Prospective memory is a form of memory that involves remembering to perform a planned action or recall a planned intention at some future point in time. Prospective memory tasks are common in daily life and range from the relatively simple to extreme life-or-death situations. Examples of simple tasks include remembering to put the toothpaste cap back on, remembering to reply to an email, or remembering to return a rented movie. Examples of highly important situations include a patient remembering to take medication or a pilot remembering to perform specific safety procedures during a flight. In contrast to prospective memory, retrospective memory involves remembering people, events, or words that have been encountered in the past. Whereas retrospective memory requires only the recall of past events, prospective memory requires the exercise of retrospective memory at a time that has not yet occurred. Prospective memory is thus considered a form of "memory of the future". Retrospective memory involves the memory of what we know, containing informational content; prospective memory focuses on when to act, rather than focusing on informational content. There is some evidence demonstrating the role of retrospective memory in the successful execution of prospective memory, but this role seems to be relatively small.
Types
Event-based vs. time-based
There are two types of prospective memory: event-based and time-based prospective memory. Event-based prospective memory involves remembering to perform certain actions when specific circumstances occur. For example, driving past the local library cues the remembrance of the need to return an overdue book. Time-based prospective memory involves remembering to perform an action at a particular point in time. For example, seeing that it is 10:00 PM acts as a cue to watch a favorite television show. Research performed by Sellen et al. compared event-based and time-based cues on prospective memory tasks. The experimenters gave participants a place and a time and were told to press a button each time those cues appeared during the study. It was found that performance on event-based tasks was better than performance on time-based tasks, even when participants took more time to think about their responses. The difference in task performance between the two types of prospective memory suggests that the intended action was better triggered by external cues of the event-based task than internal cues of the time-based task. External cues, as opposed to internal cues, act as a prompt for better performance, making it easier to complete event-based tasks.
Types of event-based prospective memory: Immediate-execute vs. delayed-execute
McDaniel et al. further distinguished event-based prospective memory into immediate-execute tasks and delayed-execute tasks. Immediate-execute tasks involve a response as soon as a particular cue is noticed, while delayed-execute tasks involve delays between the perception of the relevant cue and the performance of the intended action. Delayed-execute tasks more commonly occur in real life when circumstances of a situation prevent intermediate action once the cue has been perceived. Research was performed by McDaniel et al., in which participants completed tasks involving various delays and interruptions between cues and responses. It was demonstrated that correct performance suffered when there was a delay or interruption during a task. However, it was further shown that the use of reminders for participants eliminated the effects of the interruption task.
History and theoretical perspectives
There is great interest about the possible mechanisms and resources that underlie the workings of prospective memory.
The preparatory attentional and memory (PAM) theory
The preparatory attentional and memory theory proposes two types of processes involved in successful prospective memory performance. The first component of this theory involves a monitoring process that begins when a person constructs an intention that is then maintained until it is performed. This monitoring component involves a capacity-consuming process, similar to those used when maintaining attention, because there is a need for the intention to be stored and maintained in memory. The second component involves the use of elements of retrospective memory processes. These elements are used to differentiate between the wanted prospective memory intention and unwanted thoughts, in an attempt to keep focus on the goal and not the other options surrounding it. Retrospective memory is also used to remember specifically what intention is supposed to be performed in the future, and the monitoring process is needed to be able to remember to perform this action at the correct condition or time. According to this theory, prospective memory should be enhanced when complete attention is given to the desired task than when attention is divided among multiple tasks. Research conducted by McDaniel et al. attempted to prove that prospective memory performance is better on focused tasks as opposed to those where attention is split. Subjects completed a prospective memory task in either a condition where full attention was given or a condition where attention was divided on other tasks. The results were consistent with the PAM theory, showing that participants' prospective memory performance was better with full attention. However, there is a lot of scepticism that the rather complex mechanisms of the PAM theory are required for all, sometimes mundane, prospective memory tasks. In research by Reese and Cherry, participants formed an intention to act in the future, but were interrupted prior to acting on their intention when the cue was present. When participants were asked their thoughts at the moment of interruption, only 2% reported that they were thinking of the original intention. This demonstrated evidence against the PAM theory, that there is constant maintenance from the time of constructing the intention to acting upon it at the right circumstance.
Reflexive-associative theory
Further research conducted by Einstein and McDaniel in 1990, found that subjects during prospective memory tasks reported that their intention often "popped" into mind, instead of being constantly monitored and consciously maintained. Another theory was proposed in 2000, called the reflexive-associative theory, which states that when people create an intention for a prospective memory task, they make an association between the target cue and the intended action. Later when the target cue occurs, the automatic associative-memory system triggers the retrieval of the intended action and brings it back into conscious awareness. Therefore, as long as the target cue occurs, the association previously made will initiate the retrieval of the intended action, regardless of whether the intention is in consciousness.
Multi-process model
Another theory that has been used to explain the mechanisms of prospective memory is the multi-process model proposed by McDaniel and Einstein. This theory states that prospective memory retrieval does not always need an active monitoring process but can occur spontaneously. Therefore, multiple processes can be used for successful prospective memory. Further, it was believed that it would be maladaptive to rely solely on active monitoring because it requires a lot of attentional resources. This may potentially interfere with other forms of processing that are required for different tasks during the retention interval. Prospective memory cues will lead to spontaneous retrieval of an intention when at least one of four conditions is met: the cue and target action are highly associated with each other, the cue is salient, the other processes performed during the period between cue and action of the prospective memory task direct attention to relevant cue features, or the intended action is simple. Further research has found that although many aspects of prospective memory tasks are automatic, they do involve a small amount of processing. An experiment conducted by Einstein et al. found that some participants performed slower on a filler task when performing a prospective memory task at the same time. Even though some of the participants did not engage in active monitoring, they showed nearly the same rate of success on the task, demonstrating the use of multiple processes for prospective memory performance.
Neuroanatomy
Frontal lobe
As prospective memory involves remembering and fulfilling an intention, it requires episodic memory, declarative memory, and retrospective memory, followed by supervisory executive functions. All of these are controlled by the frontal lobe which is situated at the front of the cerebral hemisphere. Studies using positron emission tomography trace a slight increase in blood flow to the frontal lobe in participants completing prospective memory tasks involving remembering a planned action, while performing other tasks. During these procedures, sites of brain activation include the prefrontal cortex, specifically the right dorsolateral, ventrolateral, and medial regions, as well as the median frontal lobe. The prefrontal cortex is responsible for holding the intention in consciousness and suppressing other internal thoughts. The median frontal lobe keeps attention focused on the planned action instead of the other tasks. The prefrontal cortex is involved mainly in event-based as opposed to time-based prospective memory. Cheng et al. had participants with lesions in the prefrontal cortex perform event-based and time-based prospective memory tasks. They found that performance was impaired in the event-based tasks, which use event cues to trigger intentions, but not in the time-based tasks which use time cues to trigger intentions. Other lesion studies have also shown the use of the frontal lobe in remembering and focusing on intentions. Burgess et al. studied patients with lesions to areas in the frontal lobe such as Brodmann's area 10, finding that these patients failed to follow instructions and switch attention during prospective memory tasks.
Parietal lobe
The parietal lobe is typically involved in processing sensory information and is situated in the superior region of the brain. For prospective memory, the parietal lobe is important for recognizing cues that trigger an intended action, especially when the cues are visual or spatial. The parietal lobe is also responsible for maintaining attention on the intended action and inhibiting other activities during performance. Studies using PET have shown that the parietal lobe is activated when participants engage in prospective memory tasks involving visual information such as remembering a series of numbers. Activation of the parietal lobe is also evident in studies using magnetoencephalography which traces electric activity of the brain. Harrington et al. found that neural areas ranging from the inferior parietal cortex to the frontal gyri are involved in temporal monitoring during time-based prospective memory tasks. Patients with damage to these areas of the brain had difficulty judging duration and frequency of auditory tones that were presented. Keeping track of information over time is important for prospective memory, remembering intentions to perform in the future.
Limbic system
Much of the limbic system, which contains primitive brain structures relating to emotion and motivation, are involved in memory.
Testing methods
Methods that test prospective memory require the distinction between retrospective memory, which is remembering information, and prospective memory, which is remembering information for the future. Prospective memory requires retrospective memory because one must remember the information itself in order to act in the future. For example, remembering to buy groceries after work requires the ability to remember what type of groceries are needed. While prospective memory and retrospective memory are connected, they are distinguishable. This makes it possible to separate these two processes during tests.
Self-report
Prospective memory tasks
Prospective memory tasks can be used in a variety of ways to assess prospective memory. Firstly, results from these tasks can directly assess prospective memory. Furthermore, these tasks can be performed while experimenters use PET, magnetic resonance imaging, or MEG to monitor brain activation. Finally, these tasks can be followed by questionnaires about prospective memory. Combining different assessments can confirm or deny experimental findings, making sure that conclusions about prospective memory are accurate. All tasks can assess individual stages of prospective memory such as the formation or execution of an intention, or access prospective memory as a whole by looking at overall performance.
Technological assessments
Technological assessments were created in order to more appropriately evaluate prospective memory by combining real life intentions with experimental control.
Factors affecting prospective memory
Age
There is an increasing amount of research on the effect of age on prospective memory where typical studies compare groups of people from different ages. A study by Smith et al. comparing event-based prospective memory in schoolchildren and young adults found that adults had better memory performance. Another study by Kvavilashvili et al. comparing time-based prospective memory among young adults, young-old adults and old-old adults showed that young adults had better performance. Event-based prospective memory was further compared between young-old and old-old adults and findings were that young-old adults performed better than old-old adults. These studies suggested that there is continual improvement of prospective memory from childhood into young adulthood but that a decline begins in late adulthood.
Genetics
A study comparing prospective memory of non-psychotic first-degree relatives of patients with schizophrenia and control participants showed that the relatives performed significantly worse on time-based and event-based prospective memory tasks. Since schizophrenia has a heritable component, this suggested that genetics may play a role in affecting prospective memory.
Substance use
Diseases and disorders
Many diseases and disorders negatively affect prospective memory, as well as source memory, item recognition, and temporal order memory. The effects range from mild cognitive impairments to more detrimental impairments such as early onset dementia.
Pregnancy
The effect of pregnancy on prospective memory is still under current study. Rendell et al. tested the prospective memory of 20 pregnant women in the laboratory. There were no significant differences observed between pregnant and non-pregnant women for event-based prospective memory tasks, but there were clear hindrances in performance for pregnant women in time-based prospective memory tasks such as a job deadline. Pregnant women are more likely to remember to perform an intention after the cue has already passed. Further, women tested a few months after giving birth were found to forget intentions entirely. Both these findings may be related to stress encountered during pregnancy or child rearing and lack of sleep.
Emotional target cues
Emotional target cues have been shown to eliminate age differences in prospective memory. For older participants, emotional prospective memory cues were better remembered than neutral cues. Whether the cues are positive or negative, strong emotional attachment makes the cue more self-relevant and easier to remember. For example, an aversive picture of a snake biting a person or a positive picture of a dog licking its owner are easier to remember because they evoke emotional responses, as opposed to a neutral picture of an animal that does not evoke an emotional response. It is speculated that the amygdala and hippocampus may play a role in this emotionally enhanced memory effect.
Motivational incentives
In a study by Kliegel et al., it was shown that motivational state affected performance in two age groups completing the same prospective memory task. There was no difference for the two age groups when motivation was high but performance of the three years old was reduced when motivation was low. If a person considers a task to be unimportant or is affected by fatigue, they will not be motivated to remember the intention. Less attention will be given to relevant cues and the memory is more likely to be forgotten. Therefore, prospective memory can be enhanced by avoiding low motivational states.
Everyday prospective memory
Various studies have reported that 50-80% of all everyday memories are, at least in part, related to prospective memory. Prospective memory is crucial for normal functioning since people form future intentions and remember to carry out past intentions on a daily basis. Numerous aspects of daily life require prospective memory, ranging from ordinary activities such as remembering where to meet a friend, to more important tasks such as remembering what time to take medication.
Time management
There is a complicated relationship between prospective memory and time management skills which include making lists, scheduling activities, and avoiding interruptions. Studies have not identified distinct cause and effect relationships between prospective memory and time management, but many consistent correlations have been observed. For example, people who reported better prospective memory according to the Prospective and Retrospective Memory Questionnaire also indicated a higher likelihood of setting goals and priorities and being more organized. There may be a cyclical effect between prospective memory and time management: better memory may lead to better organization, and better organization may further lead to better memory.
Aviation
Aviation controllers are often occupied with multiple tasks at the same time, and hazardous effects can occur when prospective memory fails. In an accident in 1991, a tower controller in an airport forgot a step in a simple procedure and that led to two planes crashing into each other, killing a number of passengers and crew. An analysis of over 1300 fatal aviation accidents from 1950-2009 showed that the majority were due to pilot error: 50% attributed to pilot error, 6% due to non-pilot human error, 22% to mechanical failure, 12% to bad weather, 9% to sabotage, and 1% to other causes.
Nursing
The nursing environment is full of event-based and time-based prospective memory tasks. Simple tasks such as remembering to order a drug or calling patient's family and remembering when to switch shifts are just some examples of a nurse's reliance on prospective memory. It is surprising that not much research has been done concerning the importance of prospective memory in nurses since they face many life-threatening tasks.
Contraception
Prospective memory is required to remember when to take oral contraceptive pills. A study performed by Matter and Meier showed that women who self-reported higher prospective memory ability were more satisfied with oral contraceptive use and experienced lower stress levels. Having better memory makes it is easier for these women to remind themselves to take their contraceptives at the required time of the day.
Smartphones
With advancements in technology, Smartphones can serve as prospective memory aids. Electronic calendars are of great use in time-based prospective memory tasks and recently they have been shown to also cue event-based tasks. The iPhone, as well as phones using the Android operating system, can track the user's location using the phone's Global Positioning System and send reminders based on the current location. For example, when a parent is near their children's school, the phone can send a reminder for them to pick up their children after school.
Education
Prospective memory has been implicated in the steering cognition model of how children coordinate their attention and response to learning tasks in school. Walker and Walker showed that pupils able to adjust their prospective memory most accurately for different curriculum learning tasks in maths, science and English were more effective learners than pupils whose prospective memory was fixed or inflexible.
Prospective person memory
Attempts to find wanted or missing individuals through public alert systems sometimes make use of a type of event based prospective memory called prospective person memory. In prospective person memory, a picture of a wanted or missing person is presented to the public with instructions to report any sightings of the individual to authorities. Field experiments show that prospective person memory is often quite poor.
