Testing the Limits of Optimizing Dual-Task Performance in Younger and Older Adults

Strobach, Tilo; Frensch, Peter; Mueller, Hermann Josef; Schubert, Torsten

doi:10.3389/fnhum.2012.00039

ORIGINAL RESEARCH article

Front. Hum. Neurosci., 05 March 2012
Sec. Cognitive Neuroscience
Volume 6 - 2012 | https://doi.org/10.3389/fnhum.2012.00039

Testing the limits of optimizing dual-task performance in younger and older adults

Tilo Strobach¹* Peter Frensch² Herrmann Josef Müller¹ Torsten Schubert^1,2

¹ Department Psychology, Ludwig-Maximilians University Munich, Munich, Germany
² Department of Psychology, Humboldt-University Berlin, Berlin, Germany

Impaired dual-task performance in younger and older adults can be improved with practice. Optimal conditions even allow for a (near) elimination of this impairment in younger adults. However, it is unknown whether such (near) elimination is the limit of performance improvements in older adults. The present study tests this limit in older adults under conditions of (a) a high amount of dual-task training and (b) training with simplified component tasks in dual-task situations. The data showed that a high amount of dual-task training in older adults provided no evidence for an improvement of dual-task performance to the optimal dual-task performance level achieved by younger adults. However, training with simplified component tasks in dual-task situations exclusively in older adults provided a similar level of optimal dual-task performance in both age groups. Therefore through applying a testing the limits approach, we demonstrated that older adults improved dual-task performance to the same level as younger adults at the end of training under very specific conditions.

Introduction

In recent years, a number of studies have examined the effect of practice on dual-task performance across different age groups in order to better understand the basic cognitive mechanisms underlying dual-task performance and cognitive aging (Maquestiaux et al., 2004, 2010; Bherer et al., 2005, 2006, 2008; Göthe et al., 2007; Allen et al., 2009; Hartley et al., 2011). In younger adults, some studies have even demonstrated perfect time sharing of two tasks after practice defined by zero performance costs in dual compared to single-tasks (i.e., dual-task costs; Van Selst et al., 1999; Ruthruff et al., 2001, 2003; Schumacher et al., 2001). However, such findings of perfect or near perfect time sharing are lacking in the aging literature on older adults. The aim of the present study was to close this gap. Therefore, we investigated the limits of dual-task performance optimization in older adults (i.e., near perfect or perfect time sharing) and, furthermore, tested the conditions of such optimization.

Investigations of (near) perfect time sharing with practice are interesting for aging research because they may provide more conclusive evidence about cognitive plasticity, its range, and developmental mechanisms in older adults (Bherer et al., 2006). In particular, testing the limits of optimizing dual-task performance in older adults should demonstrate, in the case of complex task situations, the maximum cognitive performance potential or the “latent” reserve capacity of older adults in a more appropriate way than investigating cognitive abilities of older people without extensive practice. Baltes, Lindenberger and colleagues (e.g., Lindenberger et al., 1992; Lindenberger and Baltes, 1995) have argued that the testing the limits approach can lead to an identification of true age-related cognitive decline, rather than overestimate age-related differences due to non-optimized testing conditions, assuming that age-related differences in reserve capacity are more accurately assessed near the limits of performance. Rephrased in the testing the limits terminology, we test the developmental reserve when assessing the limits of optimized dual-task performance in older adults. This test is essential because older adults’ difficulty in performing concurrent tasks is one of the most well documented executive control deficits in cognitive aging literature (e.g., Allen et al., 1998; Hartley and Little, 1999; Glass et al., 2000; McDowd and Shaw, 2000; Hartley, 2001; Verhaeghen et al., 2003; Hein and Schubert, 2004; Verhaeghen, 2011). This deficit may result from age-impaired attentional control processes that are related to the substantial modifications observed in the frontal and prefrontal areas of the cerebral cortex during aging (e.g., West, 1996; Raz, 2000).

Optimizing Dual-Task Performance in Younger and Older Adults

Meyer and Kieras (1999) outlined conditions for optimal dual-task performance. In particular, the authors listed five prerequisites, which should be fulfilled in order to achieve such performance: “(Condition 1) participants are encouraged to give the tasks equal priority; (Condition 2) participants are expected to perform each task quickly; (Condition 3) there are no constraints on temporal relations or serial order among responses; (Condition 4) different tasks use different perceptual and motor processors; and (Condition 5) participants receive enough practice to compile complete production rule sets for performing each task” (p. 54).

Previous attempts to compare practice-related improvements in older and younger adults’ dual-task processing have provided impressive findings concerning cognitive plasticity in old age; however, unfortunately, they have not yet considered all of the conditions mentioned above and consequently their findings may not be fully conclusive regarding the limits of practice-related changes in older adults’ dual-task performance. For instance, Maquestiaux et al. (2010) applied a dual-task practice situation that emphasizes response speed for and extensive practice of only one component task (i.e., Conditions 1, 3, and 5 of Meyer and Kieras, 1999, were not implemented). In a different line of research, Bherer et al. (2005, 2006) did not include all conditions when applying similar perceptual and motor processors on the component tasks (i.e., Condition 4 was not implemented).

One such situation including conditions for optimal dual-task performance was applied in younger and older adults by Strobach et al. (2012b), see also Schumacher et al., 2001). The authors asked participants to perform a training paradigm that consisted of tasks with different perception and motor components (Condition 4): a visual-manual (i.e., the visual task) and an auditory-vocal choice RT task (i.e., the auditory task). During training with these tasks (Condition 5), three different trial types were performed: participants performed only one of the two tasks in single-task blocks (single-task trials); in mixed blocks, participants either responded to only one task (i.e., mixed single-task trials) or actually executed two motor responses to simultaneously presented stimuli in two different tasks (dual-task trials with stimulus onset asynchrony, SOA, of 0 ms). Participants were instructed to respond as quickly and as accurately as possible with equal priority and with no pre-specified serial order to both stimuli in these trials (Conditions 1, 2, and 3). They received adaptive and continuous on-screen feedback as well as performance-based monetary bonuses for optimized RT and error performance.

The training RTs in single-task, mixed single-tasks, and dual-task trials up to Session 8 (younger adults) and Session 12 (older adults) are summarized in Figure 1 and Tables 1 and 2. Dual-task costs are illustrated by the mean difference of dual-task and mixed single-task trials. At the end of eight training sessions, these dual-task costs were extremely reduced in younger adults (see also Schumacher et al., 2001; Hazeltine et al., 2002; Tombu and Jolicoeur, 2004; Strobach et al., 2008; Liepelt et al., 2011; Strobach et al., in press; for an adaption to a task switching situation see Strobach et al., 2012a). These findings demonstrate that in dual-task situations, implementing the prerequisite conditions for optimal dual-task performance (Meyer and Kieras, 1999), younger adults show nearly eliminated dual-task costs, i.e., near perfect time sharing, at the end of training; for the reasons for the not perfect but near perfect time sharing in this design see “General Discussion.” In older adults, however, dual-task costs were still relatively high after the same amount of training (i.e., eight sessions) and even after four additional training sessions when compared to the reduced costs in younger adults. Thus, these data provide no evidence for near perfect time sharing in older adults when testing the limits of cognitive functioning in this age group (e.g., Baltes and Kliegl, 1992).

FIGURE 1

Figure 1. Mean reaction times (RTs) in milliseconds (ms) on single-task trials in single-task blocks, single-task trials in mixed blocks (mixed single-task trials), and dual-task trials for (A) the visual task and (B) the auditory task across Sessions 2–21 (older adults) or Sessions 2–8 (younger adults). Session 2–12: Strobach et al. (2012b), Session 13–16: Experiment 1, Session 17–21: Experiment 2.

TABLE 1

Table 1. Error rates in percent in single-task trials of single-task blocks, single-task trials in mixed blocks (mixed single-task trials), and dual-task trials for the visual task in older and younger adults across Sessions 2–12 (older adults) or Sessions 2–8 (younger adults).

TABLE 2

Table 2. Error rates in percent in single-task trials of single-task blocks, single-task trials in mixed blocks (mixed single-task trials), and dual-task trials for the auditory task in older and younger adults across Session 2–21 (older adults) or Session 2–8 (younger adults).

However, it is conceivable that the defined conditions of Meyer and Kieras (1999) enable near perfect time sharing in younger and older adults. However, these conditions were not appropriately set to the requirements of older adults’ cognitive processing and learning functions in Strobach et al. (2012b). Consequently, the aim of the present paper is to modulate task and training characteristics in the dual-tasks of Strobach et al. to create conditions for near perfect time sharing in older adults. In fact, we modulated two crucial factors of this dual-task training procedure: (1) we increased the amount of training (Experiment 1) and (2) we simplified the included component tasks (Experiment 2).

Experiment 1

One potential explanation for the difference in the dual-task costs between younger and older adults at the end of practice could be the different initial costs in both age groups (e.g., also see Allen et al., 2009). The reduction of these higher costs in older adults to the level of reduced costs in younger adults may require an increased amount of practice in the former group. For instance, this requirement may result from the older adults’ slower speed in automatizing task sets during practice (e.g., Kramer et al., 1995). Thus, one possible method to optimize the level of dual-task performance in older adults is to increase the amount of training in this group of participants. This increase in the amount of training is similar to a strategy applied by Maquestiaux et al. (2008) in younger adults. Participants in that study conducted an increased amount of task training compared to a previous study by Ruthruff et al. (2006) to provide optimal conditions for eliminating dual-task performance costs. After this increase, a larger proportion of younger adults performed dual-tasks at an optimized level. Findings also exist about the effects of an increased training amount on dual-task performance through our study with older adults (Strobach et al., 2012b) demonstrating that such increase leads to improved dual-task performance even with prior training, i.e., after four additional sessions following eight prior training sessions.

In fact, in Experiment 1, we increased the amount of dual-task training by adding four sessions immediately after the end of the 12 sessions with the identical older adults of Strobach et al. (2012b). Such a prolongation of training with an identical group of participants makes a novel contribution of the present experiment by testing whether prolonged practice after 12 training sessions enabled older adults to arrive at a performance limit (i.e., near perfect time sharing). This prolongation is plausible because dual-task performance improved until Session 12 and the limit was not attained in this session. Note that we refer to the additional sessions in the following sections of Experiment 1 as Session 13–16 as these sessions immediately followed the Sessions 1–12 reported in Strobach et al. The performance after combined training of 16 sessions with these older adults were compared with the optimized dual-task “target” performance, i.e., reduced dual-task costs reflecting near perfect time sharing, in younger adults after the eighth training session. In this way, we doubled the amount of training in older compared to younger adults before we assessed dual-task performance. If this doubling produces conditions for near perfect time sharing in older adults, we would expect the same level of dual-task performance, i.e., (extremely) reduced dual-task costs, at the end of training in younger adults (i.e., Session 8) and older adults (i.e., Session 16). However, if such doubling is not sufficient to produce conditions for near perfect time sharing in older adults, dual-task costs should not be (extremely) reduced in this age group and should increase the amount of younger adults’ costs.

Method

Participants

Ten older adults (mean age = 63.3 years, SD = 3.4, range 57–68, 5 female) were recruited from university courses for senior adults at LMU Munich. Alternatively, the 10 younger adults (mean age = 22.7 years, SD = 3.3, range 19–29, 5 female) were recruited from the university’s bachelor and diploma courses. Older and younger adults were paid eight Euros per session plus performance-based monetary bonuses for their participation (for bonus details see Procedure and Design). All participants were generally well educated, with older adults reporting more years of education (M = 18.0 years, SD = 3.9 years) than younger adults (M = 14.2 years, SD = 1.4 years), t(18) = 2.962, p < 0.01; this higher number years of education in older adults may follow from the prerequisite condition of university courses for senior adults of a prior, completed study. On a five-point health rating scale (1 = poor health; 5 = excellent health), older and younger adults gave similar mean self-ratings of 4.4 (SD = 0.7) and 3.7 (SD = 1.3), respectively, t(18) = 1.544, p = 0.14. Participants were screened for normal or corrected to normal vision and hearing via self-report. Older adults also had no history of neurological diseases, diabetes or coronary disease, and did not take any medication that might have affected cognition. The Mini-Mental State Examination (MMSE; Folstein et al., 1975) indicated no impaired cognitive abilities among the older participants (M = 29.8, SD = 0.4, range = 29–30). A handedness test (Oldfield, 1971) indicated that participants in both groups were right-handed.

In order to further characterize the participants, we conducted paper-and-pencil tests on attention performance (d2 Test; Brickenkamp and Zillmer, 1998), a non-verbal intelligence test [Culture Fair Intelligence Test (CFT 20-R); Weiß, 2006], and a vocabulary test [Wortschatztest (WST); Anger et al., 1968]. As illustrated in Table 3, performance in the d2 Test in the overall and concentration scores was higher in younger adults compared with older adults, t(18) = 3.192, p < 0.01 and t(18) = 2.335, p < 0.05, respectively. Similarly, non-verbal intelligence was optimized in younger adults in contrast with older adults, t(18) = 2.373, p < 0.05. The vocabulary test indicated similar vocabulary knowledge in both groups of participants, t(18) = 1.864, p > 0.08; such findings demonstrate the typical finding of impaired fluid processing functions but robust crystallized knowledge across aging (e.g., Cavanaugh and Blanchard-Fields, 2006).

TABLE 3

Table 3. Age, formal education, general health status, attention performance, non-verbal intelligence, and vocabulary knowledge for older and younger adults; MMSE (mini-mental state examination) scores for older adults only; CFT 20-R, cultural fair intelligence test, WST, Wortschatztest (vocabulary test).

Apparatus and stimuli

Stimuli were presented on a 17″ color monitor that was connected to a Pentium 1 PC. Experiments were carried out using ERTS software (Experimental Runtime System; Beringer, 2000).

A visual and an auditory task were performed. In the visual task, a circle appeared in one of three possible locations on the screen (left, middle, or right). Participants responded manually, indicating the location of the circle with the corresponding index, middle or ring finger of the right hand. The circles were white and were horizontally arranged on a black background on the computer screen. Each circle subtended approximately 2.5 cm which corresponds to a 2.38° visual angle, from a viewing distance of 60 cm. Three horizontal white lines served as placeholders at the possible left, middle, and right locations of the screen. The distance between the circles was 1 cm, which corresponded to approximately 0.95°. All circles subtended approximately 8.99°. Responses were recorded with a response board connected to the computer.

On the auditory task, participants verbally responded to one of three possible sine wave tones played on headphones with a sound level of 75 dB. They responded by saying “ONE” to the low frequency tone (350 Hz), “TWO” to the middle frequency tone (900 Hz) or “THREE” to the high frequency tone (1,650 Hz; German: “EINS,” “ZWEI,” and “DREI”). Verbal reactions were recorded with a Sony microphone connected to a voice key.

Procedure and design

A single-task trial started with three white lines serving as placeholders signaling the beginning of a trial for 500 ms. After this period had elapsed, an additional circle appeared in the visual task and remained visible until the participant responded or until a maximum of 2,000 ms had elapsed. A tone lasting for 40 ms was played in the auditory task. In dual-task trials, a circle and a tone were presented simultaneously. RTs were given as feedback after each trial for 1,500 ms followed by a blank screen for 700 ms. In dual-task trials, only the faster of the two RTs was given as feedback at the end of the trial to minimize the load. When participants committed an error or 2,000 ms had elapsed, the RT feedback was replaced by the German word for error (“Fehler”) for the same amount of time.

There were two types of blocks: single-task blocks and mixed blocks. In the single-task blocks, participants performed either 45 single-task trials of the visual task or of the auditory task. During mixed blocks, participants performed a mixture of 30 single-task trials (mixed single-task trials), 15 of the visual task and 15 of the auditory task, and 18 dual-task trials. All trials were randomly intermixed, requiring participants to switch between processing different single-task and dual-task trials. Participants were instructed to respond to both stimuli as quickly and accurately as possible during all blocks, to give these their full concentration and to give both tasks equal priority.

In order to familiarize the participants with the characteristics of the visual and auditory task and so that these could learn these tasks before presenting dual-task trials, participants exclusively performed six visual and six auditory single-task blocks in Session 1; these blocks were presented in an alternating order. Session 2 included six single-task blocks (three visual and three auditory single-task blocks) and eight mixed blocks. After two initial single-task blocks (one visual and one auditory single-task block), sequences of two mixed blocks and one single-task block followed in this session. The design in Sessions 3–16 was identical to that in Session 2 but these sessions included two additional mixed blocks at the end. In the Sessions 2–16, half of the participants always started with a visual single-task block and the other half always with an auditory single-task block; subsequently, the type of single-task block (i.e., visual or auditory) alternated. While Session 1 lasted around 45 min the following sessions took about 60 min. Sessions were conducted on successive days (excluding weekends). In this way, all sessions were completed within 2 weeks.

To maximize participants’ motivation for achieving fast performance, reward was given in the form of a monetary performance-based payoff (see also Schumacher et al., 2001; Tombu and Jolicoeur, 2004). The payoff matrix was based on an adaptive comparison between participant’s RT in a current block and a reference RT; this reference RT represents the individual best mean block RT and is adjusted separately for the visual and the auditory task and for task conditions (single-task trials in single-task blocks vs. dual-task trials). Participants could earn the more money the nearer the current RTs were to the reference RTs or if current block RTs were faster than the reference RTs; in the latter case reference RTs were adjusted to current block RTs. Bonus payments were also made on the basis of accuracy rates: A bonus was given for each correct response while there was a deduction from this bonus for each incorrect response.

Results

RT analyses

For the analysis of the training effects in the older adults, we compared the data of the final training session before the present training phase started (i.e., Session 12 of Strobach et al., 2012b) with the final session in the present training phase (i.e., Session 16). Therefore, we conducted 2 × 3 mixed measures ANOVAs with the within-subject factors SESSION (Session 12 vs. 16) and TRIALTYPE (single-task trials, mixed single-task trials, and dual-task trials) separately for each component task. Following the RT training data, we analyzed the performance in older adults’ Session 16 and younger adults’ Session 8. Our primary indicator of dual-task performance was the RT difference between dual-task trials and mixed single-task trials that reflects dual-task costs. In addition, we report the difference between mixed single-task trials and single-task trials that reflects task-set costs; this measure demonstrates the requirement to prepare for and maintain multiple task sets in mixed single-task conditions as compared with the condition of single-task blocks (Rogers and Monsell, 1995; Kray and Lindenberger, 2000; Bherer et al., 2005).

During older adults’ training, RTs in the visual task declined considerably, F(1, 9) = 11.041, p < 0.01, partial yes In addition, RTs differed between trial types, F(2, 18) = 19.420, p < 0.001, partial yes indicating higher RTs in dual-task trials followed by mixed single-task trials and single-task trials (dual-task vs. mixed single-task trials and mixed single-task trials vs. single-task trials: all ps < 0.05). TRIALTYPE was qualified by an interaction with SESSION, F(2, 18) = 10.310, p < 0.001, partial yes A decomposition of this interaction into comparisons of mixed single-task trials vs. dual-task trials and single-task trials vs. mixed single-task trials in Session 12 and 16 showed that dual-task costs, F(1, 9) = 9.558, p < 0.05, partial yes and task-set costs, F(1, 9) = 9.292, p < 0.05, partial yes decreased during training (Figure 1). The comparison of the dual-task performance of younger adults in Session 8 and this performance of older adults in Session 16 showed larger dual-task costs in the latter group [older adults: 43 ms, t(9) = 3.484, p < 0.01; younger adults: 15 ms, t(9) = 3.815, p < 0.01; between group comparison: F(1, 18) = 8.022, p < 0.01, partial yes ] while task-set costs showed no statistical group difference [older adults: 10 ms, t(9) = 3.550, p < 0.01; younger adults: 12 ms, t(9) = 2.955, p < 0.05; between group comparison: F(1, 18) < 1] as illustrated in Figure 2.

FIGURE 2

Figure 2. Dual-task and task-set costs in younger adults (Session 8) and older adults (Session 12, 16, 21) in RTs. Session 8 and 12: Strobach et al. (2012b), Session 16: Experiment 1, Session 21: Experiment 2.

In the auditory task, RTs were slower in Session 12 than in Session 16, F(1, 9) = 33.333, p < 0.001, partial yes Also, RTs were slower in dual-task trials than in mixed single-task trials (p < 0.001) followed (by trend) in single-task trials (p < 0.077), F(2, 18) = 17.457, p < 0.001, partial yes A non-significant interaction of SESSION and TRIALTYPE, F(6, 54) = 1.897, p > 0.179, partial yes indicated similar training effects on all types of trials. Similar to the visual task, we found increased dual-task costs in older adults’ Session 16 when contrasted with these costs of younger adults in Session 8 [older adults: 57 ms, t(9) = 4.889, p < 0.001; younger adults: 22 ms, t(9) = 4.787, p < 0.001; between group comparison: F(1, 18) = 8.022, p < 0.01, partial yes ] while task-set costs showed no group difference [older adults: 20 ms, t(9) = 1.935, p > 0.08; younger adults: 20 ms, t(9) = 3.529, p < 0.01; between group comparison: F(1, 18) < 1] as illustrated in Figure 2.

In the preceding analyses we used a strong and reliable criterion for measuring dual-task performance, by assessing dual-task costs in the RT comparison of dual-task trials and mixed single-task trials (i.e., dual-task costs = dual-task RTs − mixed single-task RTs; Bherer et al., 2006; Hazeltine et al., 2002). However, this criterion may lead to interpretative difficulties if there were baseline differences in performance due to the general slowing of processing in older adults (Somberg and Salthouse, 1982; Guttentag, 1989; Riby et al., 2004); in fact, this might have obscured possible differences between younger adults’ dual-task performance in Session 8 and older adults’ dual-task performance in Session 16 in the visual and auditory RT data. Therefore, we additionally assessed dual-task performance in terms of proportional dual-task costs to control for baseline differences between the age groups: proportional dual-task costs = (dual-task RTs − mixed single-task RTs)/mixed single-task RTs (Riby et al., 2004). The analyses of proportional dual-task costs corroborated the findings in the analyses of dual-task costs: Older adults showed larger proportional dual-task costs in the visual task, t(18) = 2.174, p < 0.05, and the auditory task, t(18) = 2.429, p < 0.05, than younger adults at the end of training. Thus, the appearance of dual-task cost differences between both aging groups is not confounded by possible differences in single-task performance between groups; therefore a general slowing in older adults cannot explain the observed differences in dual-task costs of Session 16 (older adults) and Session 8 (younger adults; Verhaeghen et al., 2003).

In addition, we analyzed whether the mean dual-task advantage in younger adults compared with older adults also holds at an individual level of data analysis (Schumacher et al., 2001; Hartley et al., 2011). For this purpose, we plotted the dual-task costs of the visual and the auditory task for each individual older and younger adult in Session 8 and 16, respectively (Figure 3). In this Brinley plot, data points for individual participants with lower costs in both tasks are located in the lower left corner while participants with larger costs are located in the upper right corner. Data points for younger adults are mostly in the lower left corner that represents relatively low dual-task costs of both tasks in Session 8. Data points of most individual older adults are at positions that represent larger costs and impaired dual-task performance relative to younger adults. Only some older adults showed data at a performance level approaching that of the younger adults. Thus, the observed difference in mean dual-task costs between the age groups at the end of training, therefore largely holds at an individual level.

FIGURE 3

Figure 3. Individual dual-task reaction time (RT) costs in younger and older adults at the end of practice (i.e., younger adults: Session 8, older adults: Session 16, 21). The x-axis represents the costs in the visual task while the y-axis represents the costs in the auditory task.

Summarizing the RT data, older adults showed a benefit of training on dual-task and task-set costs from Session 12 to 16 in the visual task, but not in the auditory task. Important for the present question of near perfect time sharing, dual-task costs in older adults were still increased after doubling the number of their training sessions compared to younger adults. Task-set costs were similar across both age groups.

Error analyses

Similar to the training RT data, error rates in Session 12–16 were analyzed for single-task, mixed single-task, and dual-task trials in older adults. In the visual task, there were lower error rates in dual-task than in mixed single-task, and in single-task trials, F(2, 18) = 9.509, p < 0.01, partial yes (dual-task vs. mixed single-task trials and mixed single-task trials vs. single-task trials: all ps < 0.05; Table 1). The effect of SESSION were marginally significant, F(1, 9) = 5.102, p = 0.05, partial yes revealing increased error rates in Session 16 than in Session 12. The interaction of both factors was non-significant, F(2, 18) = 1.274, p > 0.31, partial yes The findings of increased single-task error rates and increased error rates at the end of training are consistent with previous findings in younger adults using a similar task situation (Schumacher et al., 2001; Hazeltine et al., 2002; Tombu and Jolicoeur, 2004; Liepelt et al., 2011; Strobach et al., in press) and may be explained by a reduced degree of attentiveness in the visual task due to reduced processing demands. The analysis of older adults’ error rates during training revealed no effects or interaction in the auditory task (Table 2).

Discussion

In Experiment 1, we tested whether an increased amount of dual-task training in older adults (i.e., 16 sessions) improved dual-task performance in these learners to a level of near perfect time sharing in younger adults with only eight training sessions. The analyses of mean RT data revealed that older adults still show increased dual-task costs. Thus, this group did not optimize dual-task performance to a level achieved by younger adults. This difference even remained after we controlled for baseline differences in processing speed (i.e., proportional dual-task costs; e.g., Riby et al., 2004). Observations on a participant level demonstrated that most individual older adults performed on a lower dual-task level than the individuals of the younger adult group.

Could it be that older adults simply need even more training sessions to arrive at the same level of dual-task performance as younger adults? Generally, there is no way to rule out this conjecture; for any finite amount of training given to older adults, the possibility remains that more training would eventually eliminate dual-task cost differences between groups. However, we believe that moderately more training for older adults would not have changed the results concerning the level of dual-task performance because training had no RT effect on the dual-task costs of the visual and the auditory task, Fs(1, 9) < 1, across the last two training sessions. That is, the dual-task costs of the older adults were not further reduced at the end of training in Experiment 1.

Alternative to an increased amount of training, older adults may achieve the level of near perfect time sharing of younger adults due to training with component tasks that are simpler when compared to the tasks applied in Experiment 1. We tested this assumption in the following Experiment 2.

Experiment 2

A number of dual-task studies reported that simplified component tasks lead to a reduced impairment of one or both tasks in dual-task situations and, therefore, a reduction of dual-task costs (e.g., Frith and Done, 1986; Pashler, 1994; Van Selst and Jolicoeur, 1997; Schubert, 1999, 2008). This was particularly demonstrated for practiced dual-task performance in older adults (Maquestiaux et al., 2004).

One way to simplify tasks and, as a result, optimize dual-task performance/reduce dual-task costs is to reduce the number of stimulus-response mappings in the component tasks when contrasted with more difficult tasks. There exist two sources to explain reduced dual-task costs in situations with tasks that include a reduced number of stimulus-response mappings. First, the number of these mappings particularly affects the processing time of a central response selection stage (e.g., Schubert, 1999). When the processing time of the response selection stage is shortened, the likelihood of an overlap of the potential capacity-limited bottleneck stages in two concurrent tasks is reduced; in this way, the interference between concurrently presented tasks and the resulting dual-task costs are reduced. An additional source to explain reduced dual-task costs with tasks including a reduced number of stimulus-response mappings is associated with task coordination processes (Logan and Gordon, 2001). For instance, such processes perform a switch between capacity-limited central stages in a first task and in a second executed task (Maquestiaux et al., 2004; Sigman and Dehaene, 2006; Liepelt et al., 2011). Operations carried out during these processes potentially include activating or instantiating the stimulus-response mapping rules of the second task. To do so, it may be that the rules must be reactivated in or moved back into working-memory. The important point is that this switching stage functions more efficiently with a reduced amount of information handled in the case of simpler component tasks. This efficient functioning may result in faster switching between tasks and in a reduction of dual-task costs; previous findings of Maquestiaux et al. (2004) are consistent with this assumption. Taken together, we hypothesize that the reduction of the number of mapping rules may lead to shortened bottleneck stages within the component tasks and/or facilitate a switching process in older adults which may lead to reduced age differences in dual-task performance.

To investigate the effects after dual-task training with simplified component tasks, the older adults of Experiment 1 continued training after changing from three-choice to two-choice versions of the visual and the auditory tasks. In these two-choice versions, the visual task exclusively included presentations of circles at a left or a right position while low and high tones were exclusively presented in the auditory task. These tasks were trained for five sessions; note that we refer to these sessions in the following sections of Experiment 2 as Sessions 17–21, as older adults continued this training after the end of training in Session 1–16 (Strobach et al., 2012b; the present Experiment 1). The performance in the older adults’ Session 21 was compared with performance of near perfect time sharing in younger adults after eight training sessions. If training with simplified tasks produces conditions for near perfect time sharing in older adults, we expect the same level of dual-task performance across the included age groups, i.e., similarly reduced dual-task costs in younger and older adults. However, if training with such simplified tasks is not sufficient to produce conditions for near perfect time sharing in older adults, dual-task costs should not be reduced in this age group and should increase the amount of younger adults’ costs in Session 8.