Short Answer Questions for Chapter 2,3,4

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University of Toronto Scarborough
Matthias Niemeier

Chapter 2 Summary 1. This chapter provided some insight into the complex journey that is required for us to see stars and other spots of light. The path of the light was traced from a distant star through the eyeball and to its absorption by photoreceptors and its transduction into neural signals. In subsequent chapters well learn how those signals are transmitted to the brain and translated into the experience of perception. 2. Light, on its way to becoming a sensation (a visual sensation, that is), can be absorbed, scattered, reflected, transmitted, or refracted. It can become a sensation only when its absorbed by a photoreceptor in the retina. 3. Vision begins in the retina, when light is absorbed by rods or cones. The retina is like a minicomputer that transduces light energy into neural energy. 4. Retinal ganglion cells have centersurround receptive fields and are concerned with changes in contrast (the difference in intensity between adjacent bits of the scene). 5. The retina sends information to the brain via the ganglion cells; neurons whose axons make up the optic nerves. 6. The visual system deals with large variations in overall light intensity by (a) regulating the amount of light entering the eyeball, (b) using different types of photoreceptors in different situations, and (c) effectively throwing away photons we dont need. 7. Retinitis pigmentosa (RP) is a family of hereditary diseases characterized by the progressive death of photoreceptors and degeneration of the pigment epithelium. In the most common form of the disease, patients first notice vision problems in their peripheral vision and under low light conditions, situations in which rods play the dominant role in collecting light. Chapter 2 Study Questions A Little Light Physics 1. Describe the two ways used to conceptualize light. 2. Describe the difference between light that is reflected and light that is transmitted. Answers: 1. One way is to think of it as a wave that travels through a medium. Another is to think of it as a stream of photons, tiny particles, each consisting of one quantum of energy. 2. Reflected light occurs when a ray of light strikes a light-colored surface and then bounces back towards its point of origin. Transmitted light occurs when light is neither reflected nor absorbed by a surface. An example is a transparent window; light passes through the surface and is transmitted to the other side. Eyes That See Light 3. What is the purpose of the cornea? The cornea is a transparent surface on the exterior of the eye. It protects the eye from the outside world. Being transparent, it allows light to be transmitted through it and into the eye. 4. What is the purpose of the retina? The retina is a light-sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve. 5. How does the process of accommodation take place in the eye? Accommodation takes place in the lens of the eye. The lens changes its refractive power by changing its shape. This causes the eye to be able to focus on a given object, whether it is near or far. 6. What is astigmatism and how can it be fixed? Astigmatism is a visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea. It can be fixed by wearing lenses that have two focal points (that provide different amounts of focusing power in the horizontal and vertical planes). 7. Why are photoreceptors important in the process of seeing? Photoreceptors are the cells that make up the backmost layer of the retina. They are sensitive to light, and as soon as they sense it, they can cause neurons in the intermediate layers to fire action potentials. Photoreceptors are important in the process of seeing because they transduce the physical energy of light into neural energy that our brains can analyze Retinal Information Processing 8. What are rods and cones? Rods and cones are photoreceptors present in the retina. Rods are specialized for night vision, while cones are specialized for daylight vision, fine visual acuity, and color. 9. Explain what happens in the process of hyperpolarization. Hyperpolarization is an increase in membrane potential in where the inner membrane surface becomes more negative than the outer membrane surface. This process is one in a sequence of events that occur once light is sensed by the photoreceptors. 10. Why cant rods signal differences in color? Rods cannot signal differences in color because they only have one type of photopigment. Cones, on the other hand, have three types of photopigments, which help them differentiate between colors. 11. What is the role of horizontal cells? Horizontal cells are specialized retinal cells that contact both photoreceptors and bipolar cells. They produce lateral inhibition, which allows the signals that reach retinal ganglion cells to be based on differences in activations between nearby photoreceptors rather than absolute levels of activation. 12. What is visual acuity? Visual acuity is a measure of the finest detail that one can resolve. 13. What is the difference between an ON midget bipolar cell and an OFF midget bipolar cell? An ON midget bipolar cell is a small cone bipolar cell that depolarizes in response to an increase in light intensity. An OFF midget bipolar cell is a small cone bipolar cell that depolarizes in response to a decrease in light intensity. These two cells have opposite reactions to light. 14. What is a receptive field? A receptive field is the region on the retina in which stimuli will activate a neuron. Receptive fields vary in size, shape, and complexity. 15. Why is the centersurround organization of retinal ganglion cells so important? The centersurround organization of retinal ganglion cells is important because it allows for sensitivity to contrast rather than absolute illumination levels. Ganglion cells are most sensitive to differences in the intensity of light in the center and in the surround, and they are relatively unaffected by the average intensity of light. This is useful because the average intensity of light falling on the retina will be quite variable, depending on whether the observer is indoors, outdoors, etc., but contrasts of light are relatively constant. 16. What is a filter and how is it important in vision? A filter is an acoustic, electrical, electronic, biological, or optical device, instrument, or computer program that allows the passage of some frequencies or digital elements and blocks others. Filter is important in vision because it allows the transformation of the raw image into a representation by the brain. The filter highlights certain important visual information while it eliminates other unimportant information. The centersurround receptive fields of retinal ganglion cells are filters. 17. What are some consequences of the differing sizes of M ganglion cell and P ganglion cell receptive fields? P ganglion cells have smaller receptive fields than M ganglion cells at all eccentricities. This allows the M ganglion cells to respond to a larger portion of the visual field. In addition, they are much more sensitive to visual stimuli under low lighting conditions than P ganglion cells. P ganglion cells, on the other hand, provide finer resolution (greater acuity) than M ganglion cells can, as long as there is enough light for them to operate. Whistling in the Dark: Dark and Light Adaptation 18. Explain how the pupil adapts to dark and light conditions. The pupil has the ability to dilate and constrict, depending on amount of light. For example, under well-lit conditions, the pupil tends to constrict to let less light into the eye. Under dark conditions, the pupil dilates to allow more light into the eye. 19. Explain why it is that we are generally not bothered by variations in overall light levels. We are generally not bothered by variations in overall light levels because we have several mechanisms for regulating how much light enters the eye. One mechanism is the pupil size. Another is the regeneration rates of pigments in our photoreceptors. Yet another is the rod/cone dichotomycones operate at moderate and high light levels while rods take over for low light levels. Finally, the neural circuitry of the retina itself helps stabilize external light variations by emphasizing contrasts in luminance rather than absolute light levels. The Man Who Could Not See Stars 20. Describe the family of diseases known as retinitis pigmentosa. Retinitis pigmentosa is a family of hereditary diseases that involves the progressive death of photoreceptors and degeneration of the pigment epithelium. The rods are usually affected before the cones. Therefore, people suffering from this disease first notice vision problems in their periphery and under low light conditions (where rods play the dominant role in collecting light). 21. Describe what happens to the visual field of a patient with retinitis pigmentosa.
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