Prospective Memory

Most studies of human memory have been on retrospective memory. The focus has been on the past, especially on people's ability to remember events they have experienced or knowledge they acquired previously. In contrast, much of everyday life is concerned with prospective memory, which involves remembering to carry out intended actions.

One of the few attempts to study prospective memory in an entirely naturalistic way was reported by Marsh, Hicks, and Landau (1998). They found that people reported an average of 15 plans for the forthcoming week. Approximately 25% of these plans were not completed, but the main reasons for these non-completions were rescheduling and reprioritisation. Overall, only about 3% of the plans were not completed because they were forgotten. Forgetting was more common for plans involving an intention to commit or to communicate than it was for commitments or appointments.

There is an important distinction between time-based and event-based prospective memory. Time-based prospective memory involves remembering to perform a given action at a particular time (e.g., arriving at the pub at 7.30 pm). In contrast, event-based prospective memory involves remembering to perform an action in the appropriate circumstances (e.g., passing on a message when you see someone).

Sellen et al. (1997) compared time-based and event-based prospective memory in a work environment. The participants were equipped with badges containing buttons. They were told to press their button at pre-specified times (time-based task) or when they were in a pre-specified place (event-based task). Performance was better in the event-based task than in the time-based task (52% vs. 33% correct, respectively), in spite of the fact that the participants thought more often about the time-based task. Sellen et al. (1997) speculated that event-based prospective memory tasks are easier than time-based tasks, because the intended actions are more likely to be triggered by external cues.

As Baddeley (1997) pointed out, retrospective and prospective memory do not differ only with respect to their past versus future time orientation. Retrospective memory tends to involve remembering what we know about something and can be high in information content. In contrast, prospective memory typically focuses on when to do something, and has a low informational content. Another difference is that prospective memory is obviously of relevance to the plans or goals we form for our daily activities in a way that is not true of retrospective memory.

Another difference between prospective and retrospective memory is that there are generally more external cues available in the case of retrospective memory, especially in comparison to time-based prospective memory. If external cues are often lacking, why is prospective memory generally successful? Morris (1992) referred to a study in which there was evidence that cues only marginally related to the to-be-remembered action could sometimes suffice to trigger a prospective memory. For example, a participant who had been told to phone the experimenter as part of an experiment was reminded by seeing a poster for another psychology experiment.

Kvavilashvili (1987) found evidence of differences between prospective and retrospective memory. Participants were told to remind the experimenter to pass on a message. Those who remembered (i.e., showing good prospective memory) were no better than those who did not remind the experimenter at remembering the content of the message. Thus, prospective memory ability may be unrelated to retrospective memory ability.

Common sense indicates that motivation helps to determine whether we remember to do things. It is easier to remember something enjoyable (e.g., visit to the theatre) than something unpleasant (e.g., visit to the dentist). According to Freud (1901, p. 157), the motive behind many of our failures of prospective memory "is an unusually large amount of unavowed contempt for other people." Freud's views (as usual) were over the top, but motivation does make a difference to prospective memory. Meacham and Singer (1977)

instructed their participants to post postcards at one-weekly intervals, and performance was better when a financial incentive was offered.

As Cohen (1989) pointed out, prospective memory should be considered with respect to the action plans we form. Action plans can be routine (e.g., have lunch) or novel (e.g., buy a new car); they can be general (e.g., organise a dinner party) or specific (e.g., buy a bottle of wine); they may form part of a network of plans (e.g., organise the arrangements for a business trip) or they may be isolated (e.g., buy a collar for the cat); and they may be high or low in priority. Prospective memory is likely to be best for plans that are routine, high in priority, and relate to a network of plans (see Cohen, 1989). Networks may be of special importance: we rarely forget to carry out actions (e.g., having lunch; catching the 8.00 am train) that are well embedded in our daily plans.

Theoretical perspectives

Prospective memory depends more than retrospective memory on spontaneous memory retrieval. This suggests that prospective memory involves top-down or conceptually driven processes, a notion that was tested by McDaniel, Robinson-Riegler, and Einstein (1998) in a study on event-based prospective memory. They contrasted conceptually driven processes (depending on the meaning or significance of stimuli) with bottom-up or data-driven processes determined by the physical characteristics of stimuli.

In their first experiment, the participants had to press a key when any of three homographic words (words such bat and chest which have more than one distinct meaning) was presented. This prospective memory task was embedded within another task. It was designed to resemble the real world, in which we have to remember to interrupt our ongoing activities to perform some action. Performance on the prospective memory task was worse when the meaning of the homograph changed than when it remained the same. The finding that prospective memory was influenced by meaning rather than purely by the physical stimulus suggests the involvement of conceptually driven processes.

Similar findings were obtained in a second experiment. The stimuli to be detected on the prospective memory task were initially presented as words or pictures, and thereafter they were presented in the same form or in the alternative form. Prospective memory performance was mostly affected by the meaning of the stimuli rather than by their physical form (word or picture). As McDaniel et al. (1998, p. 130) concluded, "Across a number of manipulations that have been exploited in the retrospective memory literature as markers of conceptually driven and data-driven processes, we obtained convergence for the conclusion that prospective remembering is conceptually based."

In a third experiment, McDaniel et al. (1998) addressed the issue of whether attentional processes are involved in prospective memory. Participants performed a prospective memory task under full or divided attention. In the latter condition, they listened for three odd numbers in a row as well as performing the prospective memory task. Prospective memory performance was much better with full attention than with divided attention, indicating that attentional processes are involved in prospective memory.

Marsh and Hicks (1998) obtained similar findings. Their participants had to remember three words on each trial, and the event-based prospective memory task was to respond whenever a type of fruit was presented. They also had to perform a third task at the same time, and this task involved one of the components of working memory (see Chapter 6). Their key findings were that a task involving the attention-like central executive (e.g., random number generation) impaired prospective memory performance, but tasks involving the phonological loop or the visuo-spatial sketchpad did not. Marsh and Hicks (1998, pp. 347-348) concluded: "These experiments suggest that event-based prospective memory requires some optimal degree of conscious, central executive processing. This point is non-trivial given people's intuitions that event-based remembering feels spontaneous as evidenced by research participants reporting that the response 'pops to mind' on seeing a target word."

Guynn, McDaniel, and Einstein (1998) developed the theoretical approach of McDaniel et al. (1998). According to their activation theory, what is crucial in event-based prospective memory is the association between the target event and the intended activity. A strong prediction of this theory is that reminders will be ineffective unless they activate this association. They tested this prediction in a study in which the participants' main task was to perform an implicit memory task. The prospective memory task involved detecting certain target words whenever they appeared on the implicit memory task. Reminders either activated the association between target words and action ("Remember what you have to do if you ever see any of those three words") or they did not ("Remember the three words you studied at the beginning of the experiment").

The findings obtained by Guynn et al. (1998) were as predicted. Reminders designed to activate the target-action association produced a significant improvement in prospective memory performance, whereas those not activating the association had no effect. Guynn et al. (1998, pp. 297-298) concluded that, "Effective rehearsal or reminding appears to be that which increases the likelihood that the appearance of a target event automatically evokes remembering of the intended activity, and that appears to be rehearsal or reminding that focuses on both the target event and the intended activity."


McDaniel et al. (1998) and Guynn et al. (1998) have made progress in understanding the processes involved in event-based prospective memory. As predicted, there are important similarities between event-based prospective memory tasks and conceptually driven retrospective memory tasks involving explicit memory. However, the processes involved in time-based prospective memory remain rather mysterious, because it is generally not possible to identify any obvious external cues that facilitate performance. People sometimes remember to perform an action at a given time because they see a watch or clock shortly beforehand, or they make use of the fact that there is a set pattern to their daily routine (Sellen et al, 1997), but this is probably the exception rather than the rule.

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