Perhaps the most remarkable thing about vision is the utter simplicity of the act of seeing. We open our eyes and a three-dimensional panorama of colored images—some stationary, others in motion—unfolds before us. In most cases, the brain makes sense of this information seemingly instantaneously, allowing us to function reasonably well under a wide range of lighting conditions. The retinal image is constantly changing as we move about and yet the objects around us are perceived as stable. This seemingly effortless nature of sight beguiles the profound complexity of the processes underlying the perception of even the simplest visual stimulus. Indeed, no machine can currently perform the myriad visual recognition tasks we normally take for granted, and it is still unclear whether such technology will become available in the foreseeable future.

Vision is the dominant sense in humans and other primates, with nearly 30% of our cortical surface representing information that is predominantly visual. Reflecting the importance of vision to the formation of human experience, more effort has gone into studying the visual system than any other sensory modality. As a consequence, we have accumulated an impressive amount of information about vision at many different levels, ranging from genes and molecules to theoretical computations. Our long-term objective is to explain how the brain transforms the spatiotemporal patterns defined by the photons impinging on the retina at any given moment into a coherent visual world. The information derived from understanding these basic processes will ultimately help us prevent and treat the many disorders that impair our ability to see.

Almost 10% of people living today suffer from a visual disorder stemming from a defect of the retina or the visual centers of the brain. Effective treatment of these visual impairments is possible in only a few types of cases because we lack the basic knowledge to understand the dysfunction underlying these disorders. Although we have made significant progress in the visual neurosciences, much remains to be done. The scope of the overall effort has intensified in recent years, reflecting in part, the advent of new technologies, ranging from those of modern molecular biology to the functional imaging of the human brain. Such methodologies have now made it possible to pursue a host of previously unanswerable questions.

There is a plethora of professional journals devoted to vision research, and a number of excellent books dealing with perception as well as the neural bases of vision. No single source, however, has attempted to provide a comprehensive and authoritative account of the visual neurosciences. In an attempt to remedy this situation, we invited 100 of the world's leading researchers in this field to summarize their area of specialization in a manner understandable to the nonspecialist. The response by our colleagues was immensely gratifying. Virtually everyone invited agreed to participate, and some suggested the inclusion of additional chapters, so the final number of contributions was increased to 114.

Each chapter was reviewed by other experts, and authors made revisions based on their feedback. As editors we strove to preserve the individual "voice" of each author, and we also agreed to tolerate a certain degree of redundancy across chapters, provided they offered valuable insights into the topic under consideration. The VisualNeurosciences is a work in progress so some disagreement was expected among authors regarding specific issues. We made little attempt to broker a compromise between dissimilar viewpoints held by different authors, as long as these were supported by empirical evidence. Controversy is what often makes science fun, and we leave it for future generations to decide the relative merits of currently held positions.

The Visual Neurosciences begins with two historical chapters and an appraisal of the prospects for single-unit approaches in neurobiology. They are followed by Chapters 3-15 on Developmental Processes. This section, as in the book as whole, is organized from molecules to pathways to systems. The section on Retinal Mechanisms and Processes (Chapters 16-29) presents the current state of knowledge on phototransduction, retinal synapses, and physiology, with authors explaining how these mechanisms ostensibly optimize the processing of visual information. These chapters set the stage for the next section on the Organization of Visual Pathways (Chapters 30-34) and the subsequent elaboration of projections for Subcortical Processing (Chapters 35-40) and for Processing in Primary Visual Cortex (Chapters 41-49). Most of the chapters in these first six sections provide an anatomical and physiological context for understanding the psychophysical, perceptual, and neurophysiological chapters that follow in the next four sections, beginning with Detection and Sampling (Chapters 50-55) and proceeding to higher-level processing of Brightness and Color (Chapters 56-67), Form, Shape, and Object Recognition (Chapters 68-79), and Motion, Depth, and Spatial Relations (Chapters 80-89). These chapters illustrate how 20th Century neuroscience unraveled many phenomenological conundrums of the 19th Century. Of course, 20th Century psychology raised still other challenges for neuroscience, including the role of nonsensory variables in perception and cognition. Sections on Eye Movements (Chapters 90-99) and Attention and Cognition (Chapters 100-104) address these questions with detailed accounts of the coordination of eye position and information processing by subcortical and cortical circuits underlying cognitive phenomena. The final section, Theoretical and Computational Perspectives (Chapters 105-114), provides an integration of ideas from neuroscience, psychology, and computer science that are likely to guide future discoveries in the visual neurosciences.

For an undertaking of this scope, the entire project went remarkably smoothly. For this we thank all of the authors for adhering good-naturedly (in most cases) to the rather tight schedule. We also thank the countless anonymous reviewers, members of the Editorial Advisory Board for their input at all stages of this undertaking, and Barbara Murphy, our editor at the MIT Press, for her support and keen professional advice. It is our hope that The Visual Neurosciences will serve to motivate and inspire the next generation of researchers, whether they are currently beginning students, clinical practitioners, or established researchers in other fields of endeavor.

Leo M. Chalupa and John S. Werner 7 January 2003

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