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\title{Sequential Analysis of Postural Control Resource Allocation During a Dual Task Test}
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             \author[1]{Chang-Hyung  Lee}

             \affil[1]{  Pusan National University School of Medicine}

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\date{\small \em Received: 16 December 2012 Accepted: 31 December 2012 Published: 15 January 2013}

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\begin{abstract}
        


To investigate postural control factors influencing automatic (reflex-controlled) and attentional (high cortical) factors on dual task.Method : We used a dual-task model to examine attentional factors affecting control of posture, subjecting test subjects to vibration stimulation, one-leg standing and verbal or nonverbal task trials. Twenty-three young, healthy participants were asked to stand on force plates and their centers of pressure (COP) were measured during dual task trials. We acquired 15 seconds of data for each volunteer during six dual task trials involving varying task combinations.Results : We observed significantly different sway patterns between early and late phases of the dual task trials that probably reflect attentional demands. Vibration stimulation perturbed sway more during the early than the late phases; with or without vibration stimulation, the addition of secondary tasks decreased sway in all phases, and greater decreases in sway were observed in late phases when subjects were assigned nonverbal tasks. Less sway was observed during nonverbal task in a sequential study.

\end{abstract}


\keywords{task performance, analysis, postural balance, attention.}

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\let\tabcellsep& 	 	 		 
\section[{Introduction}]{Introduction}\par
ensory perturbations of visual, somatosensory, and vestibular systems disrupt postural stability. Postural control can be influenced by automatic (reflex-controlled) and attentional (high cortical) factors, and previous studies have suggested that postural control systems require varying degrees of attention depending on the postural tasks involved and the age of the subjects. \hyperref[b0]{[1]}\hyperref[b1]{[2]}\hyperref[b2]{[3]}\hyperref[b3]{[4]}\hyperref[b4]{[5]} Attentional factors are thought to arise from the central nervous system (CNS), while automatic factors are reflex-controlled by somatosensory (muscle, skin and pressure receptors), visual and vestibular inputs. \hyperref[b5]{6,}\hyperref[b6]{7} Dual task paradigms are important tools for understanding balance control. The primary task is usually postural control, which involves standing on a force plate with different levels of difficulty, for example on an uneven surface or standing on one leg. Teasdale et al. \hyperref[b7]{8} showed that adults of all ages exhibit delays in reaction time as postural task complexity increases. When vibrations are applied during primary postural tasks they typically cause directional shifts in subjects, due to increasing primary (Ia) afferents that are discharged during vibration and interpreted as lengthening of the vibrated muscles. \hyperref[b6]{7} In previous studies, tendon vibration stimulation was shown to increase postural sway, and subjects frequently experience vibration-induced compensatory losses of balance, falling in the same direction as the applied vibration. \hyperref[b8]{9,}\hyperref[b9]{10} However, in several studies directional shifts were either increased or decreased according to stimulation intensity and type. \hyperref[b10]{[11]}\hyperref[b11]{[12]}\hyperref[b12]{[13]}\hyperref[b13]{[14]}\hyperref[b14]{[15]} The secondary task in dual task paradigms is usually attention demanding, and task intensity and difficulty influence postural control in various ways. \hyperref[b4]{5,}\hyperref[b15]{16,}\hyperref[b16]{17} Both verbal and nonverbal tasks have been applied as secondary tasks in dual task paradigms. \hyperref[b17]{18,}\hyperref[b18]{19} Verbal tasks are considered relatively easy for participants to complete, while nonverbal tasks are more difficult due to their attention demanding characteristics. Verbal and non-verbal working memory are thought to be associated with different regions of the brain. \hyperref[b19]{[20]}\hyperref[b20]{[21]}\hyperref[b21]{[22]} Several studies have explored the effects of tendon vibration on postural control and the ability to complete tasks. However, the relationships between these parameters and postural sway have not been investigated in a dual task study design. Furthermore, the sequential relationship between automatic control and attention factors during in dual task contexts is still unclear.\par
We examined the sequential relationships of dual task on postural control. When subjects were subjected to dual task trials, we were able to sequentially observe the demands of attention factors, how they differed depending on the combinations of tasks that were presented, and the effects of attentional factors on balance control. The clinical implications of postural control can be understood through dual task performance and resource allocation analysis. 
\section[{II.}]{II.} 
\section[{Materials and Methods}]{Materials and Methods} 
\section[{Subjects}]{Subjects}\par
Twenty-three young, healthy participants participated in this study. No participants reported neurological or orthopedic disorders, and none were receiving medications known to affect postural control. All participants provided informed consent prior to testing. 
\section[{Methods}]{Methods}\par
A total of 23 subjects participated in the study. Detailed demographic data are shown in Table \hyperref[tab_0]{1}. All participants were randomly assigned to six trials (Table \hyperref[tab_1]{2}). Three participants were excluded due to poor compliance and the other 20 participants were included in experiments.\par
Each participant stood on his or her dominant leg on a force plate while watching a computer display monitor. While standing on the force plate, each participant was subjected six successive dual task trials in random order (Table \hyperref[tab_1]{2}). Secondary tasks were given to participants via the computer display and center of pressure (CoP) values were recorded during each experiment.\par
Subjects were asked to stand with arms folded and a button in one hand, and to press the button to indicate correct answers to task questions, in order to reduce any confounding effects of articulation. \hyperref[b22]{23} All participants were allowed two practice trials, and each trial began with the 'ready' cue, followed three seconds later by the 'set' cue. When the participants were told to be 'ready', they stood on one leg and held that position for 15 seconds. Participants rested for one minute between trials.\par
A primary task involving proprioceptive vibration stimulation was given to subjects as they stood on one leg for 15 seconds. A vibratory motor (consisting of two vibratory plates: 10 gram, 10 mm) was applied to the skin overlying the Achilles tendon and tibialis anterior on the inferior third of the dominant leg. The vibratory motor (Jahwa Co., Seoul, Korea) produced a stimulus at 8000 rpm and 10 mm of motor diameter. The amplitude and intensity of vibration were controlled by NI LabVIEW 8.0 software. As soon as participants could feel the vibratory stimulation, we determined and set it at supra-threshold intensity.\par
Leg dominance was determined by ball kick tests. During the one-leg stand, the CoP was measured and recorded by a Bertec force plate ® (Bertec Inc., Columbus, USA) system that consists of four road cells and Acquire software version 5.1.\par
A cognitive task was presented to subjects either as a series of random characters (verbal) or shapes (nonverbal) displayed on a computer screen. If the presented character or shape was identical to the one that had been displayed two steps before (two back task), participants were asked to respond by pressing the button. \hyperref[b23]{24,}\hyperref[b24]{25} Cognitive performance was calculated by the number of correct responses and the response time. Sway was measured by the force plates and amplified, and the summation of the sway distance during each trial was recorded as the distance from the center of pressure (DCP). Three trials of DCP data were acquired per each test (trial I\textasciitilde VI), and the mean values were calculated for use in analyses. The recorded force information was used to derive the position time function of the CoP for each trial. 
\section[{Statistical analysis}]{Statistical analysis}\par
Data were collected during three sessions performed on three different days no more than a week apart. Acquire software was used to receive signals from the force plate, and Matlab software (The MathWorks Inc., Natick, USA) was used to analyze the data after filtering with the Butterworth method. \hyperref[b25]{[26]}\hyperref[b26]{[27]} Experimental parameters included vibration (on, off) and task context (no task, verbal task, nonverbal task). The total summations of sway distances of 15 seconds from six different trials each were compared using paired t-tests. Participant performance under different test conditions (no secondary task, verbal/nonverbal tasks without vibration and without secondary task, and nonverbal/verbal tasks with vibration) were compared using one-way ANOVA with a Bonferroni correction. The amount of sway in time sequence was analyzed using one-way repeated ANOVA. After first one second for one leg standing adjustment, the summation of sway distance in the first phase (2\textasciitilde 8 seconds) and late phase (9\textasciitilde 15) were compared using paired t test. To analyze the sway difference in time sequence, the sway differences in each second were acquired and compared with the first one second sway difference (2 \textasciitilde 3 second) using a paird t test. 
\section[{III.}]{III.} 
\section[{Results}]{Results}\par
The total summation of the DCPs is shown in Fig. \hyperref[fig_0]{1}. Participants exhibited more sway when subjected to vibration (434.99±86.73 mm) than in trials with no vibration (416.54±70.97 mm). The addition of verbal and nonverbal secondary tasks decreased sway compared to trials in which participants were not given secondary tasks. We did not observe any significant differences between trials when secondary tasks were provided with or without vibration stimulation.\par
However, during trials in which participants are asked to maintain one-leg stands, greater postural control is needed as time goes by to maintain balance and eventually the participants were forced to break posture. In our study, we observed that after 8\textasciitilde 10 seconds, sway began to increase in such trials. In addition, when secondary tasks were given to standing participants, noticeable changes of sway were observed after 8\textasciitilde 10 seconds (Fig. \hyperref[fig_1]{2}). Therefore, we split the trials into early and late phases with the distinction made when we observed noticeable sway differences (Fig. \hyperref[fig_2]{3, 6}). a) Vibration stimulation study One-leg standing tasks and/or vibration stimulations were given to participants as primary tasks. More sway was noted when supra-threshold intensity vibration stimulations were applied than in trials with one-leg standing alone (Fig. \hyperref[fig_1]{2}). Significant increases in sway were noted during early phases, but none were observed in the late phases. After an adjustment period with one leg standing (1\textasciitilde 2 seconds), more sway was noted during vibration trials than in trials requiring participants to maintain one-leg standing alone, up to 9 seconds However, after 9 seconds, we did not observe any significant differences in sway between trials. In one-leg stand trials, after a short adjustment period with one leg standing, more sway was noted during late phases compared to early phases. In trials combining vibration and one-leg standing, more sway was noted in early phases but then decreased in late phases. The greatest amounts of sway were noted at the end of both types of trials (14 \textasciitilde  15 seconds). 
\section[{b) Secondary tasks given to subjects standing on one leg}]{b) Secondary tasks given to subjects standing on one leg}\par
Sway decreased when subjects standing on one leg were given each secondary task to complete (Fig. \hyperref[fig_2]{3}). We observed significant decreases in sway during the late phases of verbal task trials when compared to trials in which participants were not given secondary tasks. Compared to trials in which participants were given verbal tasks, decreased sway was noted for all phases during nonverbal task trials, but this difference was not significant. Significant decreases in sway were noted for all phases of nonverbal task trials compared to trials in which participants were not given secondary tasks. c) Vibration stimulation applied to subjects given secondary tasks\par
For trials in which subjects were given nonverbal tasks, the application of vibration stimulation increased sway in the late phases, though this increase was not significant (Fig. \hyperref[fig_3]{4}). In trials in which participants were given verbal tasks, the application of vibration stimulation increased sway in the late phases, but this increase also was not significant. In a general sense, vibration did not increase sway when secondary tasks were given in any phases (Fig. \hyperref[fig_4]{5}). 
\section[{d) Secondary tasks given to subjects standing on one leg standing and exposed to vibration stimulation}]{d) Secondary tasks given to subjects standing on one leg standing and exposed to vibration stimulation}\par
The assignment of secondary tasks decreased sway in all phases in subjects standing on one leg and simultaneously exposed to vibration stimulation (Fig.  {\ref 6}). Trials in which subjects were given both verbal and nonverbal tasks resulted in significantly decreased sway in all phases compared to trials with no secondary tasks. Compared to trials in which participants were given verbal tasks, nonverbal task trials resulted in significantly decreased sway within an early phase.\par
IV. 
\section[{Discussion}]{Discussion}\par
The present study implemented a difficult, attention-demanding two-step recall memory task. Postural changes were less apparent in subjects given attention-demanding tasks than in subjects given a primary task only. The effects of attention-demanding tasks are similar to those of external foci. Attentiondemanding tasks divert attention away from postural control, perhaps allowing for more automatic processes and less conscious interference in the control of balance. \hyperref[b27]{28} Previous studies have suggested that requesting participants to focus on body sway induced an increase in sway and hampered neuromuscular efficiency for controlling posture during standing. \hyperref[b28]{29,}\hyperref[b29]{30} This phenomenon has been explained by either high cortical arousal \hyperref[b4]{5,}\hyperref[b15]{16} or automatic reflex caused by the total consumption of attention factors. \hyperref[b3]{4,}\hyperref[b4]{5} Previous studies suggested that stimuli used to test verbal and nonverbal working memory are received and interpreted by different regions of the brain. Based on neural networks, it has been suggested that the verbal/non-verbal dichotomy reflects ventral/dorsal or left/right domain differences in the brain. \hyperref[b19]{[20]}\hyperref[b20]{[21]}\hyperref[b21]{[22]} Prominent activation of the left hemisphere is associated with verbal coding while right prefrontal activation is associated with nonverbal coding. \hyperref[b19]{20} Therefore, differences in the area of cortical stimulation targeted by different tasks may also be related to body sway. In our study, in non-verbal task trials, which are presumably related to right prefrontal activation, directional shifts were less apparent. Perhaps right prefrontal activation allows for more automatic processes to activate and control balance without conscious interference. On the other hand, in verbal task trials, which are presumably related to left hemisphere activation, fewer automatic processes may take place.\par
We found that the application of vibration stimulation induced sway, especially in the early phases during which more automatic factors are activated. This may reflect an increase in body awareness due to the application of supra-threshold degrees of vibration stimulation. McIlroy et al. \hyperref[b16]{17} hypothesized that the processing requirements for postural control vary during the time course of stability recovery and that therefore the related attentional demands also vary. The characteristics of the time courses predicted by stability recovery theory are very intriguing, and it is not clear whether the results of McIlroy's study can be generalized to the control of human posture. \hyperref[b4]{5} Furthermore, no studies have evaluated each phase in sequence after several seconds of postural control. 
\section[{Volume XIII Issue VII Version I}]{Volume XIII Issue VII Version I} 
\section[{Year}]{Year}\par
We observed postural control response during a total period of 15 seconds per trial. We hypothesized that the action of attentional factors becomes more important as time goes on and that body awareness increases after an initial adjustment phase. This is similar to the summation of McIlroy's three phases. We divided each trial into early and late phases. After 8 seconds, the amount of sway in time sequence was significantly increased. From 0 to 8 \textasciitilde 10 seconds, the early phase includes the characteristic adjustment time for one leg standing and our results implicate the involvement of more automatic reflexes during that phase, as stance does not seem to be disturbed by attentional factors. The late phase starts after 10 seconds, during which attention factors become more prominent. When participants were given difficult memory tasks to complete, sway greatly decreased compared to the performance of participants who were not given such tasks (Fig. \hyperref[fig_2]{2, 3, 6}). From these results, it seems possible that postural response can be divided into an early phase and a more attention-demanding late phase.\par
We attempted to induce changes in postural control resource allocation by implementing dual task paradigm comprised of a postural task (one leg standing, vibration stimulation with supra-threshold intensity) and a high demand cognitive working memory task (two-word recall). Cognitive resources play a key role in maintaining postural stability in older adults, which may be due to an age-related decline in sensory and motor function. \hyperref[b7]{8,}\hyperref[b30]{[31]}\hyperref[b31]{[32]}\hyperref[b32]{[33]} In previous studies, older adults are characterized as giving greater priority to the task that they perceive to have greater importance. \hyperref[b33]{34} Given a choice between postural control and a cognitive task, older adults prioritized the former \hyperref[b34]{35} due to the high prevalence of instabilitiy and risk of falling in the elderly. \hyperref[b33]{34} In one study, young adults did not show a decrease in postural sway for either easy or difficult balance tasks. \hyperref[b35]{36} However, Swan et al. \hyperref[b35]{36} demonstrated a decrease in postural sway in older adults during difficult dual task balance conditions, but no sway reduction for relatively easy balance tasks. Since demanding tasks impose greater cognitive loads for older adults than younger adults, older adults may be better subjects in which to evaluate changes in postural control resource allocation.\par
We observed the sequential influence of automatic and attention factors in dual task paradigms in young participants. Our results suggest that optimal training strategies for patients at high risk of injury from falls, such as older adults, should prioritize automatic factors and the maintenance of external focus over postural control.\par
The shortcomings of our study include a relatively small sample size and broad vibration stimulation levels that were not sensitive enough to assess the differing effects of varying, sub-threshold vibration stimulation intensities. Future studies that include more subjects and more standardized levels of difficulty may demonstrate clearer results. Future research should focus not only on a better understanding of dual task on postural control in time sequence, but also on their detailed applications in various rehabilitation settings.\par
In conclusion, both the automatic and attentional factors are required for postural control. We observed that the attentional factors were prioritized for postural control and more dominant in the later phase during a sequential study. By controlling the postural control factors, optimal parameters and training methods for postural control can be designed for use in practical applications.\par
V. 
\section[{Conflict of Interest}]{Conflict of Interest}\par
No potential conflict of interest relevant to this article was reported.  ? *Denotes significant differences between different secondary tasks (none and verbal, none and nonverbal). ? ?Denotes significant differences between none and Vibration. ? viS stands for vibration and one-leg standing as the dual task. 
\section[{VI.}]{VI.}\par
? S stands for one-leg standing only as the primary task.\par
? COP stands for the center of pressure distance.\par
? Denotes significant differences between viS and S. ? S stands for one-leg standing as the primary task.\par
? Sv stands for one-leg standing as the primary task and a verbal task as the secondary task.\par
? Snv stands for one-leg standing as the primary task and a nonverbal task as the secondary task. ? Denotes significant differences between Sv and Snv.\par
? ?Denotes significant differences between viS and viSv. ? viS, stands for vibration and one-leg standing as the primary task.\par
? viSv, stands for vibration and one-leg standing as the primary task and a verbal task as the secondary task. ? viSnv, stands for vibration and one-leg standing as the primary task and a nonverbal task as the secondary task. ? *Denotes significant differences between viS and viSnv.\par
? ?Denotes significant differences between viS and viSv.\par
? ?Denotes significant differences between viSv and viSnv. ? viSnv, stands for vibration and one-leg standing as the primary task and a nonverbal task as the secondary task. ? Snv, stands for one-leg standing as the primary task and a nonverbal task as the secondary task.\begin{figure}[htbp]
\noindent\textbf{1}\includegraphics[]{image-2.png}
\caption{\label{fig_0}Figure 1 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{2}\includegraphics[]{image-3.png}
\caption{\label{fig_1}Figure 2 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{3}\includegraphics[]{image-4.png}
\caption{\label{fig_2}Figure 3 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{4}\includegraphics[]{image-5.png}
\caption{\label{fig_3}Figure 4 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{5}\includegraphics[]{image-6.png}
\caption{\label{fig_4}Figure 5 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{}\includegraphics[]{image-7.png}
\caption{\label{figure7}}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{1} \par 
\begin{longtable}{P{0.42646048109965634\textwidth}P{0.25996563573883164\textwidth}P{0.16357388316151203\textwidth}}
\tabcellsep Verbal task\tabcellsep Nonverbal task\\
Sex(male/female)\tabcellsep 11/9\tabcellsep \\
Age(year)\tabcellsep 28.36±4.03\tabcellsep \\
Height(cm)\tabcellsep 169.05±7.26\tabcellsep \\
Weight(kg)\tabcellsep 59.94±12.16\tabcellsep \\
Accuracy of correction (\%)\tabcellsep \tabcellsep \\
Vibration off\tabcellsep 0.84±0.09\tabcellsep 0.78±0.14\\
Vibration on\tabcellsep 0.83±0.09\tabcellsep 0.80±0.12\\
Reaction time(ms)\tabcellsep \tabcellsep \\
Vibration off\tabcellsep 531.24±74.43\tabcellsep 508.50±77.35\\
Vibration on\tabcellsep 512.53±94.54\tabcellsep 516.31±82.72\\
(p<.05)\tabcellsep \tabcellsep \end{longtable} \par
 
\caption{\label{tab_0}Table 1 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{2} \par 
\begin{longtable}{P{0.159375\textwidth}P{0.07526041666666666\textwidth}P{0.458203125\textwidth}P{0.15716145833333334\textwidth}}
Name\tabcellsep \multicolumn{2}{l}{Abbreviations Condition}\tabcellsep Sequence\\
Trial I Trial II Trial III\tabcellsep S Sv Snv\tabcellsep One leg standing One leg standing+verbal task One leg standing+nonverbal task\tabcellsep Randomized Randomized Randomized\\
Trial I-1\tabcellsep viS\tabcellsep One leg standing+vibration on foot\tabcellsep Randomized\\
\multicolumn{2}{l}{Trial II-2 viSv Trial III-3 viSnv}\tabcellsep One leg standing+vibration on foot+verbal task One leg standing+vibration on foot+nonverbal task\tabcellsep Randomized Randomized\end{longtable} \par
 
\caption{\label{tab_1}Table 2 :}\end{figure}
 			\footnote{© 2013 Global Journals Inc. (US)} 			\footnote{( ) B Sequential Analysis of Postural Control Resource Allocation During a Dual Task Test} 			\footnote{( ) B Sequential Analysis of Postural Control Resource Allocation During a Dual Task Test} 			\footnote{( ) B Sequential Analysis of Postural Control Resource Allocation During a Dual Task Test} 			\footnote{© 2013 Global Journals Inc. (US) © 2013 Global Journals Inc. (US)} 			\footnote{( ) B Sequential Analysis of Postural Control Resource Allocation During a Dual Task Test} 			\footnote{( ) B Sequential Analysis of Postural Control Resource Allocation During a Dual Task Test} 			\footnote{( ) B Sequential Analysis of Postural Control Resource Allocation During a Dual Task Test} 		 		\backmatter   			 
\subsection[{Acknowledgments}]{Acknowledgments}\par
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\end{document}
