Modeling Basal Ganglia for understanding Parkinsonian Reaching Movements

TL;DR

This paper introduces a computational model of the basal ganglia within a reinforcement learning framework to understand Parkinsonian reaching movements, highlighting exploration-exploitation dynamics and simulating PD symptoms.

Contribution

It proposes a neurobiologically grounded RL model with the indirect pathway as an explorer, and demonstrates how PD symptoms emerge from altered dopamine levels and pathway dynamics.

Findings

Reaching trajectories become less variable with training.

PD symptoms like tremor and bradykinesia are simulated by reducing dopamine and pathway complexity.

The indirect pathway acts as an explorer driving exploratory movements.

Abstract

We present a computational model that highlights the role of basal ganglia (BG) in generating simple reaching movements. The model is cast within the reinforcement learning (RL) framework with the correspondence between RL components and neuroanatomy as follows: dopamine signal of substantia nigra pars compacta as the Temporal Difference error, striatum as the substrate for the Critic, and the motor cortex as the Actor. A key feature of this neurobiological interpretation is our hypothesis that the indirect pathway is the Explorer. Chaotic activity, originating from the indirect pathway part of the model, drives the wandering, exploratory movements of the arm. Thus the direct pathway subserves exploitation while the indirect pathway subserves exploration. The motor cortex becomes more and more independent of the corrective influence of BG, as training progresses. Reaching trajectories show diminishing variability with training. Reaching movements associated with Parkinson's disease (PD) are simulated by (a) reducing dopamine and (b) degrading the complexity of indirect pathway dynamics by switching it from chaotic to periodic behavior. Under the simulated PD conditions, the arm exhibits PD motor symptoms like tremor, bradykinesia and undershoot. The model echoes the notion that PD is a dynamical disease.