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Chapter 5: Perceiving Objects

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1 Chapter 5: Perceiving Objects

2 How do we distinguish objects from their background?
Overview of Questions Why do some perceptual psychologists say “the whole differs from the sum of its parts”? How do “rules of thumb” help us in arriving at a perception of the environment? How do we distinguish objects from their background? Why are even the most sophisticated computers unable to match a person’s ability to perceive objects? The instructor can use the last question to frame the entire chapter, if he or she desires to do so. The question can be posed at the end of the lecture material to see how the students have processed the material.

3 The Challenge of Object Perception
The stimulus on the receptors is ambiguous Inverse projection problem: an image on the retina can be caused by an infinite number of objects Objects can be hidden or blurred Occlusions are common in the environment

4 Inverse projection problem.

5 The Challenge of Object Perception - continued
Objects look different from different viewpoints Viewpoint invariance: the ability to recognize an object regardless of the viewpoint The reasons for changes in lightness and darkness in the environment can be unclear


7 The Structuralist Approach
Approach established by Wundt (late 1800s) States that perceptions are created by combining elements called sensations Structuralism could not explain apparent movement Stimulated the founding of Gestalt psychology in the 1920s by Wertheimer, Koffka, and Kohler

8 Structuralist Approach

9 Gestalt Psychology Wertheimer, Koffka, and Kohler (1912)
Perceptions are often greater (or at least different) than the sum of the sensations Structuralism: Purely Bottom-Up processing Many perceptions undermine Structuralism

10 Illusory contours are another phenomenon that the structural approach could not explain since the observer sees things that are not present, such as the sides of the Necker cube.

11 The Gestalt Approach The whole differs from the sum of its parts Perception is not built up from sensations but is a result of perceptual organization Principles of perceptual organization Pragnanz - every stimulus is seen as simply as possible Similarity - similar things are grouped together

12 Pragnanz

13 Similarity


15 Principles of Perceptual Organization - continued
Good continuation - connected points resulting in straight or smooth curves belong together Lines are seen as following the smoothest path Proximity - things that are near to each other are grouped together Common fate - things moving in same direction are grouped together

16 Good Continuation

17 (a) Nearness and (b) Nearness competing with Similarity.

18 Gestalt Laws of Perceptual Organization
Common Fate – things that move together appear to be grouped together

19 Principles of Perceptual Organization - continued
Meaningfulness or familiarity - things form groups if they appear familiar or meaningful Common region - elements in the same region tend to be grouped together Uniform connectedness - connected region of visual properties are perceived as single unit Synchrony - elements occurring at the same time are seen as belonging together

20 Figure 5. 20 The Forest Has Eyes by Bev Doolittle (1985)
Figure 5.20 The Forest Has Eyes by Bev Doolittle (1985). Can you find the 13 faces in this picture?

21 Figure 5.21 Grouping by (a) common region; (b) proximity; (c) connectedness; and (d) synchrony. The yellow lights blink on and off together.

22 The Gestalt Approach - continued
Researchers have found neurons that respond maximally to displays that reflect: Good continuation Similarity Gestalt principles do not make strong enough predictions to qualify as “laws” They are better understood as heuristics - “best guess rules” The instructor can compare heuristics with algorithms and note that heuristics are not always correct, but they are quick and provide right answers enough of the time that they are effective in most cases.

23 Perceptual Segregation
Figure-ground segregation - determining what part of environment is the figure so that it “stands out” from the background Properties of figure and ground The figure is more “thinglike” and more memorable than ground The figure is seen in front of the ground The ground is more uniform and extends behind figure The contour separating figure from ground belongs to the figure

24 Figure 5.24 A version of Rubin’s reversible face-vase figure.
This slide and the next one explain reversible figure-ground objects. These can be used as an example for how the viewer actually determines the figure rather than this being something that is dictated by the environment. The instructor can make sure the students understand the term reversible figure-ground by using these two slides. Figure 5.24 A version of Rubin’s reversible face-vase figure.

25 Figure 5.25 (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.

26 Figure-Ground Segregation - continued
Factors that determine which area is figure: Elements located in the lower part of displays Units that are symmetrical Elements that are small Units that are oriented vertically Elements that have meaning

27 This slide provides the stimuli and results from an experiment on factors that affect which part of a display is seen as the figure. There was a preference for the lower part of the display over the upper, but there was not preference for left versus right. Figure 5.27 (a) Stimuli from Vecera et al. (2002). (b) Percentage of trials on which lower or left areas were seen as figure.

28 Figure 5.28 Examples of how displays that are (a) symmetrical; (b) small in size; c) oriented vertically or horizontally; or meaningful and more likely to be seen as figure.

29 Figure-Ground Segregation - Neural Evidence
Recordings from V1 in the monkey cortex show: Response to area that is figure No response to area that is ground This result is important because: V1 neurons are early in the nervous system It reveals both a “feedforward” and “feedbackward” in the system It demonstrates contextual modulation This slide contains a number of very important concepts the students should understand that are related to the last three points on the slide. Since V1 neurons are early in the system, they would not be assumed to be relevant for the upper level function of perception. However, since the system is a loop that communicates both forward (from the receptors to the further levels for the brain) and backward (from the cortex to the receptors), even a neuron like V1 with a small receptive field can play a key role in the perception of something as complex as figure-ground segregation. Contextual modulation (neurons outside of a given neuron’s receptive field having an impact on it firing rate) also shows the interconnectedness of the system and that the feedback runs in a number of directions.

30 Responses from V1 Cells (Adapted from Lamme et al., 1995.)

31 Questions Used in Modern Object Perception Research
Why does the visual system respond best to specific types of stimuli? Must a figure be separated from ground before we can recognize objects? How do we recognize objects from different viewpoints? How does the brain process information about objects? This slide is the first of series that includes the major questions asked by modern researchers in object perception. The instructor may want to keep referring to this as he or she works through this section.

32 Why Does the Visual System Respond Best to Specific Types of Stimuli?
Regularities in the environment There is a preponderance of verticals and horizontals Oblique effect - people are more sensitive to these orientations Occurs due to biology and experience Gestalt heuristics are reflected in environmental objects The Gestalt heuristics that may work the best to discuss are good continuation or uniform contentedness. This would be a good place for students to review the Gestalt heuristics and see how they could find examples of them in their everyday environment.

33 Figure Left: Photographs like the ones taken by the participants in Coppola et al.’s (1998) experiment as they walked around the Duke University campus. The results of a computer analysis of the orientation in each type of scene (indoor campus, outdoor campus, and in the forest). This slide can be used to explain that is it not simply due to our carpentered environment that we have a preference for horizontal and vertical orientations.

34 Must a Figure Be Separated from Ground Before We Can Recognize Objects?
Research has shown that objects may be recognized before or during the separation of figure from ground Stimuli with a standing woman and a less meaningful shape were used The meaningful stimulus (the woman) was recognized more often than the other When the picture of the woman was turned upside down, this effect disappeared

35 How Do We Recognize Objects From Different Viewpoints?
Structural-description models 3-D objects are based on 3-D volumes called volumetric features that are combined for a given shape Marr’s model proposed a sequence of events using simple geometrical features The sequence begins with identifying edges and proceeds to recognition of the object

36 Structural-Description Models (Think 3D)
Recognition-by-components theory by Irving Biederman Volumetric features are called geons Theory proposes there are 36 geons that combine to make all 3-D objects Geons include cylinders, rectangular solids, and pyramids

37 Structural-Description Models - continued
Properties of geons View-invariant properties - aspects of the object that remain visible from different viewpoints Accidental property - a property that appears rarely and from certain viewpoints Discriminability - the ability to distinguish geons from one another Principle of componential recovery - the ability to recognize an object if we can identify its geons The instructor can ask the students if they can come up with objects that could NOT be identified from geons alone. The book mentions horses and zebras, but the students should be able to come up with examples of their own. This illustrates one of the criticisms of the theory.

38 Can you Recover the Components?
The instructor can ask the students to think about exactly what is different between the two figures. They should notice that (a) has no vertices so the viewer cannot identify the constituent parts. It should also be noted that both images have the same amount of material missing - it is what is missing that matters, not how much. Can you Recover the Components?

39 Image-Description Models
Ability to identify 3-D objects comes from stored 2-D viewpoints from different perspectives For a familiar object, view invariance occurs For a novel object, view invariance does not occur This shows that an observer needs to have the different viewpoints encoded (stored) before recognition can occur from all viewpoints

40 How Does the Brain Process Information About Objects?
Perceiving an object - sunburst or butterfly? Experiment by Sheinberg & Logothetis Monkey was trained to pull a lever for a sunburst or a butterfly Binocular rivalry was used - each picture shown to one eye Neuron in the IT cortex was monitored Firing was vigorous for only the butterfly IT (interotemporal) cortex Important to note that the firing of the single neuron was directly related to the perception of the butterfly, not to the two physical stimuli that were always present. This suggests that there would be another neuron that would fire preferentially to the sunburst stimulus perception. It is also important to note the way the monkey pulled the levers indicated that the perception of the objects switched from one to the other.


42 Identifying an Object: Is That Harrison Ford?
Grill-Spector experiment Region-of-interest approach: the FFA for each person was determined first by: Showing participants faces and non-faces Finding the area that responded preferentially to faces Instructor can note that the FFA (fusiform face areas) are not in the exact same place for each person. Since this region responds best when perceiving a face, it makes sense that it also may respond best when someone is identifying a face.

43 Grill-Spector Experiment
FFA in each participant was monitored On each trial, participants were shown either: A picture of Harrison Ford’s face A picture of another person’s face A random texture All stimuli were shown for 50 ms followed by a random-pattern mask Participants were to indicate what they saw 60 pictures of each type were presented The instructor can explain the purpose of the masking stimulus at the end of the trial.


45 Grill-Spector Experiment - continued
For trials that only included Harrison Ford’s face, results showed that FFA activation: Was greatest when picture was correctly identified as Ford Was less when picture was identified as other object Showed little response when there was no identification of a face Neural processing is associated with both the presentation of the stimulus and with the response (recognition) to the stimulus

46 Activity in FFA is not passively processing info, It is influenced by attention and the task at hand.

47 Perceptual Intelligence
Theory of unconscious inference Created by Helmholtz (1866/1911) to explain why stimuli can be interpreted in more than one way Main Principle - perceptions are result of unconscious assumptions about the environment Likelihood principle - objects are perceived based on what is most likely to have caused the pattern Your visual system is “making decisions” about what you see before you know what you are looking at! Note that this means that what we learn about our environment becomes part of the problem solving we do when we encounter an object. This idea is related to the Gestalt heuristic because we are giving our best guess based on experience.

48 Figure 5.44 The display in (a) looks like (b) -- a blue rectangle in front of a red rectangle -- but it could be (c), a blue rectangle and an appropriately positioned 6-sided red figure.

49 Modern Ideas on Perceptual Intelligence
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 When we are not sure, we “decide” to see things that fit the scene and “decide” not to see things that are not

50 Stimuli used in Steven Palmer’s (1975) experiment
Stimuli used in Steven Palmer’s (1975) experiment. The scene at the left is presented first, then one of the objects on the right is presented. The observer is then asked to identify if one of the objects on the right was presented.

51 Modern Ideas about Perceptual Intelligence - continued
Light-from-above-heuristic Objects are generally perceived with the assumption that illumination comes from above This is consistent with our experiences from the environment

52 Figure 5.47 Why does (a) look like indentations in the sand and (b) look like mounds of sand? See text for explanation.

53 Figure 5.46 (a) Some of these discs are perceived as jutting out, and some are perceived as indentations. (b) Light coming from above will illuminate the top of a shape that is jutting out, and (c) the bottom of an indentation.




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