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Chapter 5

Chapter 5 & 6 Textbook.docx

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Wilfrid Laurier University
Laurie Manwell

Perceiving Objects and Scenes • Goldstein: Ch. 5 (p. 100-103; 105-109; 120-121) Visual Attention • Goldstein: Ch. 6 (p. 134-140; 148-150) Midnight Resurrections: Stroke Victims Learn to Move and Speak Again • Doidge: Ch. 5 (all) The Culturally Modified Brain (Part 2) • Doidge: Appendix I (p. 300-312 only) Chapter 5: The stimulus on the receptors is ambiguous - The fact that a particular image on the retina (or a computer vision machine’s sensors) can be created by many different objects is called the inverse projection problem. - Another way to state this problem is as follows: If we know an object’s shape, distance, and orientation, we can determine the shape of the object’s image on the retina. However, a particular image on the retina can be created by an infi nite number of objects - information from a single view of an object can be ambiguous. Humans solve this problem by moving to different viewpoints, and by making use of knowledge they have gained from past experiences in perceiving objects. Objects can be hidden or blurred - Although it might take a little searching, people can fi nd the pencil in the foreground, and the glasses frame sticking out from behind the computer next to the scissors, even though only a small portion of these objects is visible. - People also easily perceive the book, scissors, and paper as single objects, even though they are partially hidden by other objects. - This problem of hidden objects occurs any time one object obscures part of another object. This occurs frequently in the environment, but people easily understand that the part of an object that is covered continues to exist, and they are able to use their knowledge of the environment to determine what is likely to be present - Objects look different from different viewpoints Another problem facing any perception machine is that objects are often viewed from different angles. This means that the images of objects are continually changing, depending on the angle from which they are viewed - The ability to recognize an object seen from different viewpoints is called viewpoint invariance. - Gestalt psychologists— where Gestalt, roughly translated, means a whole confi guration that cannot be described merely as the sum of its parts. We can appreciate the meaning of this defi nition by considering how Gestalt psychology began. Gestalt laws of perceptual organization - Perceptual organization involves the grouping of elements in an image to create larger objects - Here are six of the laws of organization that the Gestalt psychologists proposed to explain how perceptual grouping VL8 such as this occurscan also occur because of similarity of shape, VL 10 size, or orientation - Grouping also occurs for auditory stimuli Good continuation - law of good continuation: Points that, when connected, result in straight or smoothly curving lines are seen as belonging together, and the lines tend to be seen in such a way as to follow the smoothest path. - Good continuation also helped us to perceive VL 11, 1the smoothly curving circles Proximity - law of proximity, or nearness: Things that are near each other appear to VL 13be grouped together. Common region - Principle of common region: Elements that are within the same region of space appear to be grouped together. Even though the circles inside the ovals are farther apart than the circles that are next to each other in neighboring ovals, we see the circles inside the ovals as belonging together. This occurs because each oval is seen as a separate region of space Uniform connectedness - The principle of uniform connectedness states: A connected region of visual properties, such as lightness, color, texture, or motion, is perceived as a single unit. Synchrony - The principle of synchrony states: Visual events that occur at the same time are perceived as belonging together. Common fate - The law of common fate states: Things that are moving in the same direction appear to be grouped together. - Notice that common fate is like synchrony in that both principles are dynamic, but synchrony can occur without movement, and the elements don’t have to change VL14 in the same direction as they do in common fate Meaningfulness or familiarity - law of familiarity, things that form patterns that are familiar or meaningful are likely to become grouped together perceptual segregation: how objects are separated from the background - The Gestalt psychologists were also interested in explaining perceptual segregation, the perceptual separation of one object from another, as Roger did when he perceived each of the buildings in Figure 5.1 as separate from one another. The question of what causes perceptual segregation is often referred to as the problem of fi gure–ground segregation. When we see a separate object, it is usually seen as a fi gure that stands out from its background, which is called the ground - The Gestalt psychologists were interested in determining the properties of the fi gure and the ground and what causes us to perceive one area as fi gure and the other as ground. What are properties of figure and ground? - Some of the properties of the fi gure and ground are: o The fi gure is more ―thinglike‖ and more memorable than the ground. Thus, when you see the vase as fi gure, it appears as an object that can be remembered later. However, when you see the same white area as ground, it does not appear to be an object and VL 15 is therefore not particularly memorable. o The fi gure is seen as being in front of the ground. Thus, when the vase is seen as fi gure, it appears to be in front of the dark background (Figure 5.22a), and when the faces are seen as fi gure, they are on top of the light background (Figure 5.22b). o The ground is seen as unformed material and seemsto extend behind the fi gure. o The contour separating the fi gure from the ground appears to belong to the fi gure. This property of fi gure, which is called border ownership, means that, although fi gure and ground share a contour, the border is associated with the fi gure. Figure 5.23 illustratesborder ownership for another display that can be perceived in two ways. If you perceive the display in Figure 5.23a as a light gray square (the fi gure) sitting on a dark background (the ground), then the border belongs to the gray square, as indicated by the dot in Figure 5.23b. But if you perceive the display as a black rectangle with a hole in it (the fi gure) through which you are viewing a gray surface (the ground), the border would be on the black rectangle, as shown in What factors determine which area is figure - regions in the lower part of a display are more likely to be perceived as fi gure than regions in the upper part Gestalt Laws of heuristics - the Gestalt principles are more accurately described as heuristics—rules of thumb that provide a best-guess solution to a problem. We can understand what heuristics are by comparing them to another way of solving a problem, called algorithms. - An algorithm is a procedure that is guaranteed to solve a problem - We say the Gestalt principles are heuristics because, like heuristics, they are best-guess rules that work most of the time, but not necessarily all of the time - The fact that heuristics are usually faster than algorithms helps explain why the perceptual system is designed to operate in a way that sometimes produces errors. Pg 120-121 The physiology of object and sense perception Neurons that respond to perceptual grouping and figure ground - Many years after the Gestalt psychologists proposed the laws of good continuation and similarity, researchers discovered neurons in the visual cortex that respond best to displays that refl ect these principles of grouping - contextual modulation, the context within which the bar appears affects the neuron’s response to the bar. How does the brain respond to objects Review of sensory coding - Three of these areas are the fusiform face area (FFA), for faces; the extrastriate body area (EBA), for bodies; and the parahippocampal place area (PPA), for buildings and places. Although neurons in these areas respond to specifi c types of stimuli, they aren’t totally specialized, so a
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