作業優先性對姿勢-上姿勢作業與大腦活動的影響：年齡效應

國立臺灣大學醫學院物理治療學系暨研究所 碩士論文

School and Graduate Institute of Physical Therapy College of Medicine

National Taiwan University Master Thesis

作業優先性對姿勢-上姿勢作業與大腦活動的影響：

年齡效應

The Effects of Task Prioritization on Postural-suprapostural Task and Cortical Activity: Age-related Differences

游舒涵 Shu-Han Yu

指導教授：黃正雅 博士 Advisor: Cheng-Ya Huang, Ph.D.

中華民國 104 年 7 月

July, 2015

I

II

致謝

人生中每一個里程碑的到達都伴隨許多人的協助，而長達兩年的研究所生

涯，也同樣接受到很多人的幫助。首先，由衷地感謝指導教授黃正雅老師的協

助，從研究架構的形成、實驗基礎的訓練、實驗困境的突破到碩士論文的撰寫，

每一個階段都受到老師很大的幫助及引導，總是在身旁給予許多建議及鼓勵，使

我的研究之路一直都相當順遂，也從中獲得很多的成就感及收穫。此外，感謝口

試委員吳瑞美醫師、周立偉老師、陸哲駒老師和張雅如老師在研究及論文上給予

許多寶貴的建議，使我能對於研究有更多不同面向的思考，而使研究與論文都能

更臻於完善。

這兩年的研究生活中，感謝好朋友郁婷、甯雅和嘉容提供能夠談心歡笑的時

間和空間，紓解準備研究和論文時的煩惱及壓力，也感謝學系老師給予許多的關

心與指導，更感謝所有受試者熱心地參與實驗，包含臺大醫院志工阿姨伯伯和醫

院員工、其他單位的志工阿姨伯伯、大學部的學弟妹、碩士班和博士班的學長

姊，給予相當大的協助與鼓勵而使實驗進度能順利推展。最後，真心感謝父母提

供無憂的環境可以專心的進行研究及撰寫論文，這兩年間默默給予強大的支援。

「學問如逆水行舟，不進則退」，感謝過去已無法細數的所有協助，成為了

推動的助力，幫助我在學問的逆流中一路前進，而能在今日邁向了新的里程碑。

III

中文摘要

研究背景與目的：姿勢-上姿勢作業為於維持身體平衡下，同時進行另一項動

作或認知活動。由於注意力資源的有限性，適當且有效率的注意力配置，亦即作

業優先性選擇，為獲得較佳姿勢-上姿勢作業表現的關鍵因素。此外，隨年齡增

長，大腦注意力資源及其注意力配置的能力會逐漸下降，更加突顯作業優先性選

擇的重要性。然而，目前關於姿勢-上姿勢控制的作業優先性(姿勢優先、上姿勢

優先)探討及其相對應的神經機制仍尚未被仔細探討。因此，本研究的主要目的為

探討年輕及老年族群，在使用不同作業優先策略下，對姿勢-上姿勢作業表現及大

腦活動的影響。

研究方法：本研究共招募 16 位健康年輕受試者(平均年齡：24.4 ± 4.6 歲)及

16 位健康年長受試者(平均年齡：69.1 ± 2.7 歲)進行姿勢-上姿勢作業測試。實驗

中受試者站立於平衡板上維持平衡(姿勢作業)，並同時執行右手大拇指與食指的

精準按壓動作(上姿勢作業)。姿勢作業之目標角度設為受試者前傾平衡板最大角

度的一半，而上姿勢作業之目標力量設為受試者執行精準按壓最大力量數值的一

半。實驗過程中須分別將主要注意力放置於姿勢平衡(姿勢優先)或精準按壓動作

(上姿勢優先)來執行姿勢-上姿勢作業。實驗過程中記錄平衡板角度變化、精準按

壓力量、右手第一背側指間肌肌電圖，並同步測量受試者之腦電圖。本研究之分

IV

析參數包含：姿勢作業角度誤差、精準按壓力量誤差、平衡板晃動之近似熵

(approximate entropy)、精準按壓反應時間及腦電圖事件相關電位(P1, N1, P2)振

幅。統計分析使用 2 × 2 混合變異數分析(2 × 2 mixed ANOVA)及最小顯著差異

法(least significant difference)進行事後檢定，分析作業優先性與年齡效應對各行為

表現參數及事件相關電位的影響。

結果與討論：相較於姿勢優先策略，於使用上姿勢優先策略時，年輕族群與

老年族群皆會有較少的姿勢作業誤差，尤其老年族群於上姿勢優先策略時，同時

會呈現較高的姿勢近似熵數值與較低的精準按壓力量誤差。於腦電圖事件相關電

位振幅結果，在使用上姿勢優先策略時，年輕與老年族群的 N1 振幅皆較使用姿

勢優先策略時小，反應上姿勢優先策略可降低姿勢作業所需之注意力資源的需求

量，代表上姿勢優先策略是個較有效率的策略。此外，相較於年輕族群，老年族

群於 N1 波與 P2 波之前，多呈現 P1 波，顯示老年族群於執行姿勢-上姿勢作業的

準備初期會先進行感覺訊息的促進與整合。

結論：在執行姿勢-上姿勢作業時，上姿勢優先策略對健康年輕族群及老年族

群皆是較佳的動作控制策略，不但能產生較高的作業精準度且有較佳的大腦注意

力資源配置情形。

V

重要性與預期貢獻：本研究結果可提供健康族群，尤其是老年族群在執行姿

勢-上姿勢作業時，一個較適當的動作控制策略，以提升整體動作表現，並可對姿

勢-上姿勢控制的神經生理機制有進一步的瞭解。未來將進一步推展至神經疾患之

患者，以期提供臨床治療時適當的訓練策略。

關鍵字：作業優先性、姿勢平衡、雙重作業、事件相關電位、年齡效應

VI

Abstract

Background and Purpose: Postural-suprapostural task is defined as achievement

of a motor or cognitive task performed simultaneously with successful postural control.

Due to limited attentional resource, appropriate task prioritization is required for better

performance during postural-suprapostural task, especially in elderly adults, who may

have decreased attentional capacity and impaired attentional allocation. However,

research on the suitable strategy of task prioritization (posture-first (PF) vs. supraposture-

first (SF)) in younger and older adults is limited and lacks direct neural evidences. The

purpose of this study was to investigate the effects of task-priority strategies on postural-

suprapostural performance and its related cortical activity in younger and older

populations.

Methods: Sixteen younger healthy and sixteen elderly healthy adults were recruited

in this study. Each participant was requested to perform a force-matching precision grip

task (suprapostural task) while maintaining balance on a stabilometer (postural task) with

postural task or suprapostural task as the first-priority task. Both behavioral and cortical

data, including task accuracy (postural error and force-matching error), postural ApEn

(approximate entropy), reaction time of precision-grip, and event-related potentials

(ERPs), including P1, N1, and P2 amplitudes, were recorded.

VII

Results and Discussions: With SF strategy, less postural error was found in both

younger and older groups. Furthermore, smaller force-matching error and larger postural

ApEn were observed under the SF condition in the older group. ERP results revealed a

task priority-dependent N1 response, which was smaller in the SF condition, indicating

that SF is an efficient strategy for postural-suprapostural control. In addition, besides N1

and P2 waves, P1 positivity was observed only in the older adults, implying more

facilitation of sensory processing was invested in the initial preparation phase of postural-

suprapostural performance for older adults.

Conclusion: SF strategy may be the adequate strategy for both healthy younger and

older adults, with better postural-suprapostural accuracy and more efficient attentional

allocation than PF strategy. Further study is needed to be confident in this conclusion for

patients with neurological disease, such as Parkinson’s disease.

Significance and Contribution: The study not only provided an optimal task-

priority strategy for healthy adults, especially older adults, to increase their movement

quality of postural-suprapostural task, but also gain a better insight to neural correlates of

concurrent postural and motor-suprapostural tasks.

Keywords: task prioritization; postural balance; dual task; event-related potential; age

effect

VIII

Contents

Page

Verification Letter from the Oral Examination Committee……… I

Acknowledgement……… II

Chinese Abstract……….. III

Abstract……… VI List of Abbreviation………. XI

List of Figures……….. XIII

List of Tables……… XV

Chapter 1 Introduction……… 1

1.1 Overview of Postural-suprapostural Task……… 1

1.1.1 Definition………... 1

1.1.2 Theoretical Framework of Postural-suprapostural Task………… 2

1.1.3 Age-related Models of Postural-suprapostural Performance……. 3

1.2 Related Literature………. 5

1.2.1 Task Prioritization on Postural-suprapostural Performance…….. 5

1.2.2 Age Difference on Postural-suprapostural Performance………… 7

IX

1.2.3 Limitation of Previous Study About Postural-suprapostural

Task……… 8

1.2.4 Characterization of Cortical activity with Event-related Potentials……… 10

1.3 Rationales ……… 12

1.4 Purpose and Significance………. 13

1.5 Hypothesis……… 14

Chapter 2 Methods……… 16

2.1 Participants……….. 16

2.2 System Set-up and Data Recording………. 17

2.3 Experimental Conditions and Procedures……… 19

2.4 Data Analysis………... 22

2.4.1 Behavioral Data………. 22

2.4.2 ERPs Data……….. 23

2.5 Statistical Analysis………... 24

Chapter 3 Results………. 26

3.1 Behavioral Performance……….. 26

3.1.1 Error and Regularity of Postural Performance……….. 26

X

3.1.2 Error and Reaction Time of Force-matching Task……… 27

3.2 ERP Amplitudes………... 29

3.2.1 Task Prioritization Effect on ERP Amplitudes……….. 29

3.2.2 Age Effect on ERP Amplitudes………. 31

Chapter 4 Discussions……….. 33

4.1 Improved Task Accuracy with SF Strategy……… 33

4.2 Facilitated P1 Wave in the Older Group in SF Condition………. 36

4.3 Age Effect on ERPs in Postural-suprapostural Tasks……… 39

4.4 Methodological Issues……… 40

Chapter 5 Conclusion………... 44

References………. 45

Figures……… 54

Tables……… 72

Appendices……… 74

Appendix 1 Mini Mental State Examination (MMSE)……….. 74

Appendix 2 Approved document form the research ethics board at the National Taiwan University Clinical Trail Center…………. 78

XI

List of Abbreviation

ANOVA analysis of variance

ApEn approximate entropy

CV_PPF coefficient of variance of peak precision grip force

EEG electroencephalography

EMG electromyogram

ERP event-related potential

FDI first dorsal interosseous

LSD least significant difference

MMSE Mini Mental State Examination

MVC maximum voluntary contraction

PF posture-first

PPF peak precision-grip force

RMS root mean square

RT reaction time

SA stabilometer tilting-angle

SF supraposture-first

XII

TA target angle

TF target force

XIII

List of Figures

Page

Figure 1. Thinking process of the study……….. 54

Figure 2. Experimental setup of the study………... 55

Figure 3. Flow diagram of the study………... 56

Figure 4. Visual information for the PF and SF conditions……….... 57

Figure 5. Means and standard errors of absolute and normalized postural error of younger and older groups in the SF and PF conditions………... 58

Figure 6. Means and standard errors of absolute and normalized ApEn of younger and older groups in the SF and PF conditions………... 59

Figure 7. Means and standard errors of absolute and normalized force-matching error of younger and older groups in the SF and PF conditions……….. 60

Figure 8. Means and standard errors of absolute and normalized force-matching RT of younger and older groups in the SF and PF conditions…………. 61

Figure 9. Typical ERP waveforms of younger and older groups……… 62

XIV

Figure 10. Task prioritization effect on ERP waveforms 63

Figure 11. Task prioritization effect on grand-averaged ERP topological plots 65

Figure 12. Age effect on grand-averaged ERP topological plots 67

Figure 13. Population means of topological plots of all task priority condition (PF

and SF conditions) and age groups (younger and older groups)………. 68

Figure 14. Force CV and postural error of pilot study……….. 70

Figure 15. Graphic summary of the study………. 71

XV

List of Tables

Page

Table 1. Baseline characteristics of the participants.………. 72

Table 2. Comparison of collected normalized postural error, postural ApEn,

force-matching error, and force-matching RT between the first and

second experimental days.………... 73

1

Chapter 1 Introduction

1.1 Overview of Postural-suprapostural Task

1.1.1 Definition

Postural task is defined as the control of body posture in a stable, upright position in

space for the purpose of balance or orientation, such as standing and walking.

^1,2

It has

been traditionally considered as an automatic controlled task which required little

attention, but recent evidences have been found significant attentional requirements for

postural control in facilitating multi-sensory integration and generation of motor

execution.

^1,3

In daily activities, upright stance is rarely undertaken without other tasks.

Any task that is superordinate to the control of posture is defined as a suprapostural task.

^2,4

The evaluation or behavioral goal of the suprapostural task is different from postural

control and information of suprapostural performance cannot be acquired from the value

of postural parameter.

⁴

Performing a postural-suprapostural task is frequent for human being in daily life,

such as using mobile phone while standing on a bus or carrying a bowl of soup while

2

walking. When postural task and suprapostural task are performed together, the two

attention-demanding tasks require common attentional resource simultaneously, which

challenges the brain for prioritizing the two tasks.

^1,5

Thus, the appropriate allocation of

attention is important when performing a postural-suprapostural task for better

performance of both tasks.

1.1.2 Theoretical Framework of Postural-suprapostural Task

Two theoretical frameworks have been commonly described to explain the allocation

of attention in postural-suprapostural task, which are resource-competition model and

adaptive resource-sharing model.

^6,7

According to the resource-competition model,

attention is assumed as a capacity-limited resource. When performing a postural-

suprapostural task, postural task and suprapostural task compete for the same attentional

resource.

⁶

With the available attentional capacity, both tasks are well performed. However,

when attentional requirements of both tasks exceed the capacity, the concurrent tasks

interfere with each other and lead to the adverse effect on the both postural and

suprapostural performance.

⁷

Similar to resource-competition model, the adaptive resource-sharing model

postulates that postural task and suprapostural task share the same capacity-limited

3

resource, but the concept of cost-benefit in the postural-suprapostural sharing situation is

included in this model. The central system prioritizes between both tasks during postural-

suprapostural task and leads the performance of both tasks to the trade-off results.

⁶

Furthermore, two possible patterns in the adaptive resource-sharing model are proposed

based on some behavioral findings of postural-suprapostural performance, which are

autonomous and facilitatory patterns. The autonomous pattern emphasizes that postural

control would be acted as the primary task (the task gets more attentional resource) and

is engaged in sway minimization automatically no matter which suprapostural task is

added to a postural task. In contrast, the facilitatory pattern (also called as facilitatory

hypothesis) emphasizes that the postural stability may improve for facilitating the

suprapostural performance, especially when the suprapostural task gets more attentional

resource.

^6,8

Both resource-competition model and adaptive resource-sharing model imply that

the attention is a critical issue for postural-suprapostural control. Specially, how the

attentional allocation (or task prioritization) operates the postural task and suprapostural

task is a worth issue to study.

1.1.3 Age-related Models of Postural-suprapostural Performance

4

Age-related structural and functional changes have been found in musculoskeletal,

neuromuscular, cardiovascular, and sensory system, which affected the ability of postural

control.

^9,10

To compensate the deterioration of postural control, older adults need more

attentional requirement for balance comparing to younger adults, even in simple postural

condition.

^1,11

However, attentional capacity has been found decreased with aging, leading

to greater age-related differences of attentional allocation in postural-suprapostural

tasks.

¹²

Lacour et al. (2008)

¹³

summarized three age-related models for explaining the poor

postural control in postural-suprapostural task, including the cross-domain competition

model, the nonlinear interaction model, and the task-prioritization model. First, the cross-

domain competition model assumed that the postural task and suprapostural task shared

and competed for the attentional resource, leading to less sufficient resource for postural

control.

^13,14

The increase of the age enlarges the adverse effect of posture during the

competition of the both tasks due to reduced attentional capacity.

¹³

Second, the linear

interaction model proposed that the postural performance depended upon the attentional

requirement of the suprapostural task.

^3,13

With adding a low demanding suprapostural

task, postural task improves in both younger and older adults. However, with adding a

high demanding suprapostural task, the beneficial effect of suprapostural task reduces

with aging.

¹³

5

Different from the two models, the task prioritization model emphasized the

importance of task-priority strategy for older adults while performing a postural-

suprapostural task. Due to decreased attentional resource with aging, the older adults may

tend to select the safer strategy for postural control, allocating more attentional resource

to postural task for responding the age-related decline.

^13,15

The model predicts that

prioritization of postural control, which is also called “posture-first” strategy, is often

selected on postural-suprapostural task in older adults as a compensatory attentional

reallocation.

^11-13

However, if the “posture-first” is the optimal control strategy for older

adults while performing a postural-suprapostural task is not completely lucid.

1.2 Related Literature

1.2.1 Task Prioritization on Postural-suprapostural Performance

In a postural-suprapostural task, accomplishing the suprapostural goal and keeping

balance as well is the basic purpose of the task. To achieve the better performance,

appropriate task prioritization becomes an important issue in postural-suprapostural task.

Recently, some previous studies manipulated participants’ major attention between

postural and suprapostural tasks by verbal instruction to examine the effect of attentional

6

allocation. Some studies showed that allocating major attention on suprapostural task

would result in better postural-suprapostural performance. For example, in Siu et al.’s

study (2007), the participants were requested to perform a visual spatial memory task

while standing with feet together with focusing on the memory task or their balance.

Participants had significantly shorter response time when prioritizing the memory task

compared to prioritizing postural task and no postural sway difference between the two

prioritizing conditions.

¹⁶

Also, in the research of Jehu et al. (2015), subjects were asked

to perform a choice reaction time task while standing on a force platform with prioritizing

the choice reaction time task or the postural task. Both less postural sway and shorter

reaction time were observed under prioritizing the choice reaction time task.

¹⁷

In Kelly et

al.’s study (2013),

¹⁸

participants were asked to perform a auditory Stroop task while

walking. The results showed that with a cognitive-focus instruction, both cognitive and

walking performance would not decrease, but with a walking-focus instruction, the

performance of cognitive task deteriorated significantly but the walking speed did not

improve, indicating focusing on a postural task may not a suitable strategy in a postural-

suprapostural task.

However, the study of Yogev-Selignmann et al. (2010) had opposite results,

reporting that a worse postural-suprapostural performance was observed under

prioritization of a cognitive task.

¹⁹

In this study, participants were asked to perform a

7

verbal fluency task while walking with focusing on the verbal fluency task or on walking.

The results showed that the number of words generated in verbal fluency task was similar

between the two conditions. But with focusing on the verbal fluency task, the walking

speed decreased relative to focusing on walking. In addition, in study of Yogev-

Seligmann et al. (2012), both word-generation number and walking speed improved when

subjects focused on walking.

²⁰

Taken together, the inconsistency in current empirical

literature on postural-suprapostural task suggests that the effects of task prioritization on

postural-suprapostural performance merits further scrutiny.

1.2.2 Age Difference on Postural-suprapostural Performance

Age-related change on postural-suprapostural dual tasking has been found in clinic

and been examined in many studies. In clinic, we may observe that older adults stop

walking while talking. In attention-related studies, impaired attention functions and

impaired working memory have been evident in older adults.

¹²

Specially, aging-related

declines in attentional capacity and resource processing efficiency are noted in multiple-

tasking conditions, such as postural-suprapostural task.

^3,21-23

Besides, decreased

flexibility and optimality of attentional allocation across tasks are also presented in aging

studies.

^23,24

For instance, Doumas and Krample (2013)

²¹

found that when performing a

8

auditory n-back task with standing on a sway-reference platform, the performance of

postural task decreased in older adults, but not in younger adults. Huxhold et al. (2006)

³

showed that increased center of pressure displacement was found in older adults when

performing more demanding cognitive task with postural task , but not in younger adults.

Moreover, it had similar findings while older adults need to walk with performing a

suprapostural task. Hollman et al. (2006)

²⁵

found slower gait velocity in older adults than

younger adults when spelling five-letter words in reverse and walking across the walkway

concurrently. Also, comparing to younger adults, older adults had less word-generation

number and less walking distance when performing a word-fluency task concurrent with

walking on a narrow track.

¹⁹

All these studies showed deterioration of both postural and

suprapostural performance in support of the view of more limited attentional capacity and

attentional control ability in older adults.

1.2.3 Limitation of Previous Study About Postural-suprapostural Task

The results about task prioritization of postural-suprapostural tasks still exited

inconsistency. The inconsistency was probably due to the instruction of how the subjects

should focus their attention and the nature of suprapostural task (cognitive-supraposture

or motor-supraposture).

²⁶

The lack of specification in instruction of prioritization has

9

been considered a major limitation of postural-suprapostural related studies,

^16,19

and little

difference of the instruction may significantly affect the performance.

¹⁷

Most previous

studies only instructed the primary task to subjects, such as “focus on the cognitive task

and perform it as quickly and accurately as possible”, or “focus on your posture and keep

balance as still as possible”, and even did not tell subjects the focused task is the primary

task. Without specific instruction for both primary and secondary tasks, subjects may

allocate their attention between the primary and secondary tasks differently and result in

inconsistency performance. Hence, the instruction of how to allocate their attention

between postural and suprapostural tasks should be more specific and clear to avoid

discrepancy in attentional allocation between subjects.

On the other hand, the type of suprapostural tasks is also one of the critical factors

that may affect the interaction between postural and suprapostural tasks. Most previous

literatures used cognitive tasks to be the suprapostural task, such as Stroop task or verbal-

fluency task.

^19,27

However, growing literatures suggested that combination of motor task

and postural task may increase the sensitivity to detect the attentional resource

capacity.

^28,29

Due to similar nature of postural control and motor task, motor task and

postural task compete for the same input and output resources, resulting in larger

interference between postural balance and motor-suprapostural performance compared

with a traditional dual tasking with a posture-cognition setup. Moreover, the greater

10

interference between postural task and motor task was found in older adults than in

younger adults, due to age-related ability decline to manipulate two similar motor tasks

concurrently.

^23,30

Thus, postural task combined with motor task may be the proper design

to observe the interaction between postural and suprapostural tasks, especially in older

adults.

Next, most of previous studies about task prioritization of postural-suprapostural

control just focused on the behavioral outcome but very were limited to examine the

related cortical activation for central resource allocation in a postural-suprapostural task.

However, only behavioral evidence is unable to well explain the brain organization for

attentional allocation between postural and suprapostural tasks.

^31,32

Thus, it appears that

the cortical activity and behavioral measurement must be integrated to examine the

interaction between postural and suprapostural tasks for providing comprehensive

information of postural-suprapostural control.

1.2.4 Characterization of Cortical activity with Event-related Potentials

Event-related potential (ERP), derived from electroencephalogram (EEG), is a

common electrophysiological technique for investigating information processing of

cognitive or motor task.

³³

As a stimulus-locked cortical potential, ERP would be labeled

11

as “N” or “P” waveform for representing negative-going or positive-going component

respectively. The number following the label represents the peak latency of the

waveform,

³⁴

such as N1 represents the negative waveform which peaks around 100 ms

after stimulus and P2 represents the positive waveform which peaks around 200 ms after

stimulus. Recently, because of precise temporal resolution, ERP components have been

used in dual tasks for investigating attention shift between the two tasks and the stage of

neural information processing.

31,32,35-37

In dual-task paradigm, early ERP (P1, N1, and P2) and late ERP (P300) amplitudes

have been known as an index of resource allocation of cognitive processing.

^32,35,36

P1

amplitude was reported associated with sensory input to attended task and arousal.

^38,39

For postural-suprapostural dual tasking, it was found that N1 amplitude was associated

with the information processing of postural control

^32,37

and P2 amplitude was related to

suprapostural (a precision-grip force-matching task) control.

³²

Both Huang and Hwang

(2013)

³²

and Little and Woollacott (2015)

³⁷

reported that the amplitude of N1 increased

when posture demand increased. Besides, P2 amplitude would be modulated by

suprapostural difficulty. With high difficulty of suprapostural task, P2 amplitude would

be decreased, representing more attentional resource allocated to the suprapostural task.

³²

Based on previous studies, P1, N1 and P2 amplitudes were known to play an important

role on attention processing in postural-suprapostural task. Therefore, both P1, N1 and

12

P2 amplitudes were focused in the ERP analysis for representing attentional allocation

between postural and suprapostural tasks in the present study.

1.3 Rationales

1. There is inconsistency on advantage and defects between posture-first (PF) strategy

and supraposture-first (SF) strategy. It is valuable to realize which task-priority

strategy is the suitable strategy when performing a postural-suprapostural task.

2. Because appropriate attentional allocation or attentional shift is a critical factor for

successful postural-suprapostural execution, ERP signals could be helpful to identify

the neural mechanism of critical level in different task-priority strategies. The

understanding of cortical activation of postural-suprapostural execution may facilitate

innovative and pertinent treatment strategy for people who are multi-tasking

disturbances and prevent them from falling.

3. Comparing to younger adults, older adults may suffer from decreased attentional

capacity and impaired attentional allocation,

³

and this may affect the applicability of

task-priority strategy between younger and older adults. In this study, both younger

and older adults would be included to investigate the effects task prioritization on

postural-suprapostural tasks.

13

4. The instruction affects the way participants allocating their attention in a postural-

suprapostural task.

¹⁷

Unclear instruction may confuse the participants, leading to

different attentional allocation between subjects. In the present study, the “optimum-

maximum method”

⁴⁰

would be used for instructing subjects and enhancing the

guidance of task prioritization.

5. Most postural-suprapostural studies use a cognitive task as the suprapostural task.

However, a motor-suprapostural task can increase the phenomenon of resource-

competition or resource-sharing.

^28,29

Besides, a motor-suprapostural task is very

common in our daily life, such as cooking on moist floor or texting on the bus. In the

present study, we would choose a motor task, precision-grip task, as the suprapostural

task.

1.4 Purpose and Significance

The purpose of this study was to investigate the effects of different task prioritization

(PF vs. SF) on postural-suprapostural performance and its related cortical activity in

younger and older populations. The significance of the present study was addressed in the

academic and clinical aspects. In the academic aspect, this study provided a better insight

of the behavioral results and neural mechanism of attentional allocation under different

14

task prioritization in both younger and older populations. Especially, through this study,

we could clarify the applicability of “facilitatory hypothesis” or “posture-first principle”

with behavioral and cortical evidences (Figure 1). In clinical aspect, the results may

provide the clinical value for the physical therapists to instruct older adults who have

multi-tasking difficulty with a suitable movement strategy in their daily life and prevent

them from falling.

1.5 Hypotheses

1. Both postural and suprapostural performance are different between a postural-

suprapostural task with PF or SF strategy. In addition, the suitable task-priority

strategy for younger and older adults is different. These hypotheses would be

systematically tested by postural and suprapostural accuracy, postural regularity and

reaction time of the suprapostural task. We expected that optimal postural-

suprapostural overall performance was found with SF strategy in younger adults,

whereas optimal postural-suprapostural overall performance was found with PF

strategy in older adults.

2. Attentional resource allocation between postural and suprapostural tasks is different

depending the participants performing a postural-suprapostural task with PF or SF

15

strategy. This hypothesis would be tested by P1, N1, and P2 amplitudes of ERP

signals, for representing the allocated attention for posture and supraposture

respectively. We expected that P1, N1, and P2 amplitudes were significantly affected

between PF and SF strategies. Moreover, frontal area was found related to

information processing of working memory under dual-task condition and motor-

type suprapostural task was found related to parietal area.

^32,41,42

Therefore,

significant effects were expected found in frontal and parietal areas when adopting

PF and SF strategies.

16

Chapter 2 Methods

2.1 Participants

Thirty two healthy right-handed volunteers (16 younger adults, mean age: 24.4 ± 4.6

years; 16 older adults, mean age: 69.1 ± 2.7 years) without history of neurological,

vestibular, orthopedic, or cardiovascular disorders were recruited in this study. All

subjects had normal or corrected-to-normal vision. For older subjects, they were able to

ambulate independently without walking aids and had no history of falling. Besides, Mini

Mental State Examination (MMSE) score was measured for older adults and only the

subjects with more than 24 points were included (Appendix 1). Because the subjects were

asked to perform an suprapostural task while standing on a stabilometer (67-cm length ×

50-cm width × 24-cm height, anterior-posterior tilting angle: 0-100 degrees), the subjects

who were pregnant, had prior experience with tasks, unable to maintain balance on the

stabilometer for at least 80 seconds, or took any medications that could affect balance

were excluded from this study. Telephone interview with the subjects was done before

recruiting. Table 1 is the demographic data of both younger and older groups.

The protocol was approved by the research ethics board at the National Taiwan

17

University Clinical Trail Center (Appendix 2). Study procedure was explained by the

researcher for each subject and an inform consent was signed by the subjects prior to

participating in this experiment.

2.2 System Set-up and Data Recording

The experiment consisted of postural task and suprapostural task. Participants were

requested to perform a force-matching precision grip task with their right index and thumb

(suprapostural task) while standing on a stabilometer (postural task) (Figure 2). For the

postural task, participants were asked to maintain their balance on the stabilometer (67-

cm length × 50-cm width × 24-cm height, anterior-posterior tilting angle: 0-100 degrees)

with an inclinometer (Model: FAS-A, MicroStrain, USA) mounted on the center of the

stabilometer plate to measure the tilting angle of the stabilometer. The maximal anterior

tilting was recorded for each participant before the experiment and the 50% of the

maximal anterior tilting angle was set as the target angle for the postural task. For the

suprapostural task, participants were asked to execute a force-matching task, and the level

of force output was recorded with a load cell (15-mm diameter × 10-mm thickness, net

weight = 7 grams; Model: LCS, Nippon Tokushu Sokki Co., Japan). Maximum voluntary

contraction (MVC) of precision grip was also recorded before the experiment and the

18

50% of the MVC force was set as the target force for the suprapostural task. The

participants needed to execute the thumb-index precision grip task in response to auditory

cues. The auditory cues consisted of 80-second sequences of tone pips, with a total of

fifteen warning-executive signal pairs. The interval between a warning tone (frequency:

800 Hz, duration: 100 ms) and an executive tone (frequency: 500 Hz, duration: 100 ms)

was 1.5 seconds for the first three warning-executive pairs, but was random presented at

different intervals of 1.5, 1.8, 2.1, 2.4, 2.7 or 3.0 seconds form the fourth to fifteenth

warning-executive pairs. The interval between the executive tone and the next warning

tone was 3.5 seconds. Participants performed a quick thumb-index precision grip (force

impulse duration < 0.5 second) to couple the peak precision force with the force target

when receiving the executive tone. In order to determine the reaction time (RT) of force-

matching, the initial activation of the first dorsal interosseous (FDI) muscle was recorded

with surface electromyogram (EMG) in a bipolar arrangement (Ag/AgCl, 1.1 cm in

diameter, Model: F-E9M-40-5, GRASS) and an AC amplifier (gain: 5000, cut-off

frequency: 1 and 300 Hz; Model: QP511, GRASS).

For recording cortical activation, electroencephalogram (EEG) data was recorded

from a 32 Ag-AgCl scalp electrodes with a NuAmps amplifier (NeuroScan, EI Paso, TX).

The placement of the EEG electrodes was according to the 10-20 International System at

the following locations: Fp

1/2

, F

z

, F

3/4

, F

7/8

, FT

7/8

, FC

z

, FC

3/4

, FC

7/8

, C

z

, C

3/4

, CP

z

, CP

3/4

,

19

P

z

, P

3/4

, T

3/4

, TP

7/8

, O

z

, and O

1/2

. The ground electrode was placed along the midline ahead

of F

z

and the recording references were placed on the mastoids of the both sides. In

addition, two electrodes were attached above the arch of the left eyebrow and below the

eye to monitor eye movements and blinks. The impedances of all electrodes were

maintained below 5 kΩ, and data was recorded with a band-pass filter set at 0.1 to 100

Hz with a notch filter at 60 Hz to remove the noise from the environment. Both behavioral

and cortical signals, including stabilometer movement, precision grip force, EMG of FDI

muscle, and EEG data, were synchronized with a sampling rate of 1 kHz.

2.3 Experimental Conditions and Procedures

This study was conducted in two separate days with one-week apart. Participants in

both age groups were randomly assigned to either PF or SF conditions in the first day and

to the other in the second day (Figure 3). In each experimental day, participants were

requested to perform three experimental tasks, including one postural-suprapostural task,

and two corresponding control tasks (a single corresponding postural task and a single

corresponding suprapostural task). There were six trials for each experimental task.

In most previous researches related to task prioritization, the lack of specification

instruction for how participants directing their attention when performing dual tasks was

20

a major limitation.

¹⁶

For the better improvement of task prioritization instruction, a

procedure derived from “optimum-maximum method” proposed by Navon (1990) was

used in this study for manipulating task prioritization.

⁴⁰

The optimum-maximum method

was used to guard subjects’ attention with specific instruction for both high-priority and

low-priority tasks.

^23,43

With this method, the high-priority task was designed the “to-be-

optimized” task, and low-priority task was the “to-be-maximized” task. Participants were

instructed to execute the high-priority task with their “optimum” level and to perform

their best on the low-priority task. Such a procedure required participants to optimize the

high-priority task and not to “give up” on the low-priority task. Besides, individually

determined performance standard and performance feedback were provided in the high-

priority task but not for low-priority task. Therefore, in this study, visual feedback about

the target and performance of stabilometer movement or force-matching task was used

for enhancing the prioritization of the attention (Figure 4). For example, participants in

the PF condition were instructed to pay their primary attention on the postural task with

maintaining the tilting angle of the stabilometer at the target angle precisely, and to

maximize the precision of force-matching task. Visual feedback of stabilometer target

angle and instantaneous stabilometer tilting angle was provided in the PF condition, but

visual information about the force-matching target and force output was not provided.

Because the visual feedback was only provided for postural performance, the

21

corresponding control tasks of the PF condition were that 1) performing the postural task

on the stabilometer with visual feedback and did not execute the force-matching task, and

2) performing the force-matching task without visual feedback on a stable box (67-cm

length × 50-cm width × 24-cm height). In contrast, participants in the SF condition were

instructed to pay their major attention on the precision grip task with coupling the force

peak with the target precisely, and to maximize the precise tilting angle of the stabilometer.

Visual feedback of the force-matching target and force output was provided in the SF

condition, but visual information about the stabilometer and its target angle was not

provided. The corresponding control tasks of the PF condition were that 1) performing

the postural task on the stabilometer without visual feedback and did not execute the

force-matching task, and 2) performing the force-matching task with visual feedback on

a stable box (67-cm length × 50-cm width × 24-cm height). Besides, in order to remind

the force-matching target for the PF condition and the tilting angle target for the SF

condition, the visual feedback about the first 3 force-matching performances and the first

10-second stabilometer tilting angle with their target was provided in each trial for the PF

and the SF conditions, respectively. All the visual information was displayed on a 22-inch

computer monitor with 60 cm in front of the subjects at eye-level.

22

2.4 Data Analysis

2.4.1 Behavioral Data

For postural performance, the inclinometer data was conditioned with 6-Hz low-pass

filter and the units were converted to degrees. The inclinometer data from every executive

tone to next warning tone was selected for calculation of absolute postural error and

absolute postural approximate entropy (ApEn). The absolute postural error was presented

by calculating the root mean square (RMS) of the mismatch between the target angle and

the stabilometer tilting angle and then divided by the target angle, presenting as

RMS(SA-TA)

×100%

TA

(SA: stabilometer tilting-angle, TA: target angle). The absolute postural ApEn of the stabilometer tilting angle’s trajectory was used to represent the

variability property of the postural performance. According to previous study, the

calculation of postural ApEn was calculated after the trajectory of stabilometer tilting

angle normalized with standard deviation of time series, presenting as ApEn (m, r) =

log[C

m

(r)/C

m+1

(r)].

⁴⁴

Where m represents the length of the compared time windows and r

represents the tolerance range of the regularity.

^44-46

If a completely predictable time-series

with high regularity, value of C

m

(r) will be very close to C

m+1

(r), yielding a log-probability

(ApEn) of zero.

⁴⁴

In this study, m equaled 2 and the tolerance range of r was 0.15× the

23

standard deviation of the time series

⁴⁴

. The value of the ApEn was between 0 and 2. An

ApEn value of closer to 2 represented higher irregularities, or larger complexity of the

postural movement changes. In contrast, an ApEn value of closer to 0 represented greater

regularity.

⁴⁷

For suprapostural performance, the absolute force-matching error was presented as PPF-TF

×100%

TF (PPF: peak precision-grip force, TF: target force). The absolute force- matching RT of suprapostural task was recorded by calculating the time delay from the

presentation of executive tone to the EMG onset of FDI muscle. All behavioral

parameters of postural-suprapostural task were normalized in reference to its

corresponding control task.

_ _

postural-suprapostural 100%

corresponding control

absolute value normalized value

absolute value

 

2.4.2 ERPs Data

The manipulation of Event-related potentials (ERPs) data mainly referred to the

previous ERP study.

³²

The recorded EEG data was processed with NeuroScan’s 4.3

software (NeuroScan Inc., EI Paso, TX, USA) and the off-line analysis was used for the

analysis. The DC shift of each channel on entire EEG data was corrected with third-order

24

correction. The eye movements and blinks were removed from the EEG data. After eye

movements were removed, the EEG data was low-pass filtered with cut-off frequency of

40 Hz (48 dB/octave), and segmented into epochs of 700 ms, including a 100 ms before

the onset of executive signals. The 100 ms-data prior the executive signals was used for

the baseline correction of each EEG epoch. A visual inspection for each epoch was

applied, and those epochs with artifacts, including excessive drift, eye movements or

blinks, were removed from analysis. Those epochs with adequate responses were

averaged. ERPs from the six trials of each task were group averaged separately at each

condition for each subject. According to the previous ERP studies, P1 amplitude was

reported associated with sensory input to attended task

³⁸

, N1 was associated with the

attention modulation related to postural control, and P2 was associated with the attention

modulation related to perceptual-motor suprapostural task,

^32,44

Therefore, in the present

study, we analyzed the peak amplitudes of P1 (70-110 ms), N1 (80-150 ms), and P2 (150-

240 ms) components across all EEG electrodes to characterize the attention allocation

between postural and precision-grip tasks.

2.5 Statistical Analysis

The task prioritization conditions (PF condition, SF condition) and age groups

25

(younger group, older group) effects on behavioral and electrophysiological parameters

of postural and suprapostural tasks, including the normalized force-matching error,

normalized force-matching RT, normalized postural error, normalized postural ApEn, and

ERP amplitudes of P1, N1, and P2 components were compared with 2 × 2 mixed analysis

of variance (ANOVA). When necessary, post hoc least significant difference (LSD)

comparisons were performed. The level of significance was set at p < 0.05. Signal

processing of behavioral data and statistical analysis was completed by using MatLab v.

R2008a (Mathworks, Natick, MA, USA) and the statistical package for SPSS statistics v.

17.0 (SPSS Inc., Chicago, IL, USA).

26

Chapter 3 Results

3.1 Behavioral Performance

3.1.1 Error and Regularity of Postural Performance

Figure 5 shows the absolute and normalized postural error of SF and PF conditions

in the younger and older groups. ANOVA results suggested that normalized postural error

was subject to task prioritization (F

1, 30

= 12.99, p < 0.01) and age difference (F

1, 30

= 11.28,

p < 0.01) without interaction (F 1, 30

= 0.30, p = 0.59). Larger normalized postural error

was observed in the PF condition than that in the SF condition for both younger and older

groups (p < 0.05). Besides, normalized postural error was larger in the older group than

that in the younger group across task prioritization conditions (p < 0.05). The normalized

postural error of SF condition in the younger group was below 100% (84.51 ± 3.86%),

but the others were above 100%, indicating that younger adults had better postural

performance during the postural-suprapostural dual-task condition than that during the

single postural task condition. For postural regularity, Figure 6 displayed the absolute and

normalized postural ApEn results of SF and PF conditions in the younger and older

27

groups. ANOVA results showed a significant main effect of task prioritization (F

1, 30

=

4.41, p < 0.05) and age difference (F

1, 30

= 18.82, p < 0.001) on the normalized ApEn

values without a significant interaction (F

1, 30

= 2.21, p < 0.15). Post-hoc testing showed

a larger normalized ApEn in the younger group than that in the older group (PF condition:

younger (102.87 ± 1.58%) > older (92.16 ± 1.65%)), p <0.01; SF condition: younger

(103.87 ± 1.70%) > older (97.99 ± 2.12%), p <0.05), indicating that younger adults had

higher postural irregularity when performed a postural-suprapostural task than older

adults. Also, we noted that normalized ApEn was above 100% in the younger for both PF

and SF conditions, but was below 100% in the older group, indicating that addition of the

force-matching task led to an opposite effect on postural regularity between younger and

older groups. On the other hand, the task prioritization effect on normalized ApEn was

only shown in the older group with larger value in the SF condition than that in the PF

condition (p < 0.05).

3.1.2 Error and Reaction Time of Force-matching Task

For suprapostural performance, force-matching error of PF and SF conditions in

younger and older groups is shown in Figure 7. ANOVA results suggested that normalized

force-matching error was subject to task prioritization (F

1, 30

= 12.31, p < 0.01), but not to

28

age effect (F

1, 30

= 2.25, p = 0.14) with no significant interaction effect (F

1, 30

= 1.69, p <

0.20). Post-hoc evaluation revealed that normalized force-matching error in older group

was higher in PF condition than that in SF condition (p < 0.05). Besides, all normalized

force-matching errors were above 100% (younger group: PF condition = 118.90 ± 5.63%,

SF condition = 103.16 ± 5.49%; older group: PF condition = 139.88 ± 11.57%, SF

condition = 105.65 ± 5.31%), indicating that force-matching error tended to increase

when subjects were requested to perform a force-matching task and kept their balance on

a stabilometer concurrently compared to perform the force-matching task in a stable

posture (stand on a stable box).

Figure 8 displays the RT of force-matching task of PF and SF conditions in younger

and older groups. Similar as force-matching error, all normalized force-matching RT

values were above 100% (younger group: PF condition = 110.79 ± 3.50%, SF condition

= 107.70 ± 1.87%; older group: PF condition = 102.51 ± 4.12%, SF condition = 102.36

± 2.80%), indicating that RT would be longer when subjects were requested to perform a

force-matching task and kept their balance on a stabilometer concurrently compared to

perform the force-matching task in a stable posture. However, the RT of force-matching

did not vary with either task-priority strategy or age difference (task-priority effect: F =

0.48, p = 0.50; age effect: F = 3.15, p = 0.09).

29

3.2 ERP Amplitudes

Figure 9 displays the typical ERP waveforms of younger group and older group in

postural-suprapostural tasks. It is interesting to find that the ERP characteristics were

different between the younger and older groups. In the younger group, only the N1 and

P2 waves presented after the presentation of the executive signals across postural-

suprapostural conditions (Figure 9(a)); however, the P1, N1, and P2 waves were all

observed in sequence after the presentation of the executive signals in the older group

(Figure 9(b)). Therefore, for statistical analysis of ERP amplitude, N1 and P2 amplitudes

were analyzed via a 2 (task prioritization: PF vs. SF) × 2 (age: younger vs. older) mixed

ANOVA, with repeated measure on the first variable, while P1 amplitudes was analyzed

via a paired t-test to examine the task prioritization effect for the older adults.

3.2.1 Task Prioritization Effect on ERP Amplitudes

Figures 10(a-e) are typical ERP recordings showing the effects of task prioritization

P1, N1, and P2 amplitudes. ANOVA results suggested that in the younger group, the N1

amplitudes of most electrodes around left frontal (F

3

: F

1, 30

= 9.34, p < 0.01; FC

3

: F

1, 30

=

9.05, p < 0.01), central (C

3

: F

1, 30

= 8.93, p < 0.01) and parietal (CP

3

: F

1, 30

= 21.26, p <

30

0.001; P

3

: F

1, 30

= 16.36, p < 0.001) cortices, and midline electrodes (FC

z

: F

1, 30

= 4.37, p

< 0.05; C

z

: F

1, 30

= 6.61, p < 0.05) were subject to a significant task prioritization effect.

Post-hoc analysis further indicated that the N1 amplitudes on these electrodes (F 3

, FC

3

,

FC

z

, C

3

, C

z

, and CP

3

,) in the PF condition was generally greater than that in the SF

condition (p < 0.05)(Figure 11(a)). On the other hand, a significant supraposture effect on

P2 amplitude was noted in the left temporal (T

5

: F

1, 30

= 6.32, p < 0.05) and parietal (P

z

:

F

1, 30

= 4.68, p < 0.05) cortices. Besides, some electrodes had significant interaction

between task prioritization and age factors on P2 amplitudes (T

5

: F

1, 30

= 4.90, p < 0.05;

P

3

: F

1, 30

= 4.28, p < 0.05; O

1

: F

1, 30

= 4.47, p < 0.05). Further post-hoc analysis indicated

that P2 amplitudes on T

5

, P

3

, P

Z

, and O

1

electrodes were greater in the SF condition than

that in the PF condition (p < 0.05)(Figure 11(b)).

For the older group, paired t-test revealed that compared to with PF strategy, P1

amplitudes were larger at frontal (FC

3

and F

8

), central (C

3

and C

Z

), parietal (CP

3

, CP

Z

, P

Z

and P

4

), and right temporal (FT

8

and T

4

) areas with SF strategy (p < 0.05)(Figure 11(c)).

ANOVA results suggested that the N1 amplitudes of the electrodes around parietal (CP

3

:

F

1, 30

= 21.26, p < 0.001; CP

Z

: F

1, 30

= 8.97, p < 0.01; P

3

: F

1, 30

= 16.36, p < 0.001; P

Z

: F

1,

30

= 7.39, p < 0.05) and temporal (T

5

: F

1, 30

= 10.81, p < 0.01) areas were subject to a significant task prioritization effect. Post-hoc testing showed that N1 amplitudes on these

electrodes (T

5

, CP

3

, CP

Z

, P

3

, and P

Z

) were larger in the PF condition than that in the SF

31

condition (p < 0.05)(Figure 11(d). On the other hand, the P2 amplitudes of electrode FT

8

had a significant main effect of task prioritization (F

1, 30

= 5.16, p < 0.05). Besides, some

electrodes showed significant interaction effect between task prioritization and age

factors around right frontal (F

8

: F

1, 30

= 4.39, p < 0.05; FT

8

: F

1, 30

= 5.26, p < 0.05) and

temporal (T

4

: F

1, 30

= 4.63, p < 0.05) areas. Further post-hoc analysis indicated that F

8

,

FT

8

, and T

4

electrodes had larger P2 amplitudes in the PF condition than that in the SF

condition (p < 0.05)(Figure 11(e)).

3.2.2 Age Effect on ERP Amplitudes

The age effect on N1 and P2 amplitudes is displayed in Figures 12(a)-(b). For the

PF condition, ANOVA results revealed a significant main effect of age difference on N1

amplitudes at frontal (F

3

: F

1, 30

= 5.60, p < 0.05; FC

3

: F

1, 30

= 4.86, p < 0.05), central (C

3

:

F

1, 30

= 5.14, p < 0.05), and parietal (CP

3

: F

1, 30

= 4.86, p < 0.05; CP

Z

: F

1, 30

= 4.22, p <

0.05; P

3

: F

1, 30

= 4.95, p < 0.05) areas. Post-hoc evaluation showed that the N1 amplitude

of these electrodes (F

3

, FC

3

, C

3

, CP

3

, CP

Z

, and P

3

) in the older group was generally greater

than that in the younger group (p < 0.05)(Figure 12(a)). However, the P2 amplitude was

independent of the age effect for all cortical areas in the PF condition (p > 0.05)(Figure

12(b)).

32

For the SF condition, ANOVA results revealed the a significant main effects of age

groups difference on N1 amplitudes at left fronto-parietal cortex (F

3

: F

1, 30

= 5.60, p <

0.05; FC

3

: F

1, 30

= 4.86, p < 0.05; C

3

: F

1, 30

= 5.14, p < 0.05; CP

3

: F

1, 30

= 4.86, p < 0.05;

P

3

: F

1, 30

= 4.95, p < 0.05) with larger N1 amplitudes in the older group (Figure 12(c)). On

the other hand, ANOVA results showed a significant main effects of age difference on P2

amplitudes at occipital area (O

1

: F

1, 30

= 4.40, p < 0.05; O

z

: F

1, 30

= 6.94, p < 0.05; O

2

: F

1,

30

= 4.55, p < 0.05) and a significant interaction between task prioritization and age factors at P

z

electrode (F

1, 30

= 4.47, p < 0.05)(Figure 12(d)). Post-hoc analysis indicated that P2

amplitudes on these electrodes (P

Z

, O

1/2

, and O

z

) were greater in the younger group than

that in the older group (p < 0.05).

Figure 13 displays the topological plots of the younger and older groups in each

postural-suprapostural condition. It seems that task prioritization affected the activation

duration of N1 and P2 waves in the younger and older groups respectively. In the younger

group, with activation duration of N1 wave was shorter in the SF condition and P1

activation of the older group seemed earlier in the SF condition than in the PF condition.

In addition, the age difference also affected the activation of N1 and P2, with greater

activation intensity and area of N1 wave in the older group but greater activation intensity

and area of P2 wave in the younger group.

33

Chapter 4 Discussions

4.1 Improved Task Accuracy with SF Strategy

The results showed significant task prioritization effect on postural and

suprapostural tasks in both younger and older adults. First, better postural/ suprapostural

performance was found in both age groups when paying major attention on force-

matching task in postural-suprapostural task (Figures 5, 7), which in line with some

studies related to task prioritization.

^17,48

Burcal et al. (2014) showed greatest postural

improvements when focusing on suprapostural task compared with focusing on balance

and no focusing instruction.

⁴⁸

Jehu et al. (2015) also reported that less postural sway was

observed when prioritizing reaction time task than prioritizing posture.

¹⁷

These researches

suggested that focusing on suprapostural task allowed attention shifted attention away

from control of posture, leading to more automatic and efficient postural control. The

results may also support the constrained-action hypothesis, which proposed that

consciously controlling posture or movement close to the body may interfere with the

automatic control processes and thus negatively affected postural performance.

⁴⁹

In

addition, the postural improvement with SF strategy was also consistent with the

34

facilitatory pattern in adaptive-resource sharing model, which proposed that postural

stability may get improved in order to facilitate suprapostural performance.

^6,8

The

facilitatory effect was especially dominant in the older adults, because both force-

matching error and postural error was less in the SF condition than that in the PF condition

(Figures 5, 7). However, Yogev-Seligmann et al.’s study (2010) reported the opposite

results.

¹⁹

In the study, subjects (younger and older adults) were requested to perform a

cognitive task (verbal fluency task) during walking with different attention instruction,

including no specific prioritization instructions, prioritization of gait and prioritization of

the verbal fluency task. They found that gait speed was reduced when prioritization was

given to the verbal fluency task in both age groups, indicating that SF strategy might

decreased postural performance. The discrepancy between our results and Yogev-

Seligmann et al.’s finding may result from different type of suprapostural task. With a

motor suprapostural task, such as force-matching, attentional resource would be enforced

to integrate for optimal outcome.

On the other hand, postural performance was found to be significantly better in the

younger group than that in the older group for both PF and SF conditions. Age-related

decline of postural performance in older adults may represent the inability to adequately

allocate attentional resource between two tasks and inefficient postural control in older

adults.

^15,50

With aging, overall structural and functional decline resulted in decreased