Effects of Parkinson's disease of heading direction during navigation and coordination of walking
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Parkinson's disease (PD), traditionally regarded as a motor system disorder, has also been found to result in visuoperceptual disorders. Individuals with PD are aware of the changes in their visual perception by reporting problems in judging the distance between objects as well as perceiving speed of moving vehicles and pedestrians on the street. In addition, outcomes from studies on line bisection and size perception tasks suggest a unilateral compression of the left visual field in PD patients with predominant motor symptoms on the left body side. An asymmetry of optic flow between left and right visual fields can affect the heading direction during navigation. However, visuoperceptual disorders in PD may also result in a reliance on egocentric reference point in determining the heading direction, by taking the midline of the body as a reference. The objective of the first study was to investigate the effects of PD on the use of optic flow for heading direction and coordination of walking. A systematic manipulation of optic flow speed and the density of texture (dot density (DD)) between the left and right walls of the virtual hallway was achieved using virtual reality technology. Three dimensional kinematic data were collected by motion capture system. Compared to young controls (N=23) and age-matched normal controls (N=18), PD subjects (N=12) veered more to the left across conditions of optic flow speed (OS) and dot density manipulation. A significant linear effect of OS on lateral drift to the wall with lower OS was observed in healthy groups. A trend of veering toward the wall with lower DD suggested that visual texture guided navigation. It is hypothesized that the leftward veering bias in PD is related to the side of handedness and the asymmetry of the dopamine level between brain hemispheres. Parkinsonian gait is characterized by low walking speed, small stride length, shuffling gait, poverty of trunk movements, and reduced or absent arm swing. Compared to healthy controls, individuals with PD demonstrate reduced ability to change coordination pattern (reduced flexibility) which is accompanied by a reduced variability (increased stability) between transverse thoracic and pelvic rotation with change in walking speed. In addition, the degeneration of the dopaminergic system has also been related to a reduced flexibility of interlimb coordination in PD. The purpose of the second study was to investigate the impact of rigidity, as a neurological symptom, on the flexibility and stability of interlimb and trunk coordination during walking. Consistent with the outcomes reported in the literature, individuals with PD had smaller amplitude of arm and leg swings, thoracic, pelvic, and trunk rotations as well as smaller mean and variability of the relative phase between thoracic and pelvic rotation compared to healthy subjects. Remarkable is the finding that an increased variability in relative phase between the left and right arm swings was observed in PD which may be related to a reduced out-of-phase forcing of the arm movements at the shoulders as a result of axial rigidity. From these outcomes it can be hypothesized that improving the flexibility and stability of the gait patterns in individuals with PD can be achieved by facilitating the counter-rotation between transverse pelvic and thoracic rotations, for example, by subjects to move both arms and legs to the beat presented by a metronome (or music) during walking.
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