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Human laterality
BOOK REVIEW brain idea has been dtstorted and blown out of proportion, lateralized adult cortical control does raise some important developmental questions”. There collapses one of the foundation stones of neuropsychology. Methodological concerns of a second section are those one would expect to be discussed in a psychology text with proper sensitivity to problems of validity, replication and research design. Vulnerable and critical periods are discussed with reference to hypotheses about neural differentiation, but no clear concept emerges of how, to evaluate plasticity in relation to morphogenetic determination of functions. Cerebral disconnection effects in adults and children (including agenesis of the corpus callosum) are not related to a positive theory of how hemispheric or other differentiations ofcortical function come about; they are left in the confusion of ideas about plasticity, cross-cueing and supposed equipotentiality of immature hemispheres. One third of the 380 page text concerns “Disorders of Development and their Consequences” (i.e. genetic, structural, or consequent on prematurity, low birth weight, infections, intoxications, poor nutrition, anoxia, brain injury and neoplasms, epilepsy or hemispherectomy). This is a well-planned introduction to paediatric disorders; but in what measure is information in this detail relevant in a text on developmental neuropsychology? The final section of the book extends further this broad treatment, covering minimal brain dysfunction, motor disorders, attention disorders and hyperactivity and then, finally, in the last 100 pages, six chapters on disorders of vision, audition, language, cognition, learning and emotion (with two more authors co-opted to help in two of these). The brief Epilogue admits that, “this volume attempts to survey a field of considerable magnitude”, and that, “it was difficult to resist the temptation to lose sight of this goal and to elaborate on the richness of theory and research in one area”. The goal referred to was to view the biopsychological development of the child as a continuing process, needing follow through to find out what the outcome ofstress or damage to the nervous system might be. Well, I feel the absence of a plain theoretical framework. Without a clear view on brain development and some scheme to interpret effects of injury or disease on a psychological growth one just doesn’t get a developmental neuropsychology, however well one covers the large and lively literature from adjunct fields. It is a pity, because many parts of the book are clear and informative. It will serve as a very useful guide to the literature. COLWY\I TREVARTHEN
Human Laterality. M. C. CORBALLIS. Perspecticrs in Neurolinguistics, Neurqs~chology, and Psycholinguisfics: Series of Monographs and Treastises. Academic Press, New York, 1983, 255 pp., $29.50. WHAT
IS ASYMMETRIC
IN THE BRAIN
AND
HOW
DOES
o
IT GET THERE?
IN 1976 CORBALLISand BEALE[2] published a readable, intelligent and up-to-date book on left/right symmetries and asymmetries in nature and human psychology. Later CORBALLISand MORGAN [3] and MORGANand CORBALLIS[6] led a peer debate about the origins and inheritance of lateral asymmetry, stimulating ideas, some clear, some confused, on the epigenetics of hand preferences and asymmetries in perception and cognition. Morgan hypothesized that all one-sided psychological elfects, in common with other morphological asymmetries up and down the animal kingdom, were due to extra- or infra-genetic principles, such as a sidedness in the surface structure of the oocyte, or a twist in the fundamental structure of matter. Genes, they argued, were left,+ight agnostic and could only regulate the degree of expression of a sidedness put into the body from another source. They proposed that organisms more often than not develop under the influence of a gradient that causes the left side to grow ahead of the right. An enjoyable feature of the first book was an easy recapitulation of the many references in folklore. mythology, mystic cosmology, religious ceremony and divinatton that have long attracted anthropologists’ attention. It also reviewed philosophical and physical speculations on symmetry. Much attention was paid to how symmetry and asymmetry are perceived, and how children develop, or fail to develop, a consistent left/right sense. The mental complementarity that has been revealed by psychological testing of brain-injured or split-brained patients, and its relation to the left-sided and right-sided cognitions of normal subjects in states ofcontrolled orientation, were lightly touched on. Nevertheless, the book cleverly caught the burgeoning professional and popular interest in possible psychic effects of brain asymmetry. Now Corballis has made another interesting review of ideas on human laterality, concentrating mainly on the latest debates about handedness and hemispheric dominance. It is a scholarly book, one that students will enjoy. But it repeats rather worn naturenurture debates and the discussion of ideas on genetics of cerebral asymmetry sits on the fence. This fault arises from a superficial treatment of brain epigenetics. The brain of a newborn has come a long way from the ectoderm of the embryo or the rind of the egg cell. Like speech, handedness is a fully psychological property of a human being; that must be why the left and right hand are given magical roles in human belief. Actions of the hands in all peoples express differences among the cerebral systems that regulate many interior motives: for manipulation, for gestural and symbolic expression and for
817
BOOK REVIEWS
both technical and communicative skills. Handedness will not be reduced to a simple surface asymmetry ofthe body or its receptors, to one dimension of psychometry or to one gene. Is the concept of one gene for right (or left) handedness really worth entertaining? If, as ANNETT [l] claims and Corballis accepts, statistics on hand preferences for a few standard skills in large populations seem to show evidence of control by a single transmissible ‘right-shift’ factor, this must reflect some watershed in the segregation of neurone systems at a late stage of brain morphogenesis. No, the voice of any gene involved, and there are probably many, must be heard transformed through many stages of negotiation and selection at the cell and system levels of the brain. The data on handedness quotients are just not sensitive enough to resolve the process. Ofcourse. we do not know and will not know soon, how development of brain tissue generates hand preferences; or, indeed, asymmetry in other mental skills, such as expressing and perceiving emotions in the face or voice. or making and perceiving messages in speech or text. So psychologists have to seek provisional rules of thumb and behavioural geneticists have to score the occurrence in families, among twins and in adopted children of whatever measures of behaviour seem the most appropriate, and make best guesses of a genetic model. The crucial thing is to avoid reduction of the motives for hand activity to either reflex responses or conditioned skills. The evidence as reviewed here is compatible with the view that segregation and selective elimination among many largely mirror symmetric neural arrays in the brain produces many possible complementary functional systems in the hemispheres, Segregation patterns in mature neural networks carry traces of events in early segmentation of the embryo, in fetal stages of neurogenesis and neural migration and in infancy when synaptic arrays are blossoming. We know, for example, that a great proportion of cells and axon collaterals may be weeded out in early infancy (INNOCENTI [4]) and this could accentuate pre-existing asymmetries or create new ones. Many genes are likely to have a voice in regulating this great choreography. Hopefully, cognitive psychologists now turning attention to the cerebral programming of movement and perceptual guidance of movement will give us more suitable concepts of what motivates hands to behave in complementary ways. First, human hands are innately part of an endowment for communication. From early infancy gestures accompany socially stimulated efforts to vocalize or speak. Asymmetries in these movements need to be charted. .A different set of motivating principles governs the use of the hands as partners in the handling of objects. Recently MACNEILAGE et al. [S] have supported a thesis that even monkeys have a consistent ‘genetic’ asymmetry in hand use for complex tasks, especially for extracting small things from a context that has to be first acquired and stabilized. They say monkeys use the left hand to maintain a frame in which smaller right-hand acts are performed, and they propose that this was the evolutionary forerunner of the mechanism that assembles and analyses phonemes in words, and words in phrases. There must, indeed, be many cerebral systems making a bid for control of the hands. Human hands, especially, are agents for many facets of mental life, linking the intersubjective with the subject’s regulation of his actions as an isolated self-controlling operator on physical surroundings. There is evidence that the cerebral systems that generate and understand American Sign Language in proficient deaf signers line up with the left-hemisphere systems that generate and understand speech in hearing speakers. This control of a highly and ‘innately’ asymmetric skill has extensive genetic roots indeed. Corballis’ book concisely reviews developmental and evolutionary aspects of laterality and congenital disorders of speech and language. He has an exceptional knowledge of the sometimes amazingly insightful comments of scholars of the past and he knows the modern literature. It will be a useful reference source. COLWYN
TREVARTHEN
REFERENCES I. ANNETT, M. The genetics of handedness. 7iierid.s Nrurosci. 3, 256 258. 19X I. 2. CORBALLIS. M. C. and BEALE. I. L. Tile P.st~+ioloy,~ of Lc/t und Ri~ghrlht. Erlbaum,
Hillsdale. N.J.. 1976. 3. CORBALLIS, M. C. and MORC;N, M. J. On the biological basis ofhuman laterality: I. Evidence for a maturational left-right gradient. Bchat~/Bruit Ser.I,261-269, 1978. 4. INNOCENTI. G. M. Exuberant callosal projections between the developing hemispheres. In AJr~nce.r in ,V~,urotrtru,,?~lro/~?cli,. R. VIL.LANI.I. PAPO. M. GIOVANELLI, S. M. GAIXI and G. TOMEI (Editors), pp. 5-- 10. Excerpta Mrdica. Amsterdam, 19X3. >r MACNEILA~E. P. F.. STUDDERT-KEN~,XDY.M. G. and LINDBLOM. B. Functional precursors to language and its lateralization. Am. J. Physiol. 246, (Regularor~ lrlteyratice Comparatiw Physiology 15) R912-R914, 1984. 6. MORGAN, M. J. and CORBALLIS, M. C. On the biological basis of human laterality: II. The mechanisms of inheritance. Brhcrr I Brtri~ .%i. I, 270-277, 1978.
Human Laterality. M. C. CORBALLIS. Perspecticrs in Neurolinguistics, Neurqs~chology, and Psycholinguisfics: Series of Monographs and Treastises. Academic Press, New York, 1983, 255 pp., $29.50. WHAT
IS ASYMMETRIC
IN THE BRAIN
AND
HOW
DOES
o
IT GET THERE?
IN 1976 CORBALLISand BEALE[2] published a readable, intelligent and up-to-date book on left/right symmetries and asymmetries in nature and human psychology. Later CORBALLISand MORGAN [3] and MORGANand CORBALLIS[6] led a peer debate about the origins and inheritance of lateral asymmetry, stimulating ideas, some clear, some confused, on the epigenetics of hand preferences and asymmetries in perception and cognition. Morgan hypothesized that all one-sided psychological elfects, in common with other morphological asymmetries up and down the animal kingdom, were due to extra- or infra-genetic principles, such as a sidedness in the surface structure of the oocyte, or a twist in the fundamental structure of matter. Genes, they argued, were left,+ight agnostic and could only regulate the degree of expression of a sidedness put into the body from another source. They proposed that organisms more often than not develop under the influence of a gradient that causes the left side to grow ahead of the right. An enjoyable feature of the first book was an easy recapitulation of the many references in folklore. mythology, mystic cosmology, religious ceremony and divinatton that have long attracted anthropologists’ attention. It also reviewed philosophical and physical speculations on symmetry. Much attention was paid to how symmetry and asymmetry are perceived, and how children develop, or fail to develop, a consistent left/right sense. The mental complementarity that has been revealed by psychological testing of brain-injured or split-brained patients, and its relation to the left-sided and right-sided cognitions of normal subjects in states ofcontrolled orientation, were lightly touched on. Nevertheless, the book cleverly caught the burgeoning professional and popular interest in possible psychic effects of brain asymmetry. Now Corballis has made another interesting review of ideas on human laterality, concentrating mainly on the latest debates about handedness and hemispheric dominance. It is a scholarly book, one that students will enjoy. But it repeats rather worn naturenurture debates and the discussion of ideas on genetics of cerebral asymmetry sits on the fence. This fault arises from a superficial treatment of brain epigenetics. The brain of a newborn has come a long way from the ectoderm of the embryo or the rind of the egg cell. Like speech, handedness is a fully psychological property of a human being; that must be why the left and right hand are given magical roles in human belief. Actions of the hands in all peoples express differences among the cerebral systems that regulate many interior motives: for manipulation, for gestural and symbolic expression and for
817
BOOK REVIEWS
both technical and communicative skills. Handedness will not be reduced to a simple surface asymmetry ofthe body or its receptors, to one dimension of psychometry or to one gene. Is the concept of one gene for right (or left) handedness really worth entertaining? If, as ANNETT [l] claims and Corballis accepts, statistics on hand preferences for a few standard skills in large populations seem to show evidence of control by a single transmissible ‘right-shift’ factor, this must reflect some watershed in the segregation of neurone systems at a late stage of brain morphogenesis. No, the voice of any gene involved, and there are probably many, must be heard transformed through many stages of negotiation and selection at the cell and system levels of the brain. The data on handedness quotients are just not sensitive enough to resolve the process. Ofcourse. we do not know and will not know soon, how development of brain tissue generates hand preferences; or, indeed, asymmetry in other mental skills, such as expressing and perceiving emotions in the face or voice. or making and perceiving messages in speech or text. So psychologists have to seek provisional rules of thumb and behavioural geneticists have to score the occurrence in families, among twins and in adopted children of whatever measures of behaviour seem the most appropriate, and make best guesses of a genetic model. The crucial thing is to avoid reduction of the motives for hand activity to either reflex responses or conditioned skills. The evidence as reviewed here is compatible with the view that segregation and selective elimination among many largely mirror symmetric neural arrays in the brain produces many possible complementary functional systems in the hemispheres, Segregation patterns in mature neural networks carry traces of events in early segmentation of the embryo, in fetal stages of neurogenesis and neural migration and in infancy when synaptic arrays are blossoming. We know, for example, that a great proportion of cells and axon collaterals may be weeded out in early infancy (INNOCENTI [4]) and this could accentuate pre-existing asymmetries or create new ones. Many genes are likely to have a voice in regulating this great choreography. Hopefully, cognitive psychologists now turning attention to the cerebral programming of movement and perceptual guidance of movement will give us more suitable concepts of what motivates hands to behave in complementary ways. First, human hands are innately part of an endowment for communication. From early infancy gestures accompany socially stimulated efforts to vocalize or speak. Asymmetries in these movements need to be charted. .A different set of motivating principles governs the use of the hands as partners in the handling of objects. Recently MACNEILAGE et al. [S] have supported a thesis that even monkeys have a consistent ‘genetic’ asymmetry in hand use for complex tasks, especially for extracting small things from a context that has to be first acquired and stabilized. They say monkeys use the left hand to maintain a frame in which smaller right-hand acts are performed, and they propose that this was the evolutionary forerunner of the mechanism that assembles and analyses phonemes in words, and words in phrases. There must, indeed, be many cerebral systems making a bid for control of the hands. Human hands, especially, are agents for many facets of mental life, linking the intersubjective with the subject’s regulation of his actions as an isolated self-controlling operator on physical surroundings. There is evidence that the cerebral systems that generate and understand American Sign Language in proficient deaf signers line up with the left-hemisphere systems that generate and understand speech in hearing speakers. This control of a highly and ‘innately’ asymmetric skill has extensive genetic roots indeed. Corballis’ book concisely reviews developmental and evolutionary aspects of laterality and congenital disorders of speech and language. He has an exceptional knowledge of the sometimes amazingly insightful comments of scholars of the past and he knows the modern literature. It will be a useful reference source. COLWYN
TREVARTHEN
REFERENCES I. ANNETT, M. The genetics of handedness. 7iierid.s Nrurosci. 3, 256 258. 19X I. 2. CORBALLIS. M. C. and BEALE. I. L. Tile P.st~+ioloy,~ of Lc/t und Ri~ghrlht. Erlbaum,
Hillsdale. N.J.. 1976. 3. CORBALLIS, M. C. and MORC;N, M. J. On the biological basis ofhuman laterality: I. Evidence for a maturational left-right gradient. Bchat~/Bruit Ser.I,261-269, 1978. 4. INNOCENTI. G. M. Exuberant callosal projections between the developing hemispheres. In AJr~nce.r in ,V~,urotrtru,,?~lro/~?cli,. R. VIL.LANI.I. PAPO. M. GIOVANELLI, S. M. GAIXI and G. TOMEI (Editors), pp. 5-- 10. Excerpta Mrdica. Amsterdam, 19X3. >r MACNEILA~E. P. F.. STUDDERT-KEN~,XDY.M. G. and LINDBLOM. B. Functional precursors to language and its lateralization. Am. J. Physiol. 246, (Regularor~ lrlteyratice Comparatiw Physiology 15) R912-R914, 1984. 6. MORGAN, M. J. and CORBALLIS, M. C. On the biological basis of human laterality: II. The mechanisms of inheritance. Brhcrr I Brtri~ .%i. I, 270-277, 1978.