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The Concept of Voluntary Motor Control in the Recent Neuroscientific Literature
The Concept of Voluntary Motor Control
in the Recent Neuroscientific Literature
Paul Tibbetts, Ph.D. (paul.tibbetts@notes.udayton.edu)
Philosophy and Cognitive Science, University of Dayton
300 College Park, Dayton, OH 45469-1546
Abstract
The concept of voluntary motor control (VMC) frequently
appears in the neuroscientific literature, specifically in the
context of cortically-mediated, intentional motor actions. For
cognitive scientists, this concept of VMC raises a number of
interesting questions: (i) Are there dedicated, modular-like
structures within the motor system associated with VMC? Or (ii)
is it the case that VMC is distributed over multiple cortical as
well as subcortical structures? (iii) Is there any one place within
the so-called hierarchy of motor control where voluntary
movements could be said to originate? And (iv) in the current
neurological literature how is the adjective voluntary in VMC
being used? These questions are here considered in the context
of how higher- and lower-levels of motor control plan, initiate,
coordinate, sequence, and modulate goal-directed motor outputs
in response to changing internal and external inputs. Particularly
relevant are the conceptual implications of current neurological
modeling of VMC concerning intentional agency.
Key Words: causal agency, intentional behavior, motor system,
volitions, voluntary motor control, will.
(Note: A more extended version of this paper has been
submitted and revised for publication in the periodical, Brain
and Mind.)
The Concept of Voluntary Motor Control
in the Recent Neuroscientific Literature
Despite many years of effort, there is still no complete
understanding of the sequence of events that leads from
thought to movement, and it is fair to say that the picture
becomes increasingly blurred the farther one moves from
the muscles themselves. (Purves et al., 1997, p. 292)
More than in sensory systems, our understanding of motor
events decreases rapidly as we move from the periphery
into the central nervous system. This is a consequence of
the fact that there are many pathways converging onto
motoneurons from higher centers, and even more onto the
higher-order cells. (Nicholls et al., 2001, p. 448)
Volition, Motor Effects, and Neuroscience
Regarding voluntary motor control (hereafter, VMC), a
primary objective of the historical and contemporary
neuroscientific literature is to model how higher- and lower-
level structures within the motor nervous system plan, initiate,
sequence, modulate, and coordinate voluntary movements
(Finger, 1994, pp. 191-239; Jordan & Wolpert, 2000;
Passingham, 1993; Purves et al., 2001, pp. 347-441).
Accordingly, there are few metaphysical claims made in this
literature concerning either the reality or non-reality of
volition, choice, or so-called acts of will. Neuroscientists
simply assume as a working hypothesis that people can initiate
and sustain volitional control over their actions, contingent on
normal signal exchange between cortical, subcortical, and
spinal-cord motor structures. Consequently, in modeling the
processes and computations associated with these structures,
we understand what makes VMC possible in the first place. So
goes the reasoning of much contemporary neuroscientific
thinking concerning motor control (Kolb & Whishaw, 1996;
Purves et al., 2001; Victor & Ropper, 2001).
How much light this strategy throws on more philosophical
concerns regarding volition and choice is, of course, another
matter (Brand, 1995). E.g., do volitions and intentions belong
to one level of analysis, and motor-system analyses to another?
Alternatively, could an account of volitions and intentions be
folded into an account of the hierarchy of motor-control
structures and computations? To explore this latter possibility
requires some understanding of the hierarchy of motor control,
from spinal-cord motoneurons to subcortical and cortical
structures.
A Preliminary Pass Through the Primate Motor
System
Complex, coordinated movements such as
reaching for,
grasping and writing with a pen require signal sequencing to
multiple muscle groups in the shoulder, arm and hand. Such
movements are initiated by signals from the primary motor
cortex (hereafter, M1) to lower motor neurons in the spinal
cord and, in turn, to skeletal muscles. However, M1 output is
modulated by input from other motor structures, some cortical
(e.g., premotor and prefrontal cortical areas), some subcortical
(e.g., basal ganglia and cerebellar inputs). Where some of
these inputs to M1 are internally generated (such as the
motivation to write this manuscript), other M1 inputs are
1158 externally cued (locating the pen and notes with which to
begin writing) (Mushiake et al., 1991). However, whether in
response to internal or external inputs, M1I outputs to spinal-
cord motor neurons and, in turn, to the trunk and limb muscles,
are constantly being modulated, updated and, in effect,
supervised by a number of motor structures. To achieve even
a rudimentary understanding of how this signaling process
works, we now examine the voluntary motor system (VMS) in
somewhat greater detail.
A Second, More Technical Pass Through the Primate
Nervous System
As with any other science, the neurological literature
regarding motor control is characterized by its own distinctive
terminology, methodologies, and causal models. This
literatures causal models are particularly complex given: (i)
the distribution of motor control over a number of cortical and
subcortical structures, (ii) converging as well as diverging
inputs and outputs between these structures, and (iii)
feedforward
and
feedback loops within the
system.
Consequently, the physical and computational complexities of
the motor system can only be hinted at in a manuscript of this
length (Churchland & Sejnowski, 1992; Georgopoulos, 2000;
Jordan & Wolpert, 2000; Tibbetts, 2002).
The corticospinal (or pyramidal) tract consists of those
axons originating in upper motor neurons (UMNs) in more
medial areas of M1 and synapse on lower motor neurons
(LMNs) in the spinal cord. On the other hand, axons of the
corticobulbar tract originate in UMNs in more lateral areas of
M1. These latter axons synapse on interneurons in the brain
stem which, in turn, synapse on cranial nerves to innervate
facial muscles (Purves et al., 2001, pp. 18 and 376-381).
While some of these descending projections from higher- to
lower-motor control levels are associated with VMC, other
projections provide the requisite stability for voluntary
movements (Purves et al., 2001, p. 347). E.g., in deciding to
retrieve my dropped pen, my posture must change as I reach
down otherwise I will lose my balance. In this example, in
response to visual and proprioceptive cues, preprogrammed
circuits in the brain stem provide the requisite postural
stability. Additionally, my intention to move across the room
to where pages of my manuscript have landed also involved
central pattern generators (CPGs) in the spinal cord to enable
the rhythmic motor patterns associated with walking by
alternatively extending and flexing lower limbs (Longstaff,
2000, pp. 213-214).
To summarize the preceding, VMC is associated with higher-
level cortical areas, particularly with M1 outputs to LMNs in
the spinal cord. [Sherrington referred to these LMNs as the
final common pathway for voluntary movement and postural
stability (Nicholls et al., 2001, p. 449).] However, actual
execution of these motor commands, as transmitted by the
corticospinal tract, is controlled by local circuitry in the spinal
cord. Given these intermediate relay circuits between motor
cortex and muscles, there is no direct translation of voluntary
motor intentions into body movements. Accordingly, while
cortical motor areas function in some sort of supervisory and
initiating role, there are a number of lower-level, middle
managers who exercise discretionary power in how to translate
supervisory intentions and commands into goal-directed
actions (Zigmond et al., 1999, pp. 931-949).
Regarding these spinal-cord motoneurons and local circuit
neurons, for Gazzaniga et al. (1998, pp. 379-383), this
arrangement:
is truly hierarchical in that the highest levels need be
concerned only with issuing commands to achieve an
action, while lower-level mechanisms translate the
commands into a movement. [E.g.,] the highest level of
the hierarchy need represent only the ultimate goal
[reaching for a cup]the elbow and hand assume a
position where the cup can be grasped with minimal
effort. How this goal is met does not have to be included
in this representation. Lower levels of the hierarchy are
concerned with translating a final goal into a certain
trajectory.
Following are examples of how different