Multiple Tasks 309must be made before that for T2. They propose that there is
no capacity limitation in processing other than a bottleneck
for response execution when the tasks require responses from
the same output system (e.g., key presses for T1 and T2). Ac-
cording to their strategic response deferment model, different
lock out strategies are adopted in specific situations to permit
performance of T1 and T2 in the manner requested. Whether
the response-selection bottleneck is due to a structural limita-
tion on information processing or a strategy adopted to sat-
isfy task demands is an issue that remains to be resolved.
According to response-selection bottleneck accounts of
the PRP effect, whether structural or strategic, response se-
lection for T2 does not begin until that for T1 is completed.
However, several recent studies have shown cross-talk
effects between T1 and T2 that imply that the T2 response is
activated before the response for T1 is selected. Hommel
(1998) had subjects make a left or right key press to the color
of a red or green rectangle for T1 and say “red” or “green” to
the letter S or H for T2. RT for both tasks showed correspon-
dence effects at short SOAs, with the response for each task
being faster when the color-naming response for T2 corre-
sponded to the color for T1. Lien and Proctor (2000) obtained
similar results when T1 involved left-right key presses with
the left hand to low or high pitch tones and T2 left-right key
presses with the right hand to left-right arrow directions.
Also, Logan and Schulkind (2000) reported correspondence
effects for the categories of T1 and T2 stimuli for a variety of
tasks. For example, when both tasks required letter-digit clas-
sifications with left-right key presses on the left and right
hands, respectively, RT was shorter when the two stimuli
were from the same category (e.g., letters) than when they
were not. The fact that, in all studies, the correspondence ef-
fects are evident in RT1, as well as RT2, implies that the stim-
ulus for T2 is translated into response activation prior to T1
response selection. Hommel has proposed that such transla-
tion of stimulus information into response activation is auto-
matic, with the bottleneck being only in the final decision
about which response to make for each task.
Stop Signals
A goal may change during the course of action selection
so that the action being selected is no longer relevant. Such
situations have been studied in the stop-signal paradigm
(Logan, 1994). In this paradigm, a choice-reaction task is ad-
ministered, but a stop signal occurs at a variable interval after
the imperative stimulus on occasional trials to indicate that a
response should not be made. Of concern is whether the
subject is able to inhibit the response for the choice task.
The response is more likely to be inhibited the shorter the
interval between the go and stop signals and the longer the
choice RT.
Performance on the stop-signal task has been interpreted
in terms of a stochastic race model: The go process and stop
process engage in a race. The response is executed if the go
process finishes before the stop process and is inhibited if the
stop process finishes first. This model predicts many features
of the results obtained in the stop-signal task, including the
probability that the response will be inhibited as a function of
go RT and stop-signal delay. The race model has been applied
to a variety of stimulus and response modes, suggesting that
it captures basic principles of action inhibition. However, it
does not provide a detailed account of the processes underly-
ing performance of specific tasks.
Logan and Irwin (2000) compared the processes involved
in inhibiting left-right key presses and left-right eye move-
ments. Subjects responded to peripheral left-right stimuli or
central left-right pointing brackets, with hand movements or
eye movements, using a compatible or incompatible S-R
mapping. Estimates of stop-signal RT for hand movements
were similar for the two stimulus types and mappings. Stop-
signal RT for eye movements was shorter than that for the
hands, being shortest for the condition in which a compatible
movement was made to a peripheral stimulus. These results
suggest that the inhibition processes for hand and eye move-
ments are different, although they follow the same basic
principles.
Research has focused on trying to identify the point of no
return, or the stage beyond which the response cannot be
stopped. De Jong, Coles, Logan, and Gratton (1990) exam-
ined this issue using left-right squeezing responses (to a cri-
terion) to measure partial responses, the lateralized readiness
potential (LRP) to measure central response activation, and
the electromyogram (EMG) to measure muscle activation.
LRP, EMG, and squeeze activity were found to occur on
stop-signal trials for which the response was successfully in-
hibited (i.e., did not reach criterion), which they interpreted
as suggesting that no stage of response preparation is ballis-
tic. However, Osman, Kornblum, and Meyer (1986) argued
that the point of no return should be defined as the point at
which the response cannot be stopped from beginning. Using
this criterion, the partial squeezes in De Jong et al.’s study are
cases of unsuccessful inhibition, indicating that muscle acti-
vation is the point of no return. The evidence in De Jong
et al.’s study favored two inhibitory mechanisms: Inhibition
of central activation processes was implicated because the
LRP was truncated on successful stop trials, but several find-
ings suggested that there was also a more peripheral mecha-
nism of inhibition that affected the transmission of activation
from central to peripheral structures.