Abstract
Experimental data show that perceptual cues can either exacerbate or ameliorate freezing of gait (FOG) in Parkinson's Disease (PD). For example, simple visual stimuli like stripes on the floor can alleviate freezing whereas complex stimuli like narrow doorways can trigger it. We present a computational model of the cognitive and motor cortico-basal ganglia loops that explains the effects of sensory and cognitive processes on FOG. The model simulates strong causative factors of FOG including decision conflict (a disagreement of various sensory stimuli in their association with a response) and cognitive load (complexity of coupling a stimulus with downstream mechanisms that control gait execution). Specifically, the model simulates gait of PD patients (freezers and non-freezers) as they navigate a series of doorways while simultaneously responding to several Stroop word cues in a virtual reality setup. The model is based on an actor-critic architecture of Reinforcement Learning involving Utility-based decision making, where Utility is a weighted sum of Value and Risk functions. The model accounts for the following experimental data: (a) the increased foot-step latency seen in relation to high conflict cues, (b) the high number of motor arrests seen in PD freezers when faced with a complex cue compared to the simple cue, and (c) the effect of dopamine medication on these motor arrests. The freezing behavior arises as a result of addition of task parameters (doorways and cues) and not due to inherent differences in the subject group. The model predicts a differential role of risk sensitivity in PD freezers and non-freezers in the cognitive and motor loops. Additionally this first-of-its-kind model provides a plausible framework for understanding the influence of cognition on automatic motor actions in controls and Parkinson's Disease.
Highlights
Deterioration of gait in Parkinson’s Disease (PD) is of major concern as it severely affects the quality of life of patients
This section is organized as follows: We explain: (1) the effect of Utility and Risk observed in both the Cognitive and Motor Modules, (2) the effect of cognitive cues on gait and their contribution to step latency under conflicting situations such as while approaching the doorways, and (3) effect of cues as a source of cognitive load
Behavioral Results The experiments reported in Matar et al (2013) suggest that the PD freezers exhibit higher step latencies in both the wide and narrow doorway cases, with the narrow doorway being more significant than the wide doorway (Figure 11A) compared to controls and non-freezers
Summary
Deterioration of gait in Parkinson’s Disease (PD) is of major concern as it severely affects the quality of life of patients. Some characteristics of gait disturbance include increased double support time, reduced stride length and velocity (Hausdorff et al, 1998; Morris et al, 1998) In addition to these features, freezing of gait (FOG) is a paroxysmal phenomenon where patients feel they are glued to the ground despite the desire to walk (Giladi et al, 2001; Nutt et al, 2011). Competitive interactions among the cortico-basal-ganglia loops are thought to be a major factor for triggering freezing (Lewis and Barker, 2009) This is due to the impaired balance in demands to resources, that is, increased demands from cognitive, motor and limbic loops under depleted dopamine resources leading to the inhibition of brainstem locomotor systems (Lewis and Barker, 2009; Shine et al, 2013d). Some examples include tasks such as the attention network task (ANT), the virtual reality gait task (Shine et al, 2013a), “timed up and go” task (TUG) (Weiss et al, 2010; Herman et al, 2011), object avoidance (Snijders et al, 2010; Pieruccini-Faria et al, 2014), and dual tasking (Yogev et al, 2005; Springer et al, 2006)
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