Laterality Functional Asymmetry in the Intact Brain

318 Pages·1982·39.558 MB·English

by  M. Bryden (Auth.)

LATERALITY
Functional Asymmetry in the Intact Brain
M. P. Bryden
Department of Psychology
University of Waterloo
Waterloo, Ontario, Canada
1982
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Library of Congress Cataloging in Publication Data
Bryden, M. P.
Laterality : functional asymmetry in the intact brain.
(Perspectives in neurolinguistics, neuropsychology,
and psycholinguistics)
Includes bibliographical references and index.
1. Cerebral dominance. 2. Laterality. I. Title.
II. Series. [DNLM: 1. Laterality. WL 335 B916L]
QP385.5.B79 612'.825 82-6692
ISBN 0-12-138180-3 AACR2
PRINTED IN THE UNITED STATES OF AMERICA
82 83 84 85 9 8 7 6 5 4 3 2 1
To my three P's
PAT, PENNY, AND PAM
who keep me going, each in her own way.
Preface
This book grew from a concern with the sweeping generalizations
about brain function and laterality that have become popular in recent
years. When I first became interested in the possibility that behavioral
techniques could be used to study the functional specialization of the
brain in normal individuals, there were only a few relevant papers.
Now, dozens of new articles appear each month, and an understanding
of the differences between the two hemispheres is seen by some re­
searchers as leading to the solution of almost all problems—from psy-
chopathology and dyslexia, to stuttering and management effective­
ness.
As I reviewed the literature, however, I found it important to adopt a
more conservative stance. Cerebral lateralization is one factor affecting
performance on a wide variety of behavioral tasks, but it is only one of
many factors. Unless we understand the demands of the specific tasks
chosen and the strategies subjects use to meet these demands, we risk
the danger of overinterpreting the results of experiments on laterality.
Thus, the central message of this book is that not all behavioral asymme­
tries are necessarily related to the differing functions of the two
hemispheres.
A major goal of the book is to provide a single source that will intro­
duce the reader to the various methods used to assess behavioral asym­
metries. The initial chapters review the literature on perceptual-cogni­
tive laterality effects in different sensory modalities, for it is research of
this type that has generated much of the enthusiasm for laterality stud­
ies. These chapters indicate some of the problems with the existing
research and offer suggestions about the direction of future research.
Chapters 7-9 deal with the areas in which laterality research has become
popular: lateralization of emotion and of motor behavior, and the elec­
trophysiological evidence.
For many people, handedness is one of the most salient asymmetries.
xi
xii Preface
General statements about brain organization and cerebral laterality are
often prefaced by the statement that they do not apply to left-handers.
Chapters 10 and 11 deal with the measurement and origins of handed­
ness and indicate the extent to which handedness must be considered to
be an important variable in laterality research.
In a broad sense, Chapters 12-16 are all concerned with problems of
individual differences in laterality. First, I examine the question of
whether laterality changes with age in young children. Second, I con­
sider the data on laterality effects in special groups, especially those
studies dealing with sex differences and with language-impaired sub­
jects. Third, I consider the evidence for hemisphericity as a cognitive
style. The concluding chapter summarizes the major points of the book
and presents a general framework for laterality research in the normal
subject.
This book is addressed primarily to neuropsychologists, experimental
psychologists, neurologists, and educators. I am convinced that experi­
mental psychology has a major contribution to make to the understand­
ing of human brain function. Experimental psychologists can typically
afford to test various hypotheses, develop precise methods, and rule out
alternative explanations by careful experimentation. The practicing neu­
ropsychologist can seldom afford such luxury: A patient with a particu­
lar type of brain damage may appear only rarely, and one cannot try a
dozen alternatives before settling on an appropriate experimental pro­
cedure. In the domain of laterality research, this volume can direct the
neuropsychologist toward those hypotheses that may be most fruitful
and away from those that are less viable.
The book should also provide a similar function for the educator
interested in special groups. Researchers in education often do not have
access to the immense literature of experimental psychology, and yet a
grasp of this literature is essential in planning good studies of dyslexia,
language impairment, and related phenomena. By providing a general
overview of laterality research, the book will give those researchers who
wish to investigate particular types of laterality effects a view of how
other researchers with different viewpoints have approached similar
problems.
The book does not, however, provide a unified theory of lateraliza­
tion of function. The more I read and wrote, the more convinced I
became that there was as yet scant evidence to support any one general
theory. There remain too many unanswered questions and too many
unsolved problems to do any more than point in the direction of an
integration of the laterality data. That will have to wait for more experi­
mentation. In the meantime, this book may suggest fruitful lines of
research.
Acknowledgments
In the preparation of this book, the contributions of two individuals
stand out. Frances Allard started this project as a collaborator, but found
it necessary to withdraw from active participation because of other de­
mands on her time. Fran wrote the initial draft of the chapter on motor
asymmetries and commented critically, often caustically, and always
helpfully, on the entire manuscript. Marion Tapley, my research techni­
cian and colleague, worked above and beyond the call of duty to super­
vise the typing of the manuscript, organize and check references, and
prepare the figures. She made my task vastly easier and prevented
innumerable disasters with her planning and organization.
I must also acknowledge the support of Harry Whitaker, the series
editor. Whit encouraged the project from the beginning, reassured me
when I was most pessimistic, tolerated the lengthy time it took to com­
plete even the first draft, and made wise suggestions about all parts of
the manuscript. I looked forward with some trepidation to the pages of
single-space commentary he sent me on each chapter. Derek Besner,
Ernie MacKinnon, Janice Murray, and Sid Segalowitz also read and
commented on my penultimate draft, and their advice has colored the
final project. Many thanks are also due to Bonnie Lee Bender, Laurie
Westlake Bruin, Bev Dakins, Diane Hagan, Paula Kovacs, and Lyn
Spaetzel who did an outstanding job of typing page upon page of man­
uscript. Finally, my own contribution was aided by a grant from the
Natural Sciences and Engineering Research Council of Canada and by
the willingness of the Department of Psychology at the University of
Waterloo to assist with both typing services and computer facilities.
xiii
Introduction
The notion that the two cerebral hemispheres have different func­
tions and that this is important in our daily lives has become very popu­
lar in the past few years. Such ideas have a long history (Wigan, 1844)
but were given new impetus by the exciting work of Sperry and his
colleagues on the behavior of patients following section of the corpus
callosum and anterior commissure (e.g., Gazzaniga, Bogen, & Sperry,
1962, 1963, 1965). This work revealed that the isolated left hemisphere
perceived, remembered, and responded in a fashion quite different from
the isolated right hemisphere (see Bogen, 1969a, 1969b; Gazzaniga, 1970
for reviews). At about the same time, work by Doreen Kimura (1961a,
1961b, 1967) in auditory perception provided some suggestion that the
different functions of the two hemispheres could be measured in the
normal, intact brain.
The popular appeal of this work has been such that it has led to a vast
number of speculations. Cerebral asymmetries are now taken as givens,
and ideas derived from the study of cerebral asymmetry are used to
explain almost every imaginable kind of behavior, from reading dis­
ability to schizophrenia, from stuttering to the gender-related difference
in spatial ability, from infantile autism to the generation gap.
The objective of the present book is to review the evidence pertaining
to cerebral asymmetries in the intact human brain. We shall be con­
cerned primarily with two issues. First, can one reliably assess the differ­
ent function of the two cerebral hemispheres with noninvasive tech­
niques? Second, do the patterns of cerebral asymmetry relate in any
consistent way to meaningful behavior? One of the viewpoints devel­
oped in this book is that many of the studies supposedly dealing with
cerebral asymmetries do not in fact do so. Nevertheless, sufficient con­
sistencies do exist for one to believe that functional cerebral asymmetries
1
2 1. Introduction
can be measured and that they do have some meaningful consequences
for normal behavior. The aim of the succeeding pages is to provide a
realistic assessment of the current state of the art.
A Simplistic View of Cerebral Asymmetry
Ever since the work of Dax (1865) and Broca (1861) in the mid-nine­
teenth century, it has been known that damage to certain portions of the
left hemisphere results in disturbances of language (aphasias) that do not
occur following damage to the right hemisphere. Thus, in some fashion
the left hemisphere is functionally specialized for at least some language
processes. In the late nineteenth and early twentieth century, there were
a number of reports of aphasias following right-hemispheric damage,
often in individuals who were left-handed or had some familial history
of left-handedness. It was an easy step from these observations to the
notion that left-handers were the reverse of right-handers, using the
right hemisphere rather than the left for speech and language processes.
While current evidence indicates that this is an oversimplification, it is
generally agreed that the incidence of right-hemispheric speech is high­
er in left-handers than in right-handers (Herron, 1980).
Perhaps because disturbances of speech and language are relatively
obvious and important in one's daily life, much of the early clinical
neuropsychological work concentrated on the aphasias and on the func­
tions of the left hemisphere. Only in the past 30 years has much atten­
tion been paid to the right hemisphere and to nonverbal skills. Although
the deficits may not be as striking or as clear-cut, recent evidence has
suggested that the right hemisphere plays an important role in a variety
of nonverbal activities, including music (Gates & Bradshaw, 1977b), spa­
tial abilities (Benton, 1979), face recognition (Geffen, Bradshaw, & Wal­
lace, 1971), and emotional expression (Ley & Bryden, 1981). At a simple
level, then, the left hemisphere can be viewed as concerned with speech
and language, whereas the right hemisphere is concerned with nonver­
bal skills. In the search for a better distinction, the left hemisphere is
often described as analytic or concerned with sequential processing,
whereas the right is considered to be concerned with the integration of
information over space and time, a holistic or gestalt processor (cf.
Bradshaw & Nettleton, 1981).
It is only a small step from this elementary description of functional
asymmetry to the view that those people who excel in verbal skills are
using their left hemispheres in preference to their right and, conversely,
A Simplistic View of Cerebral Asymmetry 3
that those who excel in spatial skills use their right hemispheres. One
might even hope that if we can find out which hemisphere people use,
then we ought to be able to say something about what kind of people
they are, and therefore some assessment of functional specialization
should be useful in the study of individual differences.
Similarly, one can argue that if an individual is poor at verbal skills,
there must be something wrong with his or her left hemisphere. If, for
example, language functions fail to lateralize clearly in one hemisphere,
then an individual might be expected to show some form of language
disturbance. Such an argument has been used to suggest that poor
readers, stutterers, the congenitally deaf, and many other special groups
suffer from a disturbance of functional lateralization. In many of these
cases, the argument also implies that functional lateralization develops
and that the particular group in question suffers from a "developmental
lag."
Not all individuals are left-hemispheric for speech and language (Hé-
caen, DeAgostini, & Monzon-Montes, 1981; Satz, 1980), and likewise, it
is presumed that not everyone is right-hemispheric for nonverbal pro­
cesses. We have known for a long time that left-handers are somewhat
more likely to have right-hemispheric speech, and, certainly, we have
often considered left-handers to be peculiar (Hardyck & Petrinovitch,
1977; L. J. Harris, 1980). Perhaps, then, there is something peculiar
about those individuals who have speech and language functionally
represented in the right hemisphere. This logic leads one to argue that
the particular pattern of functional organization is important and that at
least some left-handers are qualitatively different from the "normal"
right-handers.
Finally, one might argue that we do not use our brain as well as we
might. Most particularly, since the majority of us are left-hemispheric
for speech and since our culture seems to depend very heavily on
speech and language, perhaps we are missing a facet of our education.
There are other cultures less dependent on language, and we may very
well be missing something important by ignoring this aspect of our
experience. With training, we might give significance to integrative,
intuitive (right-hemispheric) behaviors. By implication, our training and
experience can dictate the way in which we use the two hemispheres of
our brain, and we would perhaps be better persons if we made full use
of the potential of our right hemispheres.
The preceding paragraphs should not be construed as a statement of
the position to be espoused in this book, but rather as a resume of
contemporary views concerning the relevance of lateral specialization. It
is intended to give an overview against which the remainder of this book
4 1. Introduction
can be evaluated. From this beginning, the relevant issues can be
addressed.
What Is Lateralized?
It is very easy to argue that if one hemisphere acts in a particular way,
the other hemisphere must do the opposite. The notion of a balanced or
dual asymmetry has been very popular, and it most certainly has led to
such distinctions as verbal-nonverbal or analytic-integrative. Bogen
(1969a, 1969b) and Corballis and Beale (1976) have tabled various views
as to the functional differences between the hemispheres. These tables
make it clear that there is no general agreement as to what it is that
differentiates the functions of the left hemisphere from those of the
right. Certainly, one can search for some dichotomous relationship: At
the same time, one should recognize that a dichotomy is not necessary.
If a particular region of left frontal cortex is important in the expression
of language, it does not imply that the homologous region in the right
hemisphere is important for the "opposite" function, whatever that
might be.
Issues of reliability and validity become an essential part of any dis­
cussion of cerebral laterality. In the middle 1960s, any argument that a
behaviorally measured laterality measure such as those offered by
Kimura (1961a, 1961b) or Bryden (1965) might actually be related to
cerebral asymmetry was greeted with a certain amount of derision (e.g.,
Harcum & Filion, 1963; Inglis, 1965). More recently, however, the pen­
dulum has swung the other way, and virtually any observed lateral
asymmetry is interpreted as being necessarily related to cerebral asym­
metry and virtually any deficit in spatial or verbal function is interpreted
as indicating some malfunction of one or the other cerebral
hemispheres.
One of the basic concerns of this book is that many of the tasks
employed to assess lateral specialization actually measure a diversity of
things (Bryden, 1978). Although a simple behavioral task, such as the
auditory dichotic listening procedure employed by Kimura (1961a), may
very well be related to the lateralization of language, other factors—
attentional biases, strategies of encoding or remembering the stimuli,
and the like—may influence the magnitude of the laterality effect ob­
served and may even reverse the effect under some conditions or for
some people. Without understanding and controlling these extraneous
factors, we run the danger of grossly overinterpreting our results.