Chapter 10 Summary & Outline

Vision Provides Information about the Form, Color, Location, Movement, and Identity of Objects

• The perception of forms and the recognition of objects are complex accomplishments that require processing in many parts of the visual system.
• Each photoreceptor reports only how strongly it has been excited, so at any given instant the visual nervous system receives an enormous array of quantitative information and has to determine what patterns in the outside world could have produced a particular set of “numbers.” About one-third of the human cortex is devoted to this computation. Review Figures 10.1 and 10.2
• Circuitry in the eye itself causes photoreceptors to have an inhibitory effect on the report of their neighbors, an instance of lateral inhibition that underlies simple illusions in brightness. Review Figures 10.3a and 10.4

Study questions: 1 | 2 | 3 | 4 | 5 | 6 | 7

The Visual System Extends from the Eye to the Brain

• The vertebrate eye is an elaborate structure that forms detailed and accurate optical images on the receptive cells of the retina. Review Figure 10.5, Web Activity 10.1
• Visual-information processing begins in the retina, where cells that contain photopigments capture light and initiate neural activity. Two kinds of retinal receptor cells—rods and cones—represent the initial stages of two systems: the scotopic system (dim light) and photopic system (bright light), respectively. Review Figure 10.6 and Table 10.1
• Visual acuity is greatest in the fovea, where photoreceptors are most densely packed, there is relatively little convergence of photoreceptors onto ganglion cells, and there are fewer overlying blood vessels to impede light. Review Figure 10.9 and 10.10

Study questions: 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26

Neural Signals Travel from the Retina to Several Brain Regions

• Brain pathways of the visual system include the lateral geniculate nucleus (LGN) in the thalamus, the primary visual cortex (striate cortex, or V1), and other cortical regions. Some axons of retinal ganglion cells extend to the superior colliculus in the midbrain. Review Figure 10.11, Web Activity 10.2
• The cortex contains several visual areas, each presenting a topographic map of the visual field, but each somewhat specialized for processing one or more different aspects of visual information, such as form, color, or movement. Review Figure 10.12
• Many different phyla have independently evolved photoreceptor organs; several have evolved eyes with lenses to focus light. Review Box 10.2

Study questions: 27 | 28

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

• The receptive fields of bipolar cells and gangion cells consist of a circular center and a surround that have opposing effects: either on-center/off-surround or off-center/on-surround. Review Figures 10.13 and 10.14, Web Activity 10.3
• Recordings from cells at successively higher levels in the visual system reveal that the receptive fields change in two main ways: (1) they become larger (occupy larger parts of the visual field), and (2) they require increasingly specific stimuli to evoke responses. Review Figures 10.14 and 10.16
• For the perception of visual patterns and forms, the stimulus pattern is analyzed at the primary visual cortex according to the orientation and spatial frequency of stimuli, but further analysis at specialized cortical areas is required for the recognition of objects, faces, and three-dimensional forms. Review Figures 10.17 and 10.19–10.21, Web Activity 10.4

Study questions: 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39

Area V1 Is Organized in Columns and Slabs

• The ability to locate visual stimuli in space is aided by detailed spatial maps in some regions of the visual system. Review Figure 10.22

Study questions: 40 | 41 | 42

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

• The discrimination of hue in Old World primates and in humans depends on the existence of three different cone photopigments and on the fact that retinal connections yield four different kinds of spectrally opponent retinal ganglion cells. Review Figures 10.24–10.26

Study questions: 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55

Perception of Visual Motion Is Analyzed by a Special System That Includes Cortical Area V5

• In cortical area V5, most neurons respond preferentially to objects that move in a particular direction in the visual field.

Study questions: 56 | 57

The Many Cortical Visual Areas Are Organized into Two Major Streams

• Visual cortical areas are organized into two main streams: a ventral what stream that serves in the recognition of faces and objects, and a dorsal where stream that serves in location and visuomotor skills. Review Figure 10.27
• The anterior part of the dorsal visual stream includes mirror neurons that respond to the sight of another individual’s actions.

Study questions: 58 | 59 | 60

Visual Neuroscience Can Be Applied to Alleviate Some Visual Deficiencies

• Attempts to treat amblyopia work best when retraining starts early in life, but success with some older patients demonstrates that the visual nervous system remains plastic even in adults.

Study questions: 61 | 62 | 63 | 64 | 65