Multiple Tasks 307relation between the word or ink color for successive stimuli.
For the ignored repetition list, however, the irrelevant color
word for one stimulus was the relevant color for the next
stimulus. In other words, if the color word for stimulus n-1
was red, then the ink color for stimulus n was red. The find-
ing of interest was that the time to name the colors for the ig-
nored repetition list was much longer than that for the control
list. This slowing of responses when the to-be-ignored infor-
mation on the previous trial is relevant on the current trial is
the phenomenon of negative priming.
Negative priming has subsequently been studied most
often using a method in which responses to individual stimuli
are measured. In that situation, the trials are often presented
as pairs, with the first trial called a prime and the second a
probe. Negative priming is shown when responses are slower
for trials in which the previously irrelevant information is
now relevant than for neutral trials. The negative priming ef-
fect has been found in a variety of tasks for which irrelevant
information is present (Fox, 1995; May, Kane, & Hasher,
1995), including not only tasks that require identification of
an object but also those that require localization.
The most straightforward interpretation of negative prim-
ing effects is that of selective inhibition: The irrelevant infor-
mation must be inhibited in order to respond to the relevant
information, and this inhibition carries forward to the next
trial. Consequently, the response will be slowed if the inhib-
ited information is now relevant. Although numerous find-
ings are consistent with the selective inhibition hypothesis,
they can also be accounted for without assuming inhibition.
Moreover, the situation has been shown to be much more
complex than the selective inhibition hypothesis suggests,
and alternative explanations have been proposed. The two
most prominent alternatives are feature mismatching and
episodic retrieval. According to the feature mismatch hypoth-
esis (Park & Kanwisher, 1994), symbol identities are bound
to objects and locations, and any change in the bindings from
the previous trial will produce negative priming. The
episodic retrieval hypothesis (Neill & Valdes, 1992) states
that presentation of a stimulus evokes retrieval of previous
episodes involving the stimulus. Because recent episodes are
most likely to be retrieved, if the target stimulus was a dis-
tractor on the previous trial, the episode retrieved will include
anignoretag.
One problematic finding for the inhibition account is that
negative priming effects do not appear to be short-lived.
DeSchepper and Treisman (1996) found negative priming
after a delay of 30 days between the prime and probe trials. In
addition, negative priming depends on the relation between
the prime and probe trials. For example, for the Stroop task,
the effect is not found if the probe stimulus that follows the
prime Stroop stimulus is a color patch and not a colored word
(Lowe, 1979). A simple inhibition account would seem to
predict negative priming in this situation as well as in that for
which the probe stimulus was a colored word.MULTIPLE TASKSTask SwitchingIn his classic monograph, “Mental Set and Shift,” Jersild
(1927) began by saying, “The fact of mental set is primary in
all conscious activity. The same stimulus may evoke any one
of a large number of responses depending upon the contex-
tual setting in which it is placed” (p. 5). Jersild conducted
experiments in which subjects made a series of judgments re-
garding each stimulus in a list as a function of whether a
single task was performed for all stimuli or two tasks were
performed in alternating order. The major finding was that in
many situations the time to complete the list was longer for
mixed lists than for pure lists of a single task.
Beginning in the mid-1990s, there has been a resurgence
of interest in task switching. Research conducted on task
switching, in which two tasks are presented in a fixed order
(e.g., on alternate trials), has suggested that there are two
components associated with changing the task set from the
previous trial. One component involves voluntary prepara-
tion for the forthcoming trial, with responses for the next trial
becoming progressively faster as the RSI increases. How-
ever, time to prepare for the new task cannot be the only fac-
tor contributing to the switching cost, because the cost is still
evident when the RSI is long (Allport, Styles, & Hsieh, 1994;
Rogers & Monsell, 1995). A second component, which
Allport et al. (1994) called task set inertia and Rogers and
Monsell (1995) called exogenous task set reconfiguration, is
not under the subject’s control. Apparently only a single trial
with the new task is necessary to complete configuration for
that task. Rogers and Monsell (Experiment 6) used sequences
of four task repetitions and then a switch to the alternate task
for four consecutive trials, and so on, and found that the
switch costs were eliminated after the first trial of the new
task.
Shaffer (1965) conducted a study in which trials with
compatible and incompatible spatial mappings were ran-
domly mixed. The stimulus to which the subject was to
respond occurred in a left or right location, and a centered
horizontal or vertical line signaled whether the mapping for
the trial was compatible or incompatible. When the mapping
signal occurred simultaneously with the stimulus, the stan-
dard spatial compatibility effect was eliminated. Vu and