Introduction: When Seeing Isn’t Seeing

PART I  Vision Pathways

The Visual System Extends from the Eye to the Brain

Visual processing begins in the retina

Photoreceptors respond to light by releasing less neurotransmitter

Different mechanisms enable the eyes to work over a wide range of light intensities

Acuity is best in foveal vision

Neural signals travel from the retina to several brain regions

The retina projects to the brain in a topographic fashion

PART II  Visual Analysis

Neurons at Different Levels of the Visual System Have Very Different Receptive Fields

Neurons in the retina and the LGN have concentric receptive fields

RESEARCHERS AT WORK: Neurons in the visual cortex have varied receptive fields

Spatial-frequency analysis is unintuitive but efficient

Neurons in the visual cortex beyond area V1 have complex receptive fields and help identify forms

Visual perception of motion is analyzed by a special system that includes cortical area V5

PART III  Color Vision

Color Vision Depends on Special Channels from the Retinal Cones through Cortical Area V4

Color perception requires receptor cells that differ in their sensitivities to different wavelengths

BOX 7.1: Most Mammalian Species Have Some Color Vision

Some retinal ganglion cells and LGN cells show spectral opponency

Some visual cortical cells and regions appear to be specialized for color perception

SIGNS & SYMPTOMS: Correcting “Color Blindness”?

PART IV  What versus Where

The Many Cortical Visual Areas Are Organized into Two Major Streams

Visual Neuroscience Can Be Applied to Alleviate Some Visual Deficiencies

Impairment of vision often can be prevented or reduced

Increased exercise can restore function to a previously deprived or neglected eye