot be absorbed by the slack, and, therefore,
their effects should be additive with those of SOA. These pre-
dicted patterns of results have been found for several variables
of the respective types.
Meyer and Kieras (1997) have mounted a challenge to
the response-selection bottleneck model, arguing that evi-
dence supporting it reflects a strategy adopted by subjects
when the instructions state or imply that the response for T1S1 R11A 1B 1CTime2A 2B 2CS2 R2
Figure 11.5 Illustration of response selection bottleneck model. Stage A is
stimulus identification, Stage B is response selection, and Stage C is re-
sponse initiation. Response selection for Task 2 (Stage 2B) is delayed until
response selection for Task 1 (Stage 1B) is completed. S1 and R1 are the
stimuli and responses for Task 1, and S2 and R2 are the stimuli and responses
for Task 2.