Working Paper: Basal Ganglia in the Control of Movements
Zhi-Hong Mao, MIT
July 2002
Background
Role of the basal ganglia in control of movements
The basal ganglia are masses of gray matter buried within the cerebrum, which
include four principal nuclei: the striatum, globus pallidus, substantia nigra, and
subthalamic nucleus (see Figure 1). They play a crucial role in the control of
movements. However, unlike most other components of the motor system, the
basal ganglia do not have direct connections with the spinal cord. They receive
input from the cerebral cortex and send output to the brain stem and, via the
thalamus, to the frontal cortex. Their motor functions are mostly mediated by
motor areas of the frontal cortex. The relationship of basal ganglia with
movement has been strongly suggested by clinical studies on motor disorders
related to dysfunction of the basal ganglia, e.g., Parkinson's disease, dystonia, and
Huntington’s disease.
Recent advances in knowledge of the basal ganglia have led to a hypothesis that
the basal ganglia do not generate movements. Rather, the basal ganglia act to
inhibit competing motor mechanisms that would possibly interfere with the
desired movement [Mink96]. In other words, the basal ganglia help “filter” out
competing motor patterns and keep the desired one(s) for the motor system. The
basal ganglia make the focused selection of an on-going movement or action
based on the current brain states. As brain states change, the basal ganglia may
favor an action other than the current one and then switch from the current action
to the newly selected action. In such a way, the basal ganglia can help trigger a
sequence of actions.
One example of possible motor functions of the basal ganglia is the composition
of submovements into nominal movement. Studies reveal that a movement task
can usually be decomposed into a group of simple, stereotyped submovements
with bell-shaped velocity profile [Lee97]. Based on our observation from signs of
basal ganglia dysfunctions, we have proposed a hypothesis that the basal ganglia
assist the motor system to plan and trigger a sequence of submovements during
the execution of a movement task [Massaquoi02]. We assume that several parallel
basal ganglionic circuits or “channels” monitor and control the activities of a set
of neuronal assemblies in the motor cortex. Activity of each neuronal assembly
triggers a basic action, e.g., a stereotyped simple submovement. Physiological
movements are then achieved by rapidly switching activities among those
neuronal assemblies, and the role of the basal ganglia is to organize appropriate
transitions of cortical activities.
Figure 1. Basal ganglia anatomy: (a) Internal pathways. GPe and
GPi are external and internal segments of the globus pallidus,
respectively; STN represents subthalamic nucleus; SNc and SNr
are substantia nigra pars compacta and substantia nigra pars
reticulata, respectively. (b) External pathways showing major
inputs and outputs. This figure is adapted from [Gurney01].
Approach
In this phase of research, we will explore the following two ideas borrowed from
the research of motor control and basal ganglia to the study of vehicle control.
Modular composition and decomposition
The decomposition of a complex task into simpler subtasks is a general strategy
for motor control. We have shown previously how a nominal movement is
achieved by accurate composition of a set of submovements and what the basal
ganglia contribute to the composition of submovements during the movement.
This may bring us some insight into the vehicle control strategies. For example,
given a path planning task and a set of basic maneuvers of a vehicle, how do we
“efficiently” or “optimally” realize the task for the vehicle from some
combination of the maneuvers? For this problem, we may be able to learn
something useful from the organization of submovements by the basal ganglia.
Modular composition and decomposition can also be found in group behaviors.
Useful, complex group behavior is usually based on a combination of relatively
simple reactions. These reactions exist within a collection of nominally
autonomous agents. We assume that the physiological basis for general behavioral
reactions is the same as that for the reactions (submovements) used for body
movement control and postural regulation, and the triggering of elemental
behavioral reactions depends significantly on the function of the basal ganglia.
Therefore, an accurate model of the function of the basal ganglia should provide
important insight into the control of group behavior. This would also possibly
bring ideas into the cooperative control involving multiple vehicles.
Adaptation and repair strategies
Being able to adapt to new or varied environments and able to recover wholly or
partly from damage are always amazing properties of motor systems. Clinical
studies show that slowly growing brain lesions do not necessarily betray their
presence by early signs of dysfunction, because the brain learns to adapt and to
compensate for them in movements and postures [Brooks86]. It is the suddenly
inflicted lesion like a stroke that can produce immediate and obvious
consequences. However, even for sudden and relatively severe damages of brain,
patients can sometimes regain to some extent their motor skills through training.
The abilities for the motor systems to adapt and to repair are due to (1) the
plasticity of neuronal connections, i.e., the ability to learn, and (2) redundancy in
number of neuronal modules and connections. A deep understanding of these
properties of motor systems would probably be very helpful to the control and
design of safe vehicle systems.
In our current phase of research, we are trying to understand the learning
mechanisms of the basal ganglia. It is now widely accepted that different neural
circuits are specialized for different types of learning. The basal ganglia have
been proposed as structures critical for reward-based learning [Aosaki94].
However, till now there is no consensus on the learning circuitry and neuronal
synaptic plasticity in the basal ganglia. We try to build in our model a reasonable
representation of the learning circuits and learning rules of the basal ganglia,
taking into account the cellular and molecular basis of longer term potentiation
and depression. We also hope to gain insight into some of the adaptation and
optimization problems of vehicle control, based on a study of biological version
of the reinforcement learning (by the basal ganglia).
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