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Chapter 4

Chapter 4.docx

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Elizabeth Olds

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Chapter 4: Cortical Organization The organized Visual System ­ Organization plays a central role in achieving the tasks of both processing specific information and combining information to create coherent perceptions An Exploration of Spatial Organization ­ Spatial organization refers to the way stimuli at specific locations in the environment are represented by activity at specific locations in the nervous system. The Electronic Map on V1 ­ Thus, if we record from a neuron at pointAin the cortex, its receptive field will be located at pointAon the retina; if we record from point B, the receptive field is at point B; and so on. These examples show that locations on the cortex correspond to locations on the retina. This electronic map of the retina on the cortex is called a retinotopic map . ­ Aand B take up more space on the cortex than C and D. What this means is that the map on the cortex is distorted, with more space being allotted to locations near the fovea than to locations in the peripheral retina. This apportioning to the small fovea of a large area on the cortex is called cortical magnification ­ Even though the fovea accounts for only 0.01 percent of the retina’s area, signals from the fovea account for 8 to 10 percent of the retinotopic map on the cortex ­ Brain imaging refers to a number of techniques that result in images that show which areas of the brain are active ­ The basic principle behind the PET scan is that the parts of the brain that are active will require more “fuel” from the blood than other, less active parts of the brain. Thus, changes in the activity of the brain are accompanied by changes in blood flow, so monitoring the radioactivity of the injected tracer provides a measure of brain activity. ­ Like PET, fMRI is based on the measurement of blood flow. Because hemoglobin, which carries oxygen in the blood, contains an iron molecule and therefore has magnetic properties, presenting a magnetic field to the brain causes the hemoglobin molecules to line up like tiny magnets. ­ The observer looked directly at the center of the screen, so the dot at the center fell on the fovea. During the experiment, stimulus light was presented in two places: o near the center (red area), which illuminated a small area near the fovea; and o farther from the center (blue area), which illuminated an area in the peripheral retina. ­ This activation illustrates the magnification factor because stimulation of the small area near the fovea activated a greater area on the cortex than stimulation of the larger area in the periphery The Cortex is Organized in Columns  Location and Orientation Columns  ­ When they positioned an electrode perpendicular to the surface of a cat’s cortex, they found that every neuron they encountered had its receptive field at about the same location on the retina ­ From this result, Hubel and Wiesel concluded that the striate cortex is organized into location columns that are perpendicular to the surface of the cortex, so that all of the neurons within a location column have their receptive fields at the same location on the retina. ­ Orientation columns.All of the cortical neurons encountered along trackA respond best to horizontal bars (indicated by the red lines cutting across the electrode track).All of the neurons along track B respond best to bars oriented at 45 degrees. ­ Based on this result, Hubel and Wiesel concluded that the cortex is organized into orientation columns , with each column containing cells that respond best to a particular orientation. ­ If an electrode is inserted obliquely into the cortex, it crosses a sequence of orientation columns. The preferred orientation of neurons in each column, indicated by the bars, changes in an orderly way as the electrode crosses the columns. The distance the electrode is advanced is exaggerated in this picture. One Location Column: Many Orientation Columns  ­ Alocation column that contains the full range of orientation columns.A column such as this, which contains a full array of orientation columns, was called a hypercolumn by Hubel and Wiesel.Acolumn such as this receives information about all possible orientations that fall within a small area of the retina. ­ Alocation column with all of its orientation columns, which has been called a hypercolumn by Hubel and Wiesel, receives information about all possible orientations that fall within a small area of the retina, and it is therefore well suited for processing information from a small area in the visual field How Do Feature Detectors Respond to a Scene? ­ Each of the circles represents the area served by a location column. Figure 4.11b shows the location columns in the cortex. Remember that each of these location columns contains a complete set of orientation columns (Figure 4.9). This means that the vertical tree trunk will activate the 90- degree orientation columns in each location column, as indicated by the orange areas in each column. ­ Thus, the continuous tree trunk is represented by the firing of neurons in a number of separated columns in the cortex. ­ The cortical representation of a stimulus does not have to resemble the stimulus; it just has to contain information that represents the stimulus. ­ Each circle or ellipse in the scene represents an area that sends information to one location column. Working together, these columns cover the entire visual field, an effect called tiling Streams: Pathways for What, Where, and How ­ One of the most influential ideas to come out of this research is that there are pathways, or “streams,” that transmit information from the striate cortex to other areas in the brain. Streams for Information About What and Where ­ Ablation refers to the destruction or removal of tissue in the nervous system. ­ The goal of a brain ablation experiment is to determine the function of a particular area of the brain ­ Once the animal’s perception has been measured, a particular area of the brain is ablatedAfter ablation, the monkey is retrained to determine which perceptual capacities remain and which have been affected by the ablation. ­ Object discrimination: Pick the correct shape. Lesioning the temporal lobe (shaded area) makes this task difficult. ­ Landmark discrimination: Pick the food well closer to the cylinder.
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