The Dynamics of Multi-sensory Re-weighting in Healthy and Fall-prone Older Adults

Abstract

Multi-sensory re-weighting (MSR) is an adaptive process that prioritizes the visual, vestibular and somatosensory inputs that provide the most reliable information when environmental conditions change. This process is thought to degrade with increasing age, and to be particularly deficient in fall-prone versus healthy older adults. This dissertation presents three studies designed to investigate age- and fall-related changes in MSR.

The first study examined the assumption of impaired MSR in healthy and fall-prone older adults using a two-frequency touch/vision experimental design with stimuli at varying amplitudes. Both healthy and fall-prone older adults demonstrated the same pattern of adaptive gain changes as healthy young adults. No group differences in the overall levels of vision and touch gain were found. These results suggest that, for small amplitude vision and touch stimuli, the central sensory re-weighting adaptation process remains intact in healthy and fall-prone older adults.

In the second study the effects of a sensory-challenge balance exercise program on laboratory measures of MSR and clinical measures of balance were investigated. Following the intervention the normal adaptive pattern of gain change was unaltered, while declines in overall vision and touch gains that reflect down-weighting of the sensory stimuli were seen. Improvements in four clinical balance measures were observed. These findings indicate that MSR processes in fall-prone older adults are modifiable, that sensory challenge balance exercises may facilitate the ability to down-weight unstable sensory inputs, and that these effects may generalize to other components of balance.

A third study explored the dynamics of sensory re-weighting in healthy and fall-prone older adults. Absolute levels of gain, and the rate of adaptive gain change, were examined before and after large changes in visual motion stimulus amplitude. Compared to young adults, gains in both older adult groups were higher when the stimulus amplitude was high, and gains in the fall-prone elderly were higher than both other groups when the stimulus amplitude was low. Both older groups demonstrated slowed sensory re-weighting over prolonged time periods when the stimulus amplitude was high. These results reflect age- and fall-related changes in the extent and rate of down-weighting unstable visual inputs.