Statistical Analysis - 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

Chapter 2 Methods

2.5 Statistical Analysis

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

z

: F

1, 30

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

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

), 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 1, 30

= 21.26, p < 0.001; CP

Z

: F

1, 30

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

, 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

) 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

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

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

35

attentional capacity and increased attentional requirement in postural control.

^9,10,12

Therefore, adding a seconding task secondary task to postural task may increase the

attention load and reach the limit of attentional capacity to allocate in older adults, which

consistent with the opinion of cross-domain competition model.

^12-14

In addition, when

adding a secondary task, younger adults may shift part of attention to the secondary task

and allow more automatic control of posture. However, older adults were unable to

efficiently shift attention away from posture, which lead to interference of postural

control.

⁵⁰

Second, for postural variability, the results showed a higher value of normalized

postural ApEn in the SF condition than that in the PF condition (Figure 6), which

represents more irregularity of postural control.

^47,51

Postural regularity has been found to

be positive correlated with amount of attention allocated in postural control, with higher

regularity (or lower ApEn value), more attentional resource is devoted to the postural

control.

⁴⁷

Thus, combination of the results of postural error and normalized postural

ApEn values, it could be interpret as less amount of attention required to keep postural

balance when adopting SF strategy in postural-suprapostural task, and also reflects SF

strategy could be have more efficient and automatic postural control.

^47,51

In addition, the

value of normalized postural ApEn was significantly greater in the younger group than

that in the older group when performing postural-suprapostural task, indicating that

36

younger adults could use more automatic control for keeping postural balance, and this

phenomenon may partly explain the better postural performance in the younger group

than that in the older group.

4.2 Facilitated P1 Wave in the Older Group in SF Condition

The present study appears to be the first to assess electrophysiological

correlates (P1, N1, and P2) for postural-suprapostural tasks with different task

prioritization between younger and older adults. One of our novel finding is different ERP

waves facilitated during postural-suprapostural task between age groups, with P1, N1,

and P2 waves in the older adults, whereas only N1 and P2 waves in the younger adults

(Figure 9). Specially, the facilitated P1 waves were more dominant in the SF condition

than that in the PF condition (Figure 13). According to previous literatures, although P1

and N1 were associated with sensory gain control, they reflected different aspect of

attention.

⁵²

P1 was thought to reflect the facilitation of sensory processing of task-related

stimuli.

^52-54

In addition, enhanced P1 positivity was found associated with increased

sensory input to attended task and increased arousal,

^38,39

related to high activation level

of emotion, mental and physiological system.

⁵⁵

Hence, facilitated P1 wave may imply

that more sensory processing facilitation and arousal were involved at the initial

37

preparation phase of postural-suprapostural task in older adults for compensating

decreased information processing or reduced attentional capacity. In this study, the greater

P1 positivity was observed across left primary motor cortex, sensorimotor cortex, and

frontal-parietal and right frontal-temporal area in the SF condition indicated that older

adults with SF strategy showed more arousal and sensory input facilitation than with PF

strategy (Figure 11(c)). According to previous researches, frontal-parietal cortical region

was reported related to recognition of postural instability and right frontal-temporal

cortical region was related to modulation of finger force scaling.

^56,57

The finding indicates

that SF strategy facilitated higher sensory processing for both upcoming balance and

force-matching task and results in better behavioral outcomes.

The other important finding in the present study was that N1 amplitude increased in

the PF conditions for both younger and older groups. N1 was also reported associated

with sensory processing for postural control.

³⁷

Enhance N1 negativity was found related

to high perceptual load, reflecting increased perceptual resource of sensory processing

^36,58

and reduction of N1 amplitude was associated with automatic postural control.

³⁷

According to our results, under PF conditions, N1 negativity was greater at left

frontal-parietal area in the younger group and at left central-frontal-parietal regions in the older group

respectively (Figures 11(a), (d)). Frontal-central cortical region has been found related to

action monitoring and detection of error, and activation of parietal region has been found

38 ⁵⁶

In addition, left hemisphere was reported a dominant role

in the control of movement and motor skills that are carried out with those that require

bimanual coordination.

⁵⁹

Therefore, increased N1 amplitude in these areas may imply

that more attention was required for executing postural task under the PF conditions.

However, more attention devoted to the postural task was not necessary to result in better

postural performance. According to the results of postural error, the PF conditions had

more postural error indicating that PF strategy is an ineffective strategy for postural

control in both younger and older adults.

On the other hand, it is interesting to find that there was an opposite task

prioritization effect on P2 positivity between the younger and older groups. P2 was found

related early attentional allocation for initial conscious awareness for the task

⁶⁰

and

suprapostural difficulty.

³²

Reduction of P2 amplitude was found representing more

attentional allocation to suprapostural task.

³²

In younger adults, greater P2 positive

around left temporal-parietal-occipital region (T

5

, P

3

, P

Z

, and O

1

), in the SF condition

(Figure 11 (b)), indicating that less attention for multimodal sensory integration was

allocated (or required) for the suprapostural task under SF condition than PF condition.

Although, less attention was required to perform the suprapostural task, no suprapostural

performance decline was found in behavioral results (Figure 7). Moreover, the topological

plots also showed an earlier activation of P2 wave in the SF condition than that in the PF

39

condition in younger adults (Figure 13). The early P2 activation may reflect more

effectiveness of the attention shifted from postural task to the force-matching task.

Oppositely, SF strategy would lead to less P2 positivity on right frontal-temporal cortex

in the group of older adults (Figure 11 (e)), which represents more attention allocated to

the suprapostural task in the SF condition. Right frontal-temporal cortex was reported

acting an important role in finger force scaling and right hemisphere was related constant

motor output.

⁵⁷

The results may imply that more attention was devoted for better

force-matching accuracy in older adults with SF strategy to compensate the decreased ability

of force scaling.

⁶¹

Therefore, according to behavioral and ERP results, SF strategy may

be the better strategy for both younger and older adults than PF strategy.

4.3 Age Effect on ERPs in Postural-suprapostural Tasks

Besides, N1 negativity was observed around frontal-parietal area in older adults than

in younger adults for both PF condition and SF condition (Figures 12 (a), (c)). The fact

indicates that more attentional resource was required for older adults to keep their balance

because of less automatic postural control in older adults (smaller ApEn value, Figure 6).

The topological plots also support this argument by longer activation duration and longer

activation area of N1 wave in the older group (Figure 13). Age-related changes were

40

reported in left premotor and sensorimotor cortices, which was related to postural control

and internal representation of body in space,

^52,53

especially for skilled movement.

⁵⁴

the other hand, enhanced P2 positivity on occipital area was found in the younger adults

under SF condition (Figure 12(d)), indicating that less attentional resource was required

for performing the suprapostural task in younger adults. An functional magnetic

resonance imaging study showed that the occipital area was related to sensory

processing.

³⁸

Hence, the results may represent increased attentional requirement of

suprapostural task in older adults for compensating the decline of sensory processing.

4.4 Methodological Issues and Limitation

First, in the current experimental paradigm, a force-matching task with 50% MVC

force was used as the suprapostural task. In order to choose an adequate level of force

target, we executed a pilot study to examine the variability of force output in different

force-intensity and the effects of force-intensity on postural balance. With the same

apparatus and postural-suprapostural task design as the current experiment, twelve

healthy right-handed volunteers (4 males, 8 females; mean age: 24.5 ± 3.0 years)

without past neurological or neuromuscular impairment were recruited to perform a

force-matching task with 25%, 50% and 75% of MVC force while standing on a

41

stabilometer with keeping their balance at 50% of the maximal anterior tilt angle. The

twelve subjects of the pilot study were different to that of the main experiment. Subjects

were instructed to performed both postural and force-matching tasks as precision as

possible with providing online visual feedback of both targets and their performance.

Coefficient of variance of peak precision grip force (CV_PPF) and postural error were

measured in each condition. A one-way repeated-measures analysis of variance with

Bonferroni adjustments were used to contrast force-matching variability (CV_PPF) and

postural error differences among 25%, 50%, and 75% of MVC force conditions. The

level of significance was set as p < 0.05. ANOVA statistics suggested that CV_PPF

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