Despite the documented health hazards associated with stair descent, the mechanisms of falling on stairs remain relatively unexamined. The objectives of this study were to define kinematic variables that could be used to describe foot-stair spatial relationships during the mid-stair phase of stair descent, and to investigate the effects of various visual and environmental conditions on those variables in a group of 36 healthy women between the ages of 55 and 70. Foot clearance and foot placement were measured through high-speed film analysis. Clearance between the foot and the stair during swing phase was small under all visual conditions. Degraded visual acuity had a significant effect on cadence, foot placement, and foot clearance, but visual surround conditions did not. The kinematic variables used in this experiment may be helpful in future studies to assess the results of interventions aimed at reducing the frequency of falls on stairs.
Experimental evidence is presented that the focus of the eye tends to return passively to an individually characteristic intermediate resting position or dark-focus whenever (1) the stimulus to accommodation is degraded or (2) when the quality of the image is independent of focus. Based on measurements of the dark-focus with a laser optometer, it is possible to predict the magnitude of night, empty field, and instrument myopia on an individual basis. The role of the intermediate dark-focus as a factor in accommodation also provides an explanation for the paradoxical variation of visual acuity with observation distance. Applications to night myopia and night driving are described, and implications for clinical practice are discussed.
The phenomenon of night myopia, the tendency to overaccommodate for distant objects as luminance is decreased, results from the passive return of accommodation to an individually determined intermediate resting or dark focus. More generally, accommodation is viewed as a compromise between the subject's individual resting focus and the accommodative stimulus. Under optimum viewing conditions, accommodation tends to correspond to the distance of the stimulus, but is biased progressively toward the dark focus as the adequacy of the accommodative stimulus is degraded by decreased luminance. Control experiments suggest that optical aberrations are not major factors that contribute to this effect.
Automobile accidents among older adults may be related to difficulties in judging the speed of other vehicles. To examine this possibility, 3 groups of observers in the young adult, middle-aged, and older adult age ranges were asked to estimate the velocity o fan isolated automobile traveling at 15-50 mph (24-80 kph). Across all age groups, perceived and actual velocity were related by a power function with an exponent of 1.36. Age was significantly and positively correlated with intercepts, but negatively correlated with exponents; that is, older observers showed less sensitivity to changes in actual velocity. Results bear on the issues ofontogenetic changes in accident involvement and sensitivity to motion.Although nonvisual factors play a role in effective driving performance, driving is predominantly a visual task. Hills (1980) estimated that more than 90% of the information impinging on the driver is visual in nature. Among the many visual tasks required by the driver, estimating the velocity of other vehicles appears to be one of the more salient. Velocity estimation plays a role in various traffic maneuvers, including intersection crossing, merging, and overtaking. Underestimations of vehicle velocity may place drivers attempting these maneuvers at greater risk of accident involvement. This may be particularly true for older persons, in whom a disproportionate number of both pedestrian and driver accidents may involve errors of velocity estimation (Faulkner, 1975; Hills & Johnson, cited in Hills, 1980;Sheppard & Pattinson, 1986). The data reported in this article were gathered to determine whether the ability to estimate vehicle velocity changes systematically over the adult life span. Speed Estimation and ScalingThe manner in which human observers process velocity information remains unclear, but evidence suggests that such information is used to make decisions about some driving maneuvers. For example, increasing traffic velocity leads to reductions in the minimum time gaps allowed when crossing against traffic flow (Bottom & Ashworth, 1978). Velocity estimation is often inaccurate, however. Many drivers err in estimating the last moment to safely overtake a vehicle in the presence of oncoming traffic (Jones & Heimstra, 1964;Kaukinen, 1972), and at This research was supported by a grant from the National Institute on Aging (AG-00110-02).We thank the Pennsylvania Transportation Institute for the use of their test track; the Oneonta, New York, Police Department for the use of their radar unit; and especially Joyce A. Blake-Guzy for her invaluable assistance in data collection.Lawrence T. Guzy is now at the State University of New York at Oneonta.Correspondence concerning this article should be addressed to Charles T. Scialfa, who is now at the Department o fPsychology, University of Calgary, Calgary, Alberta, Canada T2N 1N4. 60 least some authors (Leibowitz, 1985;Mackie, 1972) have argued that overtaking accidents are due in part to misjudgments of the oncoming vehicle's velocity. Such an argument raises...
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