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European Journal of Ageing logoLink to European Journal of Ageing
. 2018 Feb 27;16(1):63–71. doi: 10.1007/s10433-018-0461-8

Examining the role of rehearsal in old–old adults’ working memory

Alexandra Hering 1,✉, Mirjam Rautenberg 2, Paula von Bloh 2, Katharina Schnitzspahn 3, Nicola Ballhausen 1,4,5, Andreas Ihle 4,5, Prune Lagner 1, Matthias Kliegel 1,4,5, Katharina Zinke 6
PMCID: PMC6397115  PMID: 30886561

Abstract

We investigated the role of rehearsal in verbal working memory (WM) and whether WM capacity can be improved by a rehearsal instruction in very old age. In two experiments, we tested a total of 78 old–old adults (75 years and above) in one experimental session consisting of three assessment phases. First, participants worked on three different WM span tasks to assess their baseline performance. In the next phase, half of the participants received a rehearsal instruction to practice on two of the WM tasks, whereas the other half received no strategy instruction (Experiment 1) or worked on a filler task (Experiment 2). In the final phase, participants again worked on the three WM tasks. In Experiment 1, we found significant improvements for the WM tasks over time in both groups. However, we could not find a specific improvement for the rehearsal instruction due to a high spontaneous strategy use in the control group. When minimizing spontaneous strategy use in Experiment 2 by changing the task material, we found larger improvements in the instruction compared to the control group. However, we still found substantial spontaneous strategy use in the control group. The results indicate that rehearsal, as an essential component of verbal WM, is still intact and efficient in old–old adults. Furthermore, the spontaneous strategy use indicates that old–olds use their existing skills to cope with increasing WM demands. Finally, old–old adults benefited from an explicit rehearsal instruction showing potentials to boost WM capacity in this age group.

Keywords: Working memory, Strategy, Rehearsal, Ageing, Old–olds

Introduction

Working memory (WM) is a central neurocognitive processing resource that is involved in most conscious everyday mental activities (e.g. reading, calculating). The term WM describes the ability to maintain (store) and manipulate (process) information over short periods of time (Baddeley 2003). WM has been shown to support a wide range of complex cognitive functions, including logical reasoning and problem solving, and to be strongly related to measures of fluid intelligence (Conway et al. 2002; Engle et al. 1999). From an ageing perspective, it is crucial to note that WM is among those cognitive processes that are prone to age-related decline: Research has revealed substantial mean level decreases in WM performance in old age (e.g. Borella et al. 2008; Hale et al. 2011; Park et al. 2002). This decline is already evident in young–old adults (60–75 years), but is particularly pronounced in old–old adults (75 years and above; Gilinsky and Judd 1994; Hale et al. 2011). Considering the importance of WM for cognitive functioning in general as well as for autonomy and wellbeing until a very old age (e.g. Tomaszewski Farias et al. 2009), the understanding of its functioning is of highly relevance and would provide a foundation for tailored interventions.

The architecture of WM has been conceptualized, for example, as a multicomponent model (Baddeley 1986; for an overview on alternative WM models see Baddeley 2015) with two subcomponents that store verbal or visual information and a central executive that regulates between the subcomponents. The verbal subcomponent is called phonological loop and holds speech-related information for temporary storage. The content can be refreshed before its decay with the help of rehearsal. The second subcomponent, the visuospatial sketchpad stores visual and spatial information (e.g. objects). The central executive represents an attentional control system that coordinates the two subcomponents but is not a separate storage itself. Furthermore, in a later version of the model, Baddeley (2000) suggested an episodic buffer, a multimodal storage, that allows for the interaction of the subcomponents with long-term memory (Baddeley 2015). In the present study, we focus on verbal WM and the role of rehearsal for successful WM functioning in old–old adults. Verbal WM is essential in language comprehension such as following conversations or reading information, which is not only crucial in everyday life but especially in health behaviour in the growing population of old and old–old adults.

Rehearsal, that is the subvocal or spoken repetition of memory content, is not only considered as a process component in WM, but it is also investigated in light of mnemonic strategies to improve memory or WM performance. For example, participants are instructed to use strategies to help compensating for their WM deficits. Strategy use supports the storage component in WM by enhancing the available capacity (Kintsch 1994). Hence, strategies should increase WM performance (Ericsson and Kintsch 1995) or should make WM tasks less attention-demanding (Engle et al. 1999). Although rehearsal is often considered a less effective encoding strategy (Bailey et al. 2009), studies on individual differences in strategy use and WM performance suggest a more differentiated role of rehearsal for WM performance. Morrison et al. (2016) showed that rehearsal was the most frequent reported strategy in seven different WM tasks. However, it is to note that complex WM measures differ in their processing demands, which influences the suitability of rehearsal for the specific tasks (see Morrison et al. 2016 for an overview of different task dimensions). For example, in some WM span tasks the processing component is designed to have a secondary task to prevent rehearsal of the span items (e.g. De Beni et al. 1998). Nevertheless, Turley-Ames and Whitfield (2003) found that low-span (i.e. individuals with a lower WM capacity) but not high span (i.e. higher WM capacity) individuals benefited from a rehearsal strategy on a WM span task. As available literature mostly focused on young–old adults, it remains still open if rehearsal is still effectively supporting WM span in very old age and if old–old adults would demonstrate a general utilization deficit in strategy use that also affects the functioning of rehearsal in WM span tasks.

To investigate WM span in old–old adults, we selected two previously published WM tasks that were used in a study on WM training in old–old adults (Buschkuehl et al. 2008; additionally see Loosli et al. 2012 for a study on children using a similar WM training). One task resembled a serial recall task with increasing WM load, the second task represented a WM capacity task including a storage and processing component. In the study by Buschkuehl et al. (2008), the authors trained old–old adults on different adaptive WM and reaction time tasks twice a week for 12 weeks. Participants in the training group improved their performance in the WM training tasks and showed transfer effects to a non-trained visual WM task. Although the authors concluded that the training led to cognitive plasticity in old–old adults, they could not exclude that the observed benefits were (possibly at least partially) based on task-specific strategies used/developed by the participants.

Taken together, rehearsal is a key process in maintaining verbal information in WM. While age-related decline in WM is well documented, the impact of age in this fine-grained subcomponent remains open to investigation especially for the group of old–old adults. Furthermore, rehearsal serves as a mnemonic strategy that could be beneficial for individuals with lower WM span. Therefore, our first aim was to examine if WM span can be boosted by directly instructing old–old adults to use rehearsal in different WM span tasks compared to a control group that did not receive any rehearsal instruction. Secondly, we aimed to investigate the role of rehearsal in different WM span tasks in old–old adults. More precisely, we were interested how strategy use influences WM capacity within specific WM span tasks based on the findings by Buschkuehl et al. (2008) that indicated task-specific strategy use. In our study, we selected the two WM tasks from Buschkuehl et al. (2008) that indicated potential strategy development. We experimentally manipulated the usage of strategies in old–old adults. Either participants received an instruction to use rehearsal for the WM tasks or they worked on the tasks without any explicit instructions.

Experiment 1

Method

Participants

Thirty-nine old–old adults participated in the first experiment with an age range of 74–87 years (M = 79.9; SD = 3.55). Twenty participants, among them 17 women and 3 men, received the rehearsal instruction, whereas 19 participants belonged to the control group, among them 16 women and 3 men. Both groups did not differ regarding age (t(37) = − .048; p = .962; instruction group: M = 80.0; SD = 3.62; control group: M = 79.9; SD = 3.57) and years of education (t(37) = .058; p = .954; instruction group: M = 13.5 years; SD = 3.66; control group: M = 13.6 years; SD = 4.93). Before testing, participants were screened for current psychiatric or neurological diseases, visual or hearing impairment and colour blindness. None of the participants had to be excluded due to these criteria. Participants contributed to the study without monetary compensation.

Material

We used three different WM tasks taken from a training study on healthy old–old adults by Buschkuehl et al. (2008). We chose these specific tasks for their appropriateness in the respective age group.

Coloured squares task

Participants saw four coloured squares (each 200 × 200 pixels) in red, green, yellow and blue on the screen. During each trial, one of the squares disappeared for 1.5 s and reappeared. After another second, the next square disappeared and reappeared, and so forth. The order in which the squares disappeared was randomized. Participants were asked to recall the order in which the squares disappeared at the end of the trial by clicking on them. They received feedback whether their response was correct or not before the next trial started. Trial length (number of squares that disappeared) was adapted trial by trial to the performance of the participants. If they answered correctly, the sequence length was increased by one additional item; if they answered incorrectly, the length was reduced by one item. The number of squares on the screen was held constant to four squares. Participants performed the task for 3 min. The average span of all correctly answered trials was used as the dependent variable.

Animal task

Participants saw one of two possible cartoon pictures of a cat and a dog at a time. The picture was facing either upright or upside down, and participants had to indicate the orientation by pressing a corresponding button within 3 s. Animal pictures and their orientation were presented in a random order. If participants responded too slowly (i.e. after 3 s), they were asked to respond faster; otherwise, the next picture appeared on the screen. After a sequence of pictures, participants saw both animal pictures in the middle of the screen and were asked to recall the order of the presented animal pictures (disregarding their orientation) by clicking on them. Small icons of both animal pictures appeared at the bottom of the screen to indicate the selected order. The task was adapted to the performance of the participants. Whenever the participants answered all orientation decisions correctly and were able to recall the correct order of the pictures, the sequence length was increased by one item. Whenever participants were able to recall correctly the sequence but failed at the orientation decisions or exceeded the time limitation, the sequence length remained the same as in the previous trial. Lastly, whenever participants reproduced a wrong sequence of pictures, the sequence length was decreased by one item for the next trial. Participants performed the task for 3 min. The averaged trial length was used as the dependent variable.

Block span task

A 4 × 4 grid of white squares was displayed on the screen. A blue dot appeared within different squares of the grid in a randomized sequence. After the presented sequence, participants were asked to recall the sequence of the blue dots by clicking at the appropriate squares. The task was adapted to performance. Whenever the participant produced the correct sequence, the trial length (the number of times the dot appeared) was increased by one for the next trial; if they produced the wrong sequence, the trial length decreased by one. The task was aborted after two incorrectly reproduced sequences in a row. The average number of correctly reproduced dots per trial was used as the dependent variable.

Procedure

All participants took part in one individual session. The testing session started with signing the informed consent and assessing health information. Afterwards, participants of both groups performed the three WM tasks to assess their baseline WM performance (pre-test). This first phase of the session took 20 min and was followed by a 5-min break.

During the second phase, participants worked on the coloured squares task and the animal task with the experimental group receiving rehearsal instructions, whereas the control group did not receive such additional instructions. More precisely, the experimenter explained the rehearsal strategy to the participants before they trained the rehearsal strategy on the two WM tasks as follows: For the coloured squares task, participants were asked to repeat the colour of the current disappearing square aloud during its presentation. During recall of the sequence, they were also asked to verbalize the whole colour sequence aloud. Participants worked on five practice trials with feedback on their strategy use from the experimenter. Afterwards, the participants worked again for 3 min on the task to train the rehearsal strategy. For the animal task, participants trained the rehearsal strategy as well. Their task was to name the animal aloud and say if the orientation was right or wrong (e.g. “cat right” for the picture of the cat in upright position). Again, participants were asked to repeat the whole sequence aloud while entering it. Participants worked for five practice trials on that task and received feedback on their strategy use. Afterwards, they performed the task again for 3 min. The control group did not receive any rehearsal instruction. Participants worked quietly on the two WM tasks for 5 min each to control for task repetition effects.

After the instruction phase, all participants had another short break before starting with the final phase of the WM assessment (post-test). Participants of both groups worked on all three WM tasks. The coloured squares task and the animal task provide information on the direct use of rehearsal for these two tasks (as it had been trained before), whereas the block span task allows investigating potential rehearsal in a task that the participants know but have not been instructed to use the rehearsal strategy. At the end of the session, all participants received a questionnaire about their strategy use. Participants of the instruction group were asked if they applied the rehearsal strategy during the tasks and on which tasks they used the strategy and if it was useful to them. Participants of the control group were also asked if and on which tasks they used a strategy. Additionally, they were asked to describe what type of a strategy they applied using their own words.

Results

We conducted 2 (group: instruction group versus control group; between) × 2 (time: pre-test versus post-test; within) repeated measures ANOVAs for the three WM tasks. Prior to these analyses, we compared the baseline performance level between the two groups using independent t tests. If not otherwise stated, alpha-level was set to .05.

Baseline performance level

There were no statistically significant group differences between instruction and control group for the three WM tasks (coloured squares task: t(37) = .793; p = .433; animal task: t(37) = 1.733; p = .091; block span task: t(37) = .406; p = .687). Table 1 displays means and standard deviations for the groups and different tasks at the pre- and the post-test. During the instruction phase, the instruction group achieved an average a trial length of M = 3.46; SD = .49 for the coloured squares task and M = 2.74; SD = .55 for the animal task. The control group achieved an average a trial length of M = 3.97; SD = .64 for the coloured squares task and M = 2.95; SD = .78 for the animal task.

Table 1.

Performance of instruction group and control group for Experiments 1 and 2

Instruction group Control group
Pre-test Post-test Pre-test Post-test
M SD Range M SD Range M SD Range M SD Range
Experiment 1
Coloured squares taska 3.08 .58 2–6 3.46 .38 4–6 3.23 .67 2–7 3.74 .54 4–7
Animal taska 2.40 .56 2–6 2.81 .55 3–5 2.75 .67 2–5 2.99 .69 2–6
Block spanb 2.82 .43 3.03 .48 2.89 .57 3.17 .42
Experiment 2
Four stars taska 3.21 .57 2–7 3.47 .44 4–6 3.09 .55 3–6 3.07 .73 2–6
Two stars taska 2.43 .50 2–6 3.25 .66 3–7 2.38 .51 2–5 2.55 .62 2–6
Block spanb 3.29 .43 3.48 .37 3.31 .41 3.39 .45
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Range indicates the minimum and maximum of the maximal trial length (span) achieved

aAverage span

bAverage number of correctly reproduced dots per trial

Influence of rehearsal on WM performance

Repeated measures ANOVAs regarding the three WM tasks revealed no significant interactions or differences between the two groups on all three tasks (all p > .135). Only the main effects for the within factor time (coloured squares task: Ftime(1, 37) = 27.753; p < .001; η2p = .43; animal task: Ftime(1, 37) = 11.425; p = .002; η2p = .24; block span task: Ftime(1, 37) = 15.110; p < .001; η2p = .29) were significant. Both groups improved their performance from the pre-test to the post-test.

Strategy use

The questionnaire assessing strategy use showed that all participants of the instruction group used rehearsal at the post-test. However, 16 out of 19 (84%) participants of the control group reported using rehearsal during post-test as well.

Discussion

The results showed that old–old adults in both groups improved their performance on three different WM tasks compared to their baseline performance. The strategy use questionnaire indicated that all participants of the instruction group used the rehearsal strategy at the post-test. More interestingly, almost all control participants spontaneously came up with the same rehearsal strategy, which could be the reason for their improvement. Similar to young–old adults, the old–old adults spontaneously produced a task-appropriate strategy that benefited performance (e.g. Bailey et al. 2009 for a similar finding in young–old adults). However, the comparable performance in both experimental groups restricted the conclusions regarding our first aim to boost WM capacity through explicit instruction. We cannot rule out that the increase in performance is due to other reasons than the instructed strategy. Alternatively, the performance increase could also be due to the repeated exposure to the tasks. It remains difficult to isolate the influence of the strategy instruction on WM performance. Furthermore, we only assessed the strategy use at the post-test; thus, we cannot exclude that participants in the instruction group applied strategies spontaneously already at the pre-test. One reason for the high number of spontaneous strategy use in the control group could be that the task material was easy to verbalize (colours and animals). To minimize spontaneous verbalization, we changed the task material in Experiment 2.

Experiment 2

In Experiment 2, we followed up on our first experiment with the aim to make spontaneous strategy use in the control group more difficult in order to disentangle the specific contribution of rehearsal to WM improvements in old–old adults. For this purpose, we adapted the task material to reduce the likelihood of verbalization in the two groups and therefore to isolate the influence of rehearsal condition (instruction group) compared to the non-rehearsal condition in the control group.

Methods

Participants

Thirty-nine old adults participated in the second study with an age range of 70–88 years (M = 78.7; SD = 4.88). Nineteen participants (16 women and 3 men) received the rehearsal instruction, whereas 20 participants belonged to the control group (15 women and 5 men). The groups did not differ significantly regarding age (t(37) = − .283; p = .779; instruction group: M = 79.0; SD = 5.29; control group: M = 78.5; SD = 4.59), years of education (t(37) = − .560; p = .579; instruction group: M = 12.7; SD = 2.74; control group: M = 12.2; SD = 3.47) and verbal ability (t(33.565) = − 1.989; p = .055; instruction group: M = 32.42; SD = 3.60; control group: M = 29.55; SD = 5.30) assessed with a German vocabulary test (MWT; Lehrl et al. 1991). Before testing, participants were screened for current psychiatric or neurological diseases, visual or hearing impairment and colour blindness. None of the participants had to be excluded due to these criteria. Participants contributed to the study without monetary compensation.

Material

We used two WM tasks with different task material but similar procedure to the coloured squares task and animal task from Experiment 1. Furthermore, we included the previously used block span task. Task administration of the block span task was identical to Experiment 1.

Four stars task

The first WM task, the four stars task, followed the task procedure of the coloured squares task from Experiment 1 (Buschkuehl et al. 2008). Four different abstract pictures showing fractal shapes (3 cm × 5 cm; Floel et al. 2004) were used instead of the coloured squares to minimize spontaneous verbalization of items. Task set-up and adaptivity were identical to the coloured squares task from Experiment 1. The average span of all correctly answered trials was used as the dependent variable.

Two stars task

The second WM task—called the two stars task—was based on the animal task by Buschkuehl et al. (2008) from Experiment 1. The animal pictures were replaced by patterns of two fractal shapes (13 cm × 7 cm) with either big or thin spikes (Floel et al. 2004). Furthermore, a green or red frame surrounded the patterns. Similar to the animal task from Experiment 1, participants had to work on two tasks. The first task was to decide if the frame is green or red by pressing colour-matched keys within 3 s. For the second task, participants had to memorize the presentation order of the patterns and reproduce the sequence after each trial. The patterns were displayed in random order. The number of displayed patterns per sequence was adapted to the performance of the participant in the same way as for the animal task of Experiment 1. The average span of all correctly answered trials was used as the dependent variable.

Procedure

Every testing session started with signing the informed consent and assessing health information as well as the participant’s verbal abilities. In the first assessment phase (pre-test), participants of both groups worked on the three WM tasks to measure their baseline performance. Each task lasted approximately 3 min. After a short break, participants of the instruction group received the rehearsal instruction. Participants of the control group read articles about healthy ageing from local health magazines as a filler task to minimize spontaneous strategy development.

The strategy instruction started with the experimenter explaining the rehearsal strategy. Then, participants trained the rehearsal strategy on the four stars and two stars tasks. For the four stars task, participants were asked to number the abstract pictures and repeat the number of the disappearing picture aloud to memorize the sequence. Participants worked on the task twice for 3 min. After the first block, the experimenter made sure that the participants used the rehearsal strategy as instructed by asking them about their strategy application and gave feedback or further explanations if necessary. For the second time, the participants worked on the task on their own. For the two stars task, participants practiced the rehearsal strategy as well. This time, the experimenter asked them to name the fractal shapes according to the size of the spikes (“thin” or “big”) to better rehearse the sequence. The instruction phase lasted around 20–25 min (M = 23; SD = 4.14).

After the instruction phase or filler task, respectively, all participants had another short break before starting with the third assessment phase (post-test). Participants of both groups worked on all three WM tasks. At the end of the session, participants received a questionnaire about their strategy use. Participants of the instruction group were asked if they used a strategy on WM tasks at the pre-test, and—if yes—on which of the tasks and what strategy. For the post-test, they were asked about their use of rehearsal and the usefulness of the strategy. The participants of the control group were asked if they used a strategy at any of the tasks during pre- and post-test, and—if yes—on which tasks and what strategy they used. The session lasted for 1.5 h per participant.

Results

We conducted 2 (group: instruction group versus control group; between) × 2 (time: pre-test versus post-test; within) repeated measures ANOVAs for the three WM tasks. Prior to these analyses, we compared the baseline performance level between the two groups using independent t tests. If not otherwise stated, alpha-level was set to .05.

Baseline performance level

T tests revealed no statistical significant group differences between instruction and control group for the three WM tasks (four stars task: t(37) = − .661; p = .513; two stars task: t(37) = − .309; p = .759; block span task: t(37) = .144; p = .886). Table 1 displays means and standard deviations for the groups and different tasks at the pre- and the post-test. During the instruction phase, the instruction group achieved on average the following trial length: four stars task first trial M = 3.43; SD = .50, second trial M = 3.53; SD = .48; two stars task first trial M = 2.71; SD = .58, second trial M = 3.09; SD = .63.

Influence of rehearsal on WM performance

Repeated measures ANOVAs regarding the three WM tasks revealed a mixed pattern. For the four stars task, neither the interaction effect nor main effects for group and time were statistically significant (all p > .114), suggesting no differences between groups or changes over time. For the two stars task, all three effects of interest were statistically significant. The ANOVA revealed a significant main effect of group (Fgroup(1, 37) = 5.335; p = .027; η2p = .13), a significant main effect of time (Ftime(1, 37) = 32.191; p < .001; η2p = .47) and, importantly, a significant interaction effect between both factors (Finteraction(1, 37) = 14.085; p = .001; η2p = .28).1 Participants in the instruction group improved their WM span significantly from the pre-test to the post-test as opposed to participants in the control group. On average, the instruction group improved from 2.43 items to 3.25 items (t(18) = − 5.595; p < .001)2, whereas the control group started with 2.38 items at the pre-test and recalled on average 2.55 items at the post-test (t(19) = − 1.725; p = .101). Thus, the instruction group showed an improvement of 34%, whereas the control group only improved by 7%.

Regarding the block span task where rehearsal was not directly trained, neither the interaction nor the main effect of group was statistically significant (all ps > .354). Only the main effect for the factor time reached significance (Ftime(1, 37) = 4.487; p = .041; η2p = .11). Participants of both groups performed significantly better at the post-test compared to their baseline performance.

Strategy use

Fifty-three per cent of the participants of the instruction group reported that they had not used any strategy at the pre-test, whereas 47% reported to use a rehearsal-like strategy on either one or both of the stars tasks during the pre-test. After the post-test, 89% of the instruction group reported that the rehearsal strategy was useful for them. Fifty-five per cent of the control group report rehearsal-like strategy use at some point during the testing session, whereas 45% of participants in the control group never used a strategy. At the pre-test only 25% of the participants in the control group started to use a strategy already.

Discussion

The results extended the findings from the first experiment. Participants of the instruction group benefited from rehearsal and improved their performance in one of the WM tasks (the two stars task) more than those in the control group improved, explaining 28% of the between-group variance in terms of performance increase. However, there were no effects for the other two WM tasks (four stars task, block span task). This is surprising, given that the four stars task was considered being easier than the two stars task and performance was comparable with performance in Experiment 1. Maybe the rehearsal strategy was hindering performance gains by consuming too many resources during this specific task. The instruction to number the abstract pictures could have been too difficult, and the pictures could have looked too similar for using rehearsal efficiently. The strategy assessment showed that less participants of the control group used a strategy spontaneously compared to Experiment 1. However, the number of participants using a strategy was still high (55% of participants in Experiment 2 compared to 89% of participants in Experiment 1) indicating that even old–old adults show spontaneous strategy use dissenting a general strategy production deficit in very old age.

General discussion

In the present study, participants of the instruction group received a rehearsal strategy instruction whereas the control group received no strategy instruction. Previous research showed that rehearsal as an explicit strategy is especially beneficial for people with reduced WM capacities (i.e. low-spans), because it is an easy and cognitively little demanding strategy (Turley-Ames and Whitfield 2003). The two experiments of the present study revealed three major findings. First, old–old adults were able to improve their performance in different WM span tasks. A high proportion of rehearsal use in both the instruction and the control group that did not receive an explicit strategy instruction in Experiment 1, might suggest that this task-specific strategy use could have fostered WM capacity in old–old adults. Second, this finding indicates that old–old adults are still able to produce and apply strategies and do frequently so spontaneously. Third, old–old adults could benefit from an explicit rehearsal instruction. Even one instruction session was sufficient to improve old–old adults’ WM span in Experiment 2. In the following, we will discuss each of the main findings in more detail.

The first finding concerns the role of rehearsal for verbal WM performance in old–old adults. Rehearsal is an important prerequisite for the functioning of the phonological loop, the verbal subcomponent of WM (Baddeley 2003). In Experiment 1, participants of both groups improved over time in the three WM tasks. Importantly, also 84% of the control group reported spontaneous use of rehearsal to perform the WM tasks. It indicates that even in old–old adults, rehearsal is efficiently functioning in verbal WM and rehearsal itself might not be implied in the age-related decline of WM. Other cognitive ageing theories assume further mechanisms underlying decrements in WM, such as a deficit in inhibiting irrelevant information (e.g. Hasher and Zacks 1988) or a general slowing of processing speed (e.g. Salthouse 1996).

The second noteworthy finding was that even after changing the task material in Experiment 2 still approximately 55% of the participants used spontaneously a strategy to master the WM tasks. In older adults, there is evidence that internal strategy use switches to more external strategy use, e.g. using written notes (Bouazzaoui et al. 2010). Thus, one could expect less spontaneous strategy use in older adults, which was not the case in our study. Rehearsal is a rather easy strategy that is a beneficial strategy for participants with lower WM capacities (Turley-Ames and Whitfield 2003). As we argued previously, rehearsal could be a suitable strategy for old–old adults, which is also indicated by the frequent spontaneous use of rehearsal in the control groups in Experiments 1 and 2. Rehearsal seems to be part of the repertoire of skills that most old–old adults have available and can apply effectively. However, because we cannot quantify the link between spontaneous rehearsal use and performance in our study, we cannot entirely exclude alternative reasons for performance increase (e.g. training on the tasks, reduced test anxiety). Future studies are necessary to disentangle potential emotional and experience-related influences on WM performance in old–old adults.

The third major finding is the significant improvement of WM after the explicit rehearsal instruction. In the second experiment, the instruction group improved their WM capacity on the two stars task more than the control group. It is worth pointing out that the interaction was even qualified by a relatively large effect size. More interestingly, the task comprised two task instructions and, therefore, put higher WM demands on the participants. Nevertheless, participants benefited from the strategy instruction and could use rehearsal to improve their WM span in this task. This finding is in line with previous research in older adults (but mostly young–olds) showing beneficial effects of strategy interventions (e.g. Bailey et al. 2009; Carretti et al. 2007; Singer et al. 2003). However, studies reported reduced gains with increasing age in old–old adults (Singer et al. 2003; Verhaeghen et al. 1992), which might put a limit to possible improvements of old–olds using rehearsal.

There are limitations of the study that have to be considered when interpreting the results. Importantly, due to our experimental design, we cannot exclude the influence of task exposure effects (i.e. test–retest effects). In experiment 1, we only found a general improvement in both experimental groups but no significant interactions. In Experiment 2, the two groups differed in exposure time to the task material, which could have led to the increases in task performance as well. However, our aim was to investigate if working memory performance can be improved by using an explicit encoding strategy (i.e. rehearsal) that is suitable for old–old adults. Therefore, the instruction group needed to be trained in strategy use on the tasks. The rather large improvement in the instruction group for compared to the control group in line with the strategy reports indicates that task exposure alone does not sufficiently explain the improvement in the two stars task, but rather that the strategy use led to the improvements.

Furthermore, a significant benefit of rehearsal was only found for one WM task in Experiment 2. This could be because the suggested rehearsal instruction (i.e. numbering the fractal shapes in the four stars task versus naming the shape in the two stars task) was too difficult to achieve gains in the task or did not tap on the same task-specific processes in the other tasks to lead to improvement. More precisely, the rehearsal instruction in Experiment 2 asked for the integration of two steps, first naming or numbering the items and second rehearsing them. For example, the numbering in the four stars task could have been less informative than the description of the spikes in two stars task and therefore reduced the benefits of rehearsal. We also did not find a transfer of the rehearsal instruction to the block span task. It seemed that participants were not able to apply the strategy in a non-trained context. One reason could be that rehearsal was not a task-appropriate strategy for this rather visual WM span task. The groups in Experiment 2 varied in their exposure to the WM tasks. The instruction group trained the rehearsal strategy on the WM tasks, whereas the control group read newspaper articles. These differences might have also influenced the results.

Besides, it was not possible to completely prevent the control group from spontaneous strategy development even after changing the task material in Experiment 2. Thus, the benefit of the rehearsal instruction on performance cannot be fully disentangled between the groups. Future studies should include a more thorough online assessment of strategy use during WM span tasks (e.g. trial-by-trial assessment of strategy use) to link directly the strategy use to actual performance on an individual level. In the present study, we assessed the strategy use only after the tasks using self-reports. Still, there was an increase in rehearsal use for both the instruction and the control group in Experiment 2 and an increase in performance suggesting that rehearsal was relevant for the task processing.

Taken together, the present experiments show that old–old adults show intact rehearsal and could even benefit from an explicit instruction to improve their WM capacity. Furthermore, old–old adults used their own strategies spontaneously. This shows the potential for future training regimes in this age group. Building upon strategies and skills—which old–old adults are motivated and capable to use—and scaffolding them to more elaborated strategies and encourage old–old adults to use them might be a promising research avenue for this age group in promoting cognitive abilities and successful everyday life.

Acknowledgements

The preparation of this manuscript was in part supported by a grant from the Swiss National Science Foundation (SNSF).

Footnotes

1

The difference between the two groups in verbal abilities was close to being significant (p = .055), the instruction group tending to perform better than the control group. After covarying for verbal ability, the two main effects were no longer significant (Fgroup(1, 36) = 2.536; p = .120; η2p = .07; Ftime(1, 36) = .006; p = .941; η2p = .00). However, importantly, the interaction effect was still statistically significant (Finteraction(1, 36) = 10.807; p = .002; η2p = .23) with an almost similar effect size, suggesting that differences in verbal ability did not influence the main finding.

2

To ensure that the pre–post time effect in the instruction group was not purely due to task exposure, we conducted a mediation analysis using a multilevel linear model. We tested whether the pre–post time effect would disappear—or reduce—when adjusting for the amount of exposure received during training. As exposure variables, we extracted the duration that participants spent on the two training tasks in the instruction group. These analyses used the two stars task performance as dependent, time as the primary independent, and exposure variables as independents to be adjusted for. In addition, all models included a random subject intercept to account for within-subject correlation of the time factor. Results revealed that the pre–post time effect remained identical after adjusting for the two exposure variables (unadjusted pre–post time effect: β = 0.8221, SE = 0.1469, t(18) = 5.595, p < .001; pre–post time effect adjusted for exposure: β = 0.8221, SE = 0.1470, t(18) = 5.595, p < .001).

Responsible editor: H.-W. Wahl.

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