26: Low-Level Visual Processing: The Retina

Introduction

• The Photoreceptor Layer Samples the Visual Image

  • Ocular Optics Limit the Quality of the Retinal Image

  • There Are Two Types of Photoreceptors: Rods and Cones

• Phototransduction Links the Absorption of a Photon to a Change in Membrane Conductance

  • Light Activates Pigment Molecules in the Photoreceptors

  • Excited Rhodopsin Activates a Phosphodiesterase Through the G Protein Transducin

  • Multiple Mechanisms Shut Off the Cascade

  • Defects in Phototransduction Cause Disease

• Ganglion Cells Transmit Neural Images to the Brain

  • The Two Major Types of Ganglion Cells Are ON Cells and OFF Cells

  • Many Ganglion Cells Respond Strongly to Edges in the Image

  • The Output of Ganglion Cells Emphasizes Temporal Changes in Stimuli

  • Retinal Output Emphasizes Moving Objects

  • Several Ganglion Cell Types Project to the Brain Through Parallel Pathways

• A Network of Interneurons Shapes the Retinal Output

  • Parallel Pathways Originate in Bipolar Cells

  • Spatial Filtering Is Accomplished by Lateral Inhibition

  • Temporal Filtering Occurs in Synapses and Feedback Circuits

  • Color Vision Begins in Cone-Selective Circuits

  • Congenital Color Blindness Takes Several Forms

  • Rod and Cone Circuits Merge in the Inner Retina

• The Retina's Sensitivity Adapts to Changes in Illumination

  • Light Adaptation Is Apparent in Retinal Processing and Visual Perception

  • Multiple Gain Controls Occur Within the Retina

  • Light Adaptation Alters Spatial Processing

• An Overall View

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 diaphragm, 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 26–1). 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 26–2).