Abstract
We present a computational model of altered gait velocity patterns in Parkinson's Disease (PD) patients. PD gait is characterized by short shuffling steps, reduced walking speed, increased double support time and sometimes increased cadence. The most debilitating symptom of PD gait is the context dependent cessation in gait known as freezing of gait (FOG). Cowie et al. (2010) and Almeida and Lebold (2010) investigated FOG as the changes in velocity profiles of PD gait, as patients walked through a doorway with variable width. The former reported a sharp dip in velocity, a short distance from the doorway that was greater for narrower doorways. They compared the gait performance in PD freezers at ON and OFF dopaminergic medication. In keeping with this finding, the latter also reported the same for ON medicated PD freezers and non-freezers. In the current study, we sought to simulate these gait changes using a computational model of Basal Ganglia based on Reinforcement Learning, coupled with a spinal rhythm mimicking central pattern generator (CPG) model. In the model, a simulated agent was trained to learn a value profile over a corridor leading to the doorway by repeatedly attempting to pass through the doorway. Temporal difference error in value, associated with dopamine signal, was appropriately constrained in order to reflect the dopamine-deficient conditions of PD. Simulated gait under PD conditions exhibited a sharp dip in velocity close to the doorway, with PD OFF freezers showing the largest decrease in velocity compared to PD ON freezers and controls. PD ON and PD OFF freezers both showed sensitivity to the doorway width, with narrow door producing the least velocity/ stride length. Step length variations were also captured with PD freezers producing smaller steps and larger step-variability than PD non-freezers and controls. In addition this model is the first to explain the non-dopamine dependence for FOG giving rise to several other possibilities for its etiology.
Highlights
Altered gait behavior is a motor impairment observed in patients with Parkinson’s disease (PD), a neurodegenerative disorder that involves a loss of dopaminergic neurons in the brain
PD gait is characterized by the following features: (1) Reduced stride length, reduced walking speed, increased cadence and increased double support duration (Morris et al, 1998); (2) Exhibits flat foot strike, and in rare conditions the “toe to heel strike” gait pattern is observed (Hughes et al, 1990); (3) Intra-individual variability in foot strike patterns is lower in PD patients than in control subjects (Kimmeskamp and Hennig, 2001); (4) Vertical ground reaction force (VGRF) representing the normal force exerted on the foot during gait, has two peaks in controls—one when the foot hits the ground, and the other when it lifts off again
We simulate the results of two experimental conditions that study gait patterns of PD patients as a they walk towards a doorway (Almeida and Lebold, 2010; Cowie et al, 2010)
Summary
Altered gait behavior is a motor impairment observed in patients with Parkinson’s disease (PD), a neurodegenerative disorder that involves a loss of dopaminergic neurons in the brain. PD gait is characterized by the following features: (1) Reduced stride length, reduced walking speed, increased cadence and increased double support duration (Morris et al, 1998); (2) Exhibits flat foot strike, and in rare conditions the “toe to heel strike” gait pattern is observed (Hughes et al, 1990); (3) Intra-individual variability in foot strike patterns is lower in PD patients than in control subjects (Kimmeskamp and Hennig, 2001); (4) Vertical ground reaction force (VGRF) representing the normal force exerted on the foot during gait, has two peaks in controls—one when the foot hits the ground, and the other when it lifts off again. In advanced PD, where the patients walk with narrow shuffling steps, the two peaks in VGRF merge into one (Koozekanani et al, 1987); (5) Postural instability is a common feature in late stage PD. The degree of gait variability as seen by any of the above mentioned features, is correlated with gait severity in PD patients (Hausdorff et al, 1998)
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