Free won’t

Fronto-basal-ganglia circuits for stopping action

Ian Greenhouse, Nicole Swann & Adam Aron

In:- Neural Basis of Motivational and Cognitive Control

INTRODUCTION:  This chapter describes a neural network that can stop initiated actions. This qualifies the naïve interpretation of the Libet experiments that ascribes all choice to readiness potentials that proceed conscious awareness of the decision to make (trivial) actions. Libet himself suggested there might be a ‘free won’t‘ that could override the automaton type readiness potentials and recent neuroscience looks to support this suggestion.

The authors consider situations in which a subject has already begun an action, but stops in response to an external stimuli. Many studies suggest that a particular fronto-basal-ganglia circuit is responsible for this control. It is suggested that in this situation sensory information relative to external stimuli is projected to the prefrontal, and particularly the areas of the right inferior frontal and the presupplementary motor area, and that these areas send a ‘stop’ command via the basal ganglia. Part of the inferior frontal may relate to attention, and a part to inhibition. The subthalamic nucleus is seen as a region of the basal ganglia that is suited for a ‘stop’ signal. It is well placed to increase inhibition of thalamo-cortical output. It is thought capable of inhibiting both basal ganglia and motor system output. The subthalamic nucleus receives inputs from the inferior frontal and the presupplementary. The inferior frontal, the presupplementary and the subthalamic are viewed as a connected functional network sending a fast inhibitory signal to the motor system. Stopping an initiated action is also seen to involve increased GABA activity. A specific oscillatory frequency in the beta band also appears to be involved. The ‘stop’ process appears to require a degree of preparation and the targeting of a specific action.

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