Chapter 11: Vision
2. Describe the five types of retinal neurons. Which cells are in the outer nuclear layer?
Answer: Photoreceptors, or rods and cones, have an outer segment adjacent to the pigment epithelium and cell bodies in the outer nuclear layer. Photoreceptor cells synapse on bipolar cells, which in turn synapse on ganglion cells. The processes of horizontal cells allow for lateral interactions between photoreceptors and bipolar cells. The processes of amacrine cells are postsynaptic to bipolar cell terminals and presynaptic to the dendrites of ganglion cells. Amacrine cells can be classified into different subclasses, based on their function.
26. A ganglion cell's firing rate is proportional to what property of light? a. Wavelength b. Period c. Intensity d. Frequency e. Color
Answer: c) intensity
4. What is the role of horizontal cells?
Answer: Horizontal cells have cell bodies in the inner nuclear layer and processes that laterally connect photoreceptor cells and bipolar cells. It is thought that they regulate the amount of transmitter released from the photoreceptors to act on bipolar cells. This is important for luminance, or the ability to detect contrast over a wide range of light intensities.
27. How would the firing of an ON-center ganglion cell respond as a light moved from the edge of the receptor field to the center of the receptive field? a. It would increase. b. It would decrease. c. It would increase then decrease. d. It would decrease then increase. e. There would be no change.
Answer: a) it would increase
13. By which mechanism are rod signals transmitted in conditions of low light? a. Rod bipolar cells synapse on amacrine cells, which in turn synapse on cone bipolar cells. b. Rod bipolar cells synapse on off-center ganglion cells that then transmit the rod signals to other ganglion cells. c. Rod bipolar cells of both on and off types send outputs selectively to on- and off-center ganglion cells. d. Rod photoreceptors send their outputs directly to melanopsin-containing ganglion cells. e. Rod photoreceptors send their signals mainly through horizontal cells to the rod bipolar cells.
Answer: a) rod bipolar cells synapse on amacrine cells, which in turn synapse on cone bipolar cells.
6. Explain the steps in phototransduction in a rod, including its resting state and what happens when a photon is absorbed.
Answer: At rest, a rod photoreceptor contains high levels of cGMP in its outer segment. The cGMP binds to cGMP-gated cation channels, keeping them open, thus maintaining a depolarized state. When a photon is absorbed, cGMP levels decrease, cGMP dissociates from the channels, and the cGMP-gated channels close. This reduces the flow of Na+ and Ca2+ into the cell. However, K+ channels remain open in the presence of light, so positive charge flows out of the cell more rapidly than it flows in, leading to hyperpolarization.
10. What observations led Kuffler to define two types of retinal ganglion cells: OFF-center and ON-center? Explain how this receptive field organization is useful in detecting luminance contrast and changes in light intensity.
Answer: Kuffler discovered that each ganglion cell responds to stimulation in its receptive field by increasing or decreasing its firing rate. ON-center and OFF-center cells responded oppositely to the presence or absence of light. These receptive fields overlap in the retina, allowing for a more complex detection of contrast and changes in light intensity.
7. Why is light adaptation in the retina so important, and what does it involve?
Answer: Light adaptation occurs when photoreceptors decrease their sensitivity as illumination increases. This prevents the receptors from saturating, and it extends the range of light intensities that they can respond to.
5. What are some differences between photoreceptors and other sensory cells?
Answer: Photoreceptors exhibit a graded change in membrane potential in response to light instead of action potential. They also maintain a resting membrane potential around -40 mV, and they hyperpolarize in the presence of light.
8. What are the advantages of animals having both rods and cones, rather than just one type of photoreceptor?
Answer: Rods are very sensitive to light and thus facilitate vision in low light. Cones have very high spatial resolution (providing visual acuity) and different photopigments (allowing animals to see in color). Having both rods and cones allows animals to see and process a great variety of visual stimuli in different environments and across a broad range of light conditions.
9. What is the evidence that human color vision is trichromatic?
Answer: Studies show that any color stimulus can be duplicated by a second stimulus composed of three superimposed light sources of short, medium, and long wavelengths (as long as the intensity of the light sources can be independently adjusted).
3. Is the retina part of the central nervous system? Explain.
Answer: The retina is part of the central nervous system because it is formed via outpocketing of the diencephalon during development. The retina contains a complex neuronal circuitry, allowing it to convert the graded electrical activity of the photoreceptors into action potentials that travel along the axons of the optic nerve.
1. Are there more rods or cones in the retina? In the fovea?
Answer: There are many more rods (about 90 million) than cones (about 4.5 million) in the retina. However, cone density increases dramatically in the fovea, with the foveola being completely rod-free.
18. Refer to the figure. For an OFF-center ganglion cell, which stimulus on the cell's receptive field would cause the highest rate of action potential firing? (In the figure, black fill indicates darkness, and white fill indicates light in the receptive field.) a. a b. b c. c d. d e. Both a and d would cause the same level of activation
Answer: a
3. On its way to the retina, light passes through tissues and fluids in which order? a. Cornea, aqueous humor, lens, vitreous humor, retina b. Sclera, aqueous humor, pupil, lens, vitreous humor, retina c. Cornea, vitreous humor, lens, aqueous humor, retina d. Sclera, vitreous humor, lens, aqueous humor, retina e. Cornea, aqueous humor, pupil, vitreous humor, retina
Answer: a) Cornea, aqueous humor, lens, vitreous humor, retina
18. Which feature is responsible for the superior acuity of the fovea? a. Lack of retinal blood vessels b. Higher density of ganglion cells c. Increased concentration of rods d. Presence of the inner nuclear layer e. Maximum photon scattering before reaching the photoreceptors
Answer: a) Lack of retinal blood vessels
7. What is the most direct path that light information travels on its way to the optic nerve? a. Photoreceptor cell; bipolar cell; ganglion cell; optic nerve b. Photoreceptor cell; ganglion cell; bipolar cell; optic nerve c. Bipolar cell; photoreceptor cell; ganglion cell; optic nerve d. Bipolar cell; ganglion cell; photoreceptor cell; optic nerve e. Ganglion cell; photoreceptor cell; bipolar cell; optic nerve
Answer: a) Photoreceptor cell; bipolar cell; ganglion cell; optic nerve
6. Which sequence represents the most direct pathway for the transmission of visual information from the eye to the brain? a. Photoreceptor → bipolar cell → ganglion cell → brain b. Horizontal cell → bipolar cell → ganglion cell → brain c. Photoreceptor → bipolar cell → amacrine cell → brain d. Photoreceptor → horizontal cell → ganglion cell →brain e. Photoreceptor → bipolar cell → amacrine cell → ganglion cell → brain
Answer: a) Photoreceptor → bipolar cell → ganglion cell → brain
9. The mechanism that accounts for light-induced hyperpolarization of photoreceptors is a. the gating of ion channels by released retinal monomers. b. a sudden increase in the concentration of cAMP, leading to activation of potassium channels. c. a rapid fall in the concentration of cGMP, leading to closure of Na+/ Ca2+ channels. d. a rapid rise in the concentration of cGMP, leading to closure of Na+ / Ca2+ channels. e. light-induced photoisomerization of membrane-bound calcium channels.
Answer: c) a rapid fall in the concentration of cGMP, leading to closure of Na+/ Ca2+ channels.
23. How would an OFF-center neuron's firing rate change when a light was turned on, turned off, and then turned on again? a. Increase, increase, decrease b. Increase, decrease, increase c. Decrease, increase, decrease d. Decrease, increase, increase e. Decrease, decrease, increase
Answer: c) decrease, increase, decrease
4. The fovea a. covers approximately 40 percent of the retina. b. lies at the center of the optic disk. c. lies at the center of the macula lutea. d. is synonymous with "fundus." e. is the only part of the retina that contains xanthophyll.
Answer: c) lies at the center of the macula lutea
30. Due to a genetic modification, a mouse has no horizontal cells in its retinas. What impact will this have on the mouse's vision? a. Dichromatic color blindness b. Inability to detect light c. Reduced ability to determine relative stimulus intensity d. Increased ability to detect luminance e. There will be no effect on the mouse's vision.
Answer: c) reduced ability to determine relative stimulus intensity
12. The main reason that rods are more sensitive to light than cones is that a. the photopigment of rods is much more sensitive to light than the photopigment used in cones. b. the eye contains 1000 times as many rods as cones. c. the rod transduction mechanism provides greater signal amplification. d. rods are sensitive to a much broader range of wavelengths. e. rods have many different types of opsin proteins.
Answer: c) the rod transduction mechanism provides greater signal amplification
15. Which statement about rod and cone convergence in the retina is true? a. Many cones converge onto one cone bipolar cell. b. Convergence makes the cone system a better detector of light. c. Convergence increases the spatial resolution of rods. d. The one-to-one relationship of rods to bipolar and ganglion cells increases acuity. e. Convergence allows rods to pool signals, generating larger responses in bipolar cells.
Answer: e) Convergence allows rods to pool signals, generating larger responses in bipolar cells.
10. Which mechanism decreases sensitivity in a photoreceptor as levels of illumination increase? a. The retinoid cycle b. Accommodation c. Transducin d. Rhodopsin e. Light adaptation
Answer: e) Light adaptation
6. A 68-year-old man notices that the boxes of his crossword puzzle look wavy and blurry for the word he is working on, but the boxes in the periphery of his focus remain clear. This symptom suggests that the man has which condition? a. Presbyopia b. Emmetropia c. Cataracts d. Glaucoma e. Macular degeneration
Answer: e) Macular degeneration
12. Mesopic vision is most likely to occur in which of the the following scenarios? a. Sitting on a sunny beach b. Standing in a dark closet c. Reading in a park on a cloudy afternoon d. Shopping inside a department store e. Walking outdoors at the break of dawn
Answer: e) Walking outdoors at the break of dawn
5. Age-related macular degeneration, or AMD, a. is rising in incidence in the United States. b. can be detected by means of the Amsler grid. c. can result from gradual loss of the pigment epithelium (dry AMD). d. can be treated by laser-induced phototoxicity (wet AMD). e. All of the above
Answer: e) all of the above
16. You measure changes in membrane potential in an ON-center bipolar cell that is exposed to light in the center of its receptive field. What response would you expect to see? a. An action potential b. Depolarization due to increased release of glutamate by the photoreceptor cell c. Hyperpolarization due to decreased release of glutamate by the photoreceptor cell d. Hyperpolarization due to increased release of glutamate by the photoreceptor cell e. Depolarization due to decreased release of glutamate by the photoreceptor cell
Answer: e) depolarization due to decreased release of glutatze by the photoreceptor cell
24. A photoreceptor cell is exposed to a flash of light. How does the membrane potential of this cell and its corresponding ON-center bipolar and ganglion cells change? a. Hyperpolarize, hyperpolarize, depolarize b. Depolarize, hyperpolarize, depolarize c. Hyperpolarize, depolarize, hyperpolarize d. Depolarize, hyperpolarize, hyperpolarize e. Hyperpolarize, depolarize, depolarize
Answer: e) hyperpolarize, depolarize, depolarize
11. If a person had a mutation in the gene that codes for interphotoreceptor retinoid binding protein, what would be the most likely downstream effects? a. Arrestin would not be able to bind to rhodopsin. b. Opsin would not be able to activate the intracellular messenger transducin. c. Retinal could not be transported to and from the outer segment for the retinoid cycle. d. The conformational change from cis-retinal to trans-retinal could not take place during photoisomerization. e. PDE would not hydrolyze cGMP to reduce its concentration.
Answer: c) Retinal could not be transported to and from the outer segment for the retinoid cycle.
19. Which property best differentiates the types of cones? a. Their location in the retina b. The number of synapses on bipolar cells c. The photopigments they contain d. Their overall ratio to other types of cones e. Their shape and processes
Answer: c) The photopigments they contain
15. Which statement about color vision is false? a. Because there are several different color types of cones, they are more sensitive to low (dim) illumination levels than the population of rods is. b. Cones (in humans) come in three different "colors": blue, green, and red (or short-, medium-, and long-wavelength). c. Different wavelengths of light produce different patterns of activity in the cone population as a whole. d. Information from specific color cones can be selectively relayed to specific regions of a retinal ganglion cell's receptive field. e. The different photopigments in each cone are highly sensitive to a small range of wavelengths but can actually be activated by photons of a wide range of wavelengths.
Answer: a) because there are several different color types of cones, they are more sensory to low (dim) illumination levels thats the population of rods is.
21. A man with dichromatic vision would have the most trouble with which task? a. Determining when a red light turns green b. Differentiating his chess pieces from his opponent's c. Reading a novel d. Seeing stars in the night sky e. Adjusting to bright lights in a store
Answer: a) determining when a red light turns green
14. Which of the following correctly matches rods and cones with their properties? a. Rods: high spatial resolution; cones: color vision b. Rods: high sensitivity to light; cones: high spatial resolution c. Rods: color vision; cones: low spatial resolution d. Rods: high sensitivity to light; cones: low spatial resolution e. Rods: low sensitivity to light; cones: color vision
Answer: b) Rods: high sensitivity to light; cones: high spatial resolution
8.Which of the following statements about the location photoreceptors in the outermost layer of the retina is true. a. This placement allows light to stimulate the photoreceptors via the most direct path. b. The proximity to the pigment epithelium allows nourishment for the photoreceptor cells and recycling of photopigment. c. This placement allows more synapses to be formed between amacrine cells and ganglion cells. d. The proximity to the pigment epithelium determines if a photoreceptor cell differentiates into a rod or a cone. e. This placement allows a light stimulus to be filtered when it travels through the other layers of the retina to reach the outermost layer.
Answer: b) The proximity to the pigment epithelium allows nourishment for the photoreceptor cells and recycling of photopigment.
22. Why are males more likely than females to have red-green color deficiencies? a. Sex differences in their mitochondrial genes. b. The red and green pigment genes are located on the X chromosome. c. The red and green pigment genes are located on chromosome 7. d. Normal trichromats have one gene for red pigments. e. Normal trichromats have a varying number of genes for green pigments.
Answer: b) The red and green pigment genes are located on the X chromosome
25. What is the major anatomical difference between ON-center and OFF-center bipolar cells that explains their selective response to light increments? a. They release different neurotransmitters onto ganglion cells. b. They have different glutamate receptor types. c. OFF-center cells have larger dendritic fields. d. They have different GABA receptor types. e. ON-center cells have larger cell bodies.
Answer: b) They have different glutamate receptor types
28. In which scenario would an ON-center ganglion cell fire the most robustly? a. A small light shining in the center of the receptive field b. A light filling the center of the receptive field c. A light filling the center and surround of the receptive field d. A light filling only the surround of the receptor field e. No light shining in the receptor field
Answer: b) a light filling the center of the receptive field
8. Which mechanism is not a component of the phototransduction signaling cascade initiated by light falling on a rod? a. Absorption of a photon of light by 11-cis retinal b. Activation of protein kinase A c. A conformational change in the rhodopsin protein d. Dissociation of the α subunit of transducin from the β/γ complex e. Increased activity of PDE (phosphodiesterase)
Answer: b) activation of protein kinase A
11. The death of retinal cells in retinitis pigmentosa is most likely caused by a. infection. b. apoptosis. c. excessive light exposure. d. environmental toxins. e. injury to the eye.
Answer: b) apoptosis
4. During accommodation, the shape of the _______ is changed by the _______ in order to see objects accurately at varying distances. a. lens; zonule fibers b. lens; ciliary muscle c. pupil; ciliary muscle d. pupil; zonule fibers e. iris; zonule fibers
Answer: b) lens; ciliary muscle
1. The iris a. is responsible for the refraction of light onto the retina. b. regulates the amount of light entering the eye. c. applies tension to the lens. d. contains the pigment epithelial cells that nourish photoreceptors. e. covers the cornea, shielding it from harmful UV rays.
Answer: b) regulates the amount of light entering the eye
13. The reason that rods do not contribute to photopic vision is a. they are not being stimulated. b. their response is saturated. c. all of their membrane channels are open. d. they are not responsible for color vision. e. they have low resolution.
Answer: b) their response is saturated.
2. Our underwater vision is poor because a. water disperses light, making it impossible to focus. b. under water there is no longer a refractive index difference between the cornea and the surrounding media. c. water seeps into the iris, causing temporary cloudiness. d. the hydrostatic pressure of water changes the shape of the eyeball. e. the ions in the water cause hyperpolarization in the photoreceptor cells.
Answer: b) under water there is no longer a refractive index difference between the cornea and the surrounding media
16. What accounts for the fact that rods do not contribute to vision in daylight? a. Rods cannot be stimulated by photons in daylight. b. The occipital lobe ignores rod signaling in daylight. c. All membrane channels are closed due to saturation in daylight. d. The retinoid cycle in rods stops in daylight. e. cGMP is quickly depleted in daylight.
Answer: c) All membrane channels are closed due to saturation in daylight.
5. Which statement about the optic disc is false? a. It is a region without photoreceptors. b. It is where retinal nerves leave the eye to reach targets in the thalamus and midbrain. c. It contains a small depression known as the fovea. d. It can be an indicator for intracranial pressure. e. It creates a blind spot.
Answer: c) It contains a small depression known as the fovea.
10. Which statement regarding photoreceptor cells is true? a. Like typical sensory neurons, they fire action potentials when the cell depolarizes to threshold in response to stimuli. b. Unlike typical neurons, their resting membrane potential is more negative, usually around -100mV. c. Unlike typical neurons, cation channels are open at rest, allowing the influx of sodium and calcium. d. Unlike typical neurons, neurotransmitter release is dependent on an influx of potassium ions. e. Unlike typical neurons, they hyperpolarize in response to a stimulus due to opening of chloride channels.
Answer: c) unlike typical neurons, cation channels are open at rest, allowing the influx of sodium and calcium
9. Applying a drug that increases cGMP levels in photoreceptors would cause which effect on a photoreceptor's response to a photon of light? a. An intensified depolarization from normal b. An attenuated depolarization from normal c. An intensified hyperpolarization from normal d. An attenuated hyperpolarization from normal e. There would be no effect
Answer: d) An attenuated hyperpolarization from normal
20. A genetically engineered macaque has had the gene that codes for M pigment knocked out. Which vision impairment should the knockout macaque have? a. Blue-yellow color blindness b. Anomalous trichromat c. Protanopia d. Deuteranopia e. Complete loss of color vision
Answer: d) Deuteranopia
2. An optometrist discovers that her patient has poor drainage of his aqueous humor, and a test confirms high intraocular pressure. These symptoms suggest which condition? a. Presbyopia b. Emmetropia c. Cataracts d. Glaucoma e. Macular degeneration
Answer: d) Glaucoma
17. Which symptom would you expect a person with damage to the fovea to experience? a. Total blindness b. Poor peripheral vision c. Difficulty seeing large objects d. Trouble reading e. Increased sensitivity to light
Answer: d) Trouble reading
19. Refer to the figure. The illusion shown, in which dark rectangles are placed within a pattern of light and dark bars, is best explained by which statement? a. Light scatters between adjacent photoreceptors. b. Photoreceptors are unable to spatially resolve the intersections of the white bars. c. Photic information is conveyed only by graded electrical responses. d. Brightness percepts are generated on a statistical basis as a means of contending with the inherent ambiguity of luminance. e. All of the above are equally valid explanations.
Answer: d) brightness percepts are generated on a statistical basis as a means of contending with the inherent ambiguity of luminance
3. Why is myopia in humans thought to be more common now than it was in ancient times? a. The vigorous exercise associated with early human lifestyles better nourished the retina. b. Early humans were not exposed to the environmental toxins present in modern life. c. Chronic exposure to fire smoke had a protective effect on the vision of early humans. d. Early humans did not engage in the kinds of activities (e.g., reading and writing from an early age, watching television) characteristic of modern life. e. Myopia was not less common; it was less recognized.
Answer: d) early humans did not engage int he kinds of activities (e.g. reading and writing from and early age, watching television) characteristic of modern life.
29. Which cell type is thought to be responsible for the antagonistic surround of ganglion cells? a. Photoreceptor b. Bipolar c. Ganglion d. Horizontal e. Amacrine
Answer: d) horizontal
17. Which statement about center-surround receptive fields in the retina is false? a. Cells that are inhibited by light in their center are excited by light in their surround. b. Cells that are excited by light in their center are inhibited by light in their surround. c. The surround portion of the receptive field is typically an annulus (or doughnut-shaped ring) that surrounds the circular center. d. Light in the center of an off-center cell will increase its firing rate. e. The best stimulus for an on-center cell is light in its center and darkness in its surround.
Answer: d) light in the center of an off-center will increase its firing rate
7. The two main functions of the retinal pigment epithelium are _______ and _______. a. structural support to maintain curvature of the retina; phagocytosis of shed outer segments b. structural support to maintain curvature of the retina; synthesis of rhodopsin c. phagocytosis of shed outer segments; synthesis of rhodopsin d. phagocytosis of shed outer segments; regeneration of the photoreceptor photopigments e. synthesis of rhodopsin; regeneration of the photoreceptor photopigments
Answer: d) phagocytosis of shed outer segments; regeneration of the photoreceptors photopigments
14. Which statement about the distributions of rods and cones is true? a. Because cones carry three different color channels, there are three times as many cones as rods. b. Rods outnumber cones by 100 to 1, which explains the greater sensitivity of rods in low light. c. Rods are evenly distributed across the entire retina. d. The density of cones is 200-fold higher in the fovea than in the most eccentric retinal regions. e. The density of rods is highest in the far periphery of the retina.
Answer: d) the density of cones is 200-fold higher in the fovea than in the most eccentric retinal regions
20. Which statement about the retina's operation across different levels of ambient light is true? a. For a given level of ambient light, an ON-center ganglion cell responds proportionately to a small spot of light over an intensity range of about one log unit. b. Via adaptational mechanisms, ON-center ganglion cells can dynamically encode brightness levels in their ON-center over a range of 6 log units of ambient light levels. c. Ganglion cells generally do not report absolute light intensities, but rather encode relative intensity differences between center and surround. d. Interactions within the inner plexiform layer play an important role in modulating the photic sensitivity of ganglion cells. e. All of the above
Answer: e) All of the above
1. Which eye structure is paired with an appropriate characteristic? a. Cornea: contains neurons that are sensitive to light and transmit visual signals b. Aqueous humor: fills the space between the back of the lens and the retina c. Pupil: separates and protects the eye from the external environment d. Retina: produces melanin and capillaries to nourish photoreceptors e. Ciliary body: produces fluid to fill the front of the eye
Answer: e) Ciliary body: produces fluid to fill the front of the eye
