Chapter 5: Vision from Eye to Brain
The visual perceptions of each species depend on how their eyes and brains enveloped to process information about the spatial distribution of various wavelengths of light, and to attend to the aspects likely to be important for survival.
Contents
? Provides Information
? Optical Device and Neural Organ
? Neural Signals Travel from the Retina to Brain
? Neurons at Different Levels of the Visual System Have Very Different Receptive Fields
? Area V1 Is Organized in Columns and Slabs
? Color Vision Depends on Special Channels from the Retinal Cones through Cortical Area V4
? Perception of Visual Motion Is Analyzed by a Special System That Includes Cortical Area V5
? Visual Attention Involves Both Striate and Extrastriate Cortex ? Are the Many Cortical Visual Areas Organized into Two Major Systems?
? Visual Neuroscience Can Be Applied to Alleviate Some Visual Deficiencies
1. Vision Provides Information about the Form, Color, Location, Movement, and Identity of Objects
1.1 Perception of Form and Identification of Objects Are Complex Accomplishments ? visual field ? visual acuity ? Blind spot ? optic disc
1.2 Perception of Form and Identification of Objects Are Complex Accomplishments ? visual field ? visual acuity ? Blind spot ? optic disc
? How the nervous system processes so much data so quickly is the staggering
problem that confronts anyone who tries to understand vision
? Perception of Form and Identification of Objects Are Complex
Accomplishments
? Color Is Created by the Visual System
? color blindness (8% males; 0.5% females)
? many mammals (e.g., cats) cannot discriminate wavelengths very well
? not to equate perception of a particular hue with a particular stimulus----a
wavelength of light, depend on __(intensity of illumination\\ surrounding field\\ prior exposure to a different stimulus)
? Brightness Is Created by the Visual System ? lateral inhibition
i. Because of the lateral inhibitory connections, the rates of firing in the two central
receptor cells show a greater than the other.
ii. The Effect of Context on the Perception of Brightness ? Motion Can Enhance the Perception of Objects
? Predators and prey both must be sensitive to moving objects in order to survive
2. The Eye Is Both an Optical Device and a Neural Organ
? The Vertebrate Eye Acts in Some Ways like a Camera
? light from a point on a target object must end up as a point—rather than a
blur—in the retinal image.
? Visual Processing Begins in the Retina
Properties of the Human Photopic and Scotopic Visual System Property Receptors Photopic system Cones Scotopic system Rods 100 million Approximate 4 million numbers of receptors per eye
Photopigments Three classes of cone opsins; Rhodopsin the basis of color vision Low; needs relatively strong High; can be stimulated stimulation; used for day by weak light intensity; vision used for night vision Concentrated in and near Outside fovea fovea; present less densely throughout retina Sensitivity Location in retina Receptive field size Small in fovea, so acuity is Larger, so acuity is lower and visual acuity high Temporal responses Relatively rapid Slow; long latency 2.3 Photoreceptors Excite Some Retinal Cells and Inhibit Others ? neurotransmitter glutamate ? Depolarizes ? Hyperpolarizes
? off-center bipolar cells--Depolarizes photoreceptor cell ? on-center bipolar cells—Hyperpolarizes
? each cone connects to four types of dipolar cells
2.4 Different Mechanisms Enable the Eyes to Work over a Wide Range of Light Intensities ? pupil
? rang fractionation----different thresholds, and this system is concerned with differences, or changes, in brightness----not with the absolute level of illumination
2.5 Acuity Is Best in Foveal Vision Because of the Dense Array of Cones and Ganglion Cells in the Fovea
3. Neural Signals Travel from the Retina to Brain
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