Researchers suggest that improved performance of language, social, and behavioral skills in clinical settings does not necessarily transfer to performance during daily activities in other settings. 1–3 This lack of generalization also may hold true for the types of motor skills that are the focus of physical therapy intervention. 4–6 A child with cerebral palsy (CP) may walk successfully in the clinical setting, for example, but have difficulty walking at school or on the playground. Differences in the cognitive and perceptual demands of different settings may contribute to the lack of carryover. Most environments are characterized by background noise, obstacles, and distracting visual and auditory stimuli. 5 In addition, children may encounter situations in their everyday lives in which they must perform cognitive and motor tasks simultaneously. Because motor performance is the result of an interaction among cognitive, perceptual, mechanical, and neurologic mechanisms, 7 the role of cognitive factors should be considered in physical therapy assessment and intervention. 5–8 Cognitive factors that may be important for motor performance include arousal, attention, memory, motivation, and judgement. 9 In this article, the role of attention as it relates to motor performance is considered.
An important research paradigm for the study of attentional processes is the dual-task paradigm. 10,11 In dual-task paradigms, individuals are asked to perform two tasks simultaneously. Dual-task paradigms typically are used for two different purposes. 10 One is to investigate the attentional demands of a motor task and the other is to examine the effects of concurrent cognitive or motor tasks on motor performance. The latter is sometimes referred to as a divided attention or “time-sharing” paradigm. Researchers have used both types of dual-task methodology to examine the influence of attention on gait and balance among various age groups, subject populations, and types of concurrent tasks. 12–23
Most dual-task studies of children have been focused on developmental differences in specific cognitive skills. 24–28 Although the development of cognitive processes is of interest to physical therapists, research related to the effects of concurrent cognitive or motor tasks on motor performance may be of even greater relevance. Results of studies using the divided attention paradigm provide insight into the changes in performance that may be expected when children are required to do two things at once. Increased understanding of the effects of divided attention on motor performance may assist physical therapists in incorporating attentional factors into their examination and intervention techniques.
This review of literature begins with an overview of dual-task methodology, including background information on its use in research with adults. This overview is followed by a description of the application of dual-task methodology in pediatrics, focusing on effects of concurrent cognitive tasks on children’s motor performance. Finally, clinical implications and suggestions for future studies are discussed.
Attentional Demands of Specific Tasks
As noted above, dual-task methodology can be used in research to assess the attentional demands of a specific motor task, termed the primary task. This primary task is the main focus of this type of study. The assumptions of this paradigm are that central processing capacity is limited and that the capacity must be divided between the two concurrent tasks in various ways, depending on the strategy the subject chooses. Performance of the primary task is assumed to require some proportion of the limited processing capacity. The more demanding the primary task, the greater the proportion of the performer’s limited processing capacity that must be allocated to maintain an acceptable level of performance. Secondary task performance is considered a direct reflection of the absolute quantity of residual processing capacity. 10 In Figure 1, for example, primary task B requires a larger proportion of the processing capacity than primary task A. Consequently, secondary task performance should be worse when the secondary task is paired with task B than with task A.
When the dual-task paradigm is used to assess attentional demands, individuals usually are instructed to maintain a given level of performance on the primary task under dual-task conditions. Provided that primary task performance is maintained at baseline level under dual-task conditions, any change in the performance of the secondary task relative to baseline is taken as an indicator of the attentional demands of the primary task. When the demands of the concurrent tasks exceed the available processing capacity, deterioration in performance of one or both tasks is expected. 10
Researchers have raised several concerns related to the use of this paradigm. 10,26 One is the potential for subjects to sacrifice primary task performance to execute the secondary task. In this case, the attention demand of the primary task cannot be estimated. Although researchers attempt to minimize this problem by instructing subjects to focus on the primary task, subjects may switch attention between tasks or use other strategies that may not be easily identified or assessed.
Another concern relates to the assumption of a fixed or limited central processing capacity. Some authors have argued that the capacity for attention can change with changes in task requirements, 29 whereas others have suggested that central processing depends on multiple resources rather than on a single resource. 30 Different independent resource pools have been postulated as a function of different modalities of stimulus input and different response modes. 26 According to multiple resource theories, dual-task costs should occur only to the extent that two tasks tap into the same resources. 26,30
Kahneman 29 noted that, for certain combinations of tasks, interference may be purely structural in nature. Structural interference is considered to arise whenever concurrent tasks compete for identical input or output pathways. For example, structural interference would be likely to occur if subjects were asked to produce reaction-time responses to visual stimuli while simultaneously using a visual target to decrease their postural sway in standing. In the absence of any apparent structural effects, interference may be attributed to capacity effects. Capacity interference is thought to reflect a true overload of central processing capacity.
Researchers have used the dual-task paradigm to study the attentional demands of maintaining an upright posture. Lajoie et al 16 examined the attentional demands of sitting, standing, and walking in young adults between the ages of 20 and 30 years. Subjects performed four tasks: 1) a baseline or control sitting condition, 2) standing upright with a broad base of support, 3) standing upright with a narrow base of support, and 4) walking at the subject’s preferred speed. The secondary task was an auditory reaction-time task that required a verbal response and thus was unlikely to produce structural interference with the postural tasks. The researchers measured changes in reaction times under the dual-task conditions as indicators of the attentional demands of the standing and walking tasks.
Results of the Lajoie et al 16 experiment are illustrated in Figure 2. Reaction times were shorter for the sitting task than for the standing and walking tasks, and reaction times for both standing tasks were shorter than for the walking task. Reaction times also were shorter during double-support than during single-support phases of walking. No differences were found between reaction times during standing with a broad base of support and standing with a narrow base of support.
Lajoie et al 16 interpreted their results as indicating that attentional demands increase as the balance requirements of a task increase. Although they provided evidence that the addition of the probe reaction-time task did not affect the gait pattern of their subjects, they had no baseline (single-task) measurements of performance on the standing tasks. Consequently, we cannot assume that performance of these postural tasks was unchanged from baseline and that the differences in reaction times under dual-task conditions were truly indicative of differences in the attentional demands of the tasks.
Wright and Kemp 22 and Bardy and Laurent 31 studied the attentional demands of walking under various environmental conditions in adults. In both studies, the researchers instructed the subjects to give priority to the walking task. Wright and Kemp 22 compared the attentional demands of walking with a standard walker and a rolling walker. Subjects performed a secondary reaction-time task that included an auditory stimulus and a vocal response. The reaction time data indicated that walking with a standard walker was more attention-demanding than walking with a rolling walker or walking without an assistive device. One limitation of this study was that the subjects were young- to middle-aged adults with no apparent physical or cognitive dysfunction, thus preventing generalization of the findings to patient populations. Another limitation was that a 6.1-meter line was taped to the floor and subjects were instructed to either place one foot on each side of the line or place the right foot on the line while proceeding along the walkway. These instructions may well have altered the subjects’ natural gait patterns and the attentional demands of the walking tasks.
Bardy and Laurent 31 reported that differences in the size of a visual target affected the cognitive processing demands of walking in adults. Walking toward a small target resulted in a larger increase in reaction time on the secondary task than walking toward a large target. In addition, the reaction times increased sooner for the smaller target. Interpretation of these results is complicated by the finding that the primary (walking) task performance was altered by the addition of the secondary task. Subjects began decelerating earlier when approaching the small target under dual-task as compared with single-task conditions. Consequently, we cannot conclude with certainty that the larger and earlier increases in reaction time when walking toward the small target represented greater attentional demands for this task compared with walking toward a large target.
In general, studies of the attentional demands of various motor tasks have been limited because of small sample sizes (n = 6, 10, and 11 for Lajoie et al, 16 Wright and Kemp, 22 and Bardy and Laurent, 31 respectively) and by the assumption of a single, fixed attentional capacity. In the final analysis, knowledge of the attentional demands of particular tasks may not be as important to physical therapists as information about patterns of interference between different tasks and the strategies that subjects use under dual-task conditions. The divided attention or time-sharing paradigm described below is used to examine these interference effects.
Effects of Divided Attention on Motor Performance
When dual-task methodology is used to investigate the effects of divided attention, subjects typically are instructed to give equal priority to primary and secondary task performance. Several researchers have chosen postural control tasks as primary tasks. 18,32–34 Maylor and Wing 18 investigated the interference between five different cognitive secondary tasks and postural stability in older and younger adults. The five cognitive tasks were selected according to the involvement of a working memory component: random digit generation, Brooks’ spatial memory task, 35 backward digit recall, silent counting, and counting backward by threes. Age differences in postural stability were significantly increased under dual-task compared with single-task conditions for Brooks’ spatial memory task and backward digit recall. Both of these tasks theoretically involve use of the visuospatial sketchpad, described by Baddeley 36 as a component of working memory responsible for structuring and manipulating visuospatial images. Maylor and Wing 18 concluded that age differences in postural stability are increased only when the visuospatial sketchpad is used, not simply when overall attentional demands increase.
Along similar lines, Shumway-Cook and Wollacott 33 and Shumway-Cook et al 34 investigated the effects of cognitive tasks on postural stability in young vs older adults. In the study by Shumway-Cook and Woollacott, 33 subjects performed a choice reaction-time task while maintaining quiet standing under six different sensory conditions that manipulated the availability of accurate visual and somatosensory cues. The reaction-time task required subjects to verbally identify an auditory tone as being “high” or “low.” Addition of the reaction-time task did not affect postural stability in any of the sensory conditions in young adults, but resulted in decreased stability when both visual and somatosensory cues were removed in older adults with no known balance impairments. Older adults with a history of imbalance and falls exhibited decreased postural stability with the addition of the reaction-time task in all six sensory conditions. Although the researchers concluded that there is an age-related increase in attentional demands for postural control with a decrease in available sensory information, this interpretation is open to debate. The results also could be interpreted as simply reflecting an age-related difference in the pattern of interference of the specific reaction time and postural tasks investigated in the study.
Shumway-Cook et al 34 used a language processing task and a visual spatial orientation task to produce changes in attention during performance of standing tasks on firm vs compliant surfaces. The three groups studied were young adults, older adults without a history of falls, and older adults with a history of falls. Performance decrements were found in the postural stability measures rather than the cognitive measures for all three groups. The addition of either cognitive task to the single-task standing condition produced a significant difference in stability among the three groups, with the older adults with a history of falls exhibiting the most instability and the young adults the least.
Contrary to the expectations of the researchers and the results of Maylor and Wing, 18 interference effects in the study by Shumway-Cook et al 34 were larger for the language-processing task than the visual spatial orientation task. The researchers suggested that one explanation for the discrepant findings was that their language-processing task was a sentence completion task that was presented visually, and therefore may have placed some demands on visual processing pathways. Another explanation is that the language-processing task may have been more difficult than the spatial orientation task, creating a greater cognitive load. Evidence from other studies 32,37 supports the existence of interference effects resulting from competition for visual processing pathways when visual and postural tasks are combined.
Geurts et al 32 investigated the level of automaticity of postural control ability in individuals with lower limb amputation and an age-matched control group. Their primary interest was in the effects of a concurrent cognitive task, the modified Stroop test, 38,39 on postural stability in standing. In the modified Stroop test, subjects were presented with color names printed in different colors of ink. The color of the ink was always inconsistent with the color name (eg, the word “red” was printed in blue ink). Subjects were instructed to ignore the color name and verbally report the color of the ink. Subjects with amputation were tested before and after their participation in a rehabilitation program. Although body sway did not change significantly after rehabilitation when measured under single-task conditions, significant changes were observed under dual-task conditions. The interference effects of the Stroop test on postural control were lower after rehabilitation in subjects with amputation, although their body sway remained greater than that of control group subjects.
The finding by Geurts et al 32 that only the dual-task performance improved during the rehabilitation process suggests that imposition of secondary cognitive tasks may be one method of detecting changes in primary task performance that would otherwise go unnoticed. This suggestion is consistent with the work by Shumway-Cook and colleagues 33,34 with older adults with a history of imbalance and falls, who demonstrated much greater deficits under dual-task than single-task conditions. Geurts et al 32 stated that the interference effects in their study might have arisen from general competition for limited resources or from more specific interference related to the visual processing demands of performing the Stroop test and the standing task simultaneously. They speculated that the subjects with lower limb amputation might have decreased their dependency on visual information during the rehabilitation process, thereby reducing interference in visual processing pathways under dual-task conditions.
Research on the effects of divided attention on gait is more limited than that on the effects of divided attention on postural stability in standing. Ebersbach et al 13 investigated the influence of different concurrent tasks on gait in 10 healthy subjects between the ages of 25 and 42 years. Concurrent tasks included a cognitive task (digit span forward), a buttoning task, a finger-tapping task, and a task in which digit span was combined with buttoning. Subjects were instructed to concentrate on the concurrent task rather than on gait performance under dual-task conditions. Interference effects were observed for digit span forward, the only concurrent task for which changes between single- and dual-task conditions were quantified. Gait performance was assessed for changes in stride time and double-support time. Stride time differed between single- and dual-task conditions for the finger-tapping task only, a finding that the researchers attributed to structural interference at the subcortical level for competing rhythmic output. This explanation makes sense in that the requirements for producing fast finger tapping (frequency of five Hz or higher) were likely to “pace” or entrain the walking performance and produce a decrease in stride time. Double-support time differed between single- and dual-task conditions only in the situation in which digit span was combined with buttoning.
Interpretation of the results of the study by Ebersbach et al 13 is complicated by several methodological issues. Instructions to the subjects were not consistent with those recommended for divided attention paradigms. 10 In addition, changes in concurrent task performance were measured for the digit span task only, and this task was not administered consistently under single- and dual-task conditions. Under dual-task conditions, subjects were required to listen to and retain the digits while walking, but were not required to repeat the digits until after completion of each walking pass. However, a delay between presentation of the digits and request for digit recall was not present under single-task conditions. These methodological issues make interpretation of the findings related to changes in double-support time particularly problematic. Because the authors did not report changes in double-support time relative to overall changes in gait speed, their suggestion that the increase in double-support time was related to the attentional demands of balance control during gait seems unjustified. With the addition of two concurrent tasks, the attentional demands were probably quite high, but these may not have been specifically related to balance control.
In summary, dual-task methodology is often used to examine the role of cognition in motor performance from two perspectives. One is to assess the attentional demands of motor skills; the other is to investigate the effects of divided attention on motor skills. Some aspects of even highly practiced tasks such as postural control and gait require attention. Interference effects differ for different concurrent cognitive or motor tasks, with interference likely to occur whenever concurrent tasks compete for the same resources. Imposition of concurrent cognitive or motor tasks may be one method of detecting changes in balance or gait performance that would otherwise go unnoticed.
APPLICATION OF DUAL-TASK METHODOLOGY IN CHILDREN
Attentional Demands of Specific Tasks
The application of dual-task methodology in children frequently has involved investigation of the attentional demands of various cognitive tasks. Researchers have focused on age differences in mental resource demands of cognitive skills such as memorization, rehearsal, and reasoning ability. 24–28 These studies are important to physical therapists in understanding how age-related differences in cognitive abilities may influence a child’s motor performance or a child’s ability to combine cognitive and motor tasks. As with the research on attentional demands in adults, this research is based on the assumption of a limited processing capacity. The magnitude of decline in secondary task performance from single- to dual-task conditions is presumed to index the resource demands of the primary cognitive task.
Several researchers investigated the mental demands of cognitive skills by examining the amount of interference in finger-tapping or reaction-time performance produced by primary cognitive tasks in school-aged children. 24,25,28,40–42 In many of these studies, older children exhibited both superior performance on the primary task and less secondary task interference than younger children. 40,41 The age differences in secondary task interference presumably would have been even larger if the researchers had attempted to equate primary task performance across age. In studies in which performance on the primary task of interest was equal across age groups, the amount of interference in secondary task performance produced by the primary task declined with age. 25,42 The smaller decrement in secondary task performance under dual-task conditions in older compared to younger children may be because of greater cognitive resources or more efficient allocation of these resources in older children. Children also improve in their ability to use cognitive strategies as they get older, employing strategies spontaneously and with increasing effectiveness. 24,25,40,42
Effects of Divided Attention on Motor Performance
Most studies of the effects of attention on motor skill performance in children have used some version of the divided attention or time-sharing paradigm. Researchers have focused on developmental changes in the ability to simultaneously perform various cognitive and motor tasks. In some studies, such as studies of verbal-manual time sharing, both concurrent tasks have involved a motor component. For example, children have been asked to recite tongue twisters or animal names while concurrently performing manual skills such as finger tapping. 43–45 In other studies, researchers have used cognitive tasks in combination with more complex motor skills. 19,21
Studies of verbal-manual time sharing in children have consistently revealed asymmetric interference effects. 43–46 Speaking produces greater interference with right-hand finger tapping than with left-hand finger tapping in right-handed children. The degree of asymmetry remains constant across the age range of three to 12 years. 44 These findings support the “functional distance” principle of cerebral organization described by Kinsbourne and Hicks. 47 This principle states that the amount of interference between two incongruous activities varies inversely with the functional distance between their respective cerebral control centers. Because speech production is lateralized to the left hemisphere in right-handers, the functional distance model predicts greater interference with motor performance of the right hand. According to Kinsbourne and colleagues, 47,48 the effects should be more pronounced in children compared to adults and in younger children compared to older children because of restricted cerebral space earlier in development. Results of recent investigations with adult subjects have failed to fully support the predictions of the functional distance model, however. 49,50 Instead, these studies have revealed a large number of variables, such as hand differences in single-task performance, that may confound the effects of cerebral organization on dual-task performance.
As with the studies of developmental changes in cognition, research on motor skill performance suggests that older children exhibit less interference under dual-task conditions than younger children. 43,44,49 Presumably, the motor tasks become more automated and under more subconscious control with increasing age or practice. 11,19 Smith and Chamberlin 19 examined the effect of concurrent tasks on soccer performance for subjects with different levels of expertise. Fourteen 11- to 19-year-old soccer players were categorized as novice, intermediate, or expert players. The primary task was rapid running of a slalom course and the secondary tasks were dribbling a soccer ball and geometric shape identification. The novice group showed disproportionate decreases in running speed compared to the other two groups when both secondary tasks were performed in combination with the primary running task. Within each group, the addition of each secondary task resulted in slowing of primary task performance. The authors concluded that the addition of cognitively demanding secondary tasks had a disturbing effect on primary task performance across all levels of expertise, but the amplitude of the effect decreased as the level of expertise increased. This result supports the idea that motor task performance may become more automated with increased practice and experience.
Whitall 21 investigated developmental differences in the effects of concurrent cognitive tasks on two locomotor skills, running and galloping. Forty children (three to 10 years of age) and adults (18 to 34 years of age) completed the study. Two verbal secondary tasks, singing and letter-memorization, were used. The results indicated that coordination variables, such as temporal phasing (step time divided by stride time) and amplitude phasing (step length divided by stride length), were unaffected by the addition of a cognitive task regardless of the subject’s age. With regard to control variables such as speed, step time, and step length, subjects in all age groups tended to decrease their step-lengths when attempting to execute two tasks simultaneously. The amount of interference in performance of the control variables was age-related and task-related. The letter-memorization task caused greater effects on locomotor skills than singing, and the effect was greater in running than galloping for the younger age group.
Although characterizing her study as “time-sharing” in the title and employing time-sharing methodology, Whitall 21 interpreted her results in terms of the attentional demands of coordination and control variables in the two locomotor skills. She suggested that the interlimb coordination variables were operating at an automatized level, with the phasing pattern in place at a very early age (approximately three years). She noted that even the more complicated, less practiced, asymmetrical phasing pattern of galloping was unaffected by the imposition of the secondary tasks. In contrast to the expectations derived from other studies, this result implies that only certain locomotor performance variables, such as gait speed, step length, and step time, become more automated and thus less attention-demanding with additional practice. The basic phase relationships that distinguish different locomotor skills may not be susceptible to interference effects in typically developing children. Interestingly, Whitall 21 did not find any evidence that the singing rhythm entrained the gallop to a new phasing pattern. She concluded that either no entrainment took place or that the rhythmic pattern of galloping was dominant, so that the singing entrained to the galloping. The finding that singing slowed when paired with galloping supported the latter possibility.
In addition to investigating developmental changes in time-sharing performance in typically developing children, researchers have used dual task methodology to examine the automaticity of balance skills in children with dyslexia. 14,23 The primary interest in these studies was whether children with dyslexia were significantly impaired in balance skills under dual-task compared with single-task conditions. If balance skills were unaffected by performance of concurrent tasks, then the skills would be considered automatic. Fawcett and Nicolson 14 selected two primary task conditions, standing on a balance beam with one foot and with both feet. Two groups of subjects with dyslexia (mean age of 11 and 15 years) and age-matched children with typical development participated in the study. Performance of the primary balance task was examined with and without concurrent secondary tasks of counting or performing a choice reaction time task. The secondary tasks were “calibrated” to ensure a comparable level of performance for the subjects with dyslexia and the comparison group at baseline (ie, under single-task conditions). The results showed that children with dyslexia were significantly impaired in both balance and secondary task performance under the dual-task condition compared with the single-task condition, while typically developing children showed no significant changes in either balance or secondary task performance from single- to dual-task conditions. The researchers concluded that complex skills, such as balance, are not automatized in children with dyslexia.
Similar results were reported by Yap and van der Leij. 23 In this study, 14 subjects with dyslexia (mean age of 10 years) and age-matched comparison group subjects were asked to balance on one foot as the primary task and to perform an auditory-choice reaction time task as the secondary task. Children were instructed to give priority to the secondary task. In the single-task condition, balance performance of the children with dyslexia was not significantly different from that of children in the comparison group. However, children with dyslexia showed more balance errors, including minor foot movement or wobble, in the dual-task condition than the single task condition, while children in the comparison group did not.
Findings in the studies by Smith and Chamberlin, 19 Whitall, 21 Fawcett and Nicolson, 14 and Yap and van der Leij 23 all were interpreted in terms of the automation of motor performance. Automation is a decrease with practice or development in the quantity of resources needed to attain a given level of performance. 51 According to Wickens and Benel, 51 automation is a characteristic of the specific task as performed by the learner, and is distinct from more general time-sharing skills. Time-sharing skills are capabilities or strategies that may affect dual-task performance, such as attention switching, resource allocation, and information sampling. 51 Changes in these time-sharing skills, rather than in the level of automation of primary task performance, may at least partially account for the effects observed in the studies described above.
In summary, studies of motor performance in children, like those in adults, indicate interference effects with the imposition of secondary tasks. Older children show less interference than younger children, possibly because of increased automation of motor performance or improved time-sharing skills with maturation, practice, and experience. Certain aspects of motor skills may be more affected by secondary tasks than others. Temporal-distance variables of gait, for example, appear more susceptible to interference effects than interlimb coordination variables. Consequently, changes in temporal-distance variables from single- to dual-task conditions may be good indicators of the degree of automaticity of motor performance in children and adults.
CLINICAL IMPLICATIONS AND CONCLUSIONS
Knowledge derived from the dual-task literature can be useful for pediatric physical therapists in several ways. Increased understanding of the attentional demands of different cognitive and motor tasks should enable physical therapists to make more informed decisions about how they structure evaluation and intervention activities. Therapists can be more cognizant of the demands they are placing on clients when they provide instructions, demonstrations, or simply conversation that must be processed at the same time as on-going motor performance. The increased attentional demands of walking with various assistive devices can also be taken into consideration.
The divided attention or time-sharing paradigm is especially relevant to children’s daily activities. Children frequently encounter situations involving simultaneous performance of two tasks, such as responding to verbal instructions or manipulating an object while walking. By adding concurrent cognitive or motor tasks during clinical examinations and measuring changes in simple temporal or distance variables, therapists may be able to determine children’s abilities to perform motor tasks “automatically” or, at least, to divide attention between tasks. Evaluation of performance under dual-task conditions may reveal subtle deficits that would otherwise go unnoticed.
If therapists have information about how concurrent cognitive tasks influence motor performance, they may be able to design more effective interventions by selecting motor tasks, structuring the environment, and providing instructions and feedback in a manner that is more consistent with a child’s abilities. Children who are performing at a high level may benefit from the challenge afforded by the imposition of secondary tasks, while those who have difficulty under dual-task conditions may need a quiet environment and very limited concurrent feedback. Perhaps as a child’s motor skills improve, concurrent tasks of increasing difficulty can be systematically imposed.
Results of dual-task studies support the important role of cognitive processing in motor performance and may raise physical therapists’ awareness of the influence of cognitive factors on motor control in clinical assessment and intervention. Although this review has been limited to attentional factors, other aspects of cognitive processing play an important role as well. Additional research is needed about the motor performance effects of different concurrent tasks in children who are developing typically and in children with disabilities.
1. Du Paul GS, Eckert TL. The effects of social skills curricula: now you see them, now you don’t. Sch Psychol Q. 1994; 2: 113–132.
2. Handleman JS. Transfer of verbal responses across instructional settings by autistic-type children. J Speech Hear Disord. 1981; 46: 69–76.
3. House AE, Stambaugh EE. Transfer of therapeutic effects from institution to home: faith, hope, and behavior modification. Fam Process. 1979; 18(1): 87–93.
4. Singer RN, Chen D. A classification scheme for cognitive strategies: implications for learning and teaching psychomotor skills. Res Q Exerc Sport. 1994; 65: 143–151.
5. Mulder T, Pauwels J, Nienhuis B. Motor recovery following stroke: towards a disability-orientated assessment of motor dysfunctions. In: Harrison M, ed. Physiotherapy in Stroke Management. Edinburgh: Churchill Livingstone; 1995: 275–282.
6. Mulder T. Current ideas on motor control and learning: implications for therapy. In: Illis L, ed. Spinal Cord Dysfunction: Intervention and Treatment. New York: Oxford University Press; 1992: 187–209.
7. Campbell SK. The child
’s development of functional movement. In: Campbell SK, ed. Physical Therapy for Children. Philadelphia: WB Saunders; 1994: 3–37.
8. Mulder T, Geurts S. The assessment of motor dysfunctions: preliminaries to a disability-oriented approach. Hum Mov Sci. 1991; 10: 565–574.
9. Shumway-Cook A, Woollacott MH. Motor Control: Theory and Practical Applications. Baltimore, Md: Williams & Wilkins; 1995; 216–217.
10. Abernethy B. Dual-task methodology and motor skills
research: some applications and methodological constraints. J Hum Mov Stud. 1988; 14: 101–132.
11. Schmidt RA, Lee TD. Motor Control and Learning. 3rd ed. Champaign, Ill: Human Kinetics; 1998.
12. Camicioli R, Howieson D, Lehman S. Talking while walking: the effect of a dual task in aging and Alzheimer’s disease. Neurology. 1997; 48: 955–958.
13. Ebersbach G, Dimitrijevic MR, Porwe W. Influence of concurrent tasks on gait: a dual-task approach. Percept Mot Skills. 1995; 81: 107–113.
14. Fawcett A, Nicolson R. Automatisation deficits in balance for dyslexic children. Percept Mot Skills. 1992; 75: 507–529.
15. Geurts ACH, Mulder TW. Reorganisation of postural control following lower limb amputation: theoretical considerations and implication for rehabilitation. Physiother Theory Pract. 1992; 8: 145–157.
16. Lajoie Y, Teasdale N, Bard C. Attentional demands for static and dynamic equilibrium. Exp Brain Res. 1993; 97: 139–144.
17. Lajoie Y, Teasdale N, Bard C, et al. Upright standing and gait: are there changes in attentional requirements related to normal aging? Exp Aging Res. 1996; 22: 185–198.
18. Maylor EA, Wing AM. Age differences in postural stability are increased by additional cognitive demands. J Gerontol. 1996; 51B: P143–P154.
19. Smith M, Chamberlin C. Effect of adding cognitively demanding tasks on soccer skill performance. Percept Mot Skills. 1992; 75: 955–961.
20. Teasdale N, Bard C, LaRue J, et al. On the cognitive penetrability of postural control. Exp Aging Res. 1993; 19: 1–13.
21. Whitall J. The developmental effect of concurrent cognitive and locomotor skills: time-sharing from a dynamic perspective. J Exp Child
Psychol. 1991; 51: 245–266.
22. Wright D, Kemp T. The dual-task methodology and assessing the attentional demands of ambulation with walking devices. Phys Ther. 1992; 72: 306–315.
23. Yap RL, van der Leij A. Testing the automatization deficit hypothesis of dyslexia via a dual-task paradigm. J Learn Disab. 1994; 27: 660–665.
24. Bjorklund D, Harnishfeger K. Developmental differences in the mental effort requirements for the use of an organizational strategy in free recall. J Exp Child
Psychol. 1987; 44: 109–125.
25. Guttentag RE. The mental effort requirement of cumulative rehearsal: a developmental study. J Exp Child
Psychol. 1984; 37: 92–106.
26. Guttentag RE. Age differences in dual-task performance: procedures, assumptions, and results. Dev Rev. 1989; 9: 146–170.
27. Halford G, Maybery M, Bain J. Capacity limitation in children’s reasoning: a dual-task approach. Child
Dev. 1986; 57: 616–627.
28. Miller P, Seier W, Probert J, Aloise P. Age differences in the capacity demands of a strategy among spontaneously strategic children. J Exp Child
Psychol. 1991; 52: 149–165.
29. Kahneman D. Attention
and Effort. Englewood Cliffs, NJ: Prentice-Hall; 1973.
30. Navon D, Gopher D. On the economy of the human processing system. Psychol Rev. 1979; 86: 214–255.
31. Bardy B, Laurent M. Visual cues and attention
demand in locomotor positioning. Percept Mot Skills. 1991; 72: 915–926.
32. Geurts ACH, Mulder T, Nienhuis B, et al. Dual task assessment of reorganization of postural control in persons with lower limb amputation. Arch Phys Med Rehabil. 1991; 72: 1059–1064.
33. Shumway-Cook A, Woollacott MH. Attentional demands and postural control: the effect of sensory context. J Gerontol. 2000; 55A: M10–16.
34. Shumway-Cook A, Woollacott MH, Kerns KA, et al. The effects of two types of cognitive tasks on postural stability in older adults with and without a history of falls. J Gerontol. 1997; 52A: M232–240.
35. Brooks LR. The suppression of visualization by reading. Q J Exp Psychol. 1967; 19: 289–299.
36. Baddeley AD. Working Memory. Oxford, U.K.: Oxford University Press; 1986.
37. Kerr B, Condon SM, McDonald LA. Cognitive spatial processing and the regulation of posture. J Exp Psychol. 1985; 11: 617–622.
38. Stroop JR. Studies of interference in serial verbal reactions. J Exp Psychol. 1935; 18: 643–661.
39. Jensen AR, Rohwer WD Jr. The Stroop color-word test: a review. Acta Psychol. 1966; 25: 36–93.
40. Manis FR, Keating DP, Morrison FJ. Developmental differences in the allocation of processing capacity. J Exp Child
Psychol. 1980; 29: 156–169.
41. White N, Kinsbourne M. Does speech output control lateralize over time? Evidence from verbal-manual time sharing tasks. Brain Lang. 1980; 10: 215–223.
42. Kee DW, Davies L. Mental effort and elaboration: a developmental analysis. Contemp Educ Psychol. 1988; 13: 221–228.
43. Hiscock M. Verbal-manual time sharing in children as a function of task priority. Brain Cogn. 1982; 1: 119–131.
44. Hiscock M, Kinsbourne M. Ontogeny of cerebral dominance: evidence from time-sharing asymmetry in children. Dev Psychol. 1978; 14: 321–329.
45. Hiscock M, Kinsbourne M, Samuels M, et al. Effects of speaking upon the rate and variability of concurrent finger tapping in children. J Exp Child
Psychol. 1985; 40: 486–500.
46. Hiscock M, Kinsbourne M, Samuels M, et al. Dual task performance in children: generalized and lateralized effects of memory encoding upon the rate and variability of concurrent finger tapping. Brain Cogn. 1987; 6: 24–40.
47. Kinsbourne M, Hicks RE. Functional cerebral space: A model for overflow, transfer, and interference effects in human performance. In: Requin J,ed. Attention
and Performance VII. Hillsdale, NJ: Erlbaum; 1978: 345–362.
48. Hiscock M, Kinsbourne M. Asymmetry of verbal-manual time sharing in children: a follow-up study. Neuropsychologia. 1980; 18: 151–162.
49. Murphy K, Peters M. Right-handers and left-handers show differences and important similarities in task integration when performing manual and vocal tasks concurrently. Neuropsychologia. 1994; 32: 663–674.
50. Caroselli JS, Hiscock M, Roebuck T. Asymmetric interference between concurrent tasks: an evaluation of competing explanatory models. Neuropsychologia. 1997; 35: 457–469.
51. Wickens CD, Benel DCR. The development of time-sharing skills. In: Kelso JAS, Clark JE, eds. The Development of Movement Control and Coordination. New York: John Wiley & Sons; 1982; 253–272.