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Neurobiology and Neurology

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Discovering the Brain of Synesthesia

A person with synesthesia might feel the flavor of food on her fingertips, sense the letter “J” as shimmering magenta or the number “5” as emerald green, hear and taste her husband's voice as buttery golden brown. Synesthetes rarely talk about their peculiar sensory gift--believing either that everyone else senses the world exactly as they do, or that no one else does. Yet synesthesia occurs in one in twenty people, and is even more common among artists. One famous synesthete was novelist Vladimir Nabokov, who insisted as a toddler that the colors on his wooden alphabet blocks were “all wrong.” His mother understood exactly what he meant because she, too, had synesthesia. Nabokov’s son Dmitri, who recounts this tale in the afterword to this book, is also a synesthete--further illustrating how synesthesia runs in families.

In Wednesday Is Indigo Blue, pioneering researcher Richard Cytowic and distinguished neuroscientist David Eagleman explain the neuroscience and genetics behind synesthesia's multisensory experiences. Because synesthesia contradicted existing theory, Cytowic spent twenty years persuading colleagues that it was a real--and important--brain phenomenon rather than a mere curiosity. Today scientists in fifteen countries are exploring synesthesia and how it is changing the traditional view of how the brain works. Cytowic and Eagleman argue that perception is already multisensory, though for most of us its multiple dimensions exist beyond the reach of consciousness. Reality, they point out, is more subjective than most people realize. No mere curiosity, synesthesia is a window on the mind and brain, highlighting the amazing differences in the way people see the world.

New Perspectives

The notion that neurons in the living brain can change in response to experience—a phenomenon known as "plasticity"—has become a major conceptual issue in neuroscience research as well as a practical focus for the fields of neural rehabilitation and neurodegenerative disease. Early work dealt with the plasticity of the developing brain and demonstrated the critical role played by sensory experience in normal development. Two broader themes have emerged in recent studies: the plasticity of the adult brain (one of the most rapidly developing areas of current research) and the search for the underlying mechanisms of plasticity—explanations for the cellular, molecular, and epigenetic factors controlling plasticity. Many scientists believe that achieving a fundamental understanding of what underlies neuronal plasticity could help us treat neurological disorders and even improve the learning capabilities of the human brain.

This volume offers contributions from leaders in the field that cover all three approaches to the study of cerebral plasticity. Chapters treat normal development and the influences of environmental manipulations; cerebral plasticity in adulthood; and underlying mechanisms of plasticity. Other chapters deal with plastic changes in neurological conditions and with the enhancement of plasticity as a strategy for brain repair.

Most neurons in the brain are covered by dendritic spines, small protrusions that arise from dendrites, covering them like leaves on a tree. But a hundred and twenty years after spines were first described by Ramón y Cajal, their function is still unclear. Dozens of different functions have been proposed, from Cajal’s idea that they enhance neuronal interconnectivity to hypotheses that spines serve as plasticity machines, neuroprotective devices, or even digital logic elements. In Dendritic Spines, leading neurobiologist Rafael Yuste attempts to solve the “spine problem,” searching for the fundamental function of spines. He does this by examining many aspects of spine biology that have fascinated him over the years, including their structure, development, motility, plasticity, biophysical properties, and calcium compartmentalization. Yuste argues that we may never understand how the brain works without understanding the specific function of spines. In this book, he offers a synthesis of the information that has been gathered on spines (much of which comes from his own studies of the mammalian cortex), linking their function with the computational logic of the neuronal circuits that use them. He argues that once viewed from the circuit perspective, all the pieces of the spine puzzle fit together nicely into a single, overarching function. Yuste connects these two topics, integrating current knowledge of spines with that of key features of the circuits in which they operate. He concludes with a speculative chapter on the computational function of spines, searching for the ultimate logic of their existence in the brain and offering a proposal that is sure to stimulate discussions and drive future research.

Consciousness as a Biological Phenomenon

The question of consciousness is perhaps the most significant problem still unsolved by science. In Inner Presence, Antti Revonsuo proposes a novel approach to the study of consciousness that integrates findings from philosophy, psychology, and cognitive neuroscience into a coherent theoretical framework. Arguing that any fruitful scientific approach to the problem must consider both the subjective psychological reality of consciousness and the objective neurobiological reality, Revonsuo proposes that the best strategy for discovering the connection between these two realities is one of "biological realism," using tools of the empirical biological sciences. This approach, which he calls the "biological research program," provides a theoretical and philosophical foundation that contemporary study of consciousness lacks.

Revonsuo coins the term "world simulation metaphor" and uses this metaphor to develop a powerful way of thinking about consciousness as a biological system in the brain. This leads him to propose that the dreaming brain and visual consciousness are ideal model systems for empirical consciousness research. He offers a comprehensive overview and critical analysis of consciousness research and defends his approach against currently popular philosophical views, in particular against approaches that deny or externalize phenomenal consciousness, or claim that brain activity is not sufficient for consciousness. He systematically examines the principal issues in the science of consciousness—the contents of consciousness, the unity of consciousness and the binding problem, the explanatory gap and the neural correlates of consciousness, and the causal powers and function of consciousness.

Revonsuo draws together empirical data from a wide variety of sources, including dream research, brain imaging, neuropsychology, and evolutionary psychology, into the theoretical framework of the biological research program, thus pointing the way toward a unified biological science of consciousness. Applying imaginative thought experiments, Inner Presence reaches beyond the current state-of-the-art, revealing how the problem of consciousness may eventually be solved by future science.

The thalamus plays a critical role in perceptual processing, but many questions remain about what thalamic activities contribute to sensory and motor functions. In this book, two pioneers in research on the thalamus examine the close two-way relationships between thalamus and cerebral cortex and look at the distinctive functions of the links between the thalamus and the rest of the brain. Countering the dominant "corticocentric" approach to understanding the cerebral cortex—which does not recognize that all neocortical areas receive important inputs from the thalamus and send outputs to lower motor centers—S. Murray Sherman and R. W. Guillery argue for a reappraisal of the way we think about the cortex and its interactions with the rest of the brain.

The book defines some of the functional categories critical to understanding thalamic functions, including the distinctions between drivers (pathways that carry messages to the cortex) and modulators (which can change the pattern of transmission) and between first-order and higher-order thalamic relays—the former receiving ascending drivers and the latter receiving cortical drivers. This second edition further develops these distinctions with expanded emphasis throughout the book on the role of the thalamus in cortical function. An important new chapter suggests a structural basis for linking perception and action, supplying supporting evidence for a link often overlooked in current views of perceptual processing.

It has long been known that aspects of behavior run in families; studies show that characteristics related to cognition, temperament, and all major psychiatric disorders are heritable. This volume offers a primer on understanding the genetic mechanisms of such inherited traits. It proposes a set of tools--a conceptual basis--for critically evaluating recent studies and offers a survey of results from the latest research in the emerging fields of cognitive genetics and imaging genetics. The chapters emphasize fundamental issues regarding the design of experiments, the use of bioinformatic tools, the integration of data from different levels of analysis, and the validity of findings, arguing that associations between genes and cognitive processes must be replicable and placed in a neurobiological context for validation. The Genetics of Cognitive Neuroscience aims to give the reader a working understanding of the influence of specific genetic variants on cognition, affective regulation, personality, and central nervous system disorders. With its emphasis on general methodological points, it will remain a valuable resource in a fast-evolving field. ContributorsKristin L. Bigos, Katherine E. Burdick, Jingshan Chen, Aiden Corvin, Jeffrey L. Cummings, Ian J. Deary, Gary Donahoe, Eco J. C. de Geus, Jin Fan, Erika E. Forbes, John Fossella, Terry E. Goldberg, Ahmad R. Hariri, Lucas Kempf, Anil K. Malhotra, Venkata S. Mattay, Lauren M. McGrath. Kristin K. Nicodemus, Francesco Papaleo, Bruce F. Pennington, Michael I. Posner, Danielle Posthuma, John M. Ringman, Shelley D. Smith, Daniel R. Weinberger, Fengyu Zhang

From Libertarian Illusions to a Concept of Natural Autonomy

Neuroscientists routinely investigate such classical philosophical topics as consciousness, thought, language, meaning, aesthetics, and death. According to Henrik Walter, philosophers should in turn embrace the wealth of research findings and ideas provided by neuroscience. In this book Walter applies the methodology of neurophilosophy to one of philosophy's central challenges, the notion of free will. Neurophilosophical conclusions are based on, and consistent with, scientific knowledge about the brain and its functioning.

Walter's answer to whether there is free will is, It depends. The basic questions concerning free will are (1) whether we are able to choose other than we actually do, (2) whether our choices are made intelligibly, and (3) whether we are really the originators of our choices. According to Walter, freedom of will is an illusion if we mean by it that under identical conditions we would be able to do or decide otherwise, while simultaneously acting only for reasons and being the true originators of our actions. In place of this scientifically untenable strong version of free will, Walter offers what he calls natural autonomy—self-determination unaided by supernatural powers that could exist even in an entirely determined universe. Although natural autonomy can support neither our traditional concept of guilt nor certain cherished illusions about ourselves, it does not imply the abandonment of all concepts of responsibility. For we are not mere marionettes, with no influence over our thoughts or actions.

Emotion, Addiction, and Human Behavior

Emotion and addiction lie on a continuum between simple visceral drives such as hunger, thirst, and sexual desire at one end and calm, rational decision making at the other. Although emotion and addiction involve visceral motivation, they are also closely linked to cognition and culture. They thus provide the ideal vehicle for Jon Elster's study of the interrelation between three explanatory approaches to behavior: neurobiology, culture, and choice.

The book is organized around parallel analyses of emotion and addiction in order to bring out similarities as well as differences. Elster's study sheds fresh light on the generation of human behavior, ultimately revealing how cognition, choice, and rationality are undermined by the physical processes that underlie strong emotions and cravings. This book will be of particular interest to those studying the variety of human motivations who are dissatisfied with the prevailing reductionisms.

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Neurobiology, Ideology, and Social Change

Research shows that between birth and early adulthood the brain requires sensory stimulation to develop physically. The nature of the stimulation shapes the connections among neurons that create the neuronal networks necessary for thought and behavior. By changing the cultural environment, each generation shapes the brains of the next. By early adulthood, the neuroplasticity of the brain is greatly reduced, and this leads to a fundamental shift in the relationship between the individual and the environment: during the first part of life, the brain and mind shape themselves to the major recurring features of their environment; by early adulthood, the individual attempts to make the environment conform to the established internal structures of the brain and mind. In Brain and Culture, Bruce Wexler explores the social implications of the close and changing neurobiological relationship between the individual and the environment, with particular attention to the difficulties individuals face in adulthood when the environment changes beyond their ability to maintain the fit between existing internal structure and external reality. These difficulties are evident in bereavement, the meeting of different cultures, the experience of immigrants (in which children of immigrant families are more successful than their parents at the necessary internal transformations), and the phenomenon of interethnic violence. Integrating recent neurobiological research with major experimental findings in cognitive and developmental psychology--with illuminating references to psychoanalysis, literature, anthropology, history, and politics--Wexler presents a wealth of detail to support his arguments. The groundbreaking connections he makes allow for reconceptualization of the effect of cultural change on the brain and provide a new biological base from which to consider such social issues as "culture wars" and ethnic violence.

Recent years have seen a burst of studies on the mouse eye and visual system, fueled in large part by the relatively recent ability to produce mice with precisely defined changes in gene sequence. Mouse models have contributed to a wide range of scientific breakthroughs for a number of ocular and neurological diseases and have allowed researchers to address fundamental issues that were difficult to approach with other experimental models. This comprehensive guide to current research captures the first wave of studies in the field, with fifty-nine chapters by leading scholars that demonstrate the usefulness of mouse models as a bridge between experimental and clinical research.

The opening chapters introduce the mouse as a species and research model, discussing such topics as the mouse's evolutionary history and the mammalian visual system. Subsequent sections explore more specialized subjects, considering optics, psychophysics, and the visual behaviors of mice; the organization of the adult mouse eye and central visual system; the development of the mouse eye (including comparisons to human development); the development and plasticity of retinal projections and visuotopic maps; mouse models for human eye disease (including glaucoma and cataracts); and the application of advanced genomic technologies (including gene therapy and genetic knockouts) to the mouse visual system. Readers of this reference will see that the study of mouse models has already demonstrated real translational prowess in vision research.

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