Do Basal Ganglia Amplify Willed Action by Stochastic Resonance? A Model

V Srinivasa Chakravarthy,Derek Abbott

doi:10.1371/journal.pone.0075657

Abstract

Basal ganglia are usually attributed a role in facilitating willed action, which is found to be impaired in Parkinson's disease, a pathology of basal ganglia. We hypothesize that basal ganglia possess the machinery to amplify will signals, presumably weak, by stochastic resonance. Recently we proposed a computational model of Parkinsonian reaching, in which the contributions from basal ganglia aid the motor cortex in learning to reach. The model was cast in reinforcement learning framework. We now show that the above basal ganglia computational model has all the ingredients of stochastic resonance process. In the proposed computational model, we consider the problem of moving an arm from a rest position to a target position: the two positions correspond to two extrema of the value function. A single kick (a half-wave of sinusoid, of sufficiently low amplitude) given to the system in resting position, succeeds in taking the system to the target position, with high probability, only at a critical noise level. But for suboptimal noise levels, the model arm's movements resemble Parkinsonian movement symptoms like akinetic rigidity (low noise) and dyskinesias (high noise).

Highlights

Willed actions are a form of voluntary actions, though no rigorous definition of willed action is available [1]
We propose that by affording a combination of gradient descent and noise, basal ganglia (BG) serves as an excellent substrate for SR phenomenon, and amplifies the weak willed action signal arising from the prefrontal cortex or Supplementary Motor Area (SMA)
This dynamics of the output of BG consists of two components: 1) the dynamics of hillclimbing over a Value function, and 2) a stochastic component corresponding to exploratory behavior in reinforcement learning (RL)

Summary

Introduction

Willed actions are a form of voluntary actions, though no rigorous definition of willed action is available [1]. Voluntary actions are characterized by presence of a goal, a plan to achieve that goal, conscious awareness of the action being performed, and an intention behind the whole process. William James [2] offers a further classification of voluntary actions into ideo-motor actions and willed actions. In the former, a pre-existent idea of how the action has to be performed is executed. In willed action, there is no pre-existent idea but only the pure, direct action of will driving and shaping movement. In James’ own words, in case of willed actions, in contrast to ideo-motor actions, there is ‘‘an additional conscious element in the shape of a fiat, a mandate, or expressed consent’’ [2]

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