Chapter 6  Vision
Visual Coding and the Retinal Receptors <ul><li>Each of our senses has specialized receptors that are sensitive to a parti...
Visual Coding and the Retinal Receptors <ul><li>A  receptor potential  refers to a local depolarization or hyperpolarizati...
Visual Coding and the Retinal Receptors <ul><li>Law of specific nerve energies  states that activity by a particular nerve...
Visual Coding and the Retinal Receptors <ul><li>Light enters the eye through an opening in the center of the eye called th...
Fig. 6-1, p. 153
Visual Coding and the Retinal Receptors <ul><li>Visual receptors send messages to neurons called  bipolar cells,  located ...
Fig. 6-15, p. 167
Visual Coding and the Retinal Receptors <ul><li>Amacrine cells are additional cells that receive information from bipolar ...
Visual Coding and the Retinal Receptors <ul><li>The  optic nerve  consists of the axons of ganglion cells that band togeth...
Fig. 6-2, p. 154
Fig. 6-3, p. 154
Visual Coding and the Retinal Receptors <ul><li>The macula is the center of the human retina. </li></ul><ul><li>The centra...
Visual Coding and the Retinal Receptors <ul><li>Each receptor in the fovea attaches to a single bipolar cell and a single ...
Visual Coding and the Retinal Receptors <ul><li>In the periphery of the retina, a greater number of receptors converge int...
Visual Coding and the Retinal Receptors <ul><li>The arrangement of visual receptors in the eye is highly adaptive. </li></...
Visual Coding and the Retinal Receptors <ul><li>The vertebrate retina consist of two kind of receptors: </li></ul><ul><ul>...
Fig. 6-6, p. 156
Visual Coding and the Retinal Receptors <ul><li>Photopigments  - chemicals contained by both rods and cones that release e...
Visual Coding and the Retinal Receptors <ul><li>The perception of color is dependent upon the wavelength of the light. </l...
Fig. 6-7, p. 157
Fig. 6-8, p. 158
Visual Coding and the Retinal Receptors <ul><li>Discrimination among colors depend upon the combination of responses by di...
Visual Coding and the Retinal Receptors <ul><li>Trichromatic theory  - Color perception occurs through the relative rates ...
Visual Coding and the Retinal Receptors <ul><li>Trichromatic theory (cont.) </li></ul><ul><li>The ratio of activity across...
Visual Coding and the Retinal Receptors <ul><li>The  opponent-process theory  suggests that we perceive color in terms of ...
Fig. 6-11, p. 160
Visual Coding and the Retinal Receptors <ul><li>Both the opponent-process and trichromatic theory have limitations. </li><...
Visual Coding and the Retinal Receptors <ul><li>Color vision deficiency  is an impairment in perceiving color differences....
The Neural Basis of Visual Perception <ul><li>Structure and organization of the visual system is the same across individua...
Fig. 6-9, p. 159
The Neural Basis of Visual Perception <ul><li>Rods and cones of the retina make synaptic contact with horizontal cells and...
Fig. 6-11, p. 160
The Neural Basis of Visual Perception <ul><li>Ganglion cell  axons form the optic nerve. </li></ul><ul><li>The  optic chia...
Fig. 6-16, p. 168
The Neural Basis of Visual Perception <ul><li>The  lateral geniculate nucleus  is a nucleus in the thalamus specialized fo...
The Neural Basis of Visual Perception <ul><li>Lateral inhibition  is the reduction of activity in one neuron by activity i...
The Neural Basis of Visual Perception <ul><li>The  receptive field  refers to the part of the visual field that either exc...
Fig. 6-18, p. 170
The Neural Basis of Visual Perception <ul><li>Ganglion cells of primates generally fall into three categories: </li></ul><...
The Neural Basis of Visual Perception <ul><li>Parvocellular neurons : </li></ul><ul><ul><li>are mostly located in or near ...
The Neural Basis of Visual Perception <ul><li>Magnocellular neurons : </li></ul><ul><ul><li>are distributed evenly through...
The Neural Basis of Visual Perception <ul><li>Koniocellular neurons : </li></ul><ul><ul><li>have small cell bodies. </li><...
The Neural Basis of Visual Perception <ul><li>Cells of the lateral geniculate have a receptive field similar to those of g...
The Neural Basis of Visual Perception <ul><li>The  primary visual cortex (area V1)  receives information from the lateral ...
The Neural Basis of Visual Perception <ul><li>The  secondary visual cortex (area V2)  receives information from area V1, p...
The Neural Basis of Visual Perception <ul><li>Three visual pathways in the cerebral cortex include:  </li></ul><ul><ul><li...
Fig. 6-19, p. 172
The Neural Basis of Visual Perception <ul><li>The  ventral stream  refers to the most magnocellular visual paths in the te...
The Neural Basis of Visual Perception <ul><li>Hubel and Weisel (1959, 1998) distinguished various types of cells in the vi...
The Neural Basis of Visual Perception <ul><li>Simple cells : </li></ul><ul><ul><li>Found exclusively in the primary visual...
The Neural Basis of Visual Perception <ul><li>Complex cells : </li></ul><ul><ul><li>Located in either V1or V2. </li></ul><...
The Neural Basis of Visual Perception <ul><li>End-stopped or hypercomplex cells : </li></ul><ul><ul><li>Are similar to com...
The Neural Basis of Visual Perception <ul><li>In the visual cortex, cells are grouped together in columns. </li></ul><ul><...
The Neural Basis of Visual Perception <ul><li>Receptive fields become larger and more specialized as visual information go...
The Neural Basis of Visual Perception <ul><li>Shape constancy  is the ability to recognize an object’s shape despite chang...
The Neural Basis of Visual Perception <ul><li>Visual agnosia  is the inability to recognize objects despite satisfactory v...
The Neural Basis of Visual Perception <ul><li>Color perception depends on both the parvocellular and koniocellular paths: ...
Fig. 6-24, p. 175
The Neural Basis of Visual Perception <ul><li>Stereoscopic depth perception  or the ability to detect depth by differences...
The Neural Basis of Visual Perception <ul><li>Motion perception involves a variety of brain areas in all four lobes of the...
The Neural Basis of Visual Perception <ul><li>Several mechanisms prevent confusion or blurring of images during eye moveme...
The Neural Basis of Visual Perception <ul><li>Motion blindness  refers to the inability to determine the direction, speed ...
Development of Vision <ul><li>Vision in newborns is poorly developed at birth: </li></ul><ul><ul><li>Face recognition occu...
Fig. 6-27, p. 178
Development of Vision <ul><li>Animal studies have greatly contributed to the understanding of the development of vision. <...
Development of Vision <ul><li>Sensitive/critical periods  are periods of time during the lifespan when experiences have a ...
Development of Vision <ul><li>Stereoscopic depth perception  is a method of perceiving distance in which the brain compare...
Fig. 6-33, p. 186
Development of Vision <ul><li>Strabismus  is a condition in which the eyes do not point in the same direction. </li></ul><...
Development of Vision <ul><li>Later experience can restore the sensitivity of cortical neurons that have been deprived of ...
Fig. 6-34, p. 188
Development of Vision <ul><li>Early exposure to a limited array of patterns leads to nearly all of the visual cortex cells...
Development of Vision <ul><li>Study of people born with cataracts but removed at age 2-6 months indicate that vision can b...
Development of Vision <ul><li>Research and case studies indicate that the visual cortex is plastic but much more so early ...
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  • Chapter6 Power Point Lecture

    1. 1. Chapter 6 Vision
    2. 2. Visual Coding and the Retinal Receptors <ul><li>Each of our senses has specialized receptors that are sensitive to a particular kind of energy. </li></ul><ul><li>Receptors for vision are sensitive to light. </li></ul><ul><li>Receptors “transduce” (convert) energy into electrochemical patterns. </li></ul>
    3. 3. Visual Coding and the Retinal Receptors <ul><li>A receptor potential refers to a local depolarization or hyperpolarization of a receptor membrane. </li></ul><ul><li>The strength of the receptor potential determines how much excitation or inhibition is sent to the next neuron. </li></ul>
    4. 4. Visual Coding and the Retinal Receptors <ul><li>Law of specific nerve energies states that activity by a particular nerve always conveys the same type of information to the brain. </li></ul><ul><ul><li>Example: impulses in one neuron indicate light; impulses in another neuron indicate sound. </li></ul></ul><ul><li>The brain does not duplicate what we see; sensory coding is determined by which neurons are active. </li></ul>
    5. 5. Visual Coding and the Retinal Receptors <ul><li>Light enters the eye through an opening in the center of the eye called the pupil . </li></ul><ul><li>Light is focused by the lens and the cornea onto the rear surface of the eye known as the retina . </li></ul><ul><ul><li>The retina is lined with visual receptors. </li></ul></ul><ul><li>Light from the left side of the world strikes the right side of the retina and vice versa. </li></ul>
    6. 6. Fig. 6-1, p. 153
    7. 7. Visual Coding and the Retinal Receptors <ul><li>Visual receptors send messages to neurons called bipolar cells, located closer to the center of the eye. </li></ul><ul><li>Bipolar cells send messages to ganglion cells that are even closer to the center of the eye. </li></ul><ul><ul><li>The axons of ganglion cells join one another to form the optic nerve that travels to the brain. </li></ul></ul>
    8. 8. Fig. 6-15, p. 167
    9. 9. Visual Coding and the Retinal Receptors <ul><li>Amacrine cells are additional cells that receive information from bipolar cells and send it to other bipolar, ganglion or amacrine cells. </li></ul><ul><li>Amacrine cells control the ability of the ganglion cells to respond to shapes, movements, or other specific aspects of visual stimuli. </li></ul>
    10. 10. Visual Coding and the Retinal Receptors <ul><li>The optic nerve consists of the axons of ganglion cells that band together and exit through the back of the eye and travel to the brain. </li></ul><ul><li>The point at which the optic nerve leaves the back of the eye is called the blind spot because it contains no receptors. </li></ul>
    11. 11. Fig. 6-2, p. 154
    12. 12. Fig. 6-3, p. 154
    13. 13. Visual Coding and the Retinal Receptors <ul><li>The macula is the center of the human retina. </li></ul><ul><li>The central portion of the macula is the fovea and allows for acute and detailed vision. </li></ul><ul><ul><li>Packed tight with receptors. </li></ul></ul><ul><ul><li>Nearly free of ganglion axons and blood vessels. </li></ul></ul>
    14. 14. Visual Coding and the Retinal Receptors <ul><li>Each receptor in the fovea attaches to a single bipolar cell and a single ganglion cell known as a midget ganglion cell . </li></ul><ul><li>Each cone in the fovea has a direct line to the brain which allows the registering of the exact location of input. </li></ul>
    15. 15. Visual Coding and the Retinal Receptors <ul><li>In the periphery of the retina, a greater number of receptors converge into ganglion and bipolar cells. </li></ul><ul><ul><li>Detailed vision is less in peripheral vision. </li></ul></ul><ul><ul><li>Allows for the greater perception of much fainter light in peripheral vision. </li></ul></ul>
    16. 16. Visual Coding and the Retinal Receptors <ul><li>The arrangement of visual receptors in the eye is highly adaptive. </li></ul><ul><ul><li>Example: Predatory birds have a greater density of receptors on the top of the eye; rats have a greater density on the bottom of the eye. </li></ul></ul>
    17. 17. Visual Coding and the Retinal Receptors <ul><li>The vertebrate retina consist of two kind of receptors: </li></ul><ul><ul><li>Rods - most abundant in the periphery of the eye and respond to faint light. (120 million per retina) </li></ul></ul><ul><ul><li>Cones - most abundant in and around the fovea. (6 million per retina) </li></ul></ul><ul><ul><ul><li>Essential for color vision & more useful in bright light. </li></ul></ul></ul>
    18. 18. Fig. 6-6, p. 156
    19. 19. Visual Coding and the Retinal Receptors <ul><li>Photopigments - chemicals contained by both rods and cones that release energy when struck by light. </li></ul><ul><li>Photopigments consist of 11-cis-retinal bound to proteins called opsins. </li></ul><ul><li>Light energy converts 11-cis-retinal quickly into all-trans-retinal. </li></ul><ul><li>Light is thus absorbed and energy is released in the process, controlling cell activities. </li></ul>
    20. 20. Visual Coding and the Retinal Receptors <ul><li>The perception of color is dependent upon the wavelength of the light. </li></ul><ul><li>“Visible” wavelengths are dependent upon the species’ receptors. </li></ul><ul><li>The shortest wavelength humans can perceive is 400 nanometers (violet). </li></ul><ul><li>The longest wavelength that humans can perceive is 700 nanometers (red). </li></ul>
    21. 21. Fig. 6-7, p. 157
    22. 22. Fig. 6-8, p. 158
    23. 23. Visual Coding and the Retinal Receptors <ul><li>Discrimination among colors depend upon the combination of responses by different neurons. </li></ul><ul><li>Two major interpretations of color vision include the following: </li></ul><ul><ul><li>Trichromatic theory/Young-Helmholtz theory. </li></ul></ul><ul><ul><li>Opponent-process theory. </li></ul></ul>
    24. 24. Visual Coding and the Retinal Receptors <ul><li>Trichromatic theory - Color perception occurs through the relative rates of response by three kinds of cones. </li></ul><ul><ul><li>Short wavelength, medium-wavelength, long-wavelength. </li></ul></ul><ul><li>Each cone is maximally sensitive to a different set of wavelengths. </li></ul>
    25. 25. Visual Coding and the Retinal Receptors <ul><li>Trichromatic theory (cont.) </li></ul><ul><li>The ratio of activity across the three types of cones determines the color. </li></ul><ul><li>More intense light increases the brightness of the color but does not change the ratio and thus does not change the perception of the color itself. </li></ul>
    26. 26. Visual Coding and the Retinal Receptors <ul><li>The opponent-process theory suggests that we perceive color in terms of paired opposites. </li></ul><ul><li>The brain has a mechanism that perceives color on a continuum from red to green and another from yellow to blue. </li></ul><ul><li>A possible mechanism for the theory is that bipolar cells are excited by one set of wavelengths and inhibited by another. </li></ul>
    27. 27. Fig. 6-11, p. 160
    28. 28. Visual Coding and the Retinal Receptors <ul><li>Both the opponent-process and trichromatic theory have limitations. </li></ul><ul><li>Color constancy , the ability to recognize color despite changes in lighting, is not easily explained by these theories. </li></ul><ul><li>Retinex theory suggests the cortex compares information from various parts of the retina to determine the brightness and color for each area. </li></ul><ul><ul><li>Better explains color constancy. </li></ul></ul>
    29. 29. Visual Coding and the Retinal Receptors <ul><li>Color vision deficiency is an impairment in perceiving color differences. </li></ul><ul><li>Occurs for genetic reasons and the gene is contained on the X chromosome. </li></ul><ul><li>Caused by either the lack of a type of cone or a cone has abnormal properties. </li></ul><ul><li>Most common form is difficulty distinguishing between red and green. </li></ul><ul><ul><li>Results from the long- and medium- wavelength cones having the same photopigment. </li></ul></ul>
    30. 30. The Neural Basis of Visual Perception <ul><li>Structure and organization of the visual system is the same across individuals and species. </li></ul><ul><li>Quantitative differences in the eye itself can be substantial. </li></ul><ul><ul><li>Example: Some individuals have two or three times as many axons in the optic nerve, allowing for greater ability to detect faint or brief visual stimuli. </li></ul></ul>
    31. 31. Fig. 6-9, p. 159
    32. 32. The Neural Basis of Visual Perception <ul><li>Rods and cones of the retina make synaptic contact with horizontal cells and bipolar cells. </li></ul><ul><li>Horizontal cells are cells in the eye that make inhibitory contact onto bipolar cells. </li></ul><ul><li>Bipolar cells are cells in the eye that make synapses onto amacrine cells and ganglion cells. </li></ul><ul><li>The different cells are specialized for different visual functions. </li></ul>
    33. 33. Fig. 6-11, p. 160
    34. 34. The Neural Basis of Visual Perception <ul><li>Ganglion cell axons form the optic nerve. </li></ul><ul><li>The optic chiasm is the place where the two optic nerves leaving the eye meet. </li></ul><ul><li>In humans, half of the axons from each eye cross to the other side of the brain. </li></ul><ul><li>Most ganglion cell axons go to the lateral geniculate nucleus, a smaller amount to the superior colliculus and fewer going to other areas. </li></ul>
    35. 35. Fig. 6-16, p. 168
    36. 36. The Neural Basis of Visual Perception <ul><li>The lateral geniculate nucleus is a nucleus in the thalamus specialized for visual perception. </li></ul><ul><ul><li>Destination for most ganglion cell axons. </li></ul></ul><ul><ul><li>Sends axons to other parts of the thalamus and to the visual areas of the occipital cortex. </li></ul></ul>
    37. 37. The Neural Basis of Visual Perception <ul><li>Lateral inhibition is the reduction of activity in one neuron by activity in neighboring neurons. </li></ul><ul><li>The response of cells in the visual system depends upon the net result of excitatory and inhibitory messages it receives. </li></ul><ul><li>Lateral inhibition is responsible for heightening contrast in vision and an example of this principle. </li></ul>
    38. 38. The Neural Basis of Visual Perception <ul><li>The receptive field refers to the part of the visual field that either excites or inhibits a cell in the visual system. </li></ul><ul><li>For a receptor, the receptive field is the point in space from which light strikes it. </li></ul><ul><li>For other visual cells, receptive fields are derived from the visual field of cells that either excite or inhibit. </li></ul><ul><ul><li>Example: ganglion cells converge to form the receptive field of the next level of cells. </li></ul></ul>
    39. 39. Fig. 6-18, p. 170
    40. 40. The Neural Basis of Visual Perception <ul><li>Ganglion cells of primates generally fall into three categories: </li></ul><ul><ul><li>Parvocellular neurons </li></ul></ul><ul><ul><li>Magnocellular neurons </li></ul></ul><ul><ul><li>Koniocellular neurons </li></ul></ul>
    41. 41. The Neural Basis of Visual Perception <ul><li>Parvocellular neurons : </li></ul><ul><ul><li>are mostly located in or near the fovea. </li></ul></ul><ul><ul><li>have smaller cell bodies and small receptive fields. </li></ul></ul><ul><ul><li>connect only to the lateral geniculate nucleus </li></ul></ul><ul><ul><li>are highly sensitive to detect color and visual detail. </li></ul></ul>
    42. 42. The Neural Basis of Visual Perception <ul><li>Magnocellular neurons : </li></ul><ul><ul><li>are distributed evenly throughout the retina. </li></ul></ul><ul><ul><li>have larger cell bodies and visual fields. </li></ul></ul><ul><ul><li>mostly connect to the lateral geniculate nucleus but also connect to other visual areas of the thalamus. </li></ul></ul><ul><ul><li>are highly sensitive to large overall pattern and moving stimuli. </li></ul></ul>
    43. 43. The Neural Basis of Visual Perception <ul><li>Koniocellular neurons : </li></ul><ul><ul><li>have small cell bodies. </li></ul></ul><ul><ul><li>are found throughout the retina. </li></ul></ul><ul><ul><li>connect to the lateral geniculate nucleus, other parts of the thalamus, and the superior colliculus. </li></ul></ul>
    44. 44. The Neural Basis of Visual Perception <ul><li>Cells of the lateral geniculate have a receptive field similar to those of ganglion cells: </li></ul><ul><ul><li>An excitatory or inhibitory central portion and a surrounding ring of the opposite effect. </li></ul></ul><ul><ul><li>Large or small receptive fields. </li></ul></ul>
    45. 45. The Neural Basis of Visual Perception <ul><li>The primary visual cortex (area V1) receives information from the lateral geniculate nucleus and is the area responsible for the first stage of visual processing. </li></ul><ul><li>Some people with damage to V1 show blindsight , an ability to respond to visual stimuli that they report not seeing. </li></ul>
    46. 46. The Neural Basis of Visual Perception <ul><li>The secondary visual cortex (area V2) receives information from area V1, processes information further, and sends it to other areas. </li></ul><ul><li>Information is transferred between area V1 and V2 in a reciprocal nature. </li></ul>
    47. 47. The Neural Basis of Visual Perception <ul><li>Three visual pathways in the cerebral cortex include: </li></ul><ul><ul><li>A mostly parvocellular neuron pathway sensitive to details of shape. </li></ul></ul><ul><ul><li>A mostly magnocellular neuron pathway with a ventral branch sensitive to movement and a dorsal branch responsible for integration of vision with action. </li></ul></ul><ul><ul><li>A mixed pathway sensitive to brightness, color and shape. </li></ul></ul>
    48. 48. Fig. 6-19, p. 172
    49. 49. The Neural Basis of Visual Perception <ul><li>The ventral stream refers to the most magnocellular visual paths in the temporal cortex. </li></ul><ul><ul><li>Specialized for identifying and recognizing objects. </li></ul></ul><ul><li>The dorsal stream refers to the visual path in the parietal cortex. </li></ul><ul><ul><li>Helps the motor system to find objects and move towards them. </li></ul></ul>
    50. 50. The Neural Basis of Visual Perception <ul><li>Hubel and Weisel (1959, 1998) distinguished various types of cells in the visual cortex: </li></ul><ul><ul><li>Simple cells. </li></ul></ul><ul><ul><li>Complex cells. </li></ul></ul><ul><ul><li>End-stopped/hypercomplex cells. </li></ul></ul>
    51. 51. The Neural Basis of Visual Perception <ul><li>Simple cells : </li></ul><ul><ul><li>Found exclusively in the primary visual cortex (V1). </li></ul></ul><ul><ul><li>Fixed excitatory and inhibitory zones. </li></ul></ul><ul><ul><li>Bar-shaped or edge-shaped receptive fields with vertical and horizontal orientations outnumbering diagonal ones. </li></ul></ul>
    52. 52. The Neural Basis of Visual Perception <ul><li>Complex cells : </li></ul><ul><ul><li>Located in either V1or V2. </li></ul></ul><ul><ul><li>Have large receptive field that can not be mapped into fixed excitatory or inhibitory zones. </li></ul></ul><ul><ul><li>Responds to a pattern of light in a particular orientation and most strongly to a stimulus moving perpendicular to its access. </li></ul></ul>
    53. 53. The Neural Basis of Visual Perception <ul><li>End-stopped or hypercomplex cells : </li></ul><ul><ul><li>Are similar to complex cells but with a strong inhibitory area at one end of its bar shaped receptive field. </li></ul></ul><ul><ul><li>Respond to a bar-shaped pattern of light anywhere in its large receptive field, provided the bar does not extend beyond a certain point. </li></ul></ul>
    54. 54. The Neural Basis of Visual Perception <ul><li>In the visual cortex, cells are grouped together in columns. </li></ul><ul><li>Cells within a given column process similar information. </li></ul><ul><ul><li>Respond either mostly to the right or left eye, or respond to both eyes equally. </li></ul></ul><ul><li>Cells in the visual cortex may be feature detectors , neurons whose response indicate the presence of a particular feature/ stimuli. </li></ul>
    55. 55. The Neural Basis of Visual Perception <ul><li>Receptive fields become larger and more specialized as visual information goes from simple cells to later areas of visual processing. </li></ul><ul><li>The inferior temporal cortex contains cells that respond selectively to complex shapes but are insensitive to distinctions that are critical to other cells. </li></ul>
    56. 56. The Neural Basis of Visual Perception <ul><li>Shape constancy is the ability to recognize an object’s shape despite changes in direction or size. </li></ul><ul><li>The inferior temporal neuron’s ability to ignore changes in size and direction contributes to our capacity for shape constancy. </li></ul><ul><li>Damage to the pattern pathways of the cortex can lead to deficits in object recognition. </li></ul>
    57. 57. The Neural Basis of Visual Perception <ul><li>Visual agnosia is the inability to recognize objects despite satisfactory vision. </li></ul><ul><ul><li>Caused by damage to the pattern pathway usually in the temporal cortex. </li></ul></ul><ul><li>Prosopagnosia is the inability to recognize faces. </li></ul><ul><ul><li>Occurs after damage to the fusiform gyrus of the inferior temporal cortex. </li></ul></ul>
    58. 58. The Neural Basis of Visual Perception <ul><li>Color perception depends on both the parvocellular and koniocellular paths: </li></ul><ul><ul><li>Clusters of neurons in V1 and V2 respond selectively to color and send their output through parts of V4 to the posterior inferior temporal cortex. </li></ul></ul><ul><ul><li>Area V4 may be responsible for color constancy and visual attention. </li></ul></ul>
    59. 59. Fig. 6-24, p. 175
    60. 60. The Neural Basis of Visual Perception <ul><li>Stereoscopic depth perception or the ability to detect depth by differences in what the two eyes see. </li></ul><ul><ul><li>Mediated by certain cells in the magnocellular pathway. </li></ul></ul>
    61. 61. The Neural Basis of Visual Perception <ul><li>Motion perception involves a variety of brain areas in all four lobes of the cortex. </li></ul><ul><li>The middle-temporal cortex (MT/ V5) responds to a stimulus moving in a particular direction. </li></ul><ul><li>Cells in the dorsal part of the medial superior temporal cortex (MST) respond to expansion, contraction or rotation of a visual stimulus. </li></ul>
    62. 62. The Neural Basis of Visual Perception <ul><li>Several mechanisms prevent confusion or blurring of images during eye movements. </li></ul><ul><ul><li>Saccades are a decrease in the activity of the visual cortex during quick eye movements. </li></ul></ul><ul><ul><li>Neural activity and blood flow decrease shortly before and during eye movements. </li></ul></ul>
    63. 63. The Neural Basis of Visual Perception <ul><li>Motion blindness refers to the inability to determine the direction, speed and whether objects are moving. </li></ul><ul><ul><li>Likely caused by damage in area MT. </li></ul></ul><ul><li>Some people are blind except for the ability to detect which direction something is moving. </li></ul><ul><ul><li>Area MT probably gets some visual input despite significant damage to area V1. </li></ul></ul>
    64. 64. Development of Vision <ul><li>Vision in newborns is poorly developed at birth: </li></ul><ul><ul><li>Face recognition occurs relatively soon after birth (2 days) and is presumably centered around the fusiform gyrus. </li></ul></ul><ul><ul><li>The ability to control visual attention develops gradually after birth. </li></ul></ul><ul><ul><ul><li>An infant can shift its attention from one object to another at about 6 months and from 4-6 months, can only shift attention away briefly. </li></ul></ul></ul>
    65. 65. Fig. 6-27, p. 178
    66. 66. Development of Vision <ul><li>Animal studies have greatly contributed to the understanding of the development of vision. </li></ul><ul><li>Early lack of stimulation of one eye leads to synapses in the visual cortex becoming gradually unresponsive to input from that eye. </li></ul><ul><li>Early lack of stimulation of both eyes, cortical responses become sluggish but do not cause blindness. </li></ul>
    67. 67. Development of Vision <ul><li>Sensitive/critical periods are periods of time during the lifespan when experiences have a particularly strong and long-lasting effect. </li></ul><ul><li>Critical period begins when GABA becomes widely available in the brain. </li></ul><ul><li>Critical period ends with the onset of chemicals that inhibit axonal sprouting. </li></ul><ul><li>Changes that occur during critical period require both excitation and inhibition of some neurons. </li></ul>
    68. 68. Development of Vision <ul><li>Stereoscopic depth perception is a method of perceiving distance in which the brain compares slightly different inputs from the two eyes. </li></ul><ul><li>Relies on retinal disparity or the discrepancy between what the left and the right eye sees. </li></ul><ul><li>The ability of cortical neurons to adjust their connections to detect retinal disparity is shaped through experience. </li></ul>
    69. 69. Fig. 6-33, p. 186
    70. 70. Development of Vision <ul><li>Strabismus is a condition in which the eyes do not point in the same direction. </li></ul><ul><ul><li>Usually develops in childhood. </li></ul></ul><ul><li>Cortical cells increase responsiveness to groups of axons with synchronized activities. </li></ul><ul><li>If two eyes carry unrelated messages, cortical cell strengthens connections with only one eye. </li></ul><ul><li>Develop stereoscopic depth perception is impaired. </li></ul>
    71. 71. Development of Vision <ul><li>Later experience can restore the sensitivity of cortical neurons that have been deprived of stimulation. </li></ul><ul><ul><li>stimulation must occur before a certain period. </li></ul></ul><ul><li>Amlyopia (lazy eye) is a condition in which a child fails to attend to vision in one eye. </li></ul><ul><ul><li>Animal studies suggest it is best treated by placing a patch over the other eye to inhibit competition of input from other eye. </li></ul></ul>
    72. 72. Fig. 6-34, p. 188
    73. 73. Development of Vision <ul><li>Early exposure to a limited array of patterns leads to nearly all of the visual cortex cells becoming responsive to only that pattern. </li></ul><ul><li>Astigmatism refers to a blurring of vision for lines in one direction caused by an asymmetric curvature of the eyes. </li></ul><ul><ul><li>70 % of infants </li></ul></ul><ul><li>A strong astigmatism during critical periods can lead to permanent changes in the visual cortex. </li></ul>
    74. 74. Development of Vision <ul><li>Study of people born with cataracts but removed at age 2-6 months indicate that vision can be restored after early deprivation. </li></ul><ul><li>Subtle but lingering problems persist: </li></ul><ul><ul><li>People with left eye cataracts show mild face recognition problems. </li></ul></ul><ul><ul><li>Early in life, each hemisphere of the brain gets input almost entirely from the contralateral eye; the fusiform gyrus is located in the right hemisphere. </li></ul></ul>
    75. 75. Development of Vision <ul><li>Research and case studies indicate that the visual cortex is plastic but much more so early in life. </li></ul><ul><ul><li>Example: Early removal of cataracts leads to better improvement of various aspects of vision. </li></ul></ul>

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