Prospective Memory, Concurrent Task Management, and Pilot Error

Key Dismukes and Jessica Nowinski

In 1991 a tower controller at Los Angeles International airport cleared
a commuter aircraft to position and hold on runway 24L while she worked
to clear other aircraft to cross the other end of the runway. There were
several communications delays because one of the other aircraft was on
the wrong radio frequency. Visibility was poor at twilight because of
haze and glare. The controller's workload was considered moderate by air
traffic controllers, though laypeople might consider it quite busy. The
controller forgot to clear the commuter aircraft to take off and cleared
another aircraft to land on 24L, which it did, destroying both aircraft
and killing 34 people.

Similar errors by pilots have also led to major accidents. In 1994 an
airliner ran off the runway at LaGuardia airport after the crew rejected
the takeoff at high-speed because they observed anomalous indications on
their airspeed indicators. The NTSB determined that the anomalous
indications occurred because the crew failed to turn on the pitot heat,
a normal procedural step, that keeps the pitot input to the airspeed
indicators from freezing in cold, wet weather. Two previous major
airline accidents occurred in the 1980's when the crews forgot to extend
wing flaps and slats to takeoff position, a normal procedural step
required before takeoff. More recently, in 1996, an airliner landed
gear-up in Houston when the landing gear failed to extend because the
crew forgot to set the hydraulic pumps to the high position, which was
part of the normal procedure for preparing their type of aircraft for
landing,. Obviously multiple factors were at play in each of these
accidents, but a central aspect of each accident was the failure of the
crew to execute a simple procedural step that they had performed many
thousands of times in previous flights.

In everyday life we are all susceptible to forgetting to perform
intended actions. These everyday lapses are mainly annoying and
sometimes embarrassing, but in the operational world memory lapses can
be fatal, as these accidents testify. Memory lapses during airline
flight operations are particularly striking because the airline industry
has erected elaborate safeguards against errors, including written
standard operating procedures, checklists, and requirements for the
captain and first officer to cross check each other's actions.

Prospective memory---remembering to perform an action that cannot be
executed when the intention is formed---is a fairly new but rapidly
growing topic in cognitive psychology (see reviews in Brandimonte,
Einstein, & McDaniel, 1996; Ellis & Kvavilashvili, 2000). Prospective
memory is distinguished by three features: (1) an intention to perform
an action at some later time when circumstances permit, (2) a delay
between forming and executing the intention, typically filled with
activities not directly related to the deferred action, and (3) the
absence of an explicit prompt indicating that it is time to retrieve the
intention from memory---the individual must "remember to remember." This
third feature distinguishes prospective memory from
traditionally-studied retrospective memory. (Arguably prospective memory
has some similarity to implicit memory, which is a form of retrospective
memory). Typically, if queried after forgetting to perform an action,
individuals can recall what they intended to do. If the LAX controller
had been asked what she planned to do with the holding aircraft, she
almost certainly would have been able to report her intended sequence of
actions. Thus the critical issue in prospective memory is not how we
retain the content of our intentions but how we remember to perform
those intentions *at the appropriate moment*, and why we sometimes fail
to remember. What would have helped the controller remember to clear the
commuter aircraft to take off *before* she cleared the second aircraft
to land?

Aided by new laboratory paradigms, researchers are beginning to
elucidate the cognitive processes underlying prospective memory,
although many questions remain unanswered. The most common paradigms are
variations of a procedure developed by Einstein and McDaniel (1990):
Experimental participants are given an *ongoing task*, such as
evaluating the pleasantness of a series of words displayed on a computer
screen, and are told that if they encounter a particular word (or set of
words or class of words) they should take a specified action, such as
pressing the slash key on a keyboard. This second task is the
*prospective memory task*. This particular type of prospective memory
task, which we will call *episodic*, has been studied extensively in
recent years, however in this chapter we present evidence that this
paradigm represents only one of several types of prospective memory
situation encountered in the real world.

Our research group is attempting to link real-world prospective memory
phenomena with the emerging picture of underlying cognitive processes.
Our approach is congruent with the approach that Chris Wickens has
pioneered for many years (Wickens, 1992). We believe that understanding
human performance of complex real-world tasks requires converging
evidence from several very different types of research method. This
paper examines findings from ethnographic studies, analyses of accident
and incident reports, and laboratory studies, all of which we attempt to
pull together in a theoretical framework grounded in cognitive
psychology. Well-controlled laboratory studies are essential to
understand cognitive processes underlying human performance, but taken
by themselves often miss important phenomena and major sources of
variance in the real world. Field studies (ethnographic observations and
analyses of accident and incident reports) identify crucial phenomena
and the influence of task, individual, organizational, and social
factors, and raise theoretical issues that might not be apparent from
laboratory studies alone. Working back and forth between field and
laboratory studies enriches both approaches.

Field Studies

Airline operations lend themselves to the study of skilled human
performance and human error because these operations are highly
standardized, with formal written operating procedures that cover almost
every aspect. Because most aspects of flight operations are explicitly
scripted, we can readily observe deviations from what is prescribed. In
addition, a fair degree of consensus exists among subject matter experts
over what actions are appropriate or inappropriate in most normal
situations.

We conducted three studies that helped us to identify the kinds of tasks
involving prospective memory in airline flight operations and the most
common forms of associated error. An ethnographic study focused on a
particular aircraft type to allow in-depth analysis (the Boeing 737, one
of the most commonly used airplanes in the transport industry). We
reviewed written operating procedures, participated in classroom and
flight simulation training at two major airlines, and observed a large
number of flights from the cockpit jumpseat (Loukopoulos, Dismukes, &
Barshi, 2003; Dismukes, Loukopoulos, & Barshi, 2003). A second study
analyzed National Transportation Safety Board (NTSB) reports for the 19
major U.S. airline accidents attributed to crew error between 1990 and
2001 (Dismukes, Berman, & Loukopoulos, 2005), and a third study sampled
20 percent of all air carrier reports submitted to the Aviation Safety
Reporting System (ASRS) over a 12 month period to obtain reports
involving any type of memory error (Nowinski, Holbrook, & Dismukes,
2003).

From these studies (which also address topics beyond prospective
memory), we concluded that prospective memory demands in cockpit
operations emerge in five types of task situations:

1\) *Episodic tasks*. In these situations pilots must remember to
perform at a later time some task that is not habitually performed at
that time. For example, an air traffic controller may instruct a crew to
report passing through 10,000 feet while the crew is still at 15,000
feet, creating a delay of perhaps five minutes. Another example occurs
when circumstances force pilots to perform a habitual task out of its
normal sequence. Most laboratory research on prospective memory has
focused on these types of episodic tasks.

2\) *Habitual tasks*. Crews perform many tasks and many sub-task steps
in the course of a normal flight. On the order of a hundred action steps
are required just to prepare a large aircraft for departure. Most of
these steps are specified by written procedures, and are normally
performed in the same sequence, thus execution of tasks becomes highly
habitual for experienced crews. For example, flaps are normally set to
takeoff position after the engines have been started and before taxiing
to the runway. Pilots do not have to form an episodic intention to
perform each of these action steps, rather the intention to perform each
step is *implicit in the action schema* for the task, stored in
procedural memory. Thus pilots do not have to form an explicit intention
in advance each time they must set the flaps.

One might argue whether performing highly habitual tasks fits the
definition of prospective memory. Although habitual tasks differ
substantially from episodic tasks, we include them as a form of
prospective memory because the individual must retrieve the action to be
taken when circumstances are appropriate without receiving any explicit
prompt to retrieve the memory item. Individuals who forget to perform
habitual tasks typically report that they intended to perform the task.

3\) *Atypical actions substituted for habitual actions*. Circumstances
sometimes require crews to deviate from a well-established procedural
sequence. For example, through long experience departing from a certain
airport, a crew would come to know that the Standard Instrument
Departure procedure (a written instrument procedure) requires them to
turn left to 300 degrees upon reaching 2000 feet. This would become
habitual for the crew. If on rare occasion a controller told them to
turn to 330 degrees instead of 300, the crew would have to both form an
episodic intention to turn to 330 degrees and an intention to inhibit
their habitual response of leveling the wings at 300. Reason (1984)
discussed memory errors in such situations as *habit capture*.

4\) *Interrupted tasks*. Interruptions of procedures occur fairly
frequently, especially when crews are at the gate preparing the airplane
for departure. Flight attendants, gate agents, mechanics, and jumpseat
riders frequently interrupt the pilots as they work to complete
preflight procedures. Pilots may try to finish the immediate task they
are working on before addressing the person interrupting them, or they
may suspend the ongoing task to handle the interruption. In either case,
attention is diverted at least momentarily by the intrusion, and pilots
must remember to resume where they left off. A common form of error is
to move on to the next task in the normal procedural sequence, failing
to return to and complete the interrupted task. Pilots generally either
recognize that the interrupted task has not been completed or recognize
that they are not certain of its status.

5\) *Interleaving tasks*. Pilots must often "multitask," interleave two
or more tasks concurrently, somewhat like a circus performer twirling
plates on poles. For example, first officers must sometimes re-program
the flight management system while the airplane is taxiing to the runway
(perhaps because the original runway or the original departure clearance
has changed). But during taxi the first officer is also responsible for
other tasks, including monitoring the course of the taxi (to catch
potential errors by the captain), handling radio communications,
and---depending on the airline---various other tasks. If the
re-programming can be accomplished with a few keystrokes the first
officer may do this all at one time, but if the re-programming takes
longer it is necessary to interleave performing some programming steps
with performing other cockpit duties, switching attention back and
forth. It is easy for pilots to become preoccupied with one
attention-demanding task (for instance, if a programming glitch occurs)
and forget to interrupt themselves to check the status of other tasks
frequently enough (Dismukes, Young, & Sumwalt, 1998).

The ASRS study revealed a startling finding: Of the 75 reports with
sufficient information to clearly identify a memory failure, 74 involved
prospective memory, rather than retrospective memory. We cannot conclude
from this that prospective memory failures occur more often than
retrospective memory errors. The frequency of reporting of various error
types reflects factors beyond the frequency of occurrence. For example,
pilots are motivated to submit ASRS reports in part because submission
provides immunity from prosecution for the reporter's errors; thus
pilots are more likely to submit reports about the kinds of error that
might get them in trouble. However, this finding suggests that
prospective memory errors are more consequential, more frequent, or more
memorable than retrospective memory errors, or combine some of these
three aspects. The high level of expertise of airline pilots greatly
reduces their vulnerability to retrospective memory errors, but that
expertise appears to provide less protection against prospective memory
errors, and indeed may contribute to some forms of prospective memory
error, as will be further discussed in later sections of this chapter.
Flight operating procedures are designed to safeguard against crew
errors, but in the case of prospective memory tasks the safeguards are
themselves vulnerable to errors of omission.

Although our discussion focuses on aviation operations, we have
conducted other studies revealing that comparable prospective memory
tasks occur in everyday life situations (Holbrook, Dismukes, & Nowinski,
2005), and other workplace settings are very probably similar in
prospective memory demands.

A Theoretical Perspective

Phenomenologically, these five prototypical prospective memory
situations seem quite diverse, but we argue that they share some
cognitive features and can best be understood within a common conceptual
framework. To make that argument requires a theoretical perspective on
the cognitive processes underlying prospective memory, and this
perspective can in turn help us to understand the nature of
vulnerability to prospective memory errors and point to countermeasures
to reduce vulnerability.

Several theoretical accounts of prospective memory have been published
in recent years (e.g., McDaniel & Einstein, 2000; Smith, 2003). In some
accounts a stored intention is retrieved from memory automatically when
the individual notices some cue associated in memory with the
intention---this has been called the *automatic* view (Guynn, McDaniel,
& Einstein, 2001; McDaniel, Robinson-Riegler, & Einstein, 1998). A cue
can be a specific physical stimulus or combination of stimuli in the
external environment (I remember to take the cookies out of the oven
when I hear the timer go off), or an internal event such as a thought or
a state (I remember to go grocery shopping when I think about a recent
meal or when I feel hungry). Two critical features determine the
effectiveness of a cue. The cue must have a strong enough association to
the intention (either through rehearsal or previous experience) to bring
the intention to mind when the cue is encountered, and the cue must be
present within the window of opportunity for performing that intention.
A cue to remind pilots to extend the flaps for takeoff is not effective
if it appears after takeoff.

A competing theoretical perspective, which has been dubbed the
*strategic* view, is that retrieval requires individuals to monitor for
an opportunity to perform a delayed task (Smith, 2003; also see
discussion in McDaniel & Einstein, 2000). This monitoring makes demands
on limited cognitive resources. The critical difference between the two
theoretical perspectives is that they predict different effects of a
delayed task on ongoing task performance, and different effects of
ongoing task difficulty on prospective memory performance. The strategic
view posits that the monitoring required to identify the window of
opportunity for a prospective memory task requires cognitive resources
that must be shared with the ongoing task. Therefore performance on an
ongoing task should always be affected to some degree by a prospective
memory task, and, likewise, performance on the prospective memory task
should decline when the ongoing task is particularly difficult and
resource demanding. In contrast, the automatic view suggests that
performance on the ongoing task is not necessarily affected by the
presence of a deferred intention. Also some authors have assumed that
the demands of the ongoing task should not affect automatic retrieval of
intentions, however we argue that these demands could impair retrieval
if they prevent individuals from attending cues associated with the
intention or reduce the extent to which those cues are processed. We
believe the automatic view suggests that prospective memory performance
should vary directly with the extent to which the ongoing task directs
attention toward relevant cues when they appear. McDaniel and Einstein
(2000) combined the two perspectives in their Multi-process framework,
arguing that individuals sometimes use automatic processing and
sometimes use strategic processing, depending on the nature of the
prospective memory task.

Neither the proponents of the strategic view nor the proponents of the
automatic view have provided a detailed account of the cognitive
processes that might be involved in the retrieval of intentions. Both
views have been presented exclusively in terms of episodic intentions.
The *associative activation* model of Nowinski and Dismukes (in
preparation) is an elaboration of the automatic view that also provides
a framework for examining all five types of prospective memory
situation. This model posits that in most situations, after forming an
intention, individuals turn their attention to other tasks. The
intention resides in long-term memory, and is retrieved when the
individual processes cues associated with the intention. Thus retrieval
is dependent on the presence of adequate cues and may or may not occur
during the window of opportunity for intended execution of the deferred
task. Even if the ongoing task does not direct attention to cues that
were encoded to define the window of opportunity for execution, other
cues associated with the intention may trigger retrieval. Retrieval may
also occur at other times that are not appropriate for execution because
associated cues are present. For example, one might form an intention to
ask a colleague for a copy of his paper when seeing him, but might also
be reminded of this intention while reading another paper related to
his.

Our model posits that automatic retrieval processes are always at play,
however individuals may in some situations supplement those processes
with some strategic process, such as monitoring for opportunities to
execute intentions (though it seems unlikely that in most real-world
situations individuals could perform ongoing tasks adequately and
monitor for cues related to intentions for long periods, especially
since at any moment individuals have various and diverse intentions
stored in memory). At the heart of this model is a simple system
consisting of (1) only two separate information stores, focal attention
and long-term memory, (2) activation mechanisms that allow memory
representations to move within and between those stores, and (3) an
associative network of representations through which activation is
delivered to and distributed among those representations. Our
formulation draws directly from the ACT-R cognitive architecture
developed by Anderson and colleagues (see Anderson & Lebiere, 1998) and
Cowan's (1995) framework for integrating attention and memory processes.
Like Cowan, we do not consider working memory a separate store, but
rather treat it as a small subset of highly activated items in long term
memory.

Deferred intentions are a form of goal, however, unlike some theorists
(see, for example, Goschke & Kuhl, 1993 and Anderson & Lebiere, 1998) we
argue that goals have no special status in cognitive processes, and we
treat goals simply as memory representations consisting of actions to be
executed under specified conditions. Retrieval of deferred intentions
follows the same rules and involves cognitive processes underlying
retrieval of other types of memory items. Thus our account of retrieval
of deferred intentions is couched within an existing framework of memory
retrieval.

Following the ACT-R framework, we posit that items are stored in
long-term memory in associative networks, and that items that have been
encountered together form links through which activation can spread.
Retrieval of an item from memory occurs when the representation receives
sufficient activation to pass some threshold and enters awareness. The
activation of an item at any given time is the sum of activation from
two sources. The first, *baseline activation*, is determined by history.
It increases with rehearsal of an item and with the frequency of
retrieval, and it decreases with the length of time since the item was
last retrieved. The second type of activation, *source activation*, is
determined by the proportion of attentional resources directed to a cue
at a given moment. The source activation received by a given item
spreads to its associates, is distributed among them, and in turn
spreads from them to their associates. The level of activation spread
from one item to another is proportional to the strength of association
between the two items. Source activation is a limited resource, thus the
amount of activation reaching a given item in memory is inversely
proportional to the number of competing associates.

The content of focal attention is in a constant state of flux, and once
a cue exits attention, source activation to its associates in memory
decays rapidly. However, decay is not instantaneous, and we speculate
that this allows source activation received by a memory item from a
series of associated cues passing through attention to be summated over
brief intervals. The item retrieved from memory at a given moment is the
item with the highest total activation, baseline activation plus source
activation received through associative links.

Prospective memory, by its nature, involves dual task processing. Once
an intention is delayed, retrieval of that intention must occur during
progress of whatever task is ongoing. Thus, retrieval of intentions must
compete with retrieval of memory items directly associated with the
goals of the ongoing task. The ongoing task has an advantage in this
competition as it guides attention to environmental information needed
to achieve its goal. The overarching goal, when in focal attention,
provides activation for retrieval of sub-goals, and sub-goals, when in
focal attention, in turn provide activation to help maintain the
overarching goal as well as to retrieve specific information relevant to
the task. So how are deferred goals ever successfully retrieved when we
are in the midst of performing ongoing tasks? We attempt to answer that
question in the following section.

Sources of variance in prospective memory performance in the real world

Our theoretical account of prospective memory suggests that the
probability of retrieval of an intention (at the desired time or
otherwise) is determined by several factors, one of which is the
effectiveness of cues associated with the stored intention that may be
noticed and processed attentively. The effectiveness of a cue hinges on
the level of activation delivered to the intention from the cue.
Therefore the strength of a cue's association to the intention, the
number of intentions associated with that cue, and the number of
intermediate links though which source activation must spread before
reaching the stored intention should influence prospective memory
performance. Direct and indirect experimental evidence supports these
predictions. A number of studies have demonstrated a substantial effect
of the strength of association between a cue and an intention\--either
by choosing cues with a strong *a priori* association to the intention
(Mantyla, 1993; McDaniel & Einstein, 2000; Nowinski & Dismukes, in
press) or by encouraging participants to rehearse the association
(Guynn, McDaniel & Einstein, 1998; Passolunghi, Brandimonte & Cornoldi,
1995; Taylor, Marsh, Hicks & Hancock, 2004). Fewer studies have examined
the effect of the number of associations to the cue on prospective
memory performance, but McDaniel and Einstein (1993) demonstrated that
words that were likely to have fewer associations, unfamiliar words such
as *bole* and *monad*, were more effective prospective memory cues than
more common words. Finally, several studies have demonstrated that cues
associated only indirectly to an intention are less effective than are
directly associated cues. These studies found that prospective memory
performance was better when specific cues rather than general category
cues were used to define the conditions for executing intentions; for
example instructions to participants might read: "Press the slash key
when you see the word apple," versus "Press the slash key when you see
the name of a fruit" (Cherry, et al., 2001; Ellis & Milne, 1996). When a
general category instruction is used cue activation must spread across
two associative links from the specific target presented at retrieval to
the category concept to the intention.

The encoding of an intention is another major factor influencing
prospective memory performance. We suggest that intentions are encoded
in a form similar to IF...THEN statements, with the THEN part specifying
what is to be done and the IF part specifying the conditions under which
the intention is to be executed. The Holbrook et al. (2005) diary study
found substantial variation in the way that individuals encoded
intentions to perform everyday tasks. Often individuals encoded only a
vague notion of the window of opportunity for executing an intention,
and did not identify specific cues they were likely to encounter that
could trigger retrieval of the intention at the appropriate time. For
example, one might form an intention to go to the grocery store without
specifying when to execute the intention. In these circumstances
retrieval depends on chance encounters with cues that have preexisting
associations with the intention---for example, one might be reminded of
the need to get groceries while eating lunch in the office cafeteria.
The effects of variation in encoding of intentions have not been
explored experimentally until recently. In most laboratory paradigms the
instructions to participants specify a particular cue that narrowly
defines the condition under which the prospective task is to be executed
(e.g., encountering a particular word or category of word while
performing the ongoing task of evaluating the pleasantness of a series
of words). Later in this chapter we report an experimental study of
interruptions in which encoding was manipulated and which supports the
associative activation model.

Implicit Intentions

Our studies of interruptions and habitual tasks in the cockpit have led
us to conclude that intentions are sometimes implicit rather than
explicit. In episodic situations individuals explicitly form an
intention to perform an action at some later time when conditions become
appropriate. But interruptions of real-world tasks sometimes occur so
quickly and forcefully that individuals do not think explicitly about
the need to resume the interrupted task after the interruption. In these
situations we argue that an intention does exist, however it is implicit
in the individual's original plan to execute the interrupted task. If
queried, the individual is likely to say that they do intend to complete
the interrupted task, or if they forget to complete it, they are likely
to say they intended to do so. When individuals do not form an explicit
intention to resume an interrupted task, they may be especially
vulnerable to forgetting because they do not encode specific cues likely
to be encountered after the interruption that can trigger retrieval of
the need to go back to the interrupted task.

Implicit intentions are also involved in highly practiced tasks that are
always performed in a particular situation, for example setting flaps to
the takeoff position after starting the engines and before taxiing to
the runway. Pilots do not need to think in advance of each flight "I
must remember to set the flaps", thus no explicit advance intention is
formed for each episode of setting the flaps. However we argue that the
intention to set the flaps exists implicitly as part of the action
schema for preparing the aircraft for flight. Normally, highly-practiced
tasks such as this are performed with great reliability, but they become
vulnerable to inadvertent omission if the cues that normally trigger
execution of an action are absent.

For the sake of discussion, let us suppose that a captain normally calls
for the flaps to be set when the engine after-start checklist is
completed. Performing the checklist is strongly linked in procedural
memory to the next action, calling for flaps to be set. Contextual cues
from the environment at the airport gate may also contribute to
remembering to set the flaps. Nowinski and Dismukes (in press) reported
an experiment in which contextual cues enhanced the effectiveness of
primary cues in a prospective memory task. But what happens if the crew
must defer setting the flaps until after taxi because of freezing slush
on the taxiway? The cues that normally trigger crews to set the flaps
are removed---this action is now out of sequence, temporally separated
from completion of the after-start checklist and removed from the normal
environmental context provided by being at the gate. Unless the pilots
form an explicit intention to set the flaps at a specific point and
identify or create cues to remind them, they become vulnerable to
forgetting to perform this essential action.

Cues that normally trigger habitual action can also be removed for
reasons other than crew actions. For example, Nowinski et al. (2003)
found that landing without a clearance at a controlled airport was one
of the prospective memory errors most frequently reported by airline
pilots. Normally crews are instructed to switch radio frequencies and
contact tower immediately by approach control, and crews apparently come
to rely on this prompt. However, approach controllers on occasion tell
crews to delay switching to tower frequency until reaching a specified
distance from the airport. Nowinski et al. (2003) found that 12 of the
13 reports citing an incident of landing without clearance occurred
under these circumstances in which the normal prompt to change frequency
immediately did not occur.

Interleaving tasks

Our model predicts that any prospective memory situation in which the
ongoing task does not direct attention to cues strongly associated with
the intention is vulnerable to error. Interleaving two or more
attention-demanding tasks is an important example of this situation.
Pilots are required to monitor the state and path of the aircraft and
the actions of the other pilot while performing other tasks, and
monitoring is considered an essential defense against threats to safety
and crew errors (Sumwalt, Thomas, & Dismukes, 2002, 2003).

Although task switching has been studied extensively in fundamental
research on attention mechanisms (Pashler & Johnston, 1998), much less
research has been conducted to determine the cognitive mechanisms
involved when individuals attempt to interleave tasks in real-world
situations. In contrast to typical laboratory paradigms for studying
attention mechanisms, many real-world situations do not provide strong
environmental cues for both tasks being interleaved, and switching
between tasks occurs relatively slowly---typically on the order of
minutes. In these situations individuals cannot maintain the intention
to check the status of other tasks continuously in focal attention, and
thus must somehow retrieve that intention to interrupt the ongoing task
periodically. Pilots report becoming preoccupied with one
attention-demanding task, such as re-programming the flight management
computer, and forgetting to check the status of another task, such as
monitoring the progress of the taxi by the captain (Dismukes, Young, &
Sumwalt, 1998; Loukopoulos et al., 2003).

We suggest that interleaving may be accomplished in one of two ways.
First, individuals may attempt to remember to interrupt the ongoing task
after some period of time has passed. However, because they cannot
consciously monitor the passage of time continuously while performing
the ongoing task, and because the ongoing task often directs gaze away
from the other task(s) to be monitored, it is not clear how individuals
retrieve the intention to switch attention to the other task. (See
Cicogna, Nigro, Occhionero, & Esposito 2005; and Logie, Maylor, Sala, &
Smith, 2004, for theoretical discussions). What is clear is that
individuals are more vulnerable to forgetting to perform time-based
prospective memory tasks than they are when performing prospective
memory tasks in which salient physical cues are available (Holbrook et
al., 2005, Einstein & McDaniel, 1996).

Second, we speculate that in practice individuals do not depend entirely
on time cues to perform time-based prospective memory tasks such as
interleaving. Rather, retrieval of the intention to switch tasks may be
prompted by happenstance noticing of cues associated to some degree with
the intention, and this prompting may be facilitated by the
environmental context. Also, individuals may implicitly learn rules of
thumb to help them remember to switch tasks; for example first officers
might learn to limit the number of actions taken to program a flight
management computer before looking up to check taxi progress. Either of
these two possibilities converts the time-based prospective memory task
to an event-based task, however both processes seem rather haphazard,
and performance is not likely to be highly reliable.

An Experimental Study of Interruptions

We describe here an experimental study that illustrates some of the
issues discussed in this chapter and provides support for our
theoretical account of some of these sources of errors of omission in
prospective memory. Interruptions by external agents are a major source
of errors of omission in cockpit operations (Dismukes et al., 1998),
maintenance (Hobbs & Williamson, 2003), and everyday tasks (Holbrook et
al., 2005), and presumably contribute to errors in other domains, such
as medicine (Gawande, Studdert, Orav, Brennan, & Zinner, 2003), although
these other domains have not been studied extensively.

The associative activation model suggests that individuals may be
vulnerable to forgetting to resume interrupted tasks in large part
because of three reasons. (1) The salient intrusion of many
interruptions quickly diverts attention and discourages encoding
explicit intentions and identifying cues to resume the interrupted task.
If no explicit intention is encoded, then remembering to resume the
interrupted task will depend on noticing happenstance cues that remind
the individual of the status of the interrupted task and the implicit
intention of completing all tasks. Even if an intention is explicitly
encoded, the conditions for resuming the interrupted task are likely to
be framed only as "after the end of the interruption"; individuals are
often not in a position to identify and encode specific perceptual cues
likely to be present at the end of the interruption. (2) Cues indicating
the window of opportunity for resuming the interrupted task at the end
of the interruption may not closely match the form in which the
intention (implicit or explicit) to resume the interrupted task is
encoded. The end of the interruption is not a perceptual cue but a state
of affairs that requires cognitively interpreting diverse perceptual
cues to recognize. If the individual does not consciously monitor for
the end of the interruption, the diverse perceptual cues may fail to
trigger recognition that the interruption has ended. (3) The end of
interruptions in real-world situations is often followed immediately by
other task demands that may not allow the individual sufficient time to
fully process and interpret environmental conditions signifying that the
interruption is over or to retrieve the associated intention (Holbrook
et al., 2005; Loukopoulos et al., 2003). Further, activation from
environmental cues associated with these other task demands may support
retrieval of the goals associated with these task demands preferentially
over retrieval of the goal to resume the interrupted task.

Dodhia and Dismukes (2005) designed an experimental paradigm to
investigate these three themes. Experiment participants were required to
answer a series of questions resembling the Scholastic Aptitude Test,
arranged in blocks of different question types (e.g., analogies,
vocabulary, math). They were instructed that when blocks were
interrupted by the sudden onset of a different block of questions they
should remember to return to the interrupted block (after completing the
interrupting block) before continuing to the next block in the series.
In the baseline (control) condition, these occasional interruptions were
abrupt\--the screen with the question participants were currently
working on was suddenly replaced with a screen with a different type of
question, and the background color of the screen changed.

After the end of the interrupting block, a screen appeared for 2.5
seconds with the message "Loading next section" (this screen also
appeared between all blocks that were not interrupted) and then the next
block of questions appeared without any reference to the interrupted
block. Without receiving any explicit prompt, participants had to
remember to return to the interrupted block by pressing a key.
Participants in the baseline condition frequently forgot to resume the
interrupted task and instead continued with the next block in the series
after an interruption---the proportion of successful resumptions of the
interrupted task was 0.48. These failures to return to the interrupted
block were due to memory failures, rather than to misunderstanding the
task requirements, as shown by participants' correct description of task
requirements when debriefed after the experiment and by the distribution
of errors among the five prospective memory trials for each participant.

To address our first hypothesis, that the intrusion of a sudden
interruption discourages adequate encoding of an intention to resume the
interrupted task, we implemented an *encoding reminder* condition in
which the interruption began with a four second text message "Please
remember to return to the block that was just interrupted." This
manipulation increased the proportion of resumptions from the baseline
condition of 0.48 to 0.65, which was highly significant statistically
(as were the results of all other manipulations, discussed below). It
was not clear whether the encoding reminder manipulation was effective
at improving performance because of the explicit reminder or because of
the additional four second delay before participants had to start
performing the interrupting task. We therefore performed an *encoding
pause* manipulation in which participants saw only a blank screen for
four seconds at the beginning of the interruption. This manipulation
also improved performance to 0.65. We interpret these results to
indicate that a pause before starting to perform an interrupting task
allows individuals time to recognize the implications of being
interrupted and to encode information that helps them to remember to
resume the interrupted task. The explicit reminder to resume the
interrupted task apparently did not provide any additional encoding
advantage.

We also addressed our second hypothesis that individuals are likely to
forget to resume interrupted tasks because they do not encounter
explicit cues signaling the end of the interruption. In the *retrieval
reminder* condition, participants received a message "End of
interruption" for 2.5 seconds while the next block was loading. This
message appeared above the "Loading next section" message that appeared
in all conditions. This manipulation increased the proportion of
interruptions resumed to 0.90.

Finally, we addressed our third hypothesis, that individuals sometimes
forget to return to an interrupted task because the end of interrupting
tasks is often quickly followed by other task demands that do not allow
the individual time to fully process and interpret environmental
conditions and to retrieve the intention to resume the interrupted task.
One might imagine that the "Loading next section" message that appeared
for 2.5 seconds after the end of interrupting blocks (and between all
blocks) would give participants enough time to reflect on whether they
should do anything else before starting the block after the
interruption. But we suspected that this short pause, coupled with the
message that the next section was about to start, might orient
participants toward mentally preparing to start the next section and
make them less likely to think about the implications of the start of a
new block of questions. Thus we created a *retrieval pause* condition in
which the delay between the end of the interrupting task and the
beginning of the next block was increased to 8-12 seconds and a
countdown clock appeared to display the remaining time to the next
block. This manipulation was intended to make clear to participants that
they had plenty of time before new task demands would begin. Resumption
performance increased to 0.88, supporting the idea that people fail to
resume interrupted tasks in part because their attention is quickly
diverted to new task demands arising after interruptions end.

Conclusion: Implications and Countermeasures

The results of this experimental study of interruptions are consistent
with the associative activation model, though of course much more
empirical research is required to validate such a broad conceptual
framework. Equally important, these results suggest practical ways
individuals can reduce their vulnerability to forgetting to resume
interrupted tasks. Individuals may be able to improve performance by (1)
pausing when interrupted to form an explicit intention to resume the
interrupted task and to identify cues that may be available to remind
them after the interruption and (2) pausing after completing all tasks
to ask which task should be performed next, which may not necessarily be
the most salient task. We are currently conducting experiments to
determine whether individuals can implement these two techniques, after
being given only general instructions, without reminders on each trial
from experimenters.

More broadly, we suggest that individuals may improve their prospective
memory performance by (1) deliberately encoding information about
environmental cues that may be encountered during the window of
opportunity for executing deferred intentions, (2) by creating salient
cues they will be likely to encounter at the appropriate time, (3) by
making and consulting lists of deferred intentions, and (4) by
periodically pausing to search memory for deferred intentions.

Although a relatively new topic in cognitive psychology, prospective
memory is clearly of great importance for safe, effective human
performance in many real-world situations. Continued theoretical and
experimental studies are needed to elucidate the cognitive mechanisms
underlying prospective memory, particularly to address questions about
the form in which intentions are encoded, how intentions are retrieved,
and how prospective tasks interact and compete with ongoing tasks.
However, we hope that investigators will not forget that we must also
study prospective memory in diverse real-world situations in order to
identify the full range of phenomena and sources of variance that
theoretical and experimental studies must attempt to explain.

Acknowledgments

Different aspects of this research were supported by NASA's Aviation
Safety Program and Aviation Systems Program and by the Federal Aviation
Administration (Eleana Edens, program manager). We thank Loukia
Loukopoulos, Jon Holbrook, and Barbara Burian for helpful comments on
this manuscript, and we are most grateful to our laboratory manager, Kim
Jobe, who supports all our efforts skillfully, graciously, and
diligently.

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