Download
1 / 60
610 likes | 832 Views
Chapter 5: Perceiving Objects. 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?
E N D
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 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
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
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
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
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
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
Gestalt Laws of Perceptual Organization • Common Fate – things that move together appear to be grouped together
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
Figure 5.20 The Forest Has Eyes by Bev Doolittle (1985). Can you find the 13 faces in this picture?
Figure 5.21 Grouping by (a) common region; (b) proximity; (c) connectedness; and (d) synchrony. The yellow lights blink on and off together.
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”
Save For Exam 2 • Exam 1 • Chapters 1-4 and Chapter 5 up to page 103 • Monday we will begin with Perceptual Segregation
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
Figure 5.24 A version of Rubin’s reversible face-vase figure.
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.
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
Figure 5.27 (a) Stimuli from Vecera et al. (2002). (b) Percentage of trials on which lower or left areas were seen as figure.
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.
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
Modern Research on Object Perception • Modern research emphasizes: • Obtaining measurements over descriptions • Determining the mechanisms responsible for object perception • This is in contrast to the Gestalt approach, but builds upon Gestalt principles
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?
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
Figure 5.30 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).
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
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
Structural-Description Models – continued (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
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
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
Only one view is stored, so only one is recognized. (Supports a 2D storage model)
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
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
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
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
Activity in FFA is not passively processing info, It is influenced by attention and the task at hand.
Identifying an Object: Is That a Cat or a Dog? • Freedman et al. experiment • Stimuli preparation - images of a cat and dog were morphed in 5 steps • Physiological measurement - neurons in monkeys’ IT and PF cortex were monitored • IT cortex is a module for form perception - what pathway • PF cortex responds when an object is recognized
