The Photoreceptor Layer Samples the Visual Image
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).
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 for 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 ...
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