Right prefrontal cortex is activated for perceiving postural limits: a functional near-infrared spectroscopy study

doi:10.4236/health.2009.13039

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Vol.1, No.3, 239-2

doi:10.4236/health.2009.13039

SciRes

43 (2009) Health

Right prefrontal cortex is activated for perceiving postural

limits: a functional near-infrared spectroscopy study

Noriyuki Kamata1, Yoshimi Matsuo2, Ayako Matsuya3, Satoru Inoue1, Kazuo Abe3,4

1Rehabilitation Unit, Osaka University Hospital, Osaka, Japan; [email protected]

2Faculty of Rehabilitation, Kobe Gakuin University, Hyogo, Japan

3Faculty of Nursing and Rehabilitation, Konan Women’s University, Hyogo, Japan

4Department of Neurology and Rehabilitation Center, Konan Hospital, Hyogo, Japan

Received 25 September 2009; revised 26 October 2009; accepted 27 October 2009.

ABSTRACT

The purpose of this study was to investigate

neuronal mechanisms active during the percep-

tion of forward postural limits in a standing po-

sition and to specify fall-related brain activity

using optical functional near-infrared spectros-

copy. The study group included six right-handed,

healthy female volunteers (range: 19, 20 years).

The optical imaging device comprised 16 opto-

des designed to provide 24-channel recording of

changes in hemoglobin oxygenation. We meas-

ured the changes of oxygenated hemoglobin

levels in the frontal region when subjects per-

ceived reachability in a standing position. Com-

pared with those in other regions, the oxygen-

ated hemoglobin levels in the right frontal region

compatible with the right prefrontal cortex sig-

nificantly increased. This result suggests that

brain activities in the right prefrontal cortex are

related to perception of reachability. Overesti-

mation of postural limits has been reported as

one of the risk factor for falling. This overesti-

mation might be induced by dysfunction in the

prefrontal cortex, resulting in a failure to inhibit

a motor program that would have caused a loss

of balance in reaching. Activation of the right

prefrontal cortex may be a key factor for pre-

venting accidental falls in the elderly and in pa-

tients with neurological disorders.

Keywords: Postural Limits; Falls; Functional

Near-Infrared Spectroscopy (fNIRS); Prefrontal

Cortex (PFC)

1. INTRODUCTION

Most people can appropriately perceive reachability, which

is defined as the distance to which one could reach if actually

executing the reach. Previous studies often utilized forward

perceived reachability in a standing position as an index of

one’s own perceived postural limits [1-3]. Reaching move-

ments are programmed based on this perception, and one

can usually execute these movements safely by keeping the

center of mass within postural limits [4].

Our previous study and others have reported that eld-

erly and neurological patients, including patients with

Parkinson’s disease, tended to overestimate their reach-

ability when standing, and thus, overestimate their own

postural limits, even if a target is not within actual reach

[3,5]. In addition, this overestimation has been reported as

one of the risk factors of multiple falls [6]. Therefore,

improving the perception of reachability might decrease

the number of accidental falls in the elderly and in neu-

rological patients. For creating a rationale for this new

intervention, it is useful to investigate the neuronal

mechanisms of this abnormal estimation of reachability.

However, no research has yet examined such brain ac-

tivities with regard to perception of reachability.

Functional near-infrared spectroscopy (fNIRS) can

evaluate cortical activity by measuring the changes of

hemoglobin oxygenation of blood within a few centime-

ters of the skull surface [7]. We adapted this technique to

our study since it was suitable for use during dynamic

tasks such as walking and in a standing position [8,9].

Thus, the purpose of this study was to clarify neuronal

mechanism during the perception of forward postural

limits in a standing position by fNIRS and to specify

fall-related brain regions.

2. METHODS

2.1. Subjects

The study group included six right-handed, healthy fe-

males (range: 19, 20 years). Informed consent was ob-

tained form each subject in accordance with the Helsinki

Declaration.

N. Kamata et al. / HEALTH 1 (2009) 239-243

240

analog switch

in left hand

optodes of fNIRStarget of functional reach instrument

70cm

analog switch

in left hand

optodes of fNIRStarget of functional reach instrument

70cm

Figure 1. Position of subject and experimental devices. The

target of the functional reach instrument can be moved along the

dotted line. The distance from the target to the subject is 70cm.

9cm

24 23

20 1819

1617 15

14 131112

10 9

2 1

Cz RtLt

24 23

20 1819

1617 15

14 13 1112

10 9

2 1

(b)(a)

9cm

24 23

20 1819

1617 15

14 131112

10 9

2 1

24 23

20 1819

1617 15

14 131112

10 9

2 1

Cz RtLt

24 23

20 1819

1617 15

14 13 1112

10 9

2 1

24 23

20 1819

1617 15

14 13 1112

10 9

2 1

(b)(a)

Figure 2. (a) Schema for location of the array of optodes. The

optode array consists of 8 light fibers (white circles) and 8

detectors (black circles). The change of dortical hemoglobin

content was measured in each channel between a light fiber and

a detector (square numbers from 1 to 24). The center of the 4th

row of optodes (channel 2) was located in the Cz position of the

international 10-20 system. (b) Location of 13 regions of interest

(circles labeled A-M). Each region consists of 4 near-

est-neighbor channels For example, region of interest A was

formed by channels 1, 4, 5, and 8.

１task (35s)

Pre

(10s)

task

(15s) Post

(10s)

Pre

(10s)

Control

task

(15s) Post

(10s)

Total 350s

１task (35s)

Pre

(10s)

task

(15s) Post

(10s)

Pre

(10s)

Control

task

(15s) Post

(10s)

Total 350s

Figure 3. Task design for fNIRS. The perceived reachability tasks

(taller blocks) and control tasks (lower blocks) were measured

alternately and repeated continuously 5 times.

2.2. Experimental Devices

We used an optical imaging device (FOIRE3000, Shi-

madzu, Kyoto, Japan) and a standard device for the

functional reach test [10] (GB-200, OG Giken, Okayama,

Japan) with a wired analog switch. Subjects stood before

the target of the functional reach test instrument with

their feet positioned shoulder-width apart, holding the

analog switch in their left hand (Figure 1).

2.3. fNIRS

The schema for location of the array of optodes in opti-

cal imaging device is showed in Figure 2a. The device

consisted of 16 optodes, including 8 light-source fibers

and 8 detectors arranged in a 4 × 4 array with the inter-

optode distance set at 3.0cm. We located the center of

the 4th row of optodes in the Cz position, in accordance

with to the international electroencephalogram 10-20

system. This configuration permitted 24-channel re-

cording of changes in oxygenated hemoglobin (oxy-Hb),

deoxygenated hemoglobin (deoxy-Hb), and total hemo-

globin (total-Hb) in the motor-related cortex within the

covered 9×9-cm skull surface.

2.4. Task Design and Experimental Protocol

The task design for this fNIRS study is shown in Figure 3.

We conducted two tasks, the perceived reachability task

and the control task. Each task took a total of 35 seconds

and consisted of a 10-second pretask period, a 15-second

task period, and a 15-second posttask period. The two

tasks were measured alternately, and the protocol was

repeated 5 times. Total duration for each fNIRS measure-

ment session was 350 seconds.

During the tasks, the subject was instructed to concen-

trate on the target while in a standing position with arms at

her sides and maintaining a static posture. In the perceived

reachability task, the examiner moved the target in a direc-

tion away from the subject during the 15-second test. When

the subject judged that the target had arrived at the reach-

able limit for her right arm, she pushed the analog switch to

record her perception. In control tasks, the examiner moved

the target in a direction nearer to the subject.

2.5. Data Analysis

The hemoglobin oxygenation data was sampled every

160ms. We used the change of oxy-Hb level (mM-cm) as

the index of cortical activity. When cortical activity in-

creased, cortex needed more oxygen that increases oxy-

Hb level and it correlated better with the change of re-

gional cerebral blood flow than deoxy-Hb or total-Hb

[11]. First, the 5 repetitions of oxy-Hb measurement in

each task were grand-averaged in each channel. In addi-

tion, we obtained the data from the last 10 seconds of

each task period because hemodynamic changes as

measured by fNIRS lag behind the task stimulus by a

few seconds [12]. Second, to adjust for the influence of

N. Kamata et al. / HEALTH 1 (2009) 239-243

241

＊

２４２３

２１

２２

２０１８１９

１６１７１５

１４１３１１１２

７５

８

６４

３

１０９

２１

RtLt

-2.1

-1.6

-1.1

-0.6

-0.1

0.4

0.9

1.4

1.9

2.4

ABCDEFGHI JKLM

oxy-Hb

deoxy-Hb

total-Hb

***

*P<0.05

＊

２４２３

２１

２２

２０１８１９

１６１７１５

１４１３１１１２

７５

８

６４

３

１０９

２１

RtLt

＊

２４２３

２１

２２

２０１８１９

１６１７１５

１４１３１１１２

７５

８

６４

３

１０９

２１

２４２３

２１

２２

２０１８１９

１６１７１５

１４１３１１１２

７５

８

６４

３

１０９

２１

RtLt

-2.1

-1.6

-1.1

-0.6

-0.1

0.4

0.9

1.4

1.9

2.4

ABCDEFGHI JKLM

oxy-Hb

deoxy-Hb

total-Hb

***

*P<0.05

(a) (b)

Figure 4. Mean values of effect size in regions of interest. (a) Values are mean ± standard error. Effect size of oxy-Hb (white bars) or

total-Hb (hatched bars) significantly increased in regions A, B, D, F, and K. The effect magnitude of deoxy-Hb (black bars) tended to

show negative scores. (b) The effect size of oxy-Hb located in regions of interest A, B, D, F, and K significantly increased (black circles).

differential light-path lengths among subjects or chan-

nels and to emphasize the changes of oxy-Hb level dur-

ing perception of reachability, the effect size was calcu-

lated by the following formula: Effect size = (mean

oxy-Hb during perceived reachability task – mean

oxy-Hb during control task) / standard deviation of

oxy-Hb during the control task [8,13]. Finally, we estab-

lished 13 regions of interest, which consisted of the 4

nearest neighboring channels (Figure 2b). The data for

each region included a total of 24 effect-size values (6

subjects × 4 channels). On the basis of these values, we

used 1-sample t tests to assess the regions in which the

oxy-Hb level was significantly increased during the per-

ception of reachability. Statistical significance was de-

fined as P<0.05.

To examine changes of oxy-Hb, deoxy-Hb and to-

tal-Hb during control task, we calculated the Z scores in

each subject and each channel by following fomula: Z

score = (mean value during control task – mean value

during pretask period of control task) / standard devia-

tion during pretask period of control task [12]. All mean-

Z scores of oxy-Hb, deoxy-Hb and total-Hb in each

channel ranged from -1 to 2 and therefore, we judged

that hemoglobin levels were not changed by the control

task.

3. RESULTS

The mean values of effect size in each channel are

shown in Table 1. The effect tended to increase in the

right frontal region during the perception of reachability.

Statistical analysis also showed that the mean oxy-Hb

effect size significantly increased in regions of interest A,

B, D, F, and K (P<0.05) (Figure 4).

4. DISCUSSION

The significant change in oxy-Hb during the perception

of reachability indicated increased neural activity in the

right frontal lobe. Previous researches have reported that

the perception of reachability was estimated by internal

mental simulation or motor imagery of the reaching

movement [14-16]. Literally, motor imagery for the right

hand recruits multiple cortical areas, including the left

supplementary motor area, the premotor cortex [17], and

the primary motor cortex [18]. As previous studies have

indicated that motor imagery is related to estimation of

the right arm-reach range, neural activity would be pre-

dominantly in the left frontal lobe, not in the right frontal

lobe. However, since we observed activations in the right

frontal lobe, our results do not support the existence of a

close connection between perceived reachability and

motor imagery. The lateral area of Cz position in the

international 10-20 system corresponds with sensorimo-

tor cortex. Significant increases of oxy-Hb level in re-

gions of interest A, B, and D may be induced by the act

of pushing the analog switch in the left hand during the

perceived reachability task. Thus, we hypothesize that

regions F and K in the right frontal lobe, located in the

prefrontal cortex (PFC), are key regions in the percep-

N. Kamata et al. / HEALTH 1 (2009) 239-243

242

Table 1. The mean effect size of oxy-Hb, deoxy-Hb and total-Hb in each Channel.

Ch11.11± 1.68-0.34± 0.690.78± 0.78

Ch20.45± 1.300.12± 0.220.52± 0.50

Ch30.42± 1.730.06± 0.810.75± 0.68

Ch40.43± 1.21-0.16± 0.560.51± 0.51

Ch50.79± 1.370.12± 0.520.70± 0.54

Ch60.80± 1.800.23± 0.620.86± 0.68

Ch7-0.40± 0.720.19± 0.600.28± 0.42

Ch80.53± 1.05-0.20± 0.580.49± 0.47

Ch90.87± 2.500.26± 0.631.07± 0.94

Ch100.07± 1.01-0.09± 0.520.38± 0.43

Ch110.62± 1.400.06± 0.850.73± 0.57

Ch120.90± 1.40-0.52± 0.640.60± 0.70

Ch130.34± 1.71-0.07± 0.840.70± 0.69

Ch14-0.08± 1.560.40± 0.890.69± 0.65

Ch151.22± 2.31-0.27± 1.191.11± 1.00

Ch160.17± 1.20-0.55± 1.150.49± 0.70

Ch170.74± 2.34-0.45± 0.980.90± 1.00

Ch182.55± 3.43-3.22± 4.841.90± 2.91

Ch190.39± 3.050.08± 1.251.19± 1.20

Ch200.62± 3.95-1.06± 2.641.54± 1.88

Ch210.25± 1.89-0.31± 1.070.73± 0.83

Ch221.47± 3.11-2.76± 1.890.93± 2.04

Ch230.89± 3.03-0.92± 2.561.39± 1.56

Ch240.41± 2.61-0.89± 3.001.28± 1.58

Effect size of oxy-HbEffect size of deoxy-HbEffect size of total-Hb

Values are mean ± standerd diviation.

tion of reachability in a standing position; in other words,

perception of the subject’s own postural limits.

We also investigated the functional relationship be-

tween perceived reachability and cognitive function.

Previous studies have shown that healthy young subjects

tend to overestimate their own reachability while in a

sitting position, although they underestimate their own

reachability in a standing position. Robinovich [2] called

this underestimation a potential safety factor in reducing

the risk of loss of balance. This safety factor means that

the process of perceiving one’s own postural limits are

controlled by an inhibitory neural mechanism of a motor

program executing the reaching motion without risk of a

fall. Shallice proposed the notion of a supervisory atten-

tional system in the frontal lobe that selects appropriate

schema among activated schemas in response to various

situations, subsequently inhibiting inappropriate ones

[19]. In addition, the PFC in the right hemisphere is ac-

tivated by a GO/NO-GO task [20]. Therefore, the right

PFC activity observed in our study may represent an

inhibitory mechanism for estimating safe reaching in a

standing position. Recently, Ambrose et al. reported that

elderly fallers with impaired working memory overesti-

mated their postural limits compared with those with

preserved working memory [21]. They also reported that

an impaired executive function might promote fall risks

due to misjudgment of motor planning in daily activity.

Working memory and executive function are believed to

be conducted mainly in the PFC [22]. Consequently, our

results indicate that working memory may be associated

with perception of one’s own postural limits.

5. CONCLUSIONS

Overestimation of postural limits is a possible risk factor

for falls. Our study using fNIRS suggests that overesti-

mation of the postural limits may result from dysfunction

of the right PFC, which subsequently fails to inhibit an

inappropriate motor program. Thus, activation of the right

PFC may be a key factor for preventing accidental falls in

the elderly and in patients with neurological disorders.

Interventions to activate the right PFC could be useful for

preventing accidental falls.

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