psyb51 SA 2,3,4.pdf

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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 we’ll 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 it’s 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 center–surround 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 don’t 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 can’t 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 center–surround organization of retinal ganglion cells so important? The center–surround 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 center–surround 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 dichotomy—cones 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. Patients suffering from retinitis pigmentosa typically exhibit an overall shrinkage of their visual fields, as well as “ring scotomas,” which are bands of blindness between the relatively normal central fields and the periphery. Chapter 3 Summary 1. In this chapter we followed the path of image processing from the eyeball to the brain. Neurons in the cerebral cortex translate the array of stars perceived by retinal ganglion cells into the beginnings of forms and patterns. The primary visual cortex is organized into thousands of tiny computers, each responsible for determining the orientation, width, color, and other characteristics of the stripes in one small portion of the visual field. In Chapter 4 we will continue this story by seeing how other parts of the brain combine the outputs from these minicomputers to produce a coherent representation. 2. Perhaps the most important feature of image processing is the remarkable transformation of information from the circular receptive fields of retinal ganglion cells to the elongated receptive fields of the cortex. 3. Cortical neurons are highly selective along a number of dimensions, including stimulus orientation, size, direction of motion, and eye of origin. 4. Neurons with similar preferences are often arranged in columns in primary visual cortex. 5. Selective adaptation provides a powerful, noninvasive tool for learning about stimulus specificity in human vision. 6. The human visual cortex contains pattern analyzers that are specific to spatial frequency and orientation. 7. Normal visual development requires normal visual experience. Abnormal visual experience early in life can cause massive changes in cortical physiology that result in a devastating and permanent loss of spatial vision. Visual Acuity: Oh Say, Can You See? 1. What is visual acuity and how can it be measured? Visual acuity is the smallest spatial detail that can be seen accurately. It can be measured by doing a visual acuity test, which requires looking at figures from a distance and identifying them. 2. Explain what happens during the phenomenon of aliasing. Aliasing is the misperception of a grating due to undersampling. When looking at gratings, the visual system “samples” the grating discretely via the array of receptors at the back of the retina. If the receptors are spaced such that the lightest and darkest parts of the grating fall on separate cones, the observer can detect the grating. However, if the lightest and darkest parts of the grating both fall on the same cones, then the grating will be aliased and appear gray. 3. Explain the meaning of being able to see 20/20. Being able to see 20/20 means that the observer can identify an object at 20 feet as well as a “normal” observer would be able to identify it at 20 feet. If the observer’s vision is 20/40, that means that the observer can see at 20 feet what somebody with normal vision can see at 40 feet (meaning the observer needs glasses!). 4. What can we infer from the contrast sensitivity function? The contrast sensitivity function describes our window of visibility. Any object whose spatial frequencies and contrast fall within the region specified by the contrast sensitivity function will be visible. Those objects outside the region are outside our window of visibility. We can infer from this function that sensitivity to contrast depends on the spatial frequency of the stimulus. Retinal Ganglion Cells and Stripes 5. Explain how retinal ganglion cells respond to stripes. Each ganglion cell responds to certain types of stripes or gratings. For instance, an ON ganglion cell responds to gratings with spatial frequencies and phases that make the lightest part of the grating fall on the center of the cell and the darkest part of the grating fall on the surround. When the spatial frequency of the grating is too low, the ganglion cell responds weakly because part of the bar of the grating lands in the inhibitory surround, damping the cell’s response. Similarly, when the grating’s spatial frequency is too high, the ganglion cell responds weakly because both dark and bright stripes fall within the receptive field’s center and surround, washing out the response. When the frequency is just right, the cell responds vigorously. The Lateral Geniculate Nucleus 6. What is the role of the lateral geniculate nucleus? The lateral geniculate nucleus is a nucleus in the midbrain that shares connections with both the retina and visual cortex. 7. What are the two types of layers of the LGN and how are they different from each other? The two types of layers of the LGN are the magnocellular layers and the parvocellular layers. The magnocellular layers are the two bottom layers of the LGN, and contain neurons that are physically larger than those in the parvocellular layers. Neurons in these layers respond to large, fast-moving objects. The parvocellular layers are the top four layers of the LGN. They contain neurons that respond to details of stationary objects. 8. Explain the notion of topographical mapping. Topographical mapping is the orderly mapping of the world in the lateral geniculate nucleus and the visual cortex. Points of light that are near each other in the world will be processed by neurons that are near each other in the brain. This orderly representation provides us with a neural basis of knowing where things are in space. 9. What are two important features of the visual cortex? Explain. One important feature of the visual cortex is topographical mapping, which is the orderly mapping of the world in the brain. The second feature is the dramatic scaling of information from different parts of the visual field. Objects on or near the fovea are processed by neurons in a large part of the striate cortex, while objects imaged in the periphery are allocated a much smaller portion of the striate cortex. This feature is known as cortical magnification. Receptive Fields in Striate Cortex 10. What is orientation tuning? Orientation tuning is the tendency of neurons in striate cortex to respond optimally to certain orientations, and less to others. 11. In what way do striate cortex neurons function as filters? Each striate cortex neuron responds to a particular location and is tuned to a particular spatial frequency, orientation, and phase. These narrow tuning functions mean that each striate cortex neuron functions as a filter for the portion of the image that excites the cell. 12. What is ocular dominance? Ocular dominance is the property of the receptive fields of striate cortex neurons by which they respond more vigorously when a stimulus is presented in one eye than when it is presented in the other. 13. What is the difference between simple and complex cells? Simple cells are cortical neurons with clearly defined excitatory and inhibitory regions, while complex cells are neurons whose receptive field responds to any properly oriented bar of light, regardless of whether it is light or dark. 14. What is the role of end stopping? End stopping refers to a property of certain cortical neurons in which they respond vigorously when the end of a bar of light falls within their receptive field. It plays an important role in our ability to detect luminance boundaries and discontinuities. Columns and Hypercolumns 15. What does a hypercolumn contain? A hypercolumn contains at least two sets of columns, each covering every possible orientation, with one set preferring input from the left eye and one set preferring input from the right eye. 16. What is the enzyme cytochrome oxidase used for? This enzyme is used to reveal the regular array of “CO blobs,” which are spaced about 0.5 mm apart in the primary visual cortex. These blobs have been implicated in processing color, motion, and spatial structure. Selective Adaptation: The Psychologist’s Electrode 17. How can adaptation provide insights into the properties of cortical neurons? Adaptation is the diminishing response of a sense organ to a sustained stimulus. It is helpful in learning about the properties of cortical neurons because, by exposing an observer to a particular stimuli for an ext
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