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Jean-Marie Hanssens, Philippe Turpin-Lavallée, Roshan Soowamber, Jocelyn Faubert; Visual control of posture as a function of age and cognitive task and its relationship with subjective discomfort. Journal of Vision 2010;10(7):1022. doi: 10.1167/10.7.1022.
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Purpose: Older observers tend to be more visually dependent for posture control. It has been suggested that cognitive load can produce postural instability. Very little is known on the relationship between the cognitive tasks and postural control when posture is strongly influenced by dynamic visual inputs and whether the resulting postural reactivity relates to subjective discomfort. Methods: We used a Full Immersive Virtual Environment to simulate a virtual floor. The floor oscillated from right to left with a temporal frequency of 0.25Hz and a slope of four degrees. Three cognitive tasks were performed during the visual stimulation, (1) no mental task, (2) mental countdown task with a random starting number and (3) a Brooks's modified task that consisted in memorising series of 6 numbers. Two young and older groups were tested and asked to stand still with feet together and arms crossed. Body Sway Amplitude (BSA) and Velocity Root Mean Square (VRMS) were calculated from the electromagnetic trackers positioned on the head. After each random condition, subjects were asked their Subjective Unit Discomfort (USI) on a scale ranging from 0 to10. Results: Results show clear effects of age for all three dependent variables BSA, VRMS and USI. Cognitive task conditions had stronger effects on subjective discomfort (USI) measures (reducing subjective discomfort) than on the behavioral outcome of the visually induced posture movement (BSA and VRMS). Conclusions: Visual stimulation had a greater visual induced impact for older observers. They were more sensitive while performing the most complex mental tasks (Brooks). It suggests that older subjects are more sensitive to visually induced postural reactivity during complex multitask cognitive conditions. Subjective discomfort decreased significantly as the mental tasks were more complex suggesting that a concurrent mental task can reduce discomfort related to visual-vestibular-proprioceptive conflicts but at the risk of increasing visually-induced postural instability.
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