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Below are the abstracts of 8 forthcoming target articles for a special
issue on Movement Systems that will appear in Behavioral and Brain
Sciences (BBS), an international, interdisciplinary journal that
provides Open Peer Commentary on important and controversial current
research in the biobehavioral and cognitive sciences. This will be the
first in a new series called "Controversies in Neuroscience," done in
collaboration with Paul Cordo and the RS Dow Neurological Science
Institute.
Commentators must be current BBS Associates or nominated by a current
BBS Associate. To be considered as a commentator on any of these
articles, to suggest other appropriate commentators, or for information
about how to become a BBS Associate, please send email to:
harnad@clarity.princeton.edu or harnad@pucc.bitnet or write to:
BBS, 20 Nassau Street, #240, Princeton NJ 08542 [tel: 609-921-7771]
Please specify which article or articles you would like to comment on.
(Commentators will be allotted 1000 words to comment on one of the
articles, 750 words more to comment on two of them, 500 more for three
and then 250 more for each additional one, for a maximum of 3500 words
to comment on all eight target articles.)
To help us put together a balanced list of commentators, please give
some indication of the aspects of the topic on which you would bring
your areas of expertise to bear if you were selected as a commentator.
In the next week or so, electronic drafts of the full text of each
article will be available for inspection by anonymous ftp according to
the instructions that follow after the abstracts. These drafts are for
inspection only; please do not prepare a commentary until you are
formally invited to do so.
__________________________________________________ __________________
1. Alexander GE, MR De Long, & MD Crutcher: DO CORTICAL AND BASAL
GANGLIONIC MOTOR AREAS USE "MOTOR PROGRAMS" TO CONTROL MOVEMENT?
bbs.alexander
2. Bizzi E, N Hogan, FA Mussa-Ivaldi & S Giszter: DOES THE NERVOUS
SYSTEM USE EQUILIBRIUM-POINT CONTROL TO GUIDE SINGLE AND MULTIPLE
JOINT MOVEMENTS? bbs.bizzi
3. Bloedel JR: DOES THE ONE-STRUCTURE/ONE-FUNCTION RULE APPLY TO THE
CEREBELLUM? bbs.bloedel
4. Fetz EH: ARE MOVEMENT PARAMETERS RECOGNIZABLY CODED IN SINGLE
NEURON ACTIVITY? bbs.fetz
5. Gandevia SC & D Burke: DOES THE NERVOUS SYSTEM DEPEND ON
KINESTHETIC INFORMATION TO CONTROL NATURAL LIMB MOVEMENTS?
bbs.gandevia
6. McCrea DA: CAN SENSE BE MADE OF SPINAL INTERNEURON CIRCUITS?
bbs.mccrea
7. Robinson DA: IMPLICATIONS OF NEURAL NETWORKS FOR HOW WE THINK
ABOUT BRAIN FUNCTION bbs.robinson
8. Stein JF: POSTERIOR PARIETAL CORTEX AND EGOCENTRIC SPACE
bbs.stein
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1. DO CORTICAL AND BASAL GANGLIONIC MOTOR AREAS
USE "MOTOR PROGRAMS" TO CONTROL MOVEMENT?
Garrett E. Alexander, Mahlon R. De Long, and Michael D. Crutcher
Department of Neurology
Emory University School of Medicine
Atlanta, GA 30322
gea@vax3200.neuro.emory.edu
KEYWORDS: basal ganglia, cortex, motor system, motor program, motor
control, parallel processing, connectionism, neural network
ABSTRACT: Prevailing engineering-inspired theories of motor control
based on sequential/algorithmic or motor programming models are
difficult to reconcile with what is known about the anatomy and
physiology of the motor areas. This is partly because of certain
problems with the theories themselves and partly because of features of
the cortical and basal ganglionic motor circuits that seem ill-suited
for most engineering analyses of motor control. Recent developments in
computational neuroscience offer more realistic connectionist models of
motor processing. The distributed, highly parallel, and nonalgorithmic
processes in these models are inherently self-organizing and hence more
plausible biologically than their more traditional algorithmic or
motor-programming counterparts. The newer models also have the
potential to explain some of the unique features of natural,
brain-based motor behavior and to avoid some of the computational
dilemmas asscociated with engineering approaches.
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2. DOES THE NERVOUS SYSTEM USE EQUILIBRIUM-POINT CONTROL
TO GUIDE SINGLE AND MULTIPLE JOINT MOVEMENTS?
E. Bizzi, N. Hogan, F.A. Mussa-Ivaldi and S. Giszter
Department of Brain and Cognitive Sciences and
Department of Mechanical Engineering
Massachusetts Institute of Technology
Cambridge, MA 02139
emilio@wheaties.ai.mit.edu
KEYWORDS: spinal cord, force field, equilibrium point,
microstimulation, multi-joint coordination, contact tasks, robotics,
inverse dynamics, motor control.
ABSTRACT: The hypothesis that the central nervous system (CNS)
generates movement as a shift of the limb's equilibrium posture has
been corroborated experimentally in single- and multi-joint motions.
Posture may be controlled through the choice of muscle length tension
curves that set agonist-antagonist torque-angle curves determining an
equilibrium position for the limb and the stiffness about the joints.
Arm trajectories seem to be generated through a control signal defining
a series of equilibrium postures.
The equilibrium-point hypothesis drastically simplifies the requisite
computations for multijoint movements and mechanical interactions with
complex dynamic objects in the environment. Because the neuromuscular
system is springlike, the instantaneous difference between the arm's
actual position and the equilibrium position specified by the neural
activity can generate the requisite torques, avoiding the complex
"inverse dynamic" problem of computing the torques at the joints.
The hypothesis provides a simple unified description of posture and
movement as well as performance on contact control tasks, in which the
limb must exert force stably and do work on objects in the environment.
The latter is a surprisingly difficult problem, as robotic experience
has shown.
The prior evidence for the hypothesis came mainly from psychophysical
and behavioral experiments. Our recent work has shown that
microstimulation of the spinal cord's premotoneuronal network produces
leg movements to various positions in the frog's motor space. The
hypothesis can now be investigated in the neurophysiological machinery
of the spinal cord.
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3. DOES THE ONE-STRUCTURE/ONE-FUNCTION RULE APPLY TO THE CEREBELLUM?
James R. Bloedel
Division of Neurobiology
Barrow Neurological Institute
Phoenix, AZ
KEYWORDS: cerebellum; Purkinje cells; mossy fibres; movement;
proprioception; body image; kinesthesis; robotics; posture.
ABSTRACT: The premise explored in this target article is that the
function of the cerebellum can be best understood in terms of the
operation it performs across its structurally homogeneous subdivisions.
The functional heterogeneity sometimes ascribed to these different
regions reflects the many functions of the central targets receiving
the outputs of different cerebellar regions. Recent studies by
ourselves and others suggest that the functional unit of the cerebellum
is its sagittal zone. It is hypothesized that the climbing fiber system
produces a short-lasting modification in the gain of Purkinje cell
responses to its other principle afferent input, the mossy
fiber-granule cell-parallel fiber system. Because the climbing fiber
inputs to sagittally aligned Purkinje cells can be activated under
functionally specific conditions, they could select populations of
Purkinje neurons that were most highly modulated by the distributed
mossy fiber inputs responding to the same conditions. These operations
may be critical for the on-line integration of inputs representing
external target space with features of intended movement,
proprioceptive and kinesthetic cues, and body image.
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4. ARE MOVEMENT PARAMETERS RECOGNIZABLY CODED
IN SINGLE NEURON ACTIVITY?
Eberhard E. Fetz
Regional Primate Research Center
University of Washington
Seattle, WA 98195
fetz@locke.hs.washington.edu
KEYWORDS: neural coding; representation; neural networks;
cross-correlation; movement parameters; parallel distributed processing
ABSTRACT: To investigate neural mechanisms of movement, physiologists
have analyzed the activity of task-related neurons in behaving animals.
The relative onset latencies of neural activity have been scrutinized
for evidence of a functional hierarchy of sequentially recruited
centers, but activity appears to change largely in parallel. Neurons
whose activity covaries with movement parameters have been sought for
evidence of explicit coding of parameters such as active force, limb
displacement and behavioral set. Neurons with recognizable relations to
the task are typically selected from a larger population, ignoring
unmodulated cells as well as cells whose activity is not related to the
task in a simple, easily recognized way. Selective interpretations are
also used to support the notion that different motor regions perform
different motor functions; again, current evidence suggests that units
with similar properties are widely distributed over different regions.
These coding issues are re-examined for premotoneuronal (PreM) cells,
whose correlational links with motoneurons are revealed by
spike-triggered averages. PreM cells are recruited over long times
relative to their target muscles. They show diverse response patterns
relative to the muscle force they produce; functionally disparate PreM
cells such as afferent fibers and descending corticomotoneuronal and
rubromotoneuronal cells can exhibit similar patterns. Neural mechanisms
have been further elucidated by neural network simulations of
sensorimotor behavior; the pre-output hidden units typically show
diverse responses relative to their targets. Thus, studies in which
both the activity and the connectivity of the same units is known
reveal that units with many kinds of relations to the task, simple and
complex, contribute significantly to the output. This suggests that the
search for explicit coding may be diverting us from understanding more
distributed neural mechanisms that are more complex and less directly
related to explicit movement paremeters.
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5. DOES THE NERVOUS SYSTEM DEPEND ON KINESTHETIC
INFORMATION TO CONTROL NATURAL LIMB MOVEMENTS?
S.C. Gandevia and David Burke
Department of Clinical Neurophysiology
Institute of Neurological Sciences
The Prince Henry Hospital
P.O. Box 233
Matraville, N.S.W. 2036
Sydney, Australia
KEYWORDS: kinesthesia, motor control, muscle, joint and cutaneous
afferents, motor commands, deafferentation
ABSTRACT: This target article draws together two groups of
experimental studies on the control of human movement through
peripheral feedback and centrally generated signals of motor command.
First, during natural movement, feedback from muscle, joint and
cutaneous afferents changes; in human subjects these changes have
reflexive and kinesthetic consequences. Recent psychophysical and
microneurographic evidence suggests that joint and even cutaneous
afferents may have a proprioceptive role. Second, the role of centrally
generated motor commands in the control of normal movements and
movements following acute and chronic of deafferentation is reviewed.
There is increasing evidence that subjects can perceive their motor
commands under various conditions, but this is inadequate for normal
movement; deficits in motor performance arise when the reliance on
proprioceptive feedback is abolished, either experimentally or because
of pathology. During natural movement, the CNS appears to have access
to functionally useful input from a range of receptors as well as from
internally generated command signals. Remaining unanswered questions
suggest a number of avenues for further research.
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6. CAN SENSE BE MADE OF SPINAL INTERNEURON CIRCUITS?
David A. McCrea
The Department of Physiology
Faculty of Medicine
University of Manitoba
770 Bannatyne Avenue
Winnipeg, Manitoba, Canada R3E OW3
dave@scrc.umanitoba.ca
KEYWORDS: interneuron, motor control, reflexes, spinal cord, flexion,
muscle synery, presynaptic inhibition.
ABSTRACT: It is increasingly clear that spinal reflex systems cannot be
described in terms of simple and constant reflex actions. The extensive
convergence of segmental and descending systems onto spinal
interneurons suggests that spinal interneurons are not relay systems
but rather form a crucial component in determining which muscles are
activated during voluntary and reflex movements. The notion that
descending systems simply modulate the gain of spinal interneuronal
pathways has been tempered by the observation that spinal interneurons
gate and distribute descending control to specific motoneurons. Spinal
systems are complex, but current approaches will continue to provide
insight into motor systems. During movement, several neural mechanisms
act to reduce the functional complexity of motor systems by inhibiting
some of the parallel reflex pathways available to segmental afferents
and descending systems. The flexion reflex system is discussed as an
example of the flexibility of spinal interneuron systems and as useful
construct. Examples are provided of the kinds of experiments that can
be developed using current approaches to spinal interneuronal systems.
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7. IMPLICATIONS OF NEURAL NETWORKS
FOR HOW WE THINK ABOUT BRAIN FUNCTION
David A. Robinson
Ophthalmology, Biomedical Engineering, and Neuroscience
The Johns Hopkins University, School of Medicine
Room 355 Woods Res. Bldg.
The Wilmer Institute
Baltimore, MD 21205
KEYWORDS: Neural networks, signal processing, oculomotor system,
vestibulo-ocular reflex, pursuit eye movements, saccadic eye movements,
coordinate transformations
ABSTRACT: Engineers use neural networks to control systems too complex
for conventional engineering analysis. To examine hidden unit behavior
would defeat the purpose of this approach, because individual units
would be largely uninterpretable. Yet neurophysiologists spend their
careers doing just that! Hidden units contain bits and pieces of
signals that yield only arcane hints of network function and no
information about how the units process signals. Most of the literature
on single-unit recordings attests to this grim fact. On the other hand,
knowing system function and describing it with elegant mathematics
tells one very little about what to expect of interneuron behavior.
Examples of simple networks based on neurophysiology are taken from the
oculomotor literature to suggest how single-unit interpretability might
degrade with increasing task complexity. Trying to explain how any real
neural network works on a cell-by-cell, reductionist basis is futile;
we may have to be content with understanding the brain at higher levels
of organization.
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8. POSTERIOR PARIETAL CORTEX AND EGOCENTRIC SPACE
J.F. Stein
University Laboratory of Physiology
University of Oxford
Oxford, England OX1 3PT
stein@vax.oxford.ac.uk
KEYWORDS: posterior parietal cortex; egocentric space; space
perception; attention; coordinate transformations; distributed
systems; neural networks.
ABSTRACT: The posterior parietal cortex (PPC) is the most likely site
where egocentric spatial relationships are represented in the brain.
PPC cells receive visual, auditory, somaesthetic and vestibular sensory
inputs, oculomotor, head, limb and body motor signals, and strong
motivational projections from the limbic system. Their discharge
increases not only when an animal moves towards a sensory target, but
also when it directs its attention to it. PPC lesions have the opposite
effect: sensory inattention and neglect. PPC does not seem to contain a
"map" of the location of objects in space but a distributed neural
network for transforming one set of sensory vectors into other sensory
reference frames or into various motor coordinate systems. Which set of
transformation rules is used probably depends on attention, which
selectively enhances the synapses needed for a making particular
sensory comparison or aiming a particular movement.
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To help you decide whether you would be an appropriate commentator for
any of these articles, a (nonfinal) draft of each will soon be
retrievable by anonymous ftp from princeton.edu according to the
instructions below (filenames will be of the form bbs.alexander, based
on the name of the first author). Please do not prepare a commentary on
this draft. Just let us know, after having inspected it, what relevant
expertise you feel you would bring to bear on what aspect of the
article.
---------------------------------------------------------------
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