Inclusion Criteria:  20-30 years (Experiment 1 & 2) Mini-Mental State Examination (MMSE) score greater than 24 points (Experiment 1 & 2) ability to walk 10 meters independently without an assistive device (Experiment 2)  Exclusion Criteria:  any neurologic, orthopedic or psychological diseases (Experiment 1 & 2) color vision deficiency (Experiment 1 & 2) had the fall experiences within 6 months before the study (Experiment 2)