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MOTOR CORTEX

MOVEMENTS

A DISTRIBUTED SYSTEM

FOR DISTRIBUTED FUNCTIONS

EDITED BY

Alexa Riehle and Eilon Vaadia

CRC PRESS

Boca Raton London New York Washington, D.C.

IN VOLUNTARY

Library of Congress Cataloging-in-Publication Data

Motor cortex in voluntary movements : a distributed system for distributed functions /

edited by Alexa Riehle and Eilon Vaadia.

p. cm.

Includes bibliographical references and index.

ISBN 0-8493-1287-6 (alk. paper)

1. Motor cortex. 2. Human locomotion. I. Riehle, Alexa. II. Vaadia, Eilon. III. Series.

QP383.15.M68 2005

612.8

2004057046

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Methods & New Frontiers

in Neuroscience

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Preface

Voluntary movement is undoubtedly the overt basis of human behavior. Without

movement we cannot walk, nourish ourselves, communicate, or interact with the

environment. This is one of the reasons why the motor cortex was one of the ﬁrst

cortical areas to be explored experimentally. Historically, the generation of motor

commands was thought to proceed in a rigidly serial and hierarchical fashion. The

traditional metaphor of the piano presents the premotor cortex “playing” the upper

motoneuron keys of the primary motor cortex (M1), which in turn activate with

strict point-to-point connectivity the lower motoneurons of the spinal cord. Years of

research have taught us that we may need to reexamine almost all aspects of this

model. Both the premotor and the primary motor cortex project directly to the spinal

cord in highly complex overlapping patterns, contradicting the simple hierarchical

view of motor control. The task of generating and controlling movements appears

to be subdivided into a number of subtasks that are accomplished through parallel

distributed processing in multiple motor areas. Multiple motor areas may increase

the behavioral ﬂexibility by responding in a context-related way to any constraint

within the environment. Furthermore, although more and more knowledge is accu-

mulating, there is still an ongoing debate about what is represented in the motor

cortex: dynamic parameters (such as speciﬁc muscle activation), kinematic param-

eters of the movement (for example, its direction and speed), or even more abstract

parameters such as the context of the movement. Given the great scope of the subject

considered here, this book focuses on some new perspectives developed from con-

temporary monkey and human studies. Moreover, many topics receive very limited

treatment.

Section I , which includes the ﬁrst two chapters, uses functional neuroanatomy

and imaging studies to describe motor cortical function. The objective of Chapter 1

is to describe the major components of the structural framework employed by the

cerebral cortex to generate and control skeletomotor function.

Dum and Strick

focus on motor areas in the frontal lobe that are the source of corticospinal projec-

tions to the ventral horn of the spinal cord in primates. These cortical areas include

the primary motor cortex (M1) and the six premotor areas that project directly to it.

The results presented lead to an emerging view that motor commands can arise from

multiple motor areas and that each of these motor areas makes a specialized contri-

bution to the planning, execution, or control of voluntary movement. The purpose

of Chapter 2 is to provide an overview of the contribution of functional magnetic

resonance imaging (fMRI) to some of the prevailing topics in the study of motor

control and the function of the primary motor cortex.

Kleinschmidt and Toni

claim

that in several points the ﬁndings of functional neuroimaging seem to be in apparent

disagreement with those obtained with other methods, which cannot always be

attributed to insufﬁcient sensitivity of this noninvasive technique. In part, it may

demonstrate that rather than acting

as a somatotopic array of upper motor neurons, each controlling a single muscle

that moves a single ﬁnger, neurons in the primary motor cortex (M1) act as a spatially

distributed network of very diverse elements, many of which have outputs that

diverge to facilitate multiple muscles acting on different ﬁngers. This biological

control of a complex peripheral apparatus initially may appear unnecessarily com-

plicated compared to the independent control of digits in a robotic hand, but can be

understood as the result of concurrent evolution of the peripheral neuromuscular

apparatus and its descending control from the motor cortex. Chapter 4 deals with

simultaneous movements of the two arms, as a simple example of complex move-

ments, and may serve to test whether and how the brain generates unique represen-

tations of complex movements from their constituent elements.

Schieber, Reilly, and Lang

Vaadia and Cardoso

present evidence that bimanual representations indeed exist, both at the

level of single neurons and at the level of neuronal populations (in local ﬁeld

potentials). They further show that population ﬁring rates and dynamic interactions

between the hemispheres contain information about the bimanual movement to be

executed. In Chapter 5 ,

discusses studies with respect to the debate as to

whether the motor cortex codes the spatial aspects (kinematics) of motor output,

such as direction, velocity, and position, or primarily controls, muscles, and forces

(dynamics). Although the weight of evidence is in favor of M1 controlling spatial

output, the effect of limb biomechanics and forces on motor cortex activity is beyond

dispute. The author proposes that the motor cortex indeed codes for the most

behaviorally relevant spatial variables and that both spatial variables and limb bio-

mechanics are reﬂected in motor cortex activity. Chapter 6 starts with the important

issue of how theoretical concepts guide experimental design and data analysis.

Ashe

Scott

describes two conceptual frameworks for interpreting neural activity during reach-

ing: sensorimotor transformations and internal models. He claims that sensorimotor

transformation have been used extensively over the past 20 years to guide neuro-

physiological experiments on reaching, whereas internal models have only recently

had an impact on experimental design. Furthermore, the chapter demonstrates how

the notion of internal models can be used to explore the neural basis of movement

by describing a new experimental tool that can sense and perturb multiple-joint

planar movements. Chapter 7 deals with the function of oscillatory potentials in the

motor cortex.

MacKay

reﬂect the indirect and spatio-temporally imprecise nature of the fMRI signal, but

these studies remain informative by virtue of the fact that usually the whole brain

is covered. Not only does fMRI reveal plausible brain regions for the control of

localized effects, but the distribution of response foci and the correlation of effects

observed at many different sites can assist in the guidance of detailed studies at the

mesoscopic or microscopic spatio-temporal level. A prudently modest view might

conclude that fMRI is at present primarily a tool of exploratory rather than explan-

atory value.

Section II provides a large overview of studies about neural representations in

the motor cortex. Chapter 3 focuses on the neuromuscular evolution of individuated

ﬁnger movements.

de Oliveira

notes that from their earliest recognition, oscillatory EEG

signals in the sensorimotor cortex have been associated with stasis: a lack of move-

ment, static postures, and possibly physiological tremor. It is now established that

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