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. 2007 Jul;181(1):1-11.
doi: 10.1007/s00221-007-0905-4. Epub 2007 Mar 31.

Regularity of center-of-pressure trajectories depends on the amount of attention invested in postural control

Stella F Donker  1 , Melvyn Roerdink, An J Greven, Peter J Beek
Affiliations

Regularity of center-of-pressure trajectories depends on the amount of attention invested in postural control

Stella F Donker et al. Exp Brain Res. 2007 Jul.

Abstract

The influence of attention on the dynamical structure of postural sway was examined in 30 healthy young adults by manipulating the focus of attention. In line with the proposed direct relation between the amount of attention invested in postural control and regularity of center-of-pressure (COP) time series, we hypothesized that: (1) increasing cognitive involvement in postural control (i.e., creating an internal focus by increasing task difficulty through visual deprivation) increases COP regularity, and (2) withdrawing attention from postural control (i.e., creating an external focus by performing a cognitive dual task) decreases COP regularity. We quantified COP dynamics in terms of sample entropy (regularity), standard deviation (variability), sway-path length of the normalized posturogram (curviness), largest Lyapunov exponent (local stability), correlation dimension (dimensionality) and scaling exponent (scaling behavior). Consistent with hypothesis 1, standing with eyes closed significantly increased COP regularity. Furthermore, variability increased and local stability decreased, implying ineffective postural control. Conversely, and in line with hypothesis 2, performing a cognitive dual task while standing with eyes closed led to greater irregularity and smaller variability, suggesting an increase in the "efficiency, or "automaticity" of postural control". In conclusion, these findings not only indicate that regularity of COP trajectories is positively related to the amount of attention invested in postural control, but also substantiate that in certain situations an increased internal focus may in fact be detrimental to postural control.

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Figures

Fig. 1
Fig. 1
An example of the surrogate analysis, as applied to all time-series. Surrogate data of a measured COP trajectory (upper panel) were constructed by randomizing the Fourier phase (middle panel) and the temporal order (lower panel)
Fig. 2
Fig. 2
Interindividual averages, collapsed over x and y time-series, of sample entropy (SEn), standard deviation (σ), sway-path length of the normalized (by the standard deviation) posturogram (SPn), local stability (λmax) and dimensionality (D2) for the four experimental conditions: standing with eyes open (EO-ST), eyes closed (EC-ST), eyes open while performing a cognitive dual task (EO-DT) and eyes closed while performing a cognitive dual task (EC-DT). The asterisks indicate significant (P < 0.05) differences between conditions
Fig. 3
Fig. 3
Grand means, collapsed over all conditions, planes and participants, of sample entropy, scaling exponent, dimensionality and local stability for the original (OR) COP time-series and their phase-randomized (PHASE) and time-randomized (TIME) surrogate counterparts. The error bars represent the interindividual standard deviations. The asterisks represent significant (P < 0.05) differences between the surrogate data and the original time-series
See this image and copyright information in PMC

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