This experiment was designed to determine the effect of vertically yoked prisms on standing balance. The BALANCE SYSTEM from Chattecx Corporation was utilized to quantify center of balance position in the leftward or rightward direction, forward or rearward direction, and the amount of variability in position, postural sway. Test lenses for the conditions of plano, 12 prism diopters yoked bases-up, and 12 prism diopters yoked bases-down were fitted over the habitual lens prescriptions of 36 young adult subjects with normal vision. Pressure sensitive transducers under the ball and heel of each foot provided data at a rate of 100 samples per second during two 10-second test periods; 0-10 sec and 30-40 sec immediately after opening the eyes and fixating a muscle light at 5 meters. No differences in right/Ieft center of balance. were found in any comparative analysis. Instrument test-retest reliability was established during the 0 to 10 sec time period for right/Ieft center of balance, forward/rearward center of balance and postural sway. Differences in forward/rearward center of balance occurred during the 30 to 40 sec time period. Significant rearward shifts occurred with base-down yoked prisms when compared to base-up and plano during both the 0-1 0 sec and 30-40 sec periods. Significant increases in postural sway occurred with base-down compared with plano during the 0-10 sec epoch and base-up compared to plano during the 3D-40 sec epoch. During post-prism plano conditions, significant rearward shifts occurred With post-base-up plano and post-base-down plano compared to plano during the 30 to 40 sec period. Additionally, post-prism plano conditions showed a significant increase in postural sway during 0 to ~ 0 sec and the 30 to 40 sec periods. During the 0 to 10 sec period immediately after opening eyes, objective measurement indicates base-down vertically yoked prisms produce a significant rearward shift in and increased postural sway when compared to plano lenses. These results further support vision's strong role in standing balance.
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