Brain, Vol. 126, No. 12, 2773-2775,
December 2003
© 2003 Guarantors of Brain
doi: 10.1093/brain/awg258
Book Review |
THE ASYMMETRICAL BRAIN
Department of Psychiatry, Rudolf Magnus Institute of Neuroscience University Medical Center Utrecht, The Netherlands
THE ASYMMETRICAL BRAIN
Edited by Kenneth Hugdahl and Richard Davidson
2003. MIT Press
Price $90. ISBN 0-262-08309-4.
This book provides an update of the scientific research on cerebral asymmetry. In 1995, Brain Asymmetry, from the same editors, appeared. Since then, the field progressed rapidly by the introduction of new neuro-imaging techniques, which enabled more detailed study of anatomical and functional differences between the hemispheres. This book is not an update but a new book that consists of 21 original chapters from various authors, divided over seven major parts that include: animal models of asymmetry and basic asymmetrical functions, neuro-imaging studies, visual asymmetry, auditory asymmetry, emotional asymmetry and applications for neurological and psychiatric disorders.
In part one (animal models and basic functions), Güntürkün provides an extensive review of the visual system of birds. Several bird species have a remarkably better pattern recognition with their right eye–left hemisphere than with the left eye–right hemisphere system. The main advantage of studying birds is the ease with which each eye can be tested separately. The avian optic nerves decussate nearly completely (99.9%), so simple eye caps can be used instead of difficult hemifield projection. The anatomy of the avian visual system displays significant morphological asymmetries: the left hemisphere receives both ipsilateral and contralateral afferents from the tegmental areas, while the right hemisphere receives mainly contralateral afferents. Therefore, the left hemisphere is able to integrate information from both sides of the bird, which gives it a head start for spatial orientation. Asymmetry of the visual system in birds is induced by a tight interplay of genetic and epigenetic factors. Embryo’s of all avian species keep the head so that the right eye is exposed to the light shining through the translucent eggshell, while the left eye is occluded by the body of the embryo. This leftward turn of the head during embryogenesis is probably genetically defined, while early light stimulation is the key epigenetic factor leading to higher activity levels and greater neuronal connectivity that trigger the visual system to develop in an asymmetrical fashion. In the next chapter, Tang describes several experiments by his group in which rat pups are treated with ‘neonatal novelty exposure’, which induced a rightward shift in hippocampal volumetric asymmetry and an increase in right hippocampal neuronal plasticity. Berridge, España and Stalnaker continue this part of the book by describing dopaminergic efferents to the prefrontal cortex. In rats, the neurons projecting to the right (but not to the left) prefrontal cortex are found to be essential for anxiety-regulation and coping behavior.
At the end of the first part, Beaton presents a thorough overview of the more popular theories that try to explain why most, but not all, humans prefer to use the right hand for skilled unimanual tasks. The genetical theories by Annett and McManus are most promising, since they accurately predict the incidence of left-handedness in families. Several other factors, such as early brain trauma, width of the corpus callosum, prenatal testosterone and asymmetries of basal dopaminergic transmission could play an additional role in phenotypic handedness.
Part two focuses on neuro-imaging and brain stimulation studies. Friston discusses several statistical approaches to analyze functional imaging data. Jäncke and Steinmetz provide a comprehensive overview of the findings on structural brain asymmetry. The most pronounced asymmetry is found in the planum temporale, which coincides largely with Wernicke’s area. Asymmetry of this planum is the only anatomical asymmetry that correlates with functional asymmetry. Pascual-Leone and Walsh end part two with their chapter on the application of transcranial magnetic stimulation (TMS) to study asymmetry of cognitive functions in the brain. Single-pulse TMS can be used to probe the excitability of cortical regions and detect facilitation during cognitive tasks. Repetitive TMS can transiently disrupt the function of a cortical target and produce a ‘virtual lesion’. Repetitive TMS of the left hemisphere appears to induce transient sadness, while the same stimulation of the right prefrontal cortex produces happiness. Repetitive TMS eliciting speech arrest may be used to determine cerebral dominance, though presently accuracy may not be high enough to replace the Wada-test for presurgical research.
The third part is dedicated to visual laterality. Banich presents a series of visual half-field experiments that suggests that the hemispheres can dynamically couple and decouple their processing. When task demands are low, the hemispheres work independently. When the task becomes more complex, one hemisphere is not enough and both hemispheres become engaged in the task. It appears that division of a task over both hemispheres is a mechanism of the brain to increase cognitive brain capacity. Then, Laeng, Chabris and Kosslyn hypothesize that there is a split of incoming visual information into two networks (the hemispheres) that process different types of spatial information. The right hemisphere network preferably uses metric spatial representation, in a coordinate space. The network of the left hemisphere uses abstract relations between locations.
Saron, Foxe, Schroeder and Vaughan proceed with findings on reaction times of motor responses with the hand contralateral to hemifield visual stimuli. They found large intra-individual differences in reaction time, which proved to be a function of the route of inter-hemispheric transfer: the central route via the supplementary motor areas resulted in much faster reaction times than the posterior, visual route.
Part four is devoted to auditory laterality. Zattore shows that hemispheric specialization also exists for the processing of tonal information. He concludes from his work that the left auditory cortex is specialized for temporal processing, while the right is more suited for spectral processing. Hugdahl describes the dichotic listening paradigm: two different auditory stimuli are presented simultaneously to the two ears. The subject is required to report the best perceived stimulus. The author collected data of 1018 subjects performing this paradigm. 74% of the sample showed a right ear advantage (REA), reflecting left cerebral dominance for language, 20% showed a left ear advantage, and 6% showed no ear advantage. There were clear effects of handedness and age in the dataset, but no sex effect. O’Leary elaborates on this theme and argues that attention directed to either the left or the right ear can influence the REA. Attention probably acts at both early (perceptual) and late (response selection) stages to enhance processing of stimuli at the attended location.
Part five discusses human emotional laterality. Pizzagalli, Shackman and Davidson describe an interesting set of functional imaging studies on emotion processing. Some, but not all of these studies report lateralized activation patterns. Those studies that do find asymmetry mostly report a preponderance of left sided activation of prefrontal cortex and amygdala during positive emotions and a more right lateralized activation of the same structures (but at slightly different locations) during aversive stimuli. In addition, Heller, Koven and Miller report on EEG studies suggesting that asymmetries in activation exist between several emotional states. Two components may especially contribute to this asymmetry: the first is ‘valence’ of a stimulus. Stimuli with a positive valence generally lead to increased activation of the left hemisphere, while conditions with negative valence lead to increased activation of the right hemisphere. The second variable is ‘arousal’: conditions with high arousal (fear or sexual arousal) specifically lead to increased right sided activation, while states with low arousal (gloominess or happiness) produce more left hemisphere activation. Coan and Allen complete this part and discuss resting measures of frontal EEG asymmetry, e.g. trait asymmetry, which is associated with characteristics such as sociability, shyness, trait-anger and risk for depression.
In part six, lateralization is studied in subjects with neurological disorders. Lassonde and Sauerwein present an intriguing chapter on agenesis of the corpus callosum. Unlike patients that have undergone an operative section of the corpus callosum, subjects born with agenesis of the corpus callosum show little if any cognitive deficits. Language lateralization was found to be normal or even increased in these subjects. Eckert and Leonard studied patients with dyslexia. In contrast to earlier reports, they argue that planum temporale asymmetry is normal in dyslexic subjects. Rather, anomalous asymmetry of the planum may be associated with more global and more severe language disorders. Habib and Robichon summarize the basic asymmetrical aspects of brain anatomy, which was already done in an earlier chapter.
The final part is dedicated to lateralization in psychiatric disorders. Bruder concludes from behavioral, electrophysiological and neuro-imaging studies of his lab that depressed patients, without comorbid anxiety, generally show left prefrontal hypoactivity at rest, which could be related to decreased motivational drive. They also show right parieto-temporal hypoactivity. Patients with this typical pattern of cerebral activation may respond better to anti-depressive medication than patients with atypical activity patterns. Patients with comorbid anxiety may show hypoactivity of the left parieto-temporal region. Green, Sergi and Kern round of with their chapter on lateralization in schizophrenia. They argue that schizophrenia is associated with more left- and mixed handedness. Anatomically, there may be decreased asymmetry of the planum temporale and an atypical brain torque. Most dichotic listening studies show decreased REA in schizophrenia patients.
Altogether this book offers an extensive collection of chapters that describe lateralization of several cerebral functions in animals, in healthy humans and in patients with neurological and psychiatric disorders. Most authors provide an overview of the main findings of studies on their topic and end with their own theories and conclusions. Those were the most useful chapters. Some of the authors only gave detailed reports of their own studies, which makes it difficult to judge the scientific value of these findings because it is not clear if the data provided are consistent with findings of other groups.
A more general problem, unavoidable for most scientific books, is that science progresses faster than publishing. Especially the part on neuro-imaging findings is already outdated. The latest references are from 2000, while many new findings have become available over the last two years.
The editors have evidently made the choice to focus on neurologically oriented research, including asymmetry of motor and language functions. Apart from such studies, a large body of literature exists on psychological aspects of cerebral lateralization, such as the relationship between cerebral lateralization and cognitive functions or between lateralization and personality characteristics. Several studies suggest that left-handed subjects (who presumably have a lower degree of lateralization) may be disadvantaged for motor and language skills. On the other hand, some studies suggest that subjects with lower degrees of language lateralization may have special talents, such as increased creativity, which could explain why left-handedness has not become extinct. The stereotype left-hander presenting the latter concept is Leonardo da Vinci, who could write and paint with both hands, and had superior spatial abilities. Though this psychological literature may be older and less consistent, it has not been reviewed thoroughly although it may have been intersting to the more general readers.
A limitation of the book is that most authors fail to go beyond the level of describing the differences in qualities of the hemispheres. Few try to answer questions such as why the brain is lateralized, why hand preference and language dominance are correlated, how lateralization arose, and when it arose (was it indeed the speciation event that separated the Hominid-line from that of the chimpanzee and bonobo?). Another important question is how lateralization of the hemispheres is maintained. Is it the left hemisphere that exerts inhibiting effects on the right hemisphere via the corpus callosum? Or could it be that a subcortical structure, such as the thalamus, selectively facilitates the hemisphere that is specialized for a certain function? Furthermore, many brain researchers may have wondered why they always perceive themselves as having one mind, while in fact our higher cognitive functions are divided over two almost identical cerebral hemispheres, loosely connected by a few million fiber tracts. Lassonde and Sauerwein demonstrated in their chapter that this is also true for subjects with agenesis of the corpus callosum. However, no explanation is provided to solve this paradox. Adding more depth to the topics would make this book attractive to a wider readership, encouraging more scientists to develop a research interest in this intriguing and important aspect of the human brain.
The understandable choice of the editors to restrict the content to neurological studies has resulted in a book with high scientific merit, that can be recommended to scientists and clinicians who are affiliated with this research field. However due to this restriction it may be less attractive for readers with a more general interest in brain functioning.
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