Postural control and head stability during natural gaze behaviour in 6- to 12-year-old children

We investigated how the influence of natural exploratory gaze behaviour on postural control develops from childhood into adulthood. In a cross-sectional design, we compared four age groups: 6-, 9-, 12-year-olds and young adults. Two experimental trials were performed: quiet stance with a fixed gaze (fixed) and quiet stance with natural exploratory gaze behaviour (exploratory). The latter was elicited by having participants watch an animated short film on a large screen in front of them. 3D head rotations in space and centre of pressure (COP) excursions on the ground plane were measured. Across conditions, both head rotation and COP displacement decreased with increasing age. Head movement was greater in the exploratory condition in all age groups. In all children—but not in adults—COP displacement was markedly greater in the exploratory condition. Bivariate correlations across groups showed highly significant positive correlations between COP displacement in ML direction and head rotation in yaw, roll, and pitch in both conditions. The regularity of COP displacements did not show a clear developmental trend, which indicates that COP dynamics were qualitatively similar across age groups. Together, the results suggest that the contribution of head movement to eye-head saccades decreases with age and that head instability—in part resulting from such gaze-related head movements—is an important limiting factor in children's postural control. The lack of head stabilisation might particularly affect children in everyday activities in which both postural control and visual exploration are require

Uniform and safe metaclass composition �

www.elsevier.com/locate/cl In pure object-oriented languages, classes are objects, instances of other classes called metaclasses. In the same way as classes define the properties of their instances, metaclasses define the properties of classes. It is therefore very natural to wish to reuse class properties, utilizing them amongst several classes. However this introduced metaclass composition problems, i.e., code fragments applied to one class may break when used on another class due to the inheritance relationship between their respective metaclasses. Numerous approaches have tried to solve metaclass composition problems, but they always resort to an ad-hoc manner of handling conflicting properties, alienating the meta-programmer. We propose a uniform approach that represents class properties as traits, groups of methods that act as a unit of reuse from which classes are composed. Like all the other classes in the system, metaclasses are composed out of traits. This solution supports the reuse of class properties, and their safe and automatic composition based on explicit conflict resolution. The paper discusses traits and our solution, shows concrete examples implemented in the Smalltalk environment Squeak, and compares our approach with existing models for composing class properties

Abstract A Browser for Incremental Programming ⋆

Much of the elegance and power of Smalltalk comes from its programming environment and tools. First introduced more than 20 years ago, the Smalltalk browser enables programmers to “home in” on particular methods using a hierarchy of manually-defined classifications. By its nature, this classification scheme says a lot about the desired state of the code, but little about the actual state of the code as it is being developed. We have extended the Smalltalk browser with dynamically computed virtual categories that dramatically improve the browser’s support for incremental programming. We illustrate these improvements by example, and describe the algorithms used to compute the virtual categories efficiently

The influence of gaze behaviour on postural control from early childhood into adulthood

In the present study we aimed to track the influence of natural gaze behaviour on postural control from early childhood into adulthood. We measured time series of centre of pressure (COP) as well as head movement in three children groups aged around five (n=16), eight (n=15), and eleven (n=14) and in one group of young adults (n=15) during quiet stance with eyes closed, gaze fixed on a dot, and with gaze shifts between two dots. We adopted magnitude and irregularity of COP displacement as indexes of postural control and cross correlation between COP displacement and target oscillation as an index of the dynamical coupling between the postural and visual systems. Magnitude and irregularity of COP displacement decreased with age, which suggests a steady improvement of postural control from five to beyond eleven years of age. Cross correlations were weak and relative phases highly variable across age groups. Across conditions, and most prominently in the gaze shift conditions, 5-year-olds showed both more head movement and lower postural stability than other age groups. Finally, only in 5-year-olds did we find a marked deterioration of postural stability with gaze shifts. We thus conclude that excessive head movement, particularly during gaze shifts, may be a primary cause of lower postural stability in young children compared to older children and adults

Zwei neue Brenthiden aus der Sammlung der Exp. Stat. Of the Hawaiian Sugar Planters' Association

Encapsulation in object-oriented languages has traditionally been based on static type systems. As a consequence, dynamically-typed languages have only limited support for encapsulation. This is surprising, considering that encapsulation is one of the most fundamental and important concepts behind object-oriented programming and that it is essential for writing programs that are maintainable and reliable, and that remain robust as they evolve. In this paper we describe the problems that are caused by insufficient encapsulation mechanisms and then present object-oriented encapsulation, a simple and uniform approach that solves these problems by bringing state of the art encapsulation features to dynamically typed languages. We provide a detailed discussion of our design rationales and compare them and their consequences to the encapsulation approaches used for statically typed languages. We also describe an implementation of object-oriented encapsulation in Smalltalk. Benchmarks show that extensive use of objectoriented encapsulation results in a slowdown of less than 15 per cent