Chapter 5: Perceiving Objects and Scenes

Transcription

1 PSY382-Hande Kaynak, PhD 2/13/17 Chapter 5: Perceiving Objects and Scenes 1 2 Figure 5-1 p96 3 Figure 5-2 p96 4 Figure 5-4 p97 1

2 Why Is It So Difficult to Design a Perceiving Machine? The stimulus on the receptors is ambiguous. Inverse projection problem: An image on the retina can be caused by an infinite number of objects. 5 6 Figure 5-5 p98 A par,cular image on the re,na can be created by an infinite number of objects, among them the two rectangles and the,lted trapezoid shown here. This is why we say that the image on the re,na is ambiguous. 7 Figure 5-6 p98 8 Figure 5-7 p98 2

3 Why Is It So Difficult to Design a Perceiving Machine? Objects can be blurred. Objects can be hidden. People are also able to recognize objects that are not in sharp focus Figure 5-9 p99 Why Is It So Difficult to Design a Perceiving Machine? - continued Objects look different from different viewpoints Viewpoint invariance: the ability to recognize an object regardless of the viewpoint This is a difficult task for computers to perform Although humans continue to perceive the chair as the same chair viewed from different angles, this isn t so obvious to a computer Figure 5-10 p99 3

4 PSY382-Hande Kaynak, PhD 2/13/17 Perceptual Organization Perceptual Organization How do humans overcome these complexities? Incoming stimuli in the environment become perceptually grouped to create our perception of objects. Grouping: visual info are put together into objects. Segregation: separating object from another. Answered by perceptual organization Perceptual Organization Structuralism: Approach established by Wundt (late 1800s) States that perceptions are created by combining elements called sensations. According to structuralism, a number of sensations (represented by the dots) add up to create our perception of the face. 15 Figure 5-11 p100 Another principle of structuralism is that the combination of sensations to form perceptions is aided by the observer s past experience. 16 4

5 Perceptual Organization Structuralism could not explain apparent movement. Apparent movement can t be explained by sensations, because there is nothing in the dark space between the flashing images. (a) One light flashes, followed by (b) a short period of darkness, followed by (c) another light flashing in a different position. The resulting perception, symbolized in (d), is a light moving from left to right. Movement is seen between the two lights even though there is only darkness in 17 the space between them. 18 Figure 5-14 p101 The letters and numbers that appear to be moving smoothly across the screen are created by hundreds of small lights that are blinking on and off. Perceptual Organization Illusory contours- contours that appear real but have physical edge The illusory contours clearly visible in (b) and (c) cannot be caused by sensations, because there is only white there. Other message is: the whole is different than the sum of its parts 19 Figure 5-15 p101 Sensations can t explain illusory contours, because there aren t any sensations along the contours. 20 5

6 Gestalt Organizing Principles Example of Good continuation Gestalt psychologists proposed that perception depends on a number of organizing principles, which determine how elements in a scene become grouped together. Good continuation - connected points resulting in straight or smooth curves belong together Figure 5-17 p102 Example of Good continuation Gestalt Organizing Principles Pragnanz - every stimulus is seen as simply as possible This is usually perceived as five circles, not as the nine shapes in (b). So, also called the principle of simplicity 23 Figure 5-18 p

7 Gestalt Organizing Principles Similarity - similar things are grouped together This law causes circles of the same color to be grouped together. Waves, by Wilma Hurskainen Figure 5-21 p103 Gestalt Organizing Principles - continued Gestalt Organizing Principles - continued Proximity - things that are near to each other are grouped together Common fate - things moving in same direction are grouped together. E.g. when you see hundreds of birds all flying together, you tend to see the flock as a unit, and if some birds start flying in another direction, this creates a new unit

8 Gestalt Organizing Principles - continued Gestalt Organizing Principles - continued Common region - elements in the same region tend to be grouped together Uniform connectedness - connected region of visual properties are perceived as single unit This occurs because each oval is seen as a separate region of space Perceptual Segregation Objects (the men s legs) are partially hidden by another object (the gray boards). Figure-ground segregation - determining what part of environment is the figure so that it stands out from the background In this example, the men s legs continue in a straight line and are the same color above and below the boards, so it is highly likely that they continue behind the boards. 31 Figure 5-24 p

9 (a) When the vase is perceived as figure, it is seen in front of a homogeneous dark background. (b) When the faces are seen as figure, they are seen in front of a homogeneous light background. Perceptual Segregation - continued What causes us to perceive one area as figure and the other as ground? Elements located in the lower part of displays are perceived as figure. reversible figure ground The figure is more thinglike and more memorable 33 than the ground. Figure 5-26 p the lower part of the scene is figure and the sky is ground. They flashed stimuli like the ones on top for 150 milliseconds. And then they determined which area was seen as figure, the red area or the green one. Result: there is no left right preference for determining figure, but there is a definite preference for seeing objects lower in the display as figure 35 Figure 5-27 p Figure 5-28 p106 9

10 Subjective Factors That Determine Which are is Figure Gestalt psychologists believed that experience and meaning play a role in perceptual organization. (a) W on top of M. (b) When combined, a new pattern emerges, overriding the meaningful letters. Experiments showed that figure-ground can be affected by meaningfulness of a stimuli Figure 5-30 p107 figure ground formation can be affected by the meaningfulness of a stimulus (a) The black area is more likely to be seen as figure because it is meaningful; (b) this effect doesn t occur when meaningfulness is decreased by turning the picture upside down. At first glance, this scene appears to contain mainly trees, rocks, and water. But on closer inspection, you can see some faces in the trees 39 Figure 5-31 p107 Once you perceive a particular grouping of rocks as a face, it is often difficult not to perceive them in this way. 40 Figure 5-32 p108 10

11 Where is the iphone? J Perceiving Scenes and Objects in Scenes 41 A scene contains: background elements. objects organized in meaningful ways with each other and the background. Difference between objects and scenes A scene is acted within An object is acted upon E.g. if we are walking down the street and mail a letter, we would be acting upon the mailbox (an object) and acting within the street (the scene). 42 Perceiving Scenes and Objects in Scenes - continued You can identify most scenes after viewing them for only a fraction of a second. This general description of the type of scene is called the gist of a scene. (not be able to identify specific objects.) Research on perceiving gists of scenes Potter showed that people can do this when a picture is only presented for 1/4 second Fei-Fei presented pictures for 27 ms to 500 ms and asked observers to write a description of what they saw. He used masking to show that the overall gist is perceived first followed by details. 43 Potter first presented either a description or picture, and then rapidly presented 16 pictures for 250 ms each. The observer s task was to indicate whether the target picture/description had been presented. When the target picture was only specified by a written description, such as girl clapping, observers achieved an accuracy of almost 90 percent. On the other hand, with pictures, her observers could do this with almost 100 percent accuracy even when the pictures were flashed for only 250 ms 44 Figure 5-33 p109 11

12 Fei-Fei Masking the perception of a visual stimulus continues for about 250 ms (1/4 second) after the stimulus is extinguished. SO, when a picture is presented for 100 ms, it will be perceived as lasting about 350 ms. But the persistence of vision can be eliminated by presenting a visual masking stimulus, usually a random pattern that covers the original stimulus, so if a picture is flashed for 100 ms followed immediately by a masking stimulus, the picture is visible for just 100 ms. 45 Fei-Fei (results) The overall gist of the scene is perceived first, followed by perception of details and smaller objects within the scene. See VL. (5.10) Masking 46 Perceiving Scenes and Objects in Scenes - continued What enables observers to perceive the gist of a scene so rapidly? We used info called Global image features of scenes Degree of naturalness (Natural scenes, such as the ocean and forest) Perceiving Scenes and Objects in Scenes - continued Degree of openness (Open scenes, such as the ocean, often have a visible horizon line and contain few objects. The forest has a low degree of openness) Degree of roughness (Smooth scenes (low roughness) like the ocean contain fewer small elements. Scenes with high roughness like the forest contain many small elements and are more complex.)

13 Perceiving Scenes and Objects in Scenes - continued Degree of expansion (The convergence of parallel lines, like what you see when you look down railroad tracks that appear to vanish in the distance. Street example: indicates a high degree of expansion) holistic Color (Some scenes have characteristic colors, like the ocean scene (blue) and the forest (green and brown) Such features are holistic and perceived rapidly 49 Perceiving Scenes and Objects in Scenes - continued Our past experiences in perceiving properties of the environment play a role in determining our perceptions. We learn, for example, that blue is associated with open sky, that landscapes are often green and smooth, and that verticals and horizontals are associated with buildings. Characteristics of the environment, such as these, which occur frequently, are called regularities in the environment. 50 Regularities in the Environment: Information for Perceiving We easily use our knowledge of regularities in the environment to help us perceive, even though we may not be able to identify the specific information we are using. See VL. Physical regularities - regularly occurring physical properties Oblique effect - people perceive horizontals and vertical more easily than other orientations Uniform connectedness - objects are defined by areas of the same color or texture 51 When we pick a point on the scene (such as A) and move slightly away from that point (to B), it is likely that the two points are on the same object if the color is the same. If, however, we move to C, the color changes, which means it is likely we are looking at a different object. 52 Figure 5-36 p111 13

14 Regularities in the Environment: Information for Perceiving continued Physical regularities - regularly occurring physical properties Homogenous colors. and nearby objects have different colors Light-from-above heuristic - light in natural environment comes from above us Light coming from above will illuminate the top of a shape that is jutting out. jutting out Figure 5-37 p111 Why does (a) look like indentations in the sand and (b) look like mounds of sand? Regularities in the Environment: Information for Perceiving - continued Semantic regularities. Semantics refers to the meaning of a scene indentations mounds 55 Figure 5-38 p

15 Regularities in the Environment: Information for Perceiving - continued Semantic regularities. Palmer experiment Observers saw a context scene flashed briefly, followed by a target picture. Results showed that: Targets congruent with the context were identified 80% of the time. Targets that were incongruent were only identified 40% of the time. Stimuli used in Palmer s (1975) experiment. The scene at the left is presented first, and the observer is then asked to identify one of the objects on the right Figure 5-39 p113 the multiple personalities of a blob See rotating masks at VL. Although the blob is the same shape in all of the pictures! What we expect to see in different contexts influences our interpretation of the identity of the blob inside the circles. 59 Figure 5-40 p113 15