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Chapter 4

PSY 280 CH. 4 Summary.docx

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University of Toronto St. George
Kristie Dukewich

PSY280 CH.4 Cortical Organization • Research on the physiology of stimuli has been dominated by 2 major themes: 1) Describing the types of stimuli that cause neurons at different levels of the visual system to respond; and 2) describing how neurons in the visual system are organized. The Organized Visual System • Organization plays an important role in achieving the tasks of both processing specific information and combining information to create coherent perceptions. An Exploration of Spatial Organization • Spatial organization: How different locations in the environment and on the receptors are represented in the brain. • The organization in visual space then becomes organization of the eye. When the image is transformed into electrical signals, a new type of organization occurs in the form of ‘electrical maps’of the retina in structures higher up in the system. The Electronic Map on V1 • We can reverse the process of retinal activity in the cortex by recording from a neuron in the cortex and determining the location of its receptive field on the retina. • The electronic map of the retina on the cortex is a retinotopic map. • Retinotopic map: A map on a structure in the visual system, such as the lateral geniculate nucleus or the cortex, that indicates locations on the structure that correspond to locations on the retina. In retinotopic maps, locations adjacent to each other on the retina are usually represented by locations that are adjacent to each other on the structure. • The map is distorted in that more space is allotted to locations near the fovea. • Cortical magnification: Occurs when a disproportionately large area on the cortex is activated by stimulation of a small area on the receptor surface. One example of cortical magnification is the relatively large area of visual cortex that is activated by stimulation of the fovea. An example in the somatosensory system is the large area of somatosensory cortex activated by stimulation of the lips and fingers. • Brain imaging: Procedures that make it possible to visualize areas of the human brain that are activated by different types of stimuli, tasks, or behaviours. The two most common techniques used in perception research are position emission tomography (PET) and functional magnetic resonance imaging (fMRI). • Position emission tomography (PET): A brain mapping technique that is used in awake human subjects to determine which brain areas are activated by various tasks. • For a PET, a person is injected with a low dose of a radioactive tracer. The basic principle is that the parts of the brain that are more active will require more blood flow, thus monitoring of the radioactive tracer provides a measure of brain activity. • Functional magnetic resonance imaging (fMRI): A brain imaging technique that indicates brain activity in awake, behaving organisms. The fMRI response occurs when the response to a magnetic field changes in response to changes in blood flow in the brain. • Since hemoglobin carries oxygen to the brain, measuring it will provide a measure of activity. This is done by taking advantage of the fact that hemoglobin carries iron, which is a molecule that has magnetic properties. Presenting a magnetic field to the brain causes the hemoglobin to line up like tiny magnets. fMRI indicates the presence of brain activity because the hemoglobin molecules in areas of high brain activity lose some of the oxygen they are transporting. This makes them more magnetic, so these molecules will respond strongly to the magnetic field. • Robert Dougherty and coworkers used brain imaging to determine the magnification factor in the human visual cortex. • What the magnification factor means when you look at a scene is that the information about the part of the scene you are looking at takes up a larger space on your cortex than an area of equal size that is off to the side. The Cortex is Organized in Columns Location and Orientation Columns • Hubel and Wiesel carried out experiments in which they recorded from neurons they encountered as they lowered electrodes into the cortex. They found that when going in a perpendicular direction that every neuron they encountered had its receptive field on the same spot of the retina. • Location column: A column in the visual cortex that contains neurons with the same receptive field locations on the retina. • Hubel and Wiesel concluded that the striate cortex is organized in location columns that are perpendicular to the surface of the cortex.All neurons in a location column have the same receptive field. They also noted that the neurons responded to the same preferred orientation of stimuli. • Orientation column: A column in the visual cortex that contains neurons with the same orientation preference. • As they moved in a lateral direction, they noticed that neurons adjacent to each other prefer slightly different orientations. The change in orientation preference changes in an orderly fashion, so that a column that responds best to 90 is next to a column that prefers 85. • They noted that 1-mm was the length required to cover the range of orientations. One Location Column: Many Orientation Columns • Hypercolumn: In the striate cortex, unit proposed by Hubel and Wiesel that combines location, orientation, and ocular dominance columns that serve a specific area on the retina. • All possible orientations fall into a hypercolumn, all representing the same receptive field. How Do Feature Detectors respond to a Scene • An object, like a tree trunk, can be represented by the firing of neurons in a number of different separated columns in the cortex. • Important to note that the cortical representations of a stimulus do not have to resemble the stimulus; it just has to contain the information that represents the stimulus. • Tiling: The adjacent (and often overlapping) location columns working together to cover the entire visual field (similar to covering a floor with tiles). Streams: Pathways for What, Where, and How • One of the most influential ideas to come out of 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: Removal of an area of the brain. This is usually done in experiments on animals, to determine the function of a particular area. • Before ablation is performed, the animal is tested in its abilities. Once the animal’s perception has been measured, a particular area of the brain is ablated.After the ablation, the animal is then re-tested to see which aspects have been affected. • There were two main tasks that researchers Ungerlleider and Mishkin presented their monkeys with: an object discrimination task and a landmark discrimination task. • Object discrimination problem: The behavioural task used in Ungerleider and Mishkin’s experiment in which they provided evidence for the ventral, or what, visual processing stream. Monkeys were required to respond to an object with a particular shape. • In the object discrimination problem, the monkeys were shown one object and were then presented with 2 objects, one being the same as the one they were originally shown. If the animal pushed aside the target object, it received a food reward. • Landmark discrimination problem: The behavioural task used in Ungerleider and Mishkin’s experiments in which they provided evidence for the dorsal, or where, visual processing stream. Monkeys were required to respond to a previously indicated location. • In the landmark discrimination problem, the subject was to remove the cover of the food well that was closest to a landmark. • Lesioning the temporal lobe makes object discrimination difficult. Lesioning the parietal lobe makes the landmark discrimination problem difficult. • This indicates that the pathway that reaches the temporal lobe is responsible for determining an objects identity.Also indicates that the pathway that leads to the parietal lobe is responsible for deter
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