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CHAPTER 4: THE VISUAL CORTEX AND BEYOND 1. The Visual Pathways 2. Visual Cortex of the Cat 3. Simple Cells in the Cortex 4. Complex Cells in the Cortex 5. Contrast Sensitivity 6. Orientation Aftereffect 7. Size Aftereffect 8. Development in the Visual Cortex 9. Retinopy: Ring 10. Retinotopy: Wedge 11. What and Where Streams Chapter 4: The Visual Cortex and Beyond 1. The Visual Pathways Drag and drop the structure that corresponds to each location in the visual pathway. If you are wrong, your answer won’t stick. After you have finished, click on the dot under the left visual field to see how information from one visual field travels along the visual pathway to the brain. Pay special attention to the spatial relations of input from the left and right visual fields. Clicking on audio or script will present an explanation of this pathway. RESULTS & DISCUSSION 1. Identify, in order, the major structures in the neural pathway for vision. 2. How does the original location of a stimulus in the environment relate to where the stimulus (a) is imaged on the retina, and (b) causes activity in the cortex? Virtual Lab Manual 111 Chapter 4: The Visual Cortex and Beyond 2. Visual Cortex of the Cat In this classic 1972 film, vision researcher Colin Blakemore describes his pioneering experiments measuring response properties of neurons in the cortex of the cat. He demonstrates the mapping of receptive fields of neurons in the visual cortex of the cat. The three main types of visual cortical neurons are isolated and their activity in response to visual stimuli is recorded using a microelectrode. He also demonstrates visual neurons arranged in columns within the visual cortex. Courtesy of Colin Blakemore. RESULTS & DISCUSSION 1. Describe how Blakemore used a patterned card to determine the response properties of neurons in the visual cortex. 2. What is the significance of the noise heard when Blakemore moves the patterned card? 3. Do cortical neurons respond to changes in general illumination (turning the room lights off and on)? 4. Describe the preferred stimulus of the simple cell shown in the film. How did Blakemore demonstrate this? 5. Describe the preferred stimulus of the complex cell shown in the film. How did Blakemore demonstrate this? 6. Describe the preferred stimulus of the hypercomplex cell shown in the film (Note that hypercomplex cells are also known as end-stopped cells). How did Blakemore demonstrate this? 7. What characteristics of cell firing are observed when a microelectrode penetrates the cortex perpendicular to the surface? When the penetration is at an angle to the surface? Virtual Lab Manual 113 Chapter 4: The Visual Cortex and Beyond 3. Simple Cells in the Cortex The receptive fields for simple cortical cells have elongated side-by-side arrangement of excitatory and inhibitory areas. As a result, these cells respond best to oriented lines. This demonstration illustrates how a simple cortical cell responds to different stimulus orientations. RESULTS & DISCUSSION 1. Enter the cell’s response to different orientations. Vertical: Slanted left: Horizontal: Slanted right: 2. Why does firing rate decrease for the slanted and horizontal lines? Virtual Lab Manual 115 Chapter 4: The Visual Cortex and Beyond 4. Complex Cells in the Cortex Complex cortical cells are selective not only for line orientation, but also for the direction of motion. For each orientation, click on the arrows to move the bar. RESULTS & DISCUSSION 1. Which orientation and direction of movement resulted in the greatest response? Virtual Lab Manual 117 Chapter 4: The Visual Cortex and Beyond 5. Contrast Sensitivity The contrast threshold is the minimum intensity difference between two adjacent areas that can just be detected. Contrast sensitivity is the reciprocal of contrast threshold, so low threshold represents high sensitivity. Contrast threshold can be measured by determining the lowest contrast between bars of a grating stimulus that is barely detectible. This experiment allows you to determine the relationship between contrast sensitivity and the width of the grating bars (which is related to a measure called spatial frequency that depends on the width of the bars and the observer’s viewing distance). You can use either the method of adjustment or the method of limits. Be sure to read the instructions and do some practice trials before beginning the actual experiment, and be patient because there are 78 trials for each condition. RESULTS & DISCUSSION 1. Present your data from each psychophysical method. 2. Did your contrast sensitivity vary with spatial frequency? Which spatial frequencies have the highest contrast thresholds? Virtual Lab Manual 119 Chapter 4: The Visual Cortex and Beyond 6. Orientation Aftereffect In this demonstration you view two gratings that differ in orientation. Look at the white bar that is located between the two gratings. After the adaptation period has ended, the tilted gratings will be replaced by two vertical gratings. Keep looking at the white bar, while noting the orientations of the upper and lower gratings. RESULTS & DISCUSSION 1. Did the vertical gratings look vertical, or did they appear tilted? How did any tilt relate to the tilt of the corresponding adaptation stimulus? 2. How could this result be explained by the adaptation of orientation-selective neurons in the cortex? Virtual Lab Manual 121 Chapter 4: The Visual Cortex and Beyond 7. Size Aftereffect In this demonstration you will see two gratings, one above and one below a small white bar. Look at the white bar during the adaptation period. When adaptation ends, the wide and narrow gratings will be replaced by two identical gratings of an intermediate bar-width. Keep looking at the white bar, but compare the two gratings, and take note of differences in the sizes of the bars. RESULTS & DISCUSSION 1. Describe your perception of the gratings you viewed after the end of adaptation. 2. If you saw a difference between the gratings you viewed after adaptation, what does this result indicate about the adaptation of neurons that respond best to wide bars and to narrow bars? (Note: This is not covered in the Goldstein text.) Virtual Lab Manual 123 Chapter 4: The Visual Cortex and Beyond 8. Development in the Visual Cortex In this classic 1973 film, vision researcher Colin Blakemore describes his experiments measuring response properties of neurons in the kitten’s cortex. Blakemore discusses the role that the environment has in shaping the response properties of visual neurons. Courtesy of Colin Blakemore. RESULTS & DISCUSSION 1. What proportion of neurons in the newborn kitten and adult cat has connections to both eyes? 2. What happens to the connections to the left and right eyes when one eye is covered early in life? 3. What is the “sensitive period”? 4. Describe the environment in which Blakemore’s kittens were reared in his selective rearing experiment. 5. How do the cats behave when first exposed to a normal environment? What does Blakemore think causes their abnormal behavior? Was this initial abnormal behavior permanent? 6. What are the permanent behavioral effects of selective rearing? Describe how the kitten responds to verticals and horizontals. 7. What is the neural mechanism that is responsible for the kittens’ abnormal response to oriented lines? 8. What do the results of Blakemore’s experiments suggest about the role of acquired vs. innate properties in visual perception? Virtual Lab Manual 125 Chapter 4: The Visual Cortex and Beyond 9. Retinotopy: Ring Retinotopy is a term that refers to the mapping of the areas of the retina to which different brain regions respond. Not until recent advances were made in the field of functional magnetic resonance imaging (fMRI) have we been able to obtain detailed retinopic maps of visual cortex in humans. In fMRI studies, blood flow response to different regions of the brain is measured and is thought to reflect activity related to the processing of a stimulus. Structural anatomical images are obtained for each individual tested using fMRI and are graphically flattened in order to plot the blood flow data in a way that allows us to visualize activity within each bump and groove of the cortex clearly. This movie shows the response measured by fMRI in the visual cortex of a human who was viewing a stimulus. The stimulus shown is a flickering ring with a checkerboard pattern that slowly expanded, moving from the center of vision (the foveal region) to the periphery. Notice that the left and right hemispheres are shown separately and increases in activity are shown in white for three areas within each hemisphere: V1 in the center (red), V2 (green), and V3 (blue). The red ring represents the checkerboard ring stimulus presented in the experiment. Play the movie several times, noting the relationship between changes in the stimulus and changes in brain activity. Courtesy of Geoffrey Boynton. RESULTS & DISCUSSION 1. Describe how brain activity changes as stimulation moves from the fovea towards the periphery. 2. How does this demonstration illustrate the concept of the “retinotopic map” in visual cortex? Virtual Lab Manual 127 Chapter 4: The Visual Cortex and Beyond 10. Retinotopy: Wedge This movie is similar to the “Ring” movie, but illustrates how the cortex is activated by a moving wedge stimulus. Courtesy of Geoffrey Boynton. RESULTS & DISCUSSION 1. Describe activation of the left and right hemispheres that occurs when the wedge is in the left visual field (at 9:00 o’clock) and in the right visual field (at 3:00 o’clock). 2. What does this result indicate about how an object present in one visual field activates the cortex? Virtual Lab Manual 129 Chapter 4: The Visual Cortex and Beyond 11. What and Where Streams In this exercise you can drag and drop each label to its appropriate location. Drag and drop the structure that corresponds to each location. If you are wrong, your answer won’t stick. RESULTS & DISCUSSION 1. Where do both pathways originate? To which cortical area does the dorsal pathway go? The ventral pathway? 2. Why does it make sense that the dorsal pathway is the “action” pathway? 3. Different aspects of processing occur in different pathways, and yet we normally have a unified perception. In order to account for this, what other kinds of pathways must exist? Virtual Lab Manual 131