THE DYNAMICS OF SENSORY RE-WEIGHTING IN HEALTHY AND FALL-PRONE OLDER ADULTS

Abstract

Purpose/Hypothesis: The purpose of this study was to characterize differences in the time required to adaptively re-weight visual information following a sudden change in visual motion amplitude in healthy young, healthy older, and fall-prone older adults. Number of Subjects: Tree groups of subjects participated: healthy young adults (ages 20–30 yrs, N=21), healthy older adults (ages 70–93 yrs, N=25), and fall-prone elderly (ages 73–92 yrs, N=17). Materials/Methods: Subjects were exposed to two visual motion conditions in which the amplitude of visual motion stimuli switched from low-to-high or high-to-low. The postural sway responses to this change in visual environment were analyzed (COM gain, phase, position variability and velocity variability). We compared absolute levels of COM gain at, and adaptive gain changes across, selected time intervals and between groups. Changes in gain over time were analyzed using non-linear regression to obtain comparative prolonged rates of change before and after each amplitude switch, and rapid rates of change at each switch. Results: Absolute levels of gain, pre-and post-switch, were consistently higher in both older adult groups than in the young, when the stimulus amplitude was high. Gains were frequently higher in the fall-prone versus healthy older and young adults when the stimulus amplitude was low. For all three groups, adaptive sensory re-weighting was refected by gain changes following stimulus motion amplitude changes. Interestingly, all three groups showed evidence of equally rapid re-weighting at the time of the amplitude switch. Between-group differences were apparent in the prolonged changes in gain. Compared to young adults who usually did not reweight further after the initial rapid adaptation, both older adult groups demonstrated continued gradual changes in gain over time periods of 105 seconds. When the stimulus amplitude was high, both older adult groups demonstrated slower prolonged adaptation rates than the young. Rates of prolonged adaptation were not different between the older groups and the young when the stimulus amplitude was low. Conclusions: Rapid re-weighting when the stimulus amplitude suddenly increases is necessary to prevent instability. All three groups demonstrated functionally adaptive responses. However, down-weighting to high-amplitude stimuli is slower and to a lesser extent in healthy and fall-prone older adults compared to young adults. With low-amplitude stimuli, fall-prone older adults continued to have the highest absolute levels of gain and slowest rates of prolonged change compared to healthy older and young adults. Clinical Relevance: Older adults, especially the fall-prone elderly, may experience relatively heightened periods of postural instability and increased risk of falls until sensory adaptation is complete. It may be possible to increase the adaptation rate, and thus reduce the duration of periods of instability, via sensory-challenge balance exercises; future studies are needed to explore this possibility.