Sway regularity reflects attentional involvement in postural control: Effects of expertise, vision and cognition

Abstract

We examined the time varying (dynamic) characteristics of center-of-pressure (COP) fluctuations in a group of 14 preadolescent dancers and 16 age-matched non-dancers. The task involved maintaining balance for 20 s with eyes open or eyes closed, and with or without performing an attention demanding cognitive task (word memorization). The main finding was that the time-dependent structure of the COP trajectories of dancers exhibited less regularity than that of non-dancers, as evidenced by a higher sample entropy (decreased statistical regularity). COP irregularity also increased during secondary task performance but decreased during standing with eyes closed. The combined findings indicate that the degree of attentional involvement in postural control – as reflected in the COP dynamics – varies along an automaticity continuum, and is affected by relatively stable subject characteristics (expertise) and more transient factors related to the attentional requirements of the task at hand.

Introduction

Standing balance was recently defined as “the ability to stand with as little sway as possible” ([6], p. 501). This definition makes intuitive sense because postural sway often increases with pathology [7], [19], aging [11], or when a sensory challenge [4] is added to the standing task. On the other hand, it is highly unlikely that a situation of zero sway is desirable for postural control given that human movement is intrinsically variable and successful regulation of posture requires detection of postural sway [14].

In general, the dynamical structure of postural sway, as captured among others by the statistical regularity, dimensionality or recurrence characteristics of center-of-pressure (COP) time series, appears indicative of the underlying motor control [18], [19]. A recent measure to capture the dynamical structure of physiological data, including postural sway [3], [4], [19], is sample entropy, which indexes the statistical regularity of the time series, with low (high) values indicating more (less) regularity [10], [16]. Roerdink et al. [19] found that individuals who had suffered a stroke exhibited overall more regular sway during quiet standing than controls (comparable effects were found for a group of children with cerebral palsy [3]), and that sway regularity decreased in the course of rehabilitation. In addition, performing a mental arithmetic task during standing resulted in more irregular sway, while closing the eyes had the opposite effect. Comparable task effects were found in healthy young adults [4]. A direct relation between COP regularity and the amount of attention invested in posture was therefore proposed [3], [4], [19]. In brief, increased COP regularity in stroke patients or cerebral palsy children relative to healthy controls was interpreted as a reflection of a greater cognitive involvement in maintaining balance, whereas the increased COP irregularity with rehabilitation (in the case of stroke) was taken to suggest a progressive reduction in the amount of attention invested in posture (or inversely, increased level of automaticity of postural control) [19]. Moreover, COP regularity decreased when attention was experimentally diverted from posture [3], [4], [19], which is fully in line with the proposed relation between COP regularity and the level of automaticity of postural control.

The abovementioned studies suggest that COP regularity may be used as a marker for the amount of attention invested in postural control. In the present study we extended this line of research to the other end of the presumed automaticity continuum, namely to individuals with superior postural skills, that is, ballet dancers [2], [23]. We hypothesized that dancers, compared to non-dancers, would exhibit (1) a smaller amount of sway (i.e. smaller sway area and sway amplitude) possibly reflecting their greater postural stability [20], [23] and (2) more irregular postural sway, indicative of a greater level of postural automaticity. We further expected that removal of vision, which is an important information source for postural control, would lead to greater sway regularity. Finally, we expected that performing a secondary cognitive task, which was assumed to divert attention from postural control, would lead to more irregular postural sway. We adopted a complete factorial design, which allows us to test whether the expected attentional effects would combine in an additive (independent) or non-additive (interactive) manner. Specifically, in line with [4] we anticipated that diverting attention from postural control in standing with eyes open would have a smaller effect on sway regularity of ballet dancers than controls, reflecting the expected greater level of automaticity of dancers.