Inherent with intricate allocation of brain resource and attention shift, postural-suprapostural task is defined as postural control takes place while the other concurrent task is being performed. Recent studies have indicated that attentional focusing alters attention allocation in the brain as well as motor performance of a postural-suprapostural task. Also, most studies have demonstrated benefits of inducing an external focus relative to internal focus for motor performance. However, postural difficulty might be a critical factor for choosing an appropriate focusing strategy, and the strategy used in a particular difficulty level do not always generalize to other difficulty levels. Besides, despite a lot of studies have been done, current findings are confined to behavioral observations in young healthy adults for lacking direct neural evidence. With the uses of H-reflex, event-related potential and behavioral measures, the purpose of the 3-year research project is to investigate the differences in performance quality and intrinsic neural mechanisms of a postural-suprapostural task for older adults and patients with Parkinson's disease, by adopting external and internal strategies for suprapostural task under different posture difficulties. In the first year, the investigators will characterize suprapostural focusing effect on reciprocity of a postural-suprapostural task, with a special focus on modulation of motoneuron excitability in sitting and standing posture for healthy young and older adults using H-reflex technique. In the second year, the adaptive selection of suprapostural focusing strategy will be investigated for patients with Parkinson's disease by varying the standing surface (stable/unstable surface). In the third year, the investigators will investigate suprapostural focusing effect on brain resource allocation for patients with Parkinson's disease, in light of event-related potential and movement-related potential. The present project is expected to have significant contributions not only to gain a better insight to neural correlates of concurrent postural and motor suprapostural tasks with internal/external focusing strategy under different posture difficulty, but to optimize treatment strategy for older adults and patients with Parkinson's disease with balance or multi-tasking disturbances.

Inclusion criteria

• Independent standing balance on air pillow at least 20 sec
• No neurologic or orthopedic disorder

Exclusion criteria

• Had relative experience before
• Pregnant women