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