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Chapter 22: Low-Level Visual Processing: The Retina

Introduction

THE RETINA IS THE BRAIN’S WINDOW on the world. All visual experience is based on information processed by this neural circuit in the eye. The retina’s output is conveyed to the brain by just one million optic nerve fibers, and yet almost half of the cerebral cortex is used to process these signals. Visual information lost in the retina—by design or deficiency—can never be recovered. Because retinal processing sets fundamental limits on what can be seen, there is great interest in understanding how the retina functions.

On the surface, the vertebrate eye appears to act much like a camera. The pupil forms a variable aperture, and the cornea and lens provide the refractive optics that project a small image of the outside world onto the light-sensitive retina lining the back of the eyeball (Figure 22–1). But this is where the analogy ends. The retina is a thin sheet of neurons, a few hundred micrometers thick, composed of five major cell types that are arranged in three cellular layers separated by two synaptic layers (Figure 22–2).

Figure 22–1

The eye projects the visual scene onto the retina’s photoreceptors.

A. Light from an object in the visual field is refracted by the cornea and lens and focused onto the retina.

B. In the foveola, corresponding to the very center of gaze, the proximal neurons of the retina are shifted aside so light has direct access to the photoreceptors.

C. A letter from the eye chart used to assess normal visual acuity is projected onto the densely packed photoreceptors in the fovea. Although less sharply focused than shown here as a result of diffraction by the eye’s optics, the smallest discernible strokes of the letter are approximately one cone diameter in width. (Adapted, with permission, from Curcio and Hendrickson 1991. Copyright © 1991 Elsevier Ltd.)

Three diagrams show the refraction of light onto the retina, focusing of light in the fovea, and packing of photoreceptors in the fovea.
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Figure 22–2

The retina comprises five distinct layers of neurons and synapses.

A. A perpendicular section of the human retina seen through the light microscope. Three layers of cell bodies are evident. The outer nuclear layer contains cell bodies of photoreceptors; the inner nuclear layer includes horizontal, bipolar, and amacrine cells; and the ganglion cell layer contains ganglion cells and some displaced amacrine cells. Two layers of fibers and synapses separate these: the outer plexiform layer and the inner plexiform layer. (Reproduced, with permission, from Boycott and Dowling 1969. Permission conveyed through Copyright Clearance Center.)

B. Neurons in the retina of the macaque monkey based on Golgi staining. The cellular and synaptic layers are aligned with the image in part A. (Abbreviations: M ganglion, magnocellular ganglion cell; P ganglion, parvocellular ganglion cell.) (Reproduced, with permission, from Polyak 1941.)

A micrograph and a diagram show the neurons in the retinal layers.
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The photoreceptor cells, in the outermost layer, absorb light and convert ...

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