ReviewAge-related changes of visual contribution in posture control
Introduction
Numerous works [1], [2], [3], [4], [5], [6], [7], [8] highlighted the importance of visual information in postural control among the elderly. Various methods were used to study the role of vision in postural stabilization. Some authors evaluated the difference in postural stability under “eyes closed” and “eyes open” conditions [2], [5], [7]. Others disturbed the visual environment and assess the effect of this disturbance on postural stability [6], [9]. According to the results of these studies, it seems that postural instability induced by the disturbance of visual information increases with age, especially if proprioceptive information is deficient, for example, when the ground is unstable [2], [6], [7].
However, as several authors suggested [10], [11], because of the experimental methodology used in most of these experiments, the postural behaviour measured is barely representative of the behaviour that would be adopted by a subject outside the laboratory. In most cases, subjects were instructed to “stand upright and keep still”. These were “postural” tasks in which the observer's only goal was to maintain a stable posture while one or more of the postural control system's sensorial inputs were stimulated. By considering only this cause-and-effect relationship between the content of the visual environment and control of postural oscillations, these authors failed to take account of the fact that maintaining a stable posture is not an end in itself but enables various others tasks to be performed. Although the control of postural oscillations does depend on environmental information, it is also constrained by the others non-postural tasks performed by the subject: for example, a visual task such as reading requires accurate focusing, which is facilitated by minimizing postural oscillations, whereas looking at a landscape does not require the same degree of control over bodily movements.
As far we know, no study has specifically investigated the role of vision in posture control for people aged between 40 and 60. This population is indeed less concerned by falls but we guess that the greater postural sensitivity to visual surroundings in elderly may be the result of a progressive evolution of the postural control system, beginning before 60 years old. In a population aged over 60, Lord and colleagues [12] showed that multifocal glasses wearers were more likely to fall than nonmultifocal glasses wearers. Wearing of multifocal lenses begin from presbyopia, that is from 40 to 45 years old. So, subjects aged from 40 to 45 wearing multifocal lenses might also be concerned in “real life” by unsteadiness and sometimes falls, especially if they are sensitive to their visual environment in postural control.
That is why the present study aimed to determine whether the postural behaviour of subjects aged between 40 and 60 was more sensitive to disturbance of the visual frame of reference than young people, while performing a visual task. Our first hypothesis was that tilting the visual frame of reference would result in a reorientation of the body and an increase in postural instability. Our second hypothesis was that the postural consequences of modifying the visual frame of reference would be greater for subjects aged between 40 and 60 than for younger ones.
We also speculated that the properties of the visual task performed might modulate the postural effect induced by disturbance of the visual frame of reference. For this reason, two different visual tasks were considered. One visual task consisted of recognizing objects presented in the peripheral visual field, and the other one consisted of detecting the presence of a target among a series of small objects placed in the centre of the field of vision. During the latter task, visual attention had to be focused on a far more restricted field than during the visual recognition task. We then hypothesized that during the central detection task, subjects would be less sensitive to the tilt of the visual frame as the latter would be situated in the peripheral vision field.
Section snippets
Subjects
Forty-three subjects of 21–31 years of age (mean age 26.0 ± 2.6 years; 20 men; 23 women) and 31 subjects aged between 44 and 60 (mean age 49.7 ± 4.2 years; 15 men; 16 women) took part in the study. They did not wear glasses and their refractive error ranged between −1.50 and +1.50 diopters. Persons presenting one or more of the following criteria were not included in the study: strabismus; amblyopia; anisometropia ≥1.50 diopters; astigmatism ≥1.00 diopter; a history of falls; binocular visual
Bodily orientation
The mean bodily orientation as a function of age and the tilt of the frame is shown in Fig. 2.
Subjects were observed to reorient themselves in the direction in which the frame was tilted. This reorientation was found to be statistically different from 0 for the 44–60 years old group (44–60 group) both when the frame was tilted to the left (Student, t = −2.44, p = 0.021) and to the right (Student, t = 3.06, p = 0.005). In the case of the 21–31-year-old subjects (21–31 group), this bodily reorientation
Bodily orientation
The illusion of a tilt of one's head and body induced by perception of a tilted frame is a well-known phenomenon [15], [16]. This illusion results in reorientation of the body towards the apparent gravity vertical, i.e. in the same direction as the tilt of the frame. Our results showed a greater reorientation of the body for the 44–60-year-old subjects, indicating that they were, from a postural point of view, more sensitive to modification of the visual environment than younger subjects. The
Acknowledgement
We would like to thank Natalia DOBRESCU for software development.
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