Introduction 1984 1997 2003 1999 2002 2007 1983 1996 2006 2002 1993 1997 1999 2001 2002 2005 2006 2006 1996 1997) 2006 2007 2007 2006 2001 2001 2002 2003 1998 1991 2002 2002 1997 2003 2005 Motivated by this combination of theoretical and practical considerations, the aim of the present study was (1) to compare postural sway fluctuations of children with CP with that of typically developing (TD) children and (2) to investigate the effect of visual information on postural sway in CP children and in TD children. For this purpose, the availability of visual information was manipulated relative to standing with eyes open (EO condition) by means of visual deprivation (standing with eyes closed, EC condition) and external provision of COP feedback (FB condition). Two kinds of measures were used to analyse the COP trajectories recorded during those conditions, the one scale-dependent and the other scale-independent. The conventional, scale-dependent measures pertained to the ‘amount of sway’, whereas the more recent scale-independent measures pertained to the ‘regularity of sway’ (see Methods section for details). We hypothesised that (1) compared to TD children, CP children exhibit a greater amount of sway and more regular sway and (2) concurrent visual feedback (providing a functional external context for postural control, creating a more external focus of attention) decreases both the amount of sway and sway regularity in TD and CP children alike, while closing the eyes (increasing task difficulty through deprivation of visual information, promoting a less automatic mode of postural control) has opposite effects. Methods Participants Ten children with congenital hemiplegia or cerebral palsy (CP children; six boys, four girls; mean age = 7 years, range = 5–11 years), reduced to nine in the analyses (see below) and nine typically developing children (TD children; five boys, four girls; mean age = 8 years, range = 5–11 years) without known motor impairments or movement-related disorders volunteered to participate in the experiment. All children and their parents gave their informed consent prior to participation. The experiment was carried out in compliance with the Helsinki Declaration and was endorsed by the VU University Medical Centre Amsterdam. Procedure and apparatus 1 Children participated in three conditions that were presented in random order: standing upright with eyes open (EO condition), standing upright with eyes closed (EC condition) and standing upright with visual COP feedback (FB condition). As explained, the FB condition was introduced to provide an external functional context for postural control, creating a more external focus of attention. To this end, a 2.5 m × 2.5 m vertical screen was placed in front of the child at a distance of 1.3 m. On this screen the force plate was depicted as a 40 cm × 40 cm square onto which the COP was projected as a red dot. The children were asked to keep this dot within the target area, which consisted of a 4 cm × 4 cm square. Deprivation of visual postural sway information was effectuated by blindfolding the children in the EC condition. Trial duration was 60 s. Participants performed each condition once, with the proviso that, as part of a separate study, five CP children performed each condition twice (the first trial was used for further analyses). Because in the latter set of recordings trial duration was 30 s, the first 3,000 samples of all trials were used in the analyses to avoid differences in the reliability of the dependent measures. One CP child did not want to be blindfolded and was therefore excluded from further analyses, which rendered the number of CP children equal to the number of control subjects. Data analysis x  y x  y r \documentclass[12pt]{minimal}
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\begin{document}$$ r_{i} = {\sqrt {x_{i} ^{2} + y_{i} ^{2} } } $$\end{document} 1996 i  N N 1996 r r mean \documentclass[12pt]{minimal}
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\begin{document}$$ {\text{SP}} = {\sum\limits_{i = 1}^{N - 1} {{\sqrt {(x_{{i + 1}} - x_{i} )^{2} + (y_{{i + 1}} - y_{i} )^{2} } }} } $$\end{document} x y σ x σ y n n n 2007 2000 2002 2006 1991 m m 2000 2002 r m 2006 2002 2 2000 1992 2004 Statistics ε p 2 P P t Results F P ε p 2 1 1 r mean P r mean n 1 n n n t (17) P n t (17) P Table 1 r mean n  Group Condition Interaction F (1, 16) a P ε p 2 F (2, 32) a P ε p 2 F (2, 32) a P ε p 2 r mean 4.01 0.062 0.20 1.35 ns 0.08 1.11 ns 0.06 SP 1.86 ns 0.10 0.13 ns 0.01 0.13 ns 0.01 n 11.11 <0.005 0.41 5.12 <0.02 0.24 2.04 ns 0.11 SEn 13.49 <0.005 0.46 5.53 <0.01 0.26 3.23 0.056 0.17 ns a Fig. 1 CP TD black grey bars EO EC FB upper panels r mean SP lower panels n Error bars  1 P t (17) P t (17) P 1 F (2, 106) P ε p 2 t t (53) P Discussion In the present study we compared posturograms of children with CP with those of TD children under three conditions. Across conditions, we expected the posturograms of CP children to reflect a greater amount of sway and more regular sway than those of TD children (Hypothesis 1). Furthermore, in line with the recently postulated relation between the amount of attention invested in posture and COP regularity, we expected that concurrent visual COP feedback, providing a functional external context for postural control leading to a more external focus of attention, would result in a decrease in both the amount of sway and sway regularity, whereas the EC condition, in which visual information is absent, would have opposite results (Hypothesis 2). The data were largely in agreement with both hypotheses, as will be discussed next. Furthermore, the finding that SEn estimates were significantly increased after time- and phase-randomisation of the original data ruled out the possibility that the regular or deterministic properties observed in the original data occurred by chance. CP children exhibit greater and more regular sway than TD children 1 P r mean 1 1 1999 2002 2003 2002 2007 2005 2006 2006 1988 1995 1996 1997 2002 2003 2002 Effect of visual information 1991 2002 2001 1 2000 2003 2005 2007 2007 2006 2003 2002 2001 2001 1998 1991 2002 1 2002 1997 1999 2007 2002 2006 2005 2003 2005 Conclusion The findings of the present study indicate that postural control in CP children is deteriorated compared to TD children, most likely due to a slowed development of neural control mechanisms in CP children. Moreover, visual deprivation (EC condition) influenced sway characteristics differently in CP and TD children, testifying to the altered nature of postural control in CP children. In line with the proposed relation between the amount of attention invested in posture and COP regularity, we found in CP and TD children alike that, compared to the EO and EC condition, COP trajectories were less regular in the presence of visual COP feedback, corresponding to a functional context mediated external shift in the focus of attention. Future studies should examine the merits of adopting an external focus of attention to enhance postural control in CP children in training or therapy, for example by providing visual COP feedback leading to a posture specific functional task or by introducing an attention-demanding secondary cognitive task.