Introduction 2002 2001 2004 2000 2001 1993 2004 2000 2006 2003 2003 1999 2000 2003 2000 2004 2006 2001 1991 2001 2002 1993 1993 2005 2000 1995a 2002 2006 2002 1995 1998 1993 2001 2006 2006 2006 1999 1998 2000 2006 1 1993 1983 1988 1995 2006 2002 1999a Table 1 Abbreviations and meaning of interest of the calculated COP measures Variable Meaning of interest Sample entropy, SEn Negatively related with the regularity of COP trajectory σ Positively related with the variability of COP trajectory n −1 Positively related with the curviness of COP trajectory max Negatively related with the local stability of COP trajectory D 2 Positively related with the number of active control variables Scaling exponent, α   Methods Participants and procedures 1 After the local ethics committee had approved the study, all participants gave their informed consent prior to their participation. Data analysis 2006 x y 2 σ x y x y n n σ 2006 1 Sample entropy, SEn 1996 2002 1991 2002 2000 1991 r M M 2002 2000 3 max 4 1996 1993 max max 1992 D 2 1983 5 2001 Scaling exponent, α 1995 6 1995 1995 Surrogate analysis 1992 1 Fig. 1 upper panel middle panel lower panel  Statistical analysis 7 η 2 η 2 f \documentclass[12pt]{minimal}
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\begin{document}$$ f = {\sqrt {\frac{{\eta ^{2} }} {{1 - \eta ^{2} }}} }. $$\end{document} f 1988 Results The result section is organized as follows. We first report possible differences in the dependent variables between the eyes open and eyes closed conditions (i.e., main effect of vision, hypothesis 1). Second, we describe the effect of experimentally withdrawing attention from postural control by comparing single task and dual task conditions (i.e., main effect of dual task, hypothesis 2). Third, we report whether significant vision × dual task interaction effects were present. Finally, we present the effects of plane, which may reveal possible directional differences in control. In this context, we also report the effects of randomization on the dependent variables to ensure that the observed changes in the dynamics of COP trajectories were genuine effects. 2 x y F P f 2 Table 2 x y σ n λ max  Condition Mean Vision (EO vs. EC) Condition Mean Dual task (ST vs. DT) a F P ƒ F P ƒ F P ƒ SEn EO 0.72 3.83 * 0.36 ST 0.70 1.45 ns 0.25 6.72 <0.05 0.48 EC 0.70 DT 0.72 σ EO 3.52 11.82 <0.005 0.64 ST 3.89 2.45 ns 0.29 3.18 =0.085 0.33 EC 4.01 DT 3.64 n EO 4.27 5.28 <0.05 0.43 ST 4.13 13.57 <0.005 0.68 6.98 <0.05 0.49 EC 4.52 DT 4.66 λ max EO 1.56 36.23 <0.001 1.12 ST 1.71 0.10 ns 0.06 4.26 <0.05 0.38 EC 1.88 DT 1.73 D 2 EO 2.23 23.58 <0.001 0.90 ST 2.20 45.70 <0.001 1.26 6.15 <0.05 0.46 EC 2.48 DT 2.51 α EO 1.39 13.70 <0.001 0.69 ST 1.39 24.57 <0.001 0.92 1.80 ns 0.25 EC 1.34 DT 1.35 * F P F P a  2 Fig. 2 x y σ n λ max D 2 asterisks P  Increased postural task difficulty (EO vs. EC) 2 σ D 2 n λ max Decreased attention to posture (ST vs. DT) x y σ n D 2 2 2 Vision × dual task interaction effects 2 SEn, σ n 2 σ n 2 2 2 n σ σ 2 λ max D 2 2 D 2 Effects of plane and randomization σ λ max F P f F P f D 2 F P f F P f F P f 2 3 F P f F P f 8 F P f F P f Fig. 3 error bars asterisks P  Discussion The present experiment was conducted to investigate the role of attention in the regulation of posture. Specifically, we examined whether an increase in postural sway regularity (i.e., as indexed by a decrease in SEn) is representative of an increase in cognitive investment in postural control. We hypothesized that COP trajectories become more regular (i.e., SEn decreases) when task difficulty is increased (EC vs. EO) and, conversely, become less regular (i.e., SEn increases) when an attention-demanding cognitive dual task is introduced (DT vs. ST). We further expected that these changes in regularity of COP fluctuations would be accompanied by changes in variability, local stability, sway-path length, dimensionality and scaling exponent reflecting functional modifications of postural control. For the proper interpretation of the present findings, however, it was necessary to ascertain that the observed structure (and changes herein) of the COP fluctuations did not result from noise, but was indeed brought about by deterministic processes. Therefore, we will first discuss the results of the surrogate analyses before discussing the respective effects of vision, dual task and plane on the dynamical structure of postural sway. Surrogate analyses 1993 λ max λ max λ max 1992 2004 1999 1995b Increasing postural task difficulty (EO vs. EC) λ max 2003 2002 2 2003 2006 2002 2006 σ λ max D 2 1991 1986 1998 1985 2001 1993 2001 Withdrawing attention from posture The performance of a concurrent dual task led to changes in scaling exponents and an increase in dimensionality, reflecting cognition-invoked adjustments of postural control. These changes under dual task performance may have served to enrich the information captured in sway fluctuations without increasing the amount of sway (i.e., variability remained unaltered). This interpretation is amplified by the observation that the sway-path length of the normalized posturogram increased, indicating more twisting and turning in the COP trajectories. Interestingly, despite the fact that attention was withdrawn experimentally from postural control, local stability remained unaltered. In contrast to what we expected, no main effect of dual task was found for SEn. However, this finding does not necessarily militate against the proposed relation between the regularity of COP fluctuations and the amount of attention directed to postural control, as will be argued in the following subsection. Vision × dual task interaction 2 2 2 2002 2001 2002 2004 σ σ 2 2002 2002 2005 2000 2003 1999b Sagittal versus frontal plane 2006 2003 2003 Conclusion 2001