Chapter 6 Vision

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.

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.

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.

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.

Fig. 6-1, p. 153 Figure 6.1: Cross-section of the vertebrate eye. Note how an object in the visual field produces an inverted image on the retina. Also note that the optic nerve exits the eyeball on the nasal side (the side closer to the nose). Fig. 6-1, p. 153

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.

Fig. 6-15, p. 167 Figure 6.15: The vertebrate retina. (a) Diagram of the neurons of the retina. The top of the figure is the back of the retina. All the optic nerve fibers group together and then turn around to exit through the back of the retina, in the “blind spot” of the eye. (b) Photo of a cross-section through the retina. This section from the periphery of the retina has relatively few ganglion cells; a slice closer to the fovea would have a greater density. (Source: (a) Based on “Organization of the primate retina,” by J. E. Dowling and B. B. Boycott, Proceedings of the Royal Society of London, B, 1966, 166, pp. 80–111. Used by permission of the Royal Society of London and John Dowling.) Fig. 6-15, p. 167

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.

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.

Fig. 6-2, p. 154 Figure 6.2: Visual path within the eyeball. The receptors send their messages to bipolar and horizontal cells, which in turn send messages to the amacrine and ganglion cells. The axons of the ganglion cells loop together to exit the eye at the blind spot. They form the optic nerve, which continues to the brain. Fig. 6-2, p. 154

Figure 6.3: A bipolar cell from the retina of a carp, stained with Procion yellow. Bipolar cells get their name from the fact that a fibrous process is attached to each end (or pole) of the neuron. Fig. 6-3, p. 154

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.

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.

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.

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.

Visual Coding and the Retinal Receptors The vertebrate retina consist of two kind of receptors: Rods - most abundant in the periphery of the eye and respond to faint light. (120 million per retina) Cones - most abundant in and around the fovea. (6 million per retina) Essential for color vision & more useful in bright light.

Fig. 6-6, p. 156 Figure 6.6: Structure of rod and cone. (a) Diagram of a rod and a cone. (b) Photo of rods and a cone, produced with a scanning electron microscope. Magnification x 7000. Fig. 6-6, p. 156

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.

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).

Figure 6.7: A beam of light separated into its wavelengths. Although the wavelengths vary over a continuum, we perceive them as several distinct colors. Fig. 6-7, p. 157

Figure 6.8: Response of rods and three kinds of cones to various wavelengths of light. Note that each kind responds somewhat to a wide range of wavelengths but best to wavelengths in a particular range. (Source: From J. K. Bowmaker and H. J. A. Dartnell, “Visual pigments of rods and cones in a human retina,” Journal of Physiology, 298, 501–511. Copyright © 1980. Reprinted by permission of the author.) Fig. 6-8, p. 158

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: Trichromatic theory/Young-Helmholtz theory. Opponent-process theory.

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.

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.

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.

Fig. 6-11, p. 160 Figure 6.11: Possible wiring for one bipolar cell. Short-wavelength light (which we see as blue) excites the bipolar cell and (by way of the intermediate horizontal cell) also inhibits it. However, the excitation predominates, so blue light produces net excitation. Red, green, or yellow light inhibits this bipolar cell because they produce inhibition (through the horizontal cell) without any excitation. The strongest inhibition is from yellow light, which stimulates both the long- and medium-wavelength cones. Therefore, we can describe this bipolar cell as excited by blue and inhibited by yellow. White light produces as much inhibition as excitation and therefore no net effect. Fig. 6-11, p. 160

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.

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 medium- wavelength cones having the same photopigment.

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.

Figure 6.9: Distribution of cones in two human retinas. Investigators artificially colored these images of cones from two people’s retinas, indicating the short-wavelength cones with blue, the medium-wavelength with green, and the long-wavelength with red. Note the difference between the two people, the relative rarity of short-wavelength cones, and the patchiness of the distributions. Fig. 6-9, p. 159

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.

Fig. 6-11, p. 160 Figure 6.11: Possible wiring for one bipolar cell. Short-wavelength light (which we see as blue) excites the bipolar cell and (by way of the intermediate horizontal cell) also inhibits it. However, the excitation predominates, so blue light produces net excitation. Red, green, or yellow light inhibits this bipolar cell because they produce inhibition (through the horizontal cell) without any excitation. The strongest inhibition is from yellow light, which stimulates both the long- and medium-wavelength cones. Therefore, we can describe this bipolar cell as excited by blue and inhibited by yellow. White light produces as much inhibition as excitation and therefore no net effect. Fig. 6-11, p. 160

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.

Figure 6.16: Major connections in the visual system of the brain. (a) Part of the visual input goes to the thalamus and from there to the visual cortex. Another part of the visual input goes to the superior colliculus. (b) Axons from the retina maintain their relationship to one another—what we call their retinotopic organization—throughout their journey from the retina to the lateral geniculate and then from the lateral geniculate to the cortex. Fig. 6-16, p. 168

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.

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.

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.

Fig. 6-18, p. 170 Figure 6.18: Receptive fields. The receptive field of a receptor is simply the area of the visual field from which light strikes that receptor. For any other cell in the visual system, the receptive field is determined by which receptors connect to the cell in question. Fig. 6-18, p. 170

The Neural Basis of Visual Perception Ganglion cells of primates generally fall into three categories: Parvocellular neurons Magnocellular neurons Koniocellular neurons

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.

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.

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.

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.

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.

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.

The Neural Basis of Visual Perception Three visual pathways in the cerebral cortex include: A mostly parvocellular neuron pathway sensitive to details of shape. A mostly magnocellular neuron pathway with a ventral branch sensitive to movement and a dorsal branch responsible for integration of vision with action. A mixed pathway sensitive to brightness, color and shape.

Figure 6.19: Three visual pathways in the monkey cerebral cortex. (a) A pathway originating mainly from magnocellular neurons. (b) A mixed magnocellular/parvocellular pathway. (c) A mainly parvocellular pathway. Neurons are only sparsely connected with neurons of other pathways. (Sources: Based on DeYoe, Felleman, Van Essen, & McClendon, 1994; Ts’o & Roe, 1995; Van Essen & DeYoe, 1995) Fig. 6-19, p. 172

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.

The Neural Basis of Visual Perception Hubel and Weisel (1959, 1998) distinguished various types of cells in the visual cortex: Simple cells. Complex cells. End-stopped/hypercomplex cells.

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.

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.

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.

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.

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.

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.

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.

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.

Figure 6.24: Columns of neurons in the visual cortex. When an electrode passes perpendicular to the surface of the cortex (first part of A), it encounters a sequence of neurons responsive to the same orientation of a stimulus. (The colored lines show the preferred stimulus orientation for each cell.) When an electrode passes across columns (B, or second part of A), it encounters neurons responsive to different orientations. Column borders are shown here to make the point clear; no such borders are visible in the real cortex. (Source: From “The visual cortex of the brain,” by David H. Hubel, November 1963, Scientific American, 209, 5, p. 62. Copyright © Scientific American.) Fig. 6-24, p. 175

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.

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.

The Neural Basis of Visual Perception Several mechanisms prevent confusion or blurring of images during eye movements. Saccades are a decrease in the activity of the visual cortex during quick eye movements. Neural activity and blood flow decrease shortly before and during eye movements.

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.

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.

Fig. 6-27, p. 178 Figure 6.27: The fusiform gyrus. Many cells here are especially active during recognition of faces. Fig. 6-27, p. 178

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.

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.

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.

Figure 6.33: The anatomical basis for binocular vision in cats and primates. Light from a point in the visual field strikes points in each retina. Those two retinal areas send their axons to separate layers of the lateral geniculate, which in turn send axons to a single cell in the visual cortex. That cell is connected (via the lateral geniculate) to corresponding areas of the two retinas. Fig. 6-33, p. 186

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.

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.

Figure 6.34: Procedure for restricting a kitten’s visual experience during early development. For a few hours a day, the kitten wears goggles that show just one stimulus, such as horizontal stripes or diagonal stripes. For the rest of the day, the kitten stays with its mother in a dark room without the mask. Fig. 6-34, p. 188

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.

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.

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.