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Cover illustration: Figure 2, Chapter 1, “An Organ of Exquisite Perfection,” by George Ayoub. Adapted from a diagram from www.webvision.med.utah.edu. Modified by Nancy Fallatt. Back cover images from Figure 1, Chapter 17, “Multifocal Oscillatory Potentials of the Human Retina,” by Anne Kurtenbach and Herbert Jägle.

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PREFACE

Sensitivity to light is a near-universal attribute of living organisms. It can be seen in the tropic responses of plants, in movements of many bacteria and unicellular organisms, as well as in the more complex visual responses of most animals. While the mechanisms of light detection and the behavioral consequences of its detection in these species are a series of fascinating stories, this volume is concerned with vision in its more classical interpretation.

Although ancient philosophers, including Aristotle and Ptolemy, thought that the function of the eye was to emit light and illuminate objects, it has been over a thousand years since the Persian Alhazan (Abu Ali Hasan Ibn al-Haitham) explained that vision was the result of light coming from an object into the eyes. What happened to the light after traversing the optical path of the eyes remained unclear for many centuries. Leonardo daVinci and others of that era thought that light was channeled back to the ventricles of the brain through the optic nerves.

In the early nineteenth century, as the structure of the eye, and particularly the retina, were examined more carefully, it became apparent that vision was linked to a transformation process, which occurred in the retina and specifically in the photoreceptors. The visual pigments and their sensitivity to light were described in the mid-nineteenth century. In the first half of the twentieth century, the pioneering work of Wald showed that the visual pigment was a protein with an attached molecule that had properties of a carotenoid. The identification of this chromophore and its derivatives as retinal and retinol and the enzymatic conversion of one to the other were landmark studies carried out in the laboratories of Morton in England and Wald in the United States. This finding was the cornerstone for the next major breakthrough in our understanding of the visual transduction cascade. In a follow-up study, Wald and his coworkers Hubbard and Brown found that the active visual pigment chromophore was 11-cis retinal, and that light induced a transition in this pigment to the all-trans form. We now know that the light-induced change in the conformation of 11-cis retinal is the fundamental step in converting light energy into chemical energy in the retina.

The next major breakthrough in our understanding of the visual transduction cascade, the conversion of this cis–trans isomerization of the opsin chromophore, part of the rhodopsin complex, into changes in membrane conductance and synaptic signaling took another 30 years to understand. The work during this period showed that the lightsensitive rhodopsin machinery is primarily located in disks that are completely isolated from the plasma membrane but electrical signals involve changes in conductance at the plasma membrane of the rod photoreceptors.

Perhaps the most important realization in this story is that rod photoreceptors need an internal signal molecule. For many years, the two rival candidates for this internal signal were calcium and cyclic guanosine monophosphate (cGMP). Physiological measurements showed that changes occurred in calcium fluxes in rod photoreceptors on illumination, a finding that later led to the identification of a biochemically defined light-sensitive enzymatic machinery that hydrolyzed cGMP. The critical role of cGMP

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CONTRIBUTORS

MICHAEL C. ANTLE, PhD • Department of Psychology, University of Calgary, Calgary, Canada

GEORGE AYOUB, PhD • Neuroscience Research Institute and Department of Molecular Cellular and Developmental Biology, University of California, Santa Barbara, CA

COLIN J. BARNSTABLE, DPHIL • Department of Neural and Behavioral Sciences, Milton S. Hershey Medical Center, Penn State University College of Medicine, Hershey, PA

CAREN BELLMAN, PhD • Centre Hospitalier National d’Ophtalmogie des Quinze-Vingts, and INSERM U 592, Paris, France

JEFFREY H. BOATRIGHT, PhD • Department of Ophthalmology, Emory Eye Center, Emory University, Atlanta, GA

DIANE E. BORST, PhD • Department of Anatomy, Physiology, and Genetics, Uniformed Services University of the Health Sciences, Bethesda, MD

MICHAEL E. BOULTON, PhD • Department of Ophthalmology and Visual Sciences, University of Texas Medical Branch, Galveston, TX

KEELY M. BUMSTED O’BRIEN, PhD • Department of Optometry and Vision Science, University of Auckland, Auckland, New Zealand

YONGEN CHANG, PhD • Dyson Vision Research Institute, Weill Medical College, New York, NY

RICK H. COTE, PhD • Department of Biochemistry and Molecular Biology, University of New Hampshire, Durham, NH

CLAUDE DESPLAN, PhD • Department of Biology, New York University, New York, NY ERIKA D. EGGERS, PhD • Department of Ophthalmology and Visual Sciences,

Washington University School of Medicine, St. Louis, MO

SILVIA C. FINNEMANN, PhD • Dyson Vision Research Institute, Weill Medical College, New York, NY

SABINE FUHRMANN, PhD • Department of Ophthalmology and Visual Sciences, John A. Moran Eye Center, University of Utah, Salt Lake City, UT

SCOTT GLEIM, MS • Department of Pharmacology and Toxicology, Dartmouth Medical School, Hanover, NH

ALECIA K. GROSS, PhD • Department of Vision Sciences, School of Optometry, University of Alabama, Birmingham, AL

ARMIN HUBER, PhD • Department of Biosensorics, Institute of Physiology, University of Hohenheim, 70599 Stuttgart, Germany

JOHN HWA, MD, PhD • Department of Pharmacology and Toxicology and of Medicine, Dartmouth Medical School, Hanover, NH

TOMOMI ICHINOSE, MD, PhD • Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, MO

GERALD H. JACOBS, PhD • Neuroscience Research Institute, University of California, Santa Barbara, CA

HERBERT JÄGLE, MD • University Eye Hospital, Tübingen, Germany

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COMPANION CD

Black and White and Color illustrations are provided on the Companion CD attached to the inside back cover. The image files are organized into folders by chapter number and are viewable in most Web browsers. The CD is compatible with both Mac and PC operating systems.

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An Organ of Exquisite Perfection

George Ayoub

CONTENTS

OPTICAL PATH

RETINAL PHOTORECEPTION

RETINAL PATHWAYS

REFERENCES

OPTICAL PATH

As can be seen in Fig. 1, the eye is a nearly spherical structure, structurally limited by the sclera and cornea. The sclera is the tough white tissue that delimits the outer orbit of the eye, while the cornea is the clear portion in the front. The cornea is a focusing element for the visual path, providing more than half of the focusing power of the eye, with the lens handling the remainder. Focusing of light by the cornea and lens is necessary to create an image on the retina, which is the light-sensitive portion of the eye. This is much like the role of a camera lens in creating a clear image at the CCD (charge-coupled device) or film plane. Indeed, the cornea and lens are the two elements that focus the light, with the cornea fixed in focal length and the lens adjustable. They provide a double positive lens arrangement (i.e., two convex lenses) to accomplish this.

Light is focused based on the cornea’s shape and refractive index. The cornea is a nearly spherical structure, slightly flattened to reduce spherical aberration. The index of refraction for light (n) as it passes through the cornea is 1.376. Since the index of refraction in air is 1.0, this change in the refractive index, along with the cornea’s convex shape, causes light rays to bend in a converging manner as they pass through [1–3].

The lens of the eye has a shape that is malleable, allowing this second focusing element in the visual path to be used to adjust the focal point, which allows us to form a clear image on the retina for objects from near to distant. The shape of the lens changes due to the action of two structures. The connective tissue encompassing the lens keeps it spherical, but the suspensory ligaments that surround it pull it into a flatter shape. The ciliary muscles counteract the pull of these ligaments and allow the lens to become more round. Thus, by contracting the ciliary muscles, we are able to focus on objects that are close at hand by increasing the curvature of the lens, thereby increasing its refractive power. Relaxation of this muscle allows us to focus on objects at a distance.

From: Ophthalmology Research: Visual Transduction and Non-Visual Light Perception

Edited by: J. Tombran-Tink and C. J. Barnstable © Humana Press, Totowa, NJ

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