Evidence for the acquisition of dual-task coordination ...

Acta Psychologica 160 (2015) 104–116

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Acta Psychologica journal homepage: www.elsevier.com/ locate/actpsy

Evidence for the acquisition of dual-task coordination skills in older adults Tilo Strobach a,b,c,⁎, Peter Frensch b, Hermann Müller c, Torsten Schubert b a b c

Medical School Hamburg, Germany Humboldt University Berlin, Germany Ludwig-Maximilians-University Munich, Germany

a r t i c l e

i n f o

Article history: Received 20 February 2015 Received in revised form 9 July 2015 Accepted 13 July 2015 Available online 29 July 2015 Keywords: Dual tasks Task coordination skills Skill acquisition Cognitive aging Older adults

a b s t r a c t Practicing two simultaneously presented tasks in dual-task situations results in improved dual-task performance, in both younger and older adults. Recent findings with younger adults demonstrated that this improvement is attributable in part to improved task coordination skills acquired through practice. However, it is unclear whether practice also improves older adults' skills at dual-task coordination. To clarify this, the present study examined the acquisition of task coordination skills, reflecting one specific mechanism of practice-dependent improvement in dual-task performance for this particular age group. This examination was based on two assumptions, namely, that (1) these skills are acquired during practice of the tasks presented simultaneously (dual-task situations), but not during the separate practice of the two tasks (single-task situations), and (2), rather than being dependent on the specific properties of practiced tasks, these skills are transferable to new dual-task situations. In the context of dual-task situations with well-structured reaction-time tasks, the results indeed revealed improved dual-task performance following dual-task practice, as compared to single-task practice, in both the practiced dual-task and new dual-task (transfer) situations. These findings are consistent with the notion that, similar to younger adults, acquired task coordination skills represent one practice-related mechanism that contributes to improved dual-task performance in the age group of older adults. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Performance of two simultaneous tasks in dual-task situations is impaired in contrast to the performance with separate single tasks, leading to dual-task performance costs. For instance, these costs are indicated by increasing reaction times (RT) and error rates in dual tasks vs. single tasks (e.g., Pashler, 1994; Schubert, 1999, 2008). In younger adults, numerous studies have however demonstrated the potential to optimize dual-task performance (i.e., reduced dual-task costs) as a result of practice (e.g., Hazeltine, Teague, & Ivry, 2002; Oberauer & Kliegl, 2004; Ruthruff, Johnston, & Van Selst, 2001; Ruthruff, Van Selst, Johnston, & Remington, 2006; Van Selst, Ruthruff, & Johnston, 1999). In contrast to younger adults, the magnitude of dual-task costs is increased in older adults at low levels of practice (Allen, Smith, Vires-Collins, & Sperry, 1998; Glass et al., 2000; Hartley, 2001; Hartley & Little, 1999; Hein & Schubert, 2004; McDowd & Shaw, 2000; Verhaeghen, Steitz, Sliwinski, & Cerella, 2003). Despite these greater costs, a number of studies have provided evidence that older adults are able to improve dual-task performance with practice (e.g., Allen, ⁎ Corresponding author at: Medical School Hamburg, Department Psychology, Am Kaiserkai 1, D-20457 Hamburg, Germany. E-mail address: [email protected] (T. Strobach).

http://dx.doi.org/10.1016/j.actpsy.2015.07.006 0001-6918/© 2015 Elsevier B.V. All rights reserved.

Lien, Ruthruff, & Voss, 2014; Allen, Ruthruff, Elicker, & Lien, 2009; Baron & Mattila, 1989; Bherer et al., 2005, 2006, 2008; Hartley, Maquestiaux, & Silverman Butts, 2011; Kramer, Larish, & Strayer, 1995). For instance, Strobach, Frensch, Müller, and Schubert (2012a, Experiment 1) tested dual-task practice performance under conditions supposed to be optimal for improving this performance. These conditions included instructions of equal task priority as well as speeded, and unrelated responses of component tasks using different perceptual and motor processors (e.g., a visual–manual task [visual task] and auditory–verbal task [auditory task]; Hartley et al., 2011; Tombu & Jolicoeur, 2004; Strobach, Frensch, Müller, & Schubert, 2012b). Older adults' dual-task performance was impaired at early practice levels, compared to the younger adults' performances. However, there was a substantial benefit of practice and thus an improved dual-task performance at the end of eight sessions in older adults; this practice benefit was statistically indistinguishable from the benefits in younger adults. Evidence of a substantial practice effect on dual-task performance as demonstrated by Strobach et al. (2012a) raises the question of what kinds of learning mechanisms are responsible for this effect. Specifically, such a practice effect has led some researchers to assume that improved attentional control skills result from practice in older adults (Kramer et al., 1995) and represent one cognitive mechanism contributing to a practice-related reduction of dual-task costs. In the present study, we

T. Strobach et al. / Acta Psychologica 160 (2015) 104–116

aim at providing evidence for such skills to explain the underlying mechanisms of improved dual-task performance in older adults. We conceptualize attentional control skills in terms of task coordination skills (for an alternative conceptualization in the context of attentional allocation, see Kramer, Larish, Weber, & Bardell, 1999). This form of skills is typically associated with an optimized control of two simultaneous and independent task processing streams in dual-task situations (Hirst, Spelke, Reaves, Caharack, & Neisser, 1980). According to theoretical assumptions, coordination skills should only be acquired and improved under dual-task practice conditions, but not under single-task practice conditions. That is, task coordination skills result from practicing two tasks simultaneously, rather than being attributable to learning the component tasks (Damos & Wickens, 1980; Oberauer & Kliegl, 2004; Silsupadol et al., 2009). Furthermore, once acquired, improved task coordination skills should at least be partially independent of the specific properties of the component tasks presented during dualtask practice. Consequently, these skills should be (at least to some extent) transferable across different dual-task situations (Bherer et al., 2006; Kramer et al., 1995). There is empirical evidence for the acquisition of improved task coordination skills and their transfer to different dual tasks in younger adults (Liepelt, Strobach, Frensch, & Schubert, 2011; Strobach, Frensch, Soutschek, & Schubert, 2012). There is no direct evidence for this type of skill acquisition in older adults in the cognitive aging literature, though, from comparisons testing the consequences of single-task practice and practice including dual tasks. Indeed, the data of some studies can be interpreted as arguments against the possibility of older adults acquiring such skills (Maquestiaux, Hartley, & Bertsch, 2004), while other studies tested these skills in an arguably inappropriate manner (Kramer et al., 1995; for details see later sections). Given this status of the dual-task literature, the present study was designed to test whether task coordination skills are improved by practice in older adults. 1.1. Previous tests of task coordination skills in older adults One of the previous studies from Maquestiaux et al. (2004) failed to provide conclusive evidence for the acquisition of task coordination skills in older adults. The authors employed a dual-task situation of the psychological-refractory-period (PRP) type (e.g., Pashler, 1994; Schubert, Fischer, & Stelzel, 2008). In this paradigm, participants have to perform a first and second choice RT task in close succession (while being instructed to give priority to the first task), with varying time intervals between the onsets of the different task stimuli (i.e., stimulus onset asynchrony, SOA). Dual-task costs primarily manifest themselves in terms of an increase of RTs in the second task as the SOA becomes shorter, that is, with increasing task overlap — an effect referred to as PRP interference. Importantly, Maquestiaux et al. found dual-task practice to increase age-related differences in the PRP interference, which means that while interference was clearly reduced for practice in younger adults, it was only minimally decreased in older adults. This led Maquestiaux and colleagues to argue that task coordination processes are practice-sensitive in younger adults and can be improved as a result of skill acquisition. By contrast, older adults' capacity to coordinate dual tasks shows no or only minimal signs of being improvable by practice. However, in the study of Maquestiaux et al. (2004), a number of characteristics of the practice regime may have prevented the acquisition of improved task coordination skills in older adults. First, the component tasks included sets with a relatively high number of stimulus– response mapping rules (in their case, four plus eight rules for the first and second tasks, respectively). Given that older adults are a ‘model’ for persons with reduced capacity of working memory (Hartley & Little, 1999), arguably, this high number of rules may have prevented older adults from optimally preparing for the two tasks in working memory even at the end of their training. Moreover, Maquestiaux et al. applied a strict dual-task training strategy, which exclusively included dual-task situations with two simultaneous tasks. However,

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exclusive dual-task training may prevent or at least slow learning of the individual tasks, thus potentially hampering optimal preparation of complex tasks, particularly in older adults. Empirical evidence for impaired component task learning in a dual-task context was provided by Ahissar, Laiwand, and Hochstein (2001) who showed that improvement of an orientation feature detection task during dual-task practice occurred only after participants had fully improved a letter identification task. Further evidence for impaired task learning under conditions of divided attention comes from studies on serial motor reaction task learning (Frensch, Wenke, & Rünger, 1999; Nissen & Bullemer, 1987; Schumacher & Schwarb, 2009). In these studies a lesser amount of knowledge about sequence information was acquired after dual-task compared to single-task practice. On the other hand, isolated training of the component tasks in single-task situations can lead to automaticity of stimulus–response translation, but task coordination skills cannot be acquired through this type of training. Therefore, the optimal regime might be hybrid training combining both approaches, potentially facilitating the acquisition of task coordination skills (dual-task training) and automatization of stimulus–response mappings (single-task training). This combination seems to be the most appropriate for optimizing dual-task performance and task coordination skills. Kramer and colleagues (Bherer et al., 2006, 2008; Kramer et al., 1995, 1999) argued for a contribution of task coordination skills to the practice-related improvement of dual-task performance in older (as well as younger) adults based on their findings with such hybrid practice. In their studies, participants either received variable-task-priority or fixed-task-priority instructions while practicing a dual-task situation with, for example, a cursor monitoring task and an alphabet–arithmetic task (e.g., K + 3 = ?; Kramer et al., 1995). Participants were required to vary their response priorities between the two tasks in the variablepriority condition. In the fixed-priority condition, though, participants were to emphasize both tasks constantly and equally. They also practiced single-task situations. The results indicated that older and younger adults could learn to perform and coordinate dual tasks efficiently, though mainly when they practiced the two tasks with variable priority between them. However, the studies of Kramer and colleagues only included groups that received different types of dual-task practice (i.e., variable vs. fixed dual-task practice), but no group with exclusive single-task practice. The lack of a single-task practice group as a control condition decisively limits the conclusiveness of these studies regarding the issue of acquired task coordination skills. This is because the inclusion of a group that exclusively undergoes single-task practice and the dual-task performance comparison between single-task and hybrid (including dual tasks) groups is essential for ascertaining whether acquired task coordination skills are completely dual-task specific and could not have happened through sole training under single-task conditions (e.g., Hirst et al., 1980). Thus, on the basis of prior research in the cognitive aging literature, we cannot tell whether transferable task coordination skills (as a consequence of hybrid practice in contrast single-task practice) are improved in older adults (see also Bherer et al., 2005, 2008; Cassavaugh & Kramer, 2009; Kramer et al., 1995, 1999; Li et al., 2010; Lussier, Gagnon, & Bherer, 2012; Silsupadol et al., 2009). 1.2. Previous tests of task coordination skills in younger adults A paradigm that tests the practice-related acquisition of task coordination skills was recently proposed in two studies on younger adults (Liepelt et al., 2011; Strobach, Frensch et al., 2012). These studies included two groups of participants undergoing different types of practice and compared dual-task performance after these two practice types: (1) single-task practice alone and (2) hybrid practice. Hybrid practice included isolated single-task blocks, as well as mixed blocks, with single-task and dual-task trials. Both practice groups performed a combination of a visual and an auditory task (see also Hazeltine et al., 2002; Schumacher et al., 2001; Strobach et al., 2012a), which combines a shorter (i.e., faster RTs) and a longer (i.e., slower RTs) component task,

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respectively. While there was similar dual-task performance after both types of practice in the shorter visual task, this comparison showed a greater improvement in dual-task performance at the end of hybrid practice than at the end of single-task practice in the longer auditory task. This improvement in the longer auditory task observed after hybrid practice (including dual-task practice) is consistent with the assumption that dual-task performance is improved as a result of dualtask practice due to the acquisition of task coordination skills in younger adults. As illustrated in Fig. 1A and B, one specific realization of task coordination skills explaining the exclusive hybrid-practice advantage in the longer auditory task might be as following: the dual-task processing architecture includes (1) a capacity limitation (i.e., bottleneck process) in the faster visual task (e.g., at a central response selection stage) followed by (2) a switching operation (related with the inhibition of mapping rules of irrelevant tasks as well as activation and instantiation of such rules in relevant tasks such as the longer auditory task) and (3) the capacity limitation in the longer auditory task (Band & van Nes, 2006; Lien, Schweickert, & Proctor, 2003). After hybrid practice (Fig. 1A) in contrast to single-task practice (Fig. 1B), inhibition, activation, and instantiation processes are highly efficient, leading to a shortening of the switching operation and improved dual-task performance in the auditory task (Strobach, Salminen, Karbach, & Schubert, 2014, for a more detailed discussion). An important aspect is that the dual-task improvement after hybrid practice was evident not only for the practiced dual tasks, but also for dual-task transfer situations that introduced changes in specific properties of either the visual or the auditory task (Liepelt et al., 2011). These findings demonstrate that acquired task coordination skills are not tied to specific properties of the practiced component tasks, but are, rather, transferable across different dual-task situations. While there is thus empirical evidence for the acquisition of improved task coordination skills from studies testing the effects of hybrid vs. single-task practice in younger adults, there is (as yet) no similar evidence from studies with older adults explaining improved dual-task performance after practice such as in the case of Strobach, Frensch et al. (2012). The present study was designed to fill this gap in the cognitive literature and to isolate precisely the benefits associated with

Hybrid group Shorter task

P

RS

R S

Longer task

RS

P

R

time Single-task group Shorter task

P

RS

R S

Longer task

RS

P

practice including dual-task conditions. That is, we assessed the effects of hybrid and single-task practice on dual-task performance in older adults using the experimental set-up of Liepelt et al. (2011). In Experiment 1, practice effects were analyzed in a dual-task test situation involving the same dual task that was practiced beforehand. To conclude that hybrid practice permits the acquisition of task-independent knowledge in older adults (Bherer et al., 2008), Experiment 2 examined the transferability of task coordination skills by comparing dual-task performance between groups of older adults who had received different amounts of dual-task practice (i.e., singletask vs. hybrid practice) in different task situations following practice. 2. Experiment 1 We compared the dual-task performance of groups of participants undergoing (1) hybrid practice (including single-task trials as well as mixed single-task and dual-task trials) and (2) single-task practice alone in order to test the acquisition of task coordination skills in older adults. Similar to Strobach, Frensch et al., (2012), this comparison was realized with the task combination of a shorter visual task and a longer auditory task and was carried out in the 8th session following seven sessions of practice. With a primary focus on the longer auditory task, observing superior dual-task performance after hybrid compared to single-task practice would provide evidence of practice-related improvement of task coordination skills in older adults. By contrast, the absence of differences in dual-task performance after hybrid vs. singletask practice would add support to the view that older adults do not improve their task coordination skills through practice. 2.1. Method 2.1.1. Participants Twenty older adults were recruited from university lectures for senior adults at LMU Munich and from Munich and Berlin communities. This group was separated into 2 practice groups: a hybrid group (N = 10, mean age = 63.3 years, SD = 3.4, range 57–68, 5 female) and a single-task group (N = 10, mean age = 63.8 years, SD = 4.1, range 57–68, 5 female). A handedness test (Oldfield, 1971) indicated that all participants were right-handed. Participants were also screened for normal or corrected-to-normal vision and hearing via self-report. All participants had no history of neurological diseases, diabetes, coronary heart disease, and did not take any medication that could have affected cognition. They were paid eight Euros per session plus performancebased pecuniary bonuses for their participation. As illustrated in Table 1, all participants were generally well educated. On a 5-point health rating scale (1 = poor health; 5 = excellent health), older adults in the hybrid and single-task groups gave similar mean self-ratings. The Mini-Mental State Examination (MMSE; Folstein, Folstein, & McHugh, 1975) indicated no impaired cognitive abilities. In order to further characterize participants, we conducted paper-and-pencil tests on attention and concentration performance (d2; Brickenkamp & Zillmer, 1998), and a vocabulary knowledge test (WST; Anger et al., 1968).

R

time Fig. 1. Dual-task processing architecture after hybrid practice (Hybrid group) and singletask practice (Single-task group). According to bottleneck models, central responseselection (RS) stages are processed sequentially even with task practice (Maquestiaux et al., 2004, Maquestiaux et al., 2013; Ruthruff et al., 2003) while perception (P) and response (R) stages are processed in parallel. A potential switching stage (S) after the response-selection (RS) stage in the shorter task and before the RS stage in the longer task is shortened after hybrid in contrast to single-task practice. The latter phenomenon is a promising candidate to explain reduced dual-task costs after having experience with dual tasks (i.e., hybrid practice). Note that the latencies of the individual processing stages (i.e., P, RS, S, R) in the figure are schematic illustrations and may not represent actual stage latencies.

2.1.2. Apparatus and component tasks Visual stimuli were presented on a 17-inch color monitor and auditory stimuli were presented via headphones, controlled by a Pentium I IBM-compatible PC. RTs of manual responses were recorded using a response button box, connected to the experimental computer, and the participants' vocalization activated a voice key, also connected to the experimental computer, so that an accurate measurement of verbal RTs could be acquired. The experimental control program was based on the ERTS (Experimental Runtime System; Beringer, 2000) system. Participants carried out two speeded-choice RT tasks as quickly and as accurately as possible. In the visual task, participants responded manually by pressing a spatially compatible key with the index, middle, or

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Table 1 Age, formal education, general health status, MMSE (Mini-Mental State Examination) scores, attention/concentration performance (d2), and vocabulary knowledge (WST: Wortschatztest) in hybrid and single-task (single) groups of older adults in Experiments 1 and 2. Experiment 1

Experiment 2

Practice types: hybrid (N = 10)/single (N = 10)

Practice types: hybrid (N = 12)/single (N = 12)

M Age (in years) Education (in years) Health status (1–5) MMSE (maximum score = 30) Attention/concentration performance (D2) Overall performance Concentration performance Vocabulary test (WST) IQ

SD

Range

M

SD

Range

63.6/63.8 18.0/16.0 4.4/3.6 29.8/29.0

3.4/4.1 3.9/1.9 .7/.8 .4/.9

57–68/57–68 13–24/14–19 3–5/3–5 29–30/28–30

65.6/64.4 16.4/15.4 3.7/4.0 29.6/28.9

4.4/3.4 4.4/4.6 .9/.4 .8/1.4

59–71/60–70 8–28/9–23 2–5/3–5 28–30/26–30

410.9/406.3 144.5/152.2

90.6/67.2 46.3/44.2

284–559/332–503 62–212/44–208

401.9/381.8 151.7/141.3

58.9/87.3 22.9/40.7

304–486/306–538 121–187/118–188

114.2/116.0

8.6/9.7

97–125/99–129

109.1/106.7

6.4/11.5

99–118/95–129

ring finger of their right hand to white circles appearing at a left, central, or right position arranged horizontally on the computer screen. Three white dashes were presented as placeholders for the possible positions of the visual stimuli on visual single-task trials. These placeholders appeared as a warning signal 500 ms before the visual stimulus was presented. The stimulus remained visible until the participant responded or a 2000-ms response interval had expired. In the auditory task, participants responded to sine wave tones presented at frequencies of either 300, 950, or 1650 Hz by saying “ONE,” “TWO,” or “THREE” (German: “EINS”, “ZWEI”, or “DREI”), respectively. An auditory single-task trial started with the presentation of three dashes on the computer screen. After an interval of 500 ms, one of the 3 tones was presented for 40 ms. The trial was completed when the participant responded verbally or a 2000 ms response interval had expired. To analyze the accuracy of each response, the experimenter recorded the identity of verbal responses. After correct responses in the visual task, RTs were presented for 1500 ms on the screen. Following incorrect responses in this task, the word “ERROR” (German: “FEHLER”) appeared. Since there was no tracking of accuracy in the auditory task synchronized between the experimental software (i.e., accuracy was thus evaluated offline based on the experimenter's records), we provided RT feedback after correct and incorrect responses in the auditory task. To provide similar amounts of feedback information in single and dual tasks, we provided feedback on the first response only in dual-task trials. A blank interval of 700 ms preceded the beginning of the next trial in both component tasks. Dual-task trials included the visual and the auditory task. These trials were identical to single-task trials with the exception that a visual and an auditory stimulus were presented simultaneously (SOA = 0 ms) and participants responded to both stimuli with equal emphasis. Also, response order was free. 2.2. Procedure and design 2.2.1. Hybrid group During practice, participants in the hybrid group received singletask trials and dual-task trials. Single auditory or visual tasks were presented in visual and auditory single-task blocks of 45 trials each, respectively. In contrast, 18 dual-task trials were included in mixed blocks, together with 30 mixed single-task trials, 15 of the visual and 15 of the auditory type. These interspersed single-task trials helped to ensure that participants were equally prepared for both tasks in dual-task trials. In Session 1, participants of the hybrid group performed 6 visual and 6 auditory single-task blocks, which were presented in alternating order. As in all sessions, half of the participants started with a visual single-task block and the other half with an auditory single-task block. Session 2 included 6 single-task blocks (3 visual and 3 auditory) and 8 mixed blocks. After two initial single-task blocks (1 visual and 1 auditory), sequences of 2 mixed blocks and 1 single-task block followed. The type of single-task blocks was alternated. The design of Sessions 3 to 8

was identical to the design of Session 2, but these sessions included 2 additional mixed blocks at the end. This procedure led to a sum of 2430 single-task trials in single-task blocks, 2100 single-task trials in mixed blocks, and 1224 dual-task trials across all sessions. While Sessions 1–7 represented practice sessions, Session 8 was considered a test session; data of these 8 sessions has been published elsewhere (Strobach et al., 2012a, Experiment 1). Sessions were scheduled in a way so that the last practice session and the test session were conducted on successive days and did not interfere with weekends. The remaining practice sessions were also scheduled on successive days but could be separated by weekends. 2.2.2. Single-task group The procedure in the single-task group was similar to the procedure in the hybrid group, except that one dual-task trial (during hybrid practice) was replaced by one single-task trial of each task (during singletask practice) during the practice sessions 1 to 7. Consequently, we had single-task blocks with 45 trials (short blocks) but also single-task blocks with 66 trials (long blocks). Session 1 was identical to the same session in the hybrid group. Session 2 included 12 single-task blocks (6 visual and 6 auditory) and 2 mixed blocks. These mixed blocks were included to analyze initial dual-task performance in the singletask group at the beginning of practice and to match this performance between practice groups. In Session 2, these 2 initial mixed blocks were introduced after 2 short single-task blocks, followed by sequences of 1 short and 2 long single-task blocks. In Sessions 3 to 7, we presented 16 single-task blocks (8 visual and 8 auditory). After 2 initial short single-task blocks, sequences of 2 long single-task blocks and 1 short single-task block followed. Blocks with the visual and auditory task were alternated and the first type of block (either visual or auditory task) was counterbalanced between subjects in Sessions 1 to 7 (i.e., practice sessions). The following test session 8 was identical to this session in the hybrid group including the presentation of single and dual tasks. This procedure in the single-task group led to a sum of 6126 single-task trials in single-task blocks, 360 single-task trials in mixed blocks, and 216 dual-task trials across all sessions 2.3. Results Prior to statistical RT analyses, we excluded all trials in which responses were incorrect (7.0%). The initial session (i.e., Session 1) was devoted to help participants get acquainted with the material and was, therefore, not included in our analyses. Effects sizes were illustrated with partial ŋ2 for main effects and interactions, as well as Cohen's d on t-tests (Cohen, 1988; Morris & DeShon, 2002). 2.3.1. Performance during hybrid and single-task practice To obtain a strong and reliable parameter for dual-task costs, we compared performance in dual-task trials and single tasks of

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2.3.2. Testing skill acquisition after hybrid and single-task practice We compared the performance in single-task trials of single-task blocks and in dual-task trials in a pre- versus post-test design, i.e. at the beginning and at the end of practice, to assess single- and dualtask performance after hybrid and single-task practice. The data of Session 8 (in which both groups performed single- and dual-task trials) served as the measure for performance at the end of practice. For the pre-test, we analyzed the single- and dual-task performance at the beginning of practice by comparing the performance in the first two single-task blocks with that of the dual-task trials in the two following mixed blocks in Session 2. We performed mixed-measures ANOVAs on RT and error data with the within-subject factors Test Phase (pretest vs. post-test) and Trial Type (single-task vs. dual-task trials), as well as the between-subject factor Practice Type (hybrid vs. singletask practice). Since our primary focus is on effects resulting from the different types of practice, we exclusively report main effects of and interactions with Practice Type in the following analyses. 1 Note that the data of the dual-task trials reflect a mixture of trials with a visual– auditory task order and an auditory–visual task order. Assuming the existence of bottleneck processing between two component tasks under dual-task conditions (e.g., Pashler, 1994; Ruthruff et al., 2003; Schubert, 1999, 2008), primarily a longer component task may suffer (e.g., is delayed) from such processing. As a consequence, the dual-task data is a mixture of trials with delays (visual task: auditory–visual task order trials; auditory task: visual–auditory task order trials) and no delays (visual task: visual–auditory task order trials; auditory task: auditory–visual task order trials). However, we analyzed this mixture of trials with no reference to different task orders because the large majority of dual-task trials had a visual–auditory task order (95.9 % of the dual-task trials) which was not modulated by practice in a one-way ANOVA with the within-subjects factor Practice (Sessions 2–8), F(6, 54) = 1.771, p N .12. Therefore, the impact of the data of the auditory–visual task order trials might be negligible.

1200

(A) Visual task

Dual tasks (Hybrid practice) Dual tasks (Single-task practice) Single tasks (Hybrid practice) Single tasks (Single-task practice)

1000

800

RTs [ms]

single-task blocks: dual-task costs = Performance dual-task trials − Performance single-task trials of single-task blocks (Strobach, Gerstorf, Maquestiaux, & Schubert, 2015; Tombu & Jolicoeur, 2004). This parameter of dual-task costs is particularly essential when investigating task coordination skills (Liepelt et al., 2011; Strobach, Frensch, et al., 2012). The combination of single-task blocks and dual-task trials represents the less-related cognitive processing (those underlying pure single-task trials and those underlying dual-task trials, respectively) and therefore the most informative one for investigating these skills. As illustrated in Fig. 2A and B, respectively, the differences between single-task and dual-task RTs decreased during hybrid practice indicating a practice-related effect on dual-task costs in the visual task, F(6,54) = 4.860, p b .001, partial ŋ 2 = .35 (Session 2: M = 220 ms, t(9) = 5.212, p b .001, d = 2.619; Session 8: M = 131 ms, t(9) = 5.804, p b .001, d = 2.279), and the auditory task, F(6,54) = 13.030, p b .001, partial ŋ 2 = .59 (Session 2: M = 298 ms, t(9) = 5.238, p b .001, d = 1.751; Session 8: M = 113 ms, t(9) = 5.107, p b .001, d = 1.919). These practice effects on dual-task costs were replicated in the error data of the auditory task, F(6,54) = 2.606, p b .05, partial ŋ2 = .23 (Session 2: M = 6.0%, t(9) = 3.685, p b .01, d = 1.477; Session 8: M = 0.3%, t(9) b 1), but not in the visual task, F(6,54) = 1.904, p N .10 (Table 2); the latter might be a consequence of a floor effect.1 As illustrated in Fig. 2A and B as well as Table 2, single-task performance improved with practice in a way that was similar for the two training groups, i.e. for the group with hybrid or single-task practice. In detail, we performed mixed-measures ANOVAs on single-task RT and error data of the visual and auditory task with the within-subject factors Session (Session 2–8) and the betweensubject factor Practice Type (hybrid vs. single-task practice). Essentially, RTs and error rates were reduced with practice and revealed no significant main effect of and interaction with Practice Type in the visual task RTs, Fs(1,18) b 1.097, ps N .31, and errors, Fs(1,18) b 1, as well as the auditory task RTs, Fs(1,18) b 1.652, ps N .22, and errors, Fs(1,18) b 1.801, ps N .11.

600

400

(B) Auditory task 1000

RT [ms]

108

800

600

400

200 1

2

3

4

5

6

7

8

Sessions Fig. 2. Reaction times (RTs; for correct responses) in the hybrid group (Hybrid) and singletask group (Single) in Sessions 1 to 8 of Experiment 1. RTs are presented separately for the visual and auditory tasks, as well as for single-task trials in single-task blocks (Single tasks), single-task trials in dual-task blocks (mixed single tasks), and dual-task trials (dual tasks). Panel (A): Visual task; Panel (B): Auditory task.

Importantly, in the analysis of skill acquisition, dual-task trials with two types of task orders might obscure group differences in dual-task performance (Liepelt et al., 2011; Nino & Rickard, 2003; Strobach, Table 2 Error data in hybrid group (Hybrid) and single-task group (Single) during Sessions 1 to 8 in Experiment 1. Data are presented separately for the visual and auditory tasks, as well as for single-task trials in single-task blocks (Single tasks), single-task trials in dual-task blocks (Mixed single tasks), and dual-task trials (Dual tasks). Gray rows indicate data of single- and dual-task performances (i.e., dual-task costs) at post-test in Session 8. Session

1 2 3 4 5 6 7 8 2 3 4 5 6 7 8 2 3 4 5 6 7 8

Trial type

Single tasks Single tasks Single tasks Single tasks Single tasks Single tasks Single tasks Single tasks Mixed single tasks Mixed single tasks Mixed single tasks Mixed single tasks Mixed single tasks Mixed single tasks Mixed single tasks Dual tasks Dual tasks Dual tasks Dual tasks Dual tasks Dual tasks Dual tasks

Visual task

Auditory task

Hybrid

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Hybrid

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Schubert, Pashler & Rickard, 2014). Therefore, this analysis was limited to dual-task trials with a single response order (visual–auditory task order), as previous studies using a similar dual-task design demonstrated that the visual task is the faster and the auditory task is the slower task in most of the trials (e.g., Hazeltine et al., 2002; Schubert et al., 2008). In the present sample, 95.2% of the dual-task trials had a visual–auditory task order with some individuals producing even 100% of dual-task trials with this order (i.e., individual range across pre- and post-tests: 90.6%–100%). Therefore, we did not analyze the reversed task order trials (i.e., auditory–visual) because there was no data for some participants available and only very few trials in the remaining ones. In the visual task RTs, there were no main effect of and interactions with Practice Type, Fs(1,18) b 4.050, ps N .06 (Fig. 3A). Consistently, we did not find a main effect of and interactions with Practice Type for the error data of the visual task, Fs(1,18) b 1 (Table 3). The RT results of the auditory task, however, point to the acquisition of improved task coordination skills after hybrid practice by older adults. In fact, we found a three-way interaction between Test Phase, Trial Type, and Practice Type, F(1,18) = 5.749, p b .05, partial ŋ2 = .24. As illustrated in Fig. 3B, this interaction reflects a dual-task specific advantage of hybrid practice over single-task practice exclusively in the post-test analysis. Post-test RTs in the dual-task trials were significantly reduced in the hybrid relative to the single-task group, t(18) = 2.203, p b .05, d = .813. In contrast, post-test RTs were similar in single-task trials for the two training groups, so were single- and dual-task RTs during pre-test, ts(18) b 1.166, ps N .26. Thus, improved dual-task performance in the hybrid group at post-test cannot be explained by improved component task processing skills after practice or improved initial single-task and dual-task performance levels in this group

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relative to the single-task group. In addition, Practice Type interacted with Test Phase, F(1,18) = 8.263, p b .01, partial ŋ2 = .32, which indicates a generally increased practice benefit of the hybrid group in contrast to the single-task group. The main effect of and the remaining interactions with Practice Type were non-significant, Fs (1,18) b 2.190, ps N .16. In the corresponding analysis of the error rates of the auditory task, the main effect of and the interactions with Practice Type were non-significant, Fs (1,18) b 1 (Table 3). An extended distributional analysis (see Fig. 4A) was conducted on auditory dual-task RT costs of the two training groups. The aim of this analysis was to obtain more detailed information about the differences between the effects of hybrid vs. single-task practice. The analysis tested whether the reduced dual-task RTs after the first compared with the second type of practice were exclusively caused by some very inefficiently performed dual-task trials after single-task practice. Such pattern may result from the fact that participants in the single-task group are generally not familiar with performing the visual and auditory task in combination while participants of the hybrid group are familiar with this situation. However, except a few familiarization trials, dualtask performance does not differ after both types of practice. To test the mentioned possibility, we computed and plotted cumulative dualtask costs as following: Dual-task trials of Session 8 were rankordered for each participant from shortest to longest RT in the auditory task. This order was separated into five bins and we subtracted the session mean of auditory single-task RTs from each bin mean. A mixed repeated ANOVA with Practice Type and Bin (Bin 1–5) as factors yielded a significant main effect of and interaction with the first factor, F(1,18) = 10.098, p b .01, partial ŋ2 = .37, and, F(4,72) = 3.892, p b .01, partial ŋ2 = .20, respectively. Thus, RTs were generally decreased after hybrid practice in comparison with RTs after single-task practice and this general decrease was moderated; essentially, however, each bin showed a significant between-group difference, ts(18) N 2.566, ps b .05. This indicates that the general dual-task advantage after hybrid practice is robust and is no result of only a few dual-task trials that were inefficiently performed in the single-task group. Additionally, we conducted a nonparametric test on the dual-task RT costs to rule out the possibility that the finding of advanced dual-task performance (i.e., decreased dual-task costs) in the auditory task after hybrid practice compared to single-task practice is based on only a few participants with mean values strongly deviating from those of the rest. This test includes the rank of each participant according to its dual-task costs in the auditory task and ignores the absolute costs. In this list, a lower rank value indicates a lower amount of dual-task costs. A nonparametric Mann–Whitney U test showed a significant difference between the list ranks of the hybrid practice (mean rank = 5.90) and the single-task group (mean rank = 15.10), p b .001. In older adults, this test shows that the present finding of reduced dualtask costs after hybrid practice as compared to single-task practice is not the result of only a few outlier participants.

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Fig. 3. Reaction times (RTs; for correct responses) in single-task blocks and dual-task trials during pre-test (i.e., beginning of Session 2) and post-test (i.e., Session 8) for hybrid and single-task groups of Experiment 1. RTs are presented separately for the visual task (Panel A) and the auditory task (Panel B). Asterisks (*) indicate significant differences between hybrid and single-task groups.

Experiment 1 provided evidence for the acquisition of improved task coordination skills in older adults. This is demonstrated by faster dualtask RTs in the auditory task after hybrid practice (including dual-task trials) compared to single-task practice. The comparison of the pretest and single-task data between the practice groups showed that the dual-task RT advantage after hybrid practice does not result from group differences at the beginning of practice or from differences in task automatization. Instead, this advantage is robust and is not attributable to only a portion of the trials. In contrast to the auditory-task data, there was no evidence of improved performance in the visual task after hybrid practice in the present Experiment 1, which is consistent with previous studies (Liepelt et al., 2011, Strobach, Frensch, et al., 2012).

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Table 3 Error rates in single-task blocks (single tasks) and dual-task trials (dual tasks) during pre-test (i.e., beginning of Session 2) and post-test (i.e., Session 8) including hybrid and single-task groups in Experiment 1. Rates are illustrated in the visual and auditory tasks. Visual task

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Single-task group

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Single-task group

1.4 1.1 2.2 0.6

1.9 0.6 2.4 0.7

10.2 21.2 7.0 7.4

12.7 23.7 7.3 18.1

The postulated acquisition of improved task coordination skills as a result of hybrid practice entails the assumption that these skills are at least partially independent of the specific characteristics of the practiced (component) tasks, that is, the visual- or auditory-task properties. Thus, it is essential to test whether (acquired) task coordination skills are

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transferable to component tasks with changed properties after practice. We examined this transferability in Experiment 2. 3. Experiment 2 To conclude that hybrid practice engenders the acquisition of taskindependent and transferable knowledge in older adults (Bherer et al., 2008), evidence is required that task coordination skills are not tied to the specific properties of the component tasks. We searched for this evidence in Experiment 2 by comparing performance in new dual-task situations after hybrid vs. single-task practice in new groups of participants. The new situations were presented in a transfer session (Session 9), preceded by eight practice sessions. In the new auditorytask situation of this transfer session, the stimulus–response features in the auditory task were remapped between practice and this situation from compatible to incompatible, while keeping the visual task constant. In the new visual-task situation, the visual stimuli were changed: instead of circles, participants were presented with triangles. The stimulus–response mapping was also changed: participants responded to stimuli of different size (small, medium or large), rather than left, central or right stimuli. The auditory task remained constant from the practice to the new visual-task situation. Note that pilot titrations produced similar effects of this mapping change in the auditory task compared to the combined change of stimuli and mappings in the visual task (Liepelt et al., 2011). With a particular focus on the longer auditory task, observing better dual-task performance after hybrid compared to single-task practice in the new visual-task situation (with changes in the visual task) and new auditory-task situation (with changes in the auditory task) would point to improved task coordination skills being independent of the specific task properties of the practiced (visual and auditory) component tasks in older adults. Alternatively, lack of a differential effect in the dualtask performance following hybrid vs. single-task practice would support the assumption that, in older adults, the acquired skills are tied to the specific task properties experienced during practice.

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Dual-task costs in auditory task Fig. 4. Vincentized cumulative distribution curves of the dual-task costs in the auditory task in (A) Experiment 1, (B) the new visual task situation of Experiment 2, and (C) the new auditory task situation of Experiment 2. The curves were obtained by averaging across participants' individual distribution curves. Separate curves were drawn for the hybrid and single-task practice groups.

Participants were 24 older adults recruited from the Berlin community. These new participants were randomly assigned to the hybrid group (N = 12, mean age = 65.6 years, SD = 4.4, range 59–71, 8 female) and single-task group (N = 12, mean age = 64.5 years, SD = 3.4, range 60–70, 8 female). Identically to Experiment 1, tests and surveys further characterized both groups (Table 1). Apparatus, stimuli, procedure, and design were identical to Experiment 1, with the following exceptions. After 8 sessions of hybrid or single-task practice, transfer session 9 included two phases with 10 blocks each; the order of these 10 blocks was identical to the order of the first 10 blocks of each previous Session 2 to 8 in the hybrid group and both phases were separated by a short break. In one of the phases, the auditory task was changed while the visual task was the originally trained (i.e., unchanged) task. Alternatively, the visual task was changed and the auditory task was the originally trained task in the other phase. The order of these phases was balanced across participants. In the

T. Strobach et al. / Acta Psychologica 160 (2015) 104–116

changed auditory task, the mapping was manipulated. Participants responded by saying “TWO” to the low frequency tone, “ONE” to the middle frequency tone, and “THREE” to the high frequency tone (German: “ZWEI”, “EINS”, and “DREI”). In the changed visual task, the stimuli were changed from circles to triangles of different sizes (large, medium or small with a side length of 2.6 cm, 1.5 cm, and 1 cm/2.48°, 1.52°, and 0.95° of the visual angle, respectively). Triangles were exclusively presented at the central position of the screen. Participants had to respond according to the size of the triangles, using the right index, middle, and ring finger for the large, medium and small triangle. 3.2. Results Data handling was similar to Experiment 1. 3.2.1. Performance during hybrid and single-task practice As illustrated in Fig. 5A and B as well as consistent with other experiments (Experiment 1; Strobach et al., 2012a, Strobach, Frensch, et al., 2012), differences between single-task and dual-task RTs (i.e., dualtask costs) decreased during practice, which illustrates the practice effect on dual-task costs in the visual task, F(6,66) = 4.146, p b .001, partial ŋ2 = .274 (Session 2: M = 153 ms, t(11) = 6.088, p b .001, d = 1.996; Session 8: M = 93 ms, t(11) = 4.966, p b .001, d = 1.877) and the auditory task, F(6,66) = 22.967, p b .001, partial ŋ2 = .68 (Session 2: M = 336 ms, t(11) = 8.861, p b .001, d = 2.572; Session 8: M = 145 ms, t(11) = 6.670, p b .001, d = 2.133). These findings of practice effects on dual-task RT costs were consistent with those of the error data in the visual task, F(6,66) = 10.247, p b .001, partial ŋ2 = .48

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Sessions Fig. 5. Reaction times (RTs; for correct responses) during Hybrid and Single-task practices in Sessions 1 to 9 for Experiment 2; Session 9 includes new auditory task situations (9Vis) and new visual task situations (9Aud). RTs are presented separately for the visual and auditory tasks, as well as for single-task trials in single-task blocks (Single tasks) and dualtask trials (Dual tasks). Panel (A): Visual task; Panel (B): Auditory task.

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(Session 2: M = 13.7%, t(11) = 3.678, p b .01, d = 1.388; Session 8: M = 1.0%, t(11) = 1.461, p N .17) and the auditory task, F(6,66) = 4.411, p b .01, partial ŋ2 = .29 (Session 2: M = 9.3%, t(11) = 6.068, p b .001, d = 2.241; Session 8: M = 3.7%, t(11) = 3.199, p b .01, d = 1.798, Table 4).2 As illustrated in Fig. 5A and B as well as Table 4, the effects on single-task data were similar after hybrid and single-task practice. That is, single-task RTs and error rates equally decreased with hybrid and single-task practice: visual task RTs, F(6,132) b 1, and error rates, F(6,132) b 1.204, p N .31, as well as auditory task RTs, F(6,132) b 1, and error rates, F(6,132) b 2.124, p N .07. 3.2.2. Skill transfer test in new auditory task situation We compared dual-task performance at the beginning of practice (pre-test) with the same performance in the transfer session 9 phase, in which the auditory task was changed (i.e., the new auditory task situation), in order to test whether task coordination skills are independent from specific properties of the practiced auditory task. This skill transfer test was identical to the previous test on skill acquisition (Experiment 1) including the factors Test Phase, Trial Type, and Practice Type. Similarly to Experiment 1, we exclusively reported main effects of and interactions with Practice Type. For reasons elaborated in Experiment 1, we exclusively focused on dual-task trials with a visual– auditory task order. In the present analysis, 95.0% of the dual-task trials had a visual–auditory task order with some individuals producing even 100% of dual-task trials with this order (i.e., individual range across preand post-test: 82.9%–100%). There was no advantage in the RT and error data and no evidence for the transfer of acquired task coordination skills after hybrid practice in the visual task. Thus, the present data of the visual task provides no evidence for transfer to the new auditory task situation. In fact, the lacking transfer is substantiated by a non-significant effect of and interactions with Practice Type in the RT data, Fs(1,22) b 1.239, ps N .28 (Fig. 6A), and error data, Fs(1,22) b 2.595, ps N .12 (Table 5). Note that this data indicates that a change in the auditory task results in a disappearance of any training effect in the visual task; this disappearance may result from the increased load when activating relevant task information into working memory (Strobach et al., 2014) in the present dual-task situation with unpracticed mapping rules in the auditory task in contrast to this task's practiced mapping rules at the end of practice. Since this activation process is located in the beginning of dual-task trials, it should also show effects on the processing time of the shorter component task. Such an effect was particularly demonstrated for older adults (Hartley & Little, 1999). In the auditory task, however, findings indicate a transfer of acquired task coordination skills to the new auditory task situation. This is substantiated by the observation of a three-way interaction of Test Phase, Trial Type X Practice Type, F(1,22) = 8.436, p b .01, partial ŋ2 = .28, 2 Similar to the analysis of hybrid practice in Experiment 1, we analyzed the mixture of dual-task trials with visual–auditory and auditory–visual task trials in Experiment 2. Again, this was a plausible strategy because the large majority of dual-task trials had a visual–auditory task order (96.4% of the dual-task trials) which was not modulated by practice in a one-way ANOVA with the within-subjects factor Practice (Sessions 2–8), F(6, 54) b 1. Therefore, the impact of the data of the auditory–visual task order trials might be negligible. Furthermore, analyzing the single- and dual-task RTs in the auditory task of the hybrid practice group of Experiment 2 and the single-task practice group of Experiment 1, replicated the finding of an improved dual-task performance and supported the assumption of an acquisition of task coordination skills during hybrid practice (including dual tasks). Thus, during post-test, we analyzed the Session 8 performance of both groups. In the context of a mixed-measures ANOVA on the essential RT data with the withinsubject factors Test Phase (pre-test vs. post-test) and Trial Type (single-task vs. dualtask trials), as well as the between-subject factor Practice Type (hybrid vs. single-task practice), the 3-way interaction was significant, F(1,20) = 12.675, p b .01, partial ŋ2 = .39. This interaction resulted from lower dual-task RTs at post-test (i.e., Session 8) in the hybrid in contrast to the single-task group, t(20) = 1.947, p b .05, d = .845. This improved dual-task performance after hybrid practice cannot be explained by different component task processing skills after practice and different initial single- and dual-task performance levels at pre-test, ts(20) b 1.179, ps N .25.

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Table 4 Error data for hybrid practice (Hybrid) and single-task practice (Single) groups during Session 1 to 9 in Experiment 2. Data was presented separately for the visual and auditory tasks, as well as for single-task trials in single-task blocks (Single tasks) and dual-task trials (Dual tasks). Gray rows indicate data of single- and dual-task performances (i.e., dual-task costs) at post-test in Session 9 with the new auditory task (i.e., 9Aud) and the new visual task (i.e., 9Vis). Session

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0.7 1.5 0.9 1.2 0.9 1.3 0.9 1.2 0.7 3.3 15.2 6.8 4.7 4.0 2.2 2.3 2.2 5.7 11.6

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on the auditory task RTs. As illustrated in Fig. 6B, this interaction stems from a dual-task specific advantage of hybrid practice over single-task practice exclusively in the post-test analysis. Post-test RTs in the dual-

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Table 5 Error rates in single-task blocks (single tasks) and dual-task trials (dual tasks) during pretest (i.e., beginning of Session 2) and post-test in Session 9 with new auditory task (i.e., 9Aud) and new visual task (i.e., 9Vis) for hybrid practice (Hybrid) and single-task practice (Single) groups of Experiment 2. Rates are separately illustrated for the visual and auditory tasks.

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Fig. 6. Reaction times (RTs; for correct responses) in single-task blocks and dual-task trials during pre-test (i.e., first 4 blocks at the beginning of Session 2) and post-test in Session 9 new auditory task situations (new auditory task) and with new visual task situations (new visual task) for Hybrid and Single-task practices in Experiment 2. RTs are presented separately for the visual task (Panel A) and the auditory task (Panel B). Asterisks (*) indicate significant differences between Hybrid and Single-task practices.

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task trials were significantly reduced in the hybrid group compared to the RTs in the single-task group, t(22) = 2.787, p b .05, d = 1.139. In contrast, post-test RTs were identical in single-task trials for the two groups of participants. So were single- and dual-task RTs during pretest, ts(22) b 1. Thus, different dual-task performance in the hybrid and single-task group at post-test cannot be explained by different component task processing skills after practice and different initial singleand dual-task performance levels. The main effect of and the remaining interactions with Practice Type were non-significant, Fs(1,22) = 3.373, ps N .08. The corresponding error analysis of the auditory task provided no effect of and interaction with Practice Type, Fs(1,22) b 1.874, ps N .19 (Table 5). We tested whether the improved dual-task performance after hybrid practice results from only a few trials that are inefficiently performed after single-task practice similarly to Experiment 1 (for more theoretical and analytical details, please refer to Experiment 1's results section). The cumulated dual-task costs of the auditory task (Fig. 4B) yielded a significant difference between hybrid and single-task practice groups, F(1,22) = 5.473, p b .05, partial ŋ2 = .31, but no modulation of this difference, F(4,88) b 1. Thus, the general difference in dual-task performance was no result of only a few inefficiently performed trials. This difference was also not a result of some outlier participants: a nonparametric Mann–Whitney U test demonstrated a significant difference between the list ranks of the hybrid practice (mean rank = 9.00) and the single-task group (mean rank = 16.00), p b .05. 3.2.3. Skill transfer test in new visual task situation To test whether task coordination skills are independent from the specific properties of the practiced visual task, we compared dual-task performance at the beginning of practice (pre-test) and that performance in the transfer session 9 phase, in which the visual task was changed (i.e., the new visual task situation). This skill transfer test was identical to the previous transfer test on the changed auditory task with mixed measure ANOVAs including the factors Test Phase, Trial Type, and Practice Type. We exclusively reported main effects of and interactions with Practice Type. Again, we exclusively focused on dualtask trials with a visual-auditory task order. In the present analysis, 93.8% of the dual-task trials had a visual–auditory task order with some individuals producing even 100% of dual-task trials with this order (i.e., individual range across pre- and post-tests: 80.1%–100%). There was no advantage in the RT data of the visual task and no evidence for the transfer of acquired task coordination skills after hybrid practice (Fig. 6A). This was consistent with previous experiments. In fact, the lacking transfer effect is demonstrated by a non-significant main effect of and interactions with Practice Type, Fs(1,22) b 1.389, ps N .25. The corresponding error analyses showed an interaction of Trial Type X Practice Type, F(1,22) = 17.167, p b .001, partial ŋ2 = .44, which indicates significant dual-task costs in the single-task group across pre- and post-tests. Such costs were not evident in the hybrid group (Table 5). However, this error data provides no evidence for skill transfer as a result of hybrid practice because the main effect of

T. Strobach et al. / Acta Psychologica 160 (2015) 104–116

and crucial interactions with Practice Type (e.g., Test Phase, Trial Type, & Practice type) were non-significant, Fs(1,22) b 3.235, ps N .10. However, RT findings indicate a transfer of acquired task coordination skills in the new visual task situation during the auditory task. In fact, we found a three-way interaction of Test Phase, Trial Type X Practice Type, F(1,22) = 5.285, p b .05, partial ŋ2 = .19, as illustrated in Fig. 6B. This interaction reflects a dual-task specific advantage of hybrid practice over single-task practice exclusively in the post-test analysis. Post-test RTs in the dual-task trials were significantly reduced in the hybrid group compared to the RTs in the single-task group, t(22) = 2.10, p b .05, d = .858. In contrast, post-test RTs were similar in single-task trials for the two groups of participants and so were single- and dualtask RTs during pre-test, ts(22) b 1. Therefore, improved dual-task performance in the hybrid group at post-test, in contrast to the single-task group, cannot be explained by different component task processing skills after practice and different initial single-task and dual-task performance levels. The main effect and the remaining interactions with Practice Type were not significant, Fs(1,22) b 1. The corresponding error analysis of the auditory task provided no effect of and interactions with this factor, Fs(1,22) b 1 (Table 5). Similarly to the new auditory task situation and to Experiment 1, we tested whether improved dual-task performance after hybrid practice is a robust effect or whether this improvement results from only a few trials that are inefficiently performed after singletask practice. As illustrated in Fig. 4C, the cumulated dual-task RT costs yielded a significant difference after hybrid and single-task practice, F(1,22) = 9.683, p b .01, partial ŋ2 = .41, but no modulation of this difference across bins, F(4,88) b 1. In essence, the general difference in dual-task performance was thus substantial and does not result from distorted performance in a few trials only. Furthermore, a nonparametric Mann–Whitney U test on the dual-task costs of the auditory task RTs demonstrated that the hybrid practice advantage on dual-task performance does not result from some outlier participants only. Similarly to Experiment 1, this test revealed a significant difference between the list ranks of the hybrid (mean rank = 8.25) and the single-task groups (mean rank = 14.75), p b .05. 3.3. Discussion Replicating findings of Experiment 1, hybrid practice of the present experiment, as opposed to single-task practice, resulted in faster dualtask RTs when performing the auditory task — a demonstration of the acquisition of task coordination skills. Moreover, Experiment 2 demonstrated these faster RTs after hybrid practice even when specific properties of the auditory task (i.e., stimulus–response mapping rules) or the visual task (i.e., stimuli and stimulus–response mapping rules) were changed in the transfer session 9. The pre-test data of Session 2 and single-task data, respectively, demonstrated that these dual-task RT advantages in Session 9 did not result from differences at the beginning of practice and are not a consequence of differential levels of stimulus– response mapping automaticity following practice. Distribution analyses and non-parametric tests demonstrate the robustness of the hybrid practice advantage and the independence of task coordination skills from the specific task properties. Note that the performance advantage after hybrid practice refers to the relatively better dual-task performance as opposed to the effects of single-task practice (Fig. 6B). At the same time, evidence for this advanced dual-task performance does not require an improved dual-task performance in contrast to performance at pre-test. This assumption results from different processing baselines of the component tasks applied during pre-test and during post-test (e.g., Kramer et al., 1995). These different baselines potentially result from introducing tasks with relatively difficult and arbitrary stimulus–response mappings (e.g., a size mapping in the visual task) at post-test as opposed to the simple component tasks (e.g., a spatial-compatible mapping in the visual task) at

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pre-test (Ruthruff et al., 2006; Strobach, Frensch, et al., 2012). This influence of task difficulty on dual-task performance was demonstrated particularly well for practiced dual-task performance in older adults (Maquestiaux et al., 2004). At the same time, manipulating one task (e.g., the auditory task) can have an effect on the non-manipulated task (e.g., the visual task) because two tasks are not processed independently at the end of extensive practice, even in younger adults (e.g., Ruthruff, Johnston, Van Selst, Whitsell, & Remington, 2003). These effects between tasks could be related to longer processing to instantiate (manipulated and non-manipulated) task sets at the outset of dual-task trials. This task-set instantiation is impaired predominantly in older adults (Hartley & Little, 1999). However, these specifications (i.e., increased difficulty of component tasks at post-test, dependency between component tasks) do not affect our main conclusion of advanced dual-task performance in transfer tests after hybrid practice. 4. General discussion The present study was designed to examine the mechanisms underlying practice-related improvement of dual-task performance in older adults. We specifically examined the acquisition and transferability of improved task coordination skills after hybrid practice (including both single-task and dual-task practice). 4.1. Dual-task practice in older adults We found that hybrid practice in a task situation adapted from Strobach et al. (2012a), Strobach, Frensch, et al. (2012) strongly reduced the RT difference (i.e., dual-task costs) between single-task and dualtask performance in older participants. This result is consistent with findings based on the same paradigm in younger adults (e.g., Hartley et al., 2011; Hazeltine et al., 2002; Schumacher et al., 2001; Strobach, Frensch, et al., 2012, Strobach et al., 2014; Strobach, Liepelt, Pashler, Frensch, & Schubert, 2013; Tombu & Jolicoeur, 2004), and older adults (Strobach et al., 2012a, 2012b, Strobach et al., 2015), as well as with alternative dual-task paradigms in both age groups (e.g., Allen et al., 2009; Baron & Mattila, 1989; Kramer et al., 1995; Ruthruff et al., 2006; Van Selst et al., 1999). Furthermore, there were similar effects on singletask performance in the context of hybrid and single-task practice that mirror findings in younger adults (Liepelt et al., 2011, Strobach, Frensch, et al., 2012). Thus, in contrast to prior assumptions (Kramer et al., 1995; Schumacher & Schwarb, 2009), both types of practice are equally beneficial for the automatization of single-task skills. 4.2. Acquisition of task coordination skills in older adults The auditory-task data specifically provided evidence that hybrid practice, in contrast to single-task practice, engenders the acquisition of improved task coordination skills in older adults (Experiment 1, Footnote 1). Moreover, these skills are not tied to the specific properties of the practiced tasks, but are at least partially property-independent (Experiment 2). Note that this property-independence is tested in a context of “narrow” transfer between structurally similar tasks. This test however does not answer the question of there would still be a transfer in “far” transfer tests between structurally dissimilar tasks (Karbach & Kray, 2009). The present findings replicate previous results on acquired task coordination skills in younger adults (e.g., Liepelt et al., 2011). For the first time in the cognitive aging literature, however, these findings provide evidence for acquired skills in the age group of older adults in tests on the effects of single-task practice and practice including dual-task conditions (i.e., hybrid practice). In fact, we provided this evidence in a notable number of comparisons: In the context of the practiced dual-task situation when comparing effects of (1) hybrid practice vs. single-task practice (in Experiment 1), (2) hybrid practice (Experiment 2) vs. single-task practice (Experiment 1), (3) hybrid practice vs. single-

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task practice in a dual-task context with changed auditory task (Experiment 2), and (4) hybrid practice vs. single-task practice in a dual-task context with changed visual task (Experiment 2). Thus, our study repetitively demonstrated the potential to improve task coordination skills in older adults when provided with the appropriate type of practice. This improvements occurs despite the documented findings of general agerelated deficits in attentional control (e.g., Raz, 2000; West, 1996), particularly of divided attention (Verhaeghen, 2011). The present demonstration of one form of optimized divided attention (i.e., task coordination skills) also extends to findings in the aging literature, showing an optimization of an alternative divided attention form, attention allocation, in younger as well as older adults (Kramer et al., 1995). Improved task coordination skills in older adults could not be observed in earlier studies and is possibly due to these studies having used non-optimal practice and test conditions (Maquestiaux et al., 2004) and/or lack a single-task control group (Kramer et al., 1995). For instance, Maquestiaux et al. (2004) used complex component tasks with high numbers of stimulus–response mapping rules and applied a strict dual-task training regime (see Introduction). The present study, on the other hand, demonstrates that older adults are capable of optimizing dual-task performance with hybrid practice and of acquiring task coordination skills under conditions of less complex tasks. These findings demonstrate preserved plasticity of the aging cognitive system to optimize performance in complex situations of the dualtask type when taking into account the limitations of working memory capacity in older adults (Hartley & Little, 1999). This is at least the case in the present sample of highly scholarized participants recruited in university contexts. Future studies should investigate whether this participants' characteristic limits the generalization of our conclusions and should test the acquisition of task coordination skills in older adults recruited in contexts outside university. Future studies should also continue to investigate the transferability of task coordination skills. While the present Experiment 2 demonstrated transfer effects in tests with changes in properties of either the visual or the auditory task, it remains an open issue whether such effects are also evident in situations with changes in both tasks (Strobach, Frensch, et al., 2012). Furthermore, the introduction of the present study claimed that both hybrid practice (with a mix of single and dual tasks) and dual-task practice should differ in the consequences on the level of component task automaticity, but should be effective for the acquisition of improved task coordination skills. Future studies should test these claims empirically. 4.2.1. Specification of task coordination skills in older adults Exactly how do task coordination skills acquired by older adults improve dual-task performance? One possibility is that participants in the hybrid practice group developed a ‘daring’ strategy to perform the dual tasks and thus increased parallel processing of the component tasks (Meyer & Kieras, 1997). As a result, task processing stages would have instantiated without further postponement in dual-task situations, leading to a reduction of dual-task costs compared with processing after single-task practice. However, older adults usually tend to select a ‘cautious’ strategy for performing dual tasks (Glass et al., 2000), one which reduces (rather than increases) the parallel processing of the two tasks. Furthermore, there is an increased conservatism of attentional control in older age, making the development of a ‘daring’ strategy through practice unlikely. This conjecture is consistent with the results of Touron and Hertzog (2004) and Hertzog & Touron, (201)1, showing that despite the apparent ability to shift from a time-consuming visual-scanning strategy to a more automatic memory retrieval strategy in a word-matching task, older adults were reluctant to do so. Thus, by analogy from earlier studies, we assume that a parallel-processing “strategy” account is no reasonable realization of task coordination skills in older adults. One alternative account is that hybrid practice might serve to integrate two tasks more efficiently, to the point of combining them into a

single ‘super task’ (Hazeltine et al., 2002; Ruthruff et al., 2006). In other words, on this “combination” account, whereas two separate response selection processes are performed at the beginning of practice, one in each component task, a single selection process of the combined task is processed after hybrid practice, giving rise to the hybrid-practice advantage. The processing of only one selection process, instead of two (potentially sequential ones), would reduce dual-task RTs. In contrast, separate practice of two tasks during single-task practice would not lead to an integration of both selection processes and would, thus, prolong RTs in the dual-task situation. The second alternative, the “automatization” account, assumes that dual-task practice automatizes the tasks' stimulus–response mappings and, consequently, eliminates processing that competes for limited capacities in the cognitive system (e.g., Johnston & Delgado, 1993; Ruthruff et al., 2006). This elimination of capacity-limited processing contributes to reduced interference between two tasks in dual-task situations, and so dual-task performance is improved through practice. In the particular case of older adults, however, Maquestiaux, LaguëBeauvais, Ruthruff, Hartley, and Bherer (2010), (see also Maquestiaux, Didierjean, Ruthruff, Chauvel, & Hartley, 2013) provided evidence that this age group may have largely lost the ability to automatize task performance. But even if such automatization would still be possible, single-task practice, too, should lead to automatized stimulus– response mappings and, thus, to similar dual-task performance levels when compared with hybrid practice. This is not the case, as shown in the present experiments. Additionally, combination or automatization of two tasks would be expected to be specific to the tasks presented during practice. These two accounts would thus not predict the transfer effects actually observed in Experiment 2. Altogether, these findings are not in line with the predictions derived from the strategy, combination, and automatization accounts. As illustrated in Fig. 1, we rather assume that the present findings favor a shortened switching operation as a potential realization of improved task coordination skills in older adults (Liepelt et al., 2011; Strobach et al., 2014), which is particularly suited to explain the exclusive hybrid-practice advantage in the longer (auditory) task. A shortened switching operation may be located after the end of a response selection stage in a shorter task and before the start of this stage in a longer task (Band & van Nes, 2006; Lien et al., 2003). This shortened operation may be related with efficient inhibition of irrelevant task information (e.g., response mapping rules of completed shorter tasks or of task information relevant during practice but not during transfer) as well as activation and instantiation of response mapping rules of the longer task (De Jong, 1995). Due to its particular locus, the shortening of a switching operation after hybrid practice would influence dual-task RTs in the longer auditory task, while there should be no (or only a minimal) effect on the shorter visual task of the present dual-task situation. However, we assume that this effect pattern is even generalizable as following: the shortening of a switching operation after hybrid practice would also influence dual-task RTs in a second task (i.e., the task with the second response), while there should be no (or only a minimal) effect on the first task (i.e., the task with a first response); this generalization is based on the assumption that the order of motor responses is equivalent to the order of the tasks' response selection stages (Ruthruff et al., 2006). According to this assumption, the primary hybrid practice effect on the longer/second task should be independent from the characteristics of stimulus presentation timing and should hold for situations with simultaneous stimulus presentation (such as in the present situation) and, to some extent, to situations with successive stimulus presentations (such as in PRP dual tasks). Similarly to the younger adults, we assume that a shortened switching operation may explain the dual-task performance advantage after hybrid practice in the older adults. This would extent findings from earlier life span studies, demonstrating increased latencies of switching operations in older adults above 60 and a reduction of these latencies with practice (Cepeda, Kramer, & Gonzalez de Sather, 2001; Kray &

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Lindenberger, 2000; Mayr, 2001; Strobach, Liepelt, Schubert, & Kiesel, 2012). Moreover, the finding of a hybrid-practice advantage in the transfer tests of Experiment 2 is consistent with earlier evidence demonstrating that switching skills are transferable to new, unpracticed task situations (Karbach & Kray, 2009; Kray, Karbach, Haenig, & Freitag, 2012; Strobach, Frensch, & Schubert, 2012). Altogether, task coordination skills constitute one mechanism that contribute to improved dual-task performance resulting from practice in older adults, particularly the practice that includes dual-task situations such as a hybrid practice regime. These skills may result in a shortened switching operation between two tasks. Acknowledgments This research was supported by a grant of the German Research Foundation to T. S. (last author), P. F., and H. M. (DFG Schu 1397/5–1; DFG Fr 1493/3–2). We would like to thank Christin Arndt, Tobias Baumann, Elisabeth Bieler, Dejan Draschkow, Christina Reimer, Anja Schütz, Tilman Stephani, and Maria Wirth for their assistance in data collection. Correspondence concerning this article should be addressed to Tilo Strobach, Medical School Hamburg, Department Psychology, Am Kaiserkai 1, 20457 Hamburg, Germany. E-Mails may be sent to [email protected]. References Ahissar, M., Laiwand, R., & Hochstein, S. (2001). Attentional demands following perceptual skill training. Psychological Science, 12(1), 56–62. Allen, P.A., Lien, M. -C., Ruthruff, E., & Voss, A. (2014). Multi-tasking and aging: do older adults benefit from performing a highly practiced task? Experimental Aging Research, 40, 280–307. Allen, P.A., Ruthruff, E., Elicker, J.D., & Lien, M. (2009). Multisession, dual-task psychological refractory period practice benefits older and younger adults equally. Experimental Aging Research, 35(4), 369–399. Allen, P.A., Smith, A.F., Vires-Collins, H., & Sperry, S. (1998). The psychological refractory period: evidence for age differences in attentional time-sharing. Psychology and Aging, 13(2), 218–229. Anger, H., Hylla, E., Horn, H., Schwarz, E., Raatz, U., & Bargmann, R. (1968). Wortschatztest WST 7–8. Begabungstest für 7. und 8. Klassen. Weinheim: Verlag Julius Beltz. Band, G.P.H., & van Nes, F.T. (2006). Reconfiguration and the bottleneck: does task switching affect the refractory-period effect? European Journal of Cognitive Psychology, 18, 593–623. Baron, A., & Mattila, W.R. (1989). Response slowing of older adults: effects of timecontingencies on single and dual-task performances. Psychology and Aging, 4, 66–72. Beringer, J. (2000). Experimental runtime system. Frankfurt am Main: BeriSoft Cooperation (1987–2000). Bherer, L., Kramer, A.F., Peterson, M.S., Colcombe, S., Erickson, K., & Becic, E. (2005). Training effects on dual-task performance: are there age-related differences in plasticity of attentional control? Psychology and Aging, 20(4), 695–709. Bherer, L., Kramer, A.F., Peterson, M.S., Colcombe, S., Erickson, K., & Becic, E. (2006). Testing the limits of cognitive plasticity in older adults: application to attentional control. Acta Psychologica, 123, 261–278. Bherer, L., Kramer, A.F., Peterson, M.S., Colcombe, S., Erickson, K., & Becic, E. (2008). Transfer effects in task-set cost and dual-task cost after dual-task training in older and younger adults: further evidence for cognitive plasticity in attentional control in late adulthood. Experimental Aging Research, 34, 188–209. Brickenkamp, R., & Zillmer, E. (1998). The d2 test of attention. 1st US Ed.Seattle, WA: Hogrefe and Huber Publishers. Cassavaugh, N.D., & Kramer, A.F. (2009). Transfer of computer-based training to simulated driving in older adults. Applied Ergonomics, 40(5), 943–952. Cepeda, N.J., Kramer, A.F., & Gonzalez de Sather, J.M. (2001). Changes in executive control across the life span: examination of task-switching performance. Developmental Psychology, 37(5), 715–730. Cohen, J. (1988). Statistical power analysis for the behavioral sciences (2. Edition ). Hillsdale, NJ: Erlbaum. Damos, D.L., & Wickens, C.D. (1980). The identification and transfer of timesharing skills. Acta Psychologica, 46(1), 15–39. De Jong, R. (1995). The role of preparation in overlapping-task performance. Quarterly Journal of Experimental Psychology: Human Experimental Psychology, 48A, 2–25. Folstein, M.F., Folstein, S.E., & McHugh, P.R. (1975). “Mini Mental State” — a practical method for grading the cognitive state of patients for the clinician. Journal of Psychiatric Research, 12, 189–198. Frensch, P.A., Wenke, D., & Rünger, D. (1999). A secondary tone-counting task suppresses performance in the Serial Reaction Task. Journal of Experimental Psychology: Learning, Memory, and Cognition, 25, 260–274. Glass, J.M., Schumacher, E.H., Lauber, E.J., Zubriggen, E.L., Gmeindl, L., Kieras, D.E., et al. (2000). Aging and the psychological refractory period: task-coordination strategies in young and old adults. Psychology and Aging, 15(4), 571–595.

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