Introduction

Prospective memory (PM) is the ability to remember a delayed intention. Forgetting to complete intentions could affect school-age children’s academic performance (e.g., forgetting to bring their homework to school) and social relationships (e.g., forgetting to give back a friend’s book). In PM, delayed intentions have to be remembered in response to particular contextual situations [1]. In event-based PM tasks, retrieval of an intention requires a trigger of an associated memory from some external cue (e.g., remembering to buy bread when passing the grocery store on the way home). In contrast, in time-based PM tasks, the person intends to perform a task at a specific time, within a specific time period or when a period of time has elapsed (e.g., remembering to buy bread before 8 p.m., when the grocery store closes). Finally, activity-based tasks require that intentions be retrieved and executed upon completing other tasks (e.g., remembering to buy bread after buying vegetables from the fruit stall). Successful completion of a PM task requires remembering an intention (e.g., press a key when red words appear on a screen) while performing another ongoing task (OT; e.g., answering general knowledge questions). In addition, at the appropriate moment or when the prospective cue appears, the person must stop performing the OT to instead perform the intention [2,3].

Several studies have shown developmental increments in PM performance using event-based PM tasks (see [4–6]). Age differences in PM tasks appear when cues are not part of the OT (non-focal cues), but not when cues are part of the OT (focal cues). Focality effects in event-based tasks have been studied with children of different ages [7]; however, very few developmental studies have focused on activity-based tasks [8,9], even though results with adult participants indicate that these are more difficult to remember than event-based tasks [10]. For instance, with adult participants Brewer et al. [10] reported differences in PM performance for event-based versus activity-based PM tasks. When the PM cue was event-based (e.g., saying “now” when the OT involved numbers), participants correctly responded 60% of the time, whereas their performance dropped to 23% when the PM cue was activity- based (e.g., saying “now” when the activity involving numbers came to an end). Similarly, the few studies conducted with children using activity-based PM tasks have shown lower performance in activity than in even based tasks, with age differences being more evident in activity than event based tasks. For example, Wash et al. [8] did not find age differences in pre-schoolers when they completed an event-based task in which children were asked to catch the explicit visual cue “Elmo” when its image appeared on the screen while they were playing a computer game. By contrast, the probabilities of asking for a sticker after finishing the game were quite low for 3-and 4- compare to 5-year-old children. However, activity based performance can improve if children are motivated [9]. Thus, in a study focusing on motivation and agency, Causey et al. [8] found that retrieval of the intention was higher in a high motivation condition where pre-schoolers were asked to remember to take a sticker when finishing the task relative to a low motivation condition, in which children were asked to remember to change the sign on the door. Therefore, initial findings suggest that activity-based and event-based PM differences seem to be relevant during development, and motivation seems to be a modulating factor.

In general, motivation seems to affect the probability that children remember the PM intention [11–14]. For example, Han et al. [12] required children to correctly place coloured balls (previously associated with a specific animal) below the corresponding animals. In addition, they had to give a model bone to a dog (PM task) whenever they encountered the target dog in the OT. In their Experiment 3, Han et al. [12] manipulated the difficulty of the OT by including three or five possible animal–colour pairs. They also manipulated the children’s motivation by offering a prize to half of the children if they performed well on the OT. The results showed that the difficulty of the OT affected the PM performance only when 3-, 4- and 5-year-old children were highly motivated to complete the task. In addition, in a naturalistic study in which parents described their children’s (7–8 and 10-11-year-olds) performance in everyday PM tasks, Penningroth, Bartsch and Mcmahan [11] reported better PM performance in the tasks the children considered more important. Further, older children outperformed younger children when the task was less motivating; however, these age differences disappeared for tasks that parents evaluated as highly important for their children. Hence, these studies suggest that children's motivation might modulate PM performance by elevating performance when children are highly motivated, and reducing the effects of age, and type of tasks.

Finally, other factors that seem to affect PM performance are the difficulty and nature of the OT task. In particular, there is some evidence that the difficulty and nature of the OT might modulate some PM effects. For example, Rendell, McDaniel, Forbes and Einstein [15] found differences in PM performance between younger and older adults in a non-focal event task (Experiment 1), but found that these age differences disappeared when the OT was less complex (Experiment 2). Similarly, some observed interactions between younger ages and focality in event-based PM tasks [7] have been interpreted as being due to the greater difficulty of non-focal tasks for younger children, since the OT is usually not adapted to children’s ages (e.g.[16,17]). The nature of the task (artificial vs. natural) might also explain some of the contradictory results. For example, Kliegel et al. [7] found no age differences in a focal event-based condition consisting of a driving computer game (artificial setting), while Krasny-Pacini, Servant, Alzieu and Chevignard [18] reported age differences in 8- to 11-year-old children who performed a focal event task in a more natural environment (in which the OT consisted of baking a chocolate cake). Therefore, the nature of the task might also be an important factor to consider when exploring PM effects. However, very few studies have explored natural tasks [8,18], and their use has not been extended to the types of prospective situations that children usually confront during school (e.g., writing their name on a test or asking for a letter for their parents).

In sum, because previous studies have shown that motivation (e.g. [9,12,13]) and type of PM task (activity- vs. event-based) influence children’s performance in PM tasks [8], we sought to explore whether motivation and type of PM task influenced the prospective memory performance of school-age children and whether these variables interacted with each other and modulated their possible effects. Because the nature of the OT task seems to also influence PM performance, an important feature of our study was that we explored children’s PM performance in the context of natural tasks that were close to the types of activities that children usually perform at school and by adapting the difficulty of the OT tasks to the participants’ ages. Thus, in our study children were evaluated using school-related OT activities (e.g., working with puzzles, completing math problems, reading, finding differences between pictures, etc.) that they performed in their school context. Because we were interested in developmental effects, we assessed the 6–7 and 10-11-year-olds with the adjusted event- and activity-based PM tasks. In addition, half of our participants were included in a reward (higher motivation) group, while half were assigned to a no-reward (lower motivation) group. Overall, we expected better performance in the event-based PM task than in the activity-based PM task. However, we anticipated that these PM-task differences would be more evident in the lower-motivation condition. Importantly, we expected that age differences would be larger in the more demanding activity-based conditions than the event-based conditions. These predictions were based on the assumption that more demanding PM conditions should produce larger developmental effects, and that motivation might modulate the effects of the type of PM task.

Method

Results

To test that the OT difficulty was successfully adjusted to each age group and to explore whether the type of cue (event vs. activity) or motivation influenced the OT completion times (measured in seconds), a 2 (type of task: event vs. activity) x 2 (motivation: reward vs no reward) x 2 (age: 6–7 vs. 10–11) mixed ANOVA was performed on the completion times for the OT. The results of the analysis showed a significant effect of motivation (Table 1), F (1, 111) = 7.08, MSe = 15473.51, p< 0.01, η_p² = 0.06, indicating that the children in the reward group performed faster (M = 384.82, SD = 11.88) than the children in the no reward group (M = 428.75, SD = 11.47). All other effects and interactions were not significant: type of task x motivation, F (1, 111) = 1.14, p = 0.29, age, type of task x age, age x motivation, and type of task x age x motivation (Fs< 1). These results show that, although children in the reward group performed the OT faster than children in the no reward group, the type of cue did not influence OT completion-time performance. More relevant, no age differences were found in the completion times for OT performance, indicating that the OT was properly adapted to each age.

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Table 1. OT completion time.

https://doi.org/10.1371/journal.pone.0215845.t001

PM performance

PM scores were analysed by comparing event and activity conditions between the reward and no reward groups and the two age groups (6–7 and 10-11-year-olds). PM correct responses for each condition were calculated and submitted to a 2 (type of task: event vs. activity) x 2 (motivation: reward vs no reward) x 2 (age: 6–7 vs. 10–11) mixed ANOVA. The results of this analysis revealed a statistically significant effect of type of PM task (Table 2), F (1, 111) = 18.08, MSe = 0.11, p < 0.01, η_p² = 0.14, 1-β = .98 with better performance in the event PM (M = 0.67, SD = 0.35) than in the activity PM (M = 0.48, SD = 0.38) condition. The effect of age was also reliable: The older group was more accurate than the younger one (M = 0.69, SD = 0.31 vs. M = 0.48, SD = 0.37), F (1, 111) = 18.77, MSe = 0.13, p < 0.01, η_p² = 0.14, 1-β = .99. The motivation effect was very close to statistical significance, F (1, 111) = 3.78, MSe = 0.13, p = 0.054, η_p² = 0.03, 1-β = .49, such that higher motivation (reward) tended to produce better performance than lower motivation (no reward). None of the interactions was significant: type of task by age, F (1, 111) = 2.00, p = 0.16, 1-β = .29; type of task by motivation, F (1, 111) = 1.26, p = 0.26, 1-β = .20; age by motivation, F (1, 111) = 0.53, p = 0.47, 1-β = .11; type of cue x age x motivation, F (1, 111) = 0.14, p = 0.71, 1-β = .07. Because the distribution of PM scores was not normal, we performed supplementary non-parametric analyses (see [14] for a similar approach). A Kruskal–Wallis ANOVA confirmed that the main effects of type of PM task H(1) = 14.00, p < 0.01 and age H(1) = 16.47, p < 0.01 were significant.

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Table 2. Prospective memory performance.

https://doi.org/10.1371/journal.pone.0215845.t002

Although motivation did not interact with the other variables, because previous studies have shown that age differences in PM performance decrease with motivation, we compared children´s performance in both motivation groups (reward and non-reward, see last column in Table 2). The results indicated significant differences between the younger (M = 0.42, SD = 0.36) and older (M = 0.66, SD = 0.31) groups in the no-reward condition (t (116) = 3.63, p <0.01, d = 0.71) and also in the reward condition (t (110) = 2. 55, p <0.05, d = 0.51; M = 0.54, SD = 0.38 and M = 0.72, SD = 0.32), although the effect size was larger in the no-reward condition. In addition, because we were expecting motivation to have larger effects in the activity-based than in the event-based condition, we conducted post-hoc analyses to examine the effects of motivation on each type of cue condition. These analyses revealed no differences between the reward (M = 0.70, SD = 0.32) and no reward (M = 0.66, SD = 0.30) conditions when the cue was event-based (t (113) = 0.53, p = 0.05, d = 0.13), whereas motivation had a significant benefit (t (113) = 1.97, p = 0.05, d = 0.37) when the cue was activity-based (M = 0.56, SD = 0.38; M = 0.42, SD = 0.38).

Discussion

We explored whether age differences in remembering an intention were modulated by the type of PM task (activity- vs. event-based) and by children’s motivation. The idea was that children would perform event-based PM tasks better than activity-based tasks in line with previous results with pre-school children and adults [8,10]. We also expected that motivation would increase performance and that age differences would be more evident in more difficult conditions (activity-based tasks) and when the children were not motivated. The results partially supported our expectations.

First, performance was higher for event-based than activity-based tasks, indicating that activity-based intentions are more difficult to retrieve at the appropriate moment than are event-based tasks. This effect has been attributed to the high salience of the event-based cues since these event-based tasks provide more explicit visual cues than the activity-based tasks [8]. In line with this interpretation, Brewer et al. [10] showed that when activity-cues were made salient by asking participants to underline the last generated word (OT) and write the number of words generated, participants improved their performance in the activity based tasks. Therefore, our results are consistent with previous findings indicating that the salience of the cue (represented by event vs activity based tasks) is an important factor determining PM performance [7]. The present result was obtained when the OT task difficulty was adjusted to the children’s ages. This is important because it suggests that the type of cue effect (event-based vs activity-based) and the developmental differences cannot be attributed to the nature of the OT task—specifically, differences in difficulty of the OT. In the present paradigm the equivalence of OT-task difficulty across age groups is supported by the finding that children in both age groups were able to perform the OT tasks in similar time frames. Also of importance, the OTs and the context in which they were performed were chosen to be similar to the activities and contexts the children usually experienced at school or at home. To our knowledge, this is the first study directly comparing activity- and event-based tasks that adjusts the OT to the children’s age and uses natural tasks and school contexts.

Intriguingly, the performance of the older children was higher than that of the younger children, independent of the type of PM task and motivation. Age effects are easily explained by the fact that, within the studied age range, attentional processes and executive functions are still developing (e.g. [19]), and PM performance during childhood is likely to correlate with the development of executive functions [20,21]. However, we expected smaller or even null effects in the easier event-based condition, since event-based tasks are assumed to involve more spontaneous, less demanding processes than non-focal or activity-based tasks [22]. Previous studies have shown contradictory results, with some studies showing age differences in focal event tasks [8,18] and others showing no differences [7]. These contrasting results could be explained by differences in the tasks used. For example, Kliegel et al.[7] used a computer game as the OT, while Walsh et al. and Krasny-Pacini et al.[8,18] used more natural tasks, such as purchasing items while shopping. It is likely that computer games—in which children focus only on the computer screen—are more absorbing (i.e., more focal) than natural tasks, in which the children move more freely and information is more dispersed. However, this needs to be tested, since the effects of different type of cues on children have rarely been investigated.

We also sought to test the role of motivation in the performance of activity- and event-based tasks, as several studies have found motivation to have positive effects on PM performance during childhood (e.g. [9,11–13]). Our hypothesis was that, if children were motivated, their PM performance would increase and age differences would decrease [11]. However, our results show only marginally significant motivation effects. The fact that our motivation effects were smaller than those obtained in previous studies might be due to several factors. First, the procedures were very different. For instance, Penningroth et al. [11] based their results on parents’ recordings of their children’s activities, whereas, our study was based directly on children’s performance on PM tasks. Second, it is possible that the children were not differentially motivated by our motivation conditions. Although a previous study with adults has shown that participants’ pro-social PM performance is improved when they obtain a large reward (e.g., 20 euro) relative to a small reward (e.g., 1 euro) or no reward, in our study the children might have perceived neither reward to be sufficiently stimulating because the reward was presented inside a bag (big or small) and was unknown [23]. Despite these factors, the reward instructions induced faster responses in the OT and showed a tendency to increase PM performance in the activity-based task, but not in the event-based task. Note that it is possible, that children under the faster OT times in the reward condition might have produced lower performance in the PM task (speed/accuracy trade-off) so that if they had remained at the slower pace than children in the non-reward condition, the differences in PM performance might have been larger. In any case, it is clear that motivation had an effect on OT performance by reducing response times and that it seemed to increase PM performance in activity based tasks. This pattern of results would suggest that motivation might only benefit the effortful monitoring processes required to perform PM tasks when less salient PM cues are involved (activity-based tasks) and not the performance of tasks involving explicit visual cues (event-based PM tasks), in which retrieval would be driven by cues with less involvement of attentional control [22]. Interestingly, our results also suggest that even younger children could strategically deploy these resources when motivated to improve their performance.

In conclusion, the results show that PM during child development is affected by PM task type. PM tasks in which cues appeared as part of the OT produced better performance than PM tasks in which the intention was signalled by the end of the OT and required more cognitive resources for detection. In addition, PM performance was better in 10-11year-olds than in 6-to 7-year-olds in both the event and activity conditions. This result is in line with the suggestion that cognitive resources (e.g., working memory, inhibition and task-switching) are related to good PM performance in children [20,21]. Finally, our motivation condition had a small effect that was more evident in the activity-based tasks.

Acknowledgments

We would like to thank CEIP Tinar and the International Schoolhouse for their collaboration.