Sensation & Perception PSYC420 Thomas E. Van Cantfort, Ph.D.

Transcription

1 Sensation & Perception PSYC420 Thomas E. Van Cantfort, Ph.D. Objects & Forms When we look out into the world we are able to see things as trees, cars, people, books, etc. A wide variety of objects and forms. Given the ease with which we perceive objects, we might assume that the process of object perception is simple. Ú but the process is complex and not clearly understood. Real progress in our understanding of perceptual organization began with a group called the Gestalt psychologist, which was formed about 1912 by Max Wertheimer. Objects & Forms Gestalt Psychologist That is, they rejected the structuralist idea that perceptions are the results of addition of many elementary sensations. For example subjective contours; Ú We see whole solitary objects even when contours are missing. It is the Gestalt psychologist that argues that the whole is different than the sum of its parts. Gestalt psychologists rejected the idea that perceptions are constructed from sensations in favor of the idea that the stimulus must be considered as a whole. The Gestalt psychologists came up with a compendium of rules to explain good form. The aws of Organization. Ú Rules by which features are organized: < Principle of Proximity state that things that are close to each other seems to go together.

2 < Principle of Similarity states that we group like things together. < Prägnanz (good form) we tend to see things as belonging together if they combine to form a good figure; the better the figure the more strongly they tend to group. < Aspects of prägnanz (What makes a figure good? ) Continuity the appearance of a single entity Symmetry which is when the right half of the figure is a mirror image of the left half. Good forms are easier to see than less good forms; they are also harder to decompose into other forms. Prägnanz Prägnanz

3 Reversible Figure-Ground Figure-Ground Segregation We perceive objects when they form figures against their backgrounds This separation of figure and background, which is called figure-ground segregation, has been of great interest to Gestalt psychologists who studied reversible figure-ground pattern. When the vase is seen as figure, it is perceived in front of a black background. When the faces are seen as figure, they are in front of a white background. Reversible Figure-Ground Reversible Figure-Ground Figure-Ground Segregation Figure-Ground Segregation This figure can be seen as a cube floating in front of eight disks or as a cube seen through eight holes. In the first case the edges of the cube appear as subjective contours. Some properties of the figure and ground are: Ú The figures is more thinglike and more memorable than the ground. Ú The figure is seen as being in front of the ground. Ú The ground is seen as uniformed material and seen to extend behind the figure. Ú The contour separating the figure from the ground appears to belong to the figure. The Forest has Eyes by Bev Doolittle (1985). Can you find 13 faces in this picture?

4 Figure-Ground Segregation Figure-Ground Segregation Gestalt psychology tells us that we should pay attention to the overall stimulus pattern. It also offers a number of laws of organization that help govern the way we group parts to a stimulus together and the way we separate figure from ground. However, while the laws of organization seem to work well when applied to the examples picked to illustrate them, the operation of some of the laws is not always as straightforward. Ponies by Bev Doolittle (1979) We have good continuation (contours of the horses backs and legs) Modern Theories of Form Perception There are three major theories of perceptual processing as applied to form perception; Ú a two stage processing theory proposed by Anne Treisman in the 1980s. Ú Recognition by Components Approach proposed by Irving Biederman in the 1980s. Ú Computational Approach proposed by David Marr in the late 1970s and early 1980s. The basic approaches that the theories have in common is that the visual system identifies basic features from a sense and then constructs our perception of the scene by combining these features. Anne Treisman proposed that form perception takes place in two or more stages. In the preattentive stage, which is automatic and rapid Ú the stimulus is decomposed into a number of basic properties which she calls primitives the basic phonemes in the language of perception. < Some of the primatives are curvature, tilted lines, color, line ends and movements. Ú One of the key properties of the preattentive stage is that it requires no conscious attention. Ú That is, it occurs automatically without any effort on the part of the subject. For example, raise your hand as soon as you picked out the Q.

5 Once primitives are extracted they are combined in the focused attention stage. In contrast to preattentive stage, this stage is not automatic and requires conscious attention. Ú For example, raise your hand as soon as you picked out the T. et try one more Raise your hand as soon as you picked out the R.

6 Focused attention stage is not automatic and requires conscious attention. It is more difficult to find the stimulus R. Three factors have been identified in the processing at the focused attention stage. Ú You are going to be present a set of Xs and Os, Ú I want you to raise your hand as soon as you see the target stimulus which is a different color from the other stimuli. O OXOOXOX XOXXOXOO XXOOOXOO OXXXOXOX OOXOOXOX XXOOXXOO O OXOOXOX XOXXOXOO XXOOOXOO OXXXOXOX OOXOOXOX XXOOXXOO Ú 1) location of stimulus is important during the focused attention stage. < If subjects is told about the stimulus but not it s location < they can report the primitive of the stimulus but not it s location.

7 Raise your hand when you find the different colored O in the next slide O OXOOXOX XOXXOXOO XXOOOXOO OXXXOXOX OOXOOXOX XXOOXXOO O OXOOXOX XOXXOXOO XXOOOXOO OXXXOXOX OOXOOXOX XXOOXXOO Easier to remember the location of the target stimulus. The Gestalt principle of proximity appears to influence focused attention stage. Subjects are asked if they see t s Ú more subjects reported t s for B than for C. Ú Note that spatial separation for B and C are the same.

8 3) Top-down and bottom-up processing. Ú The analysis of objects into parts is called bottom-up processing because the processing starts with basic units < our perception is then built on the foundation laid by these units. Ú But it is clear that perception is influenced not only by the nature of the units that make up objects but also by the observer s knowledge of the world. Ú This taking into account of meaning or familiarity is called top-down processing, < because processing is based on higher-level information, < such as the meaningful context in which a stimulus is seen, < or other information that causes us to expect that another stimulus will be present. In the next slide where is the fire hydrant? More errors are made when the fire hydrant was in a strange location. Top-down processing influences our perception. Simplified flow diagram for Treisman s two-stage processing sequence.

9 The preattentive stage acts on stimuli by breaking them down into primitive. Ú This is a bottom-up process because it depends only on the physical properties of the stimuli. These primitives are then combined, in the focused attention stage, Ú which is affected both by the stimulus input from the preattentive stage Ú and by top-down processes such as the observer s knowledge of the names and functions of various objects. The result of this processing is perception of a whole object. Recognition-By-Components Treisman s work focuses on how different attributes, Ú such as shape, color, texture, and size, are integrated into single object. Irving Biederman has a mechanism called recognitionby-components that is concerned not with how attributes like color and shape are combined but, Ú how we recognize three-dimensional objects based on our perception of the components that make up these objects. The basic idea behind recognition-by-components is that we recognize an object by decomposing that object into basic components, or primitives, called geons. Recognition-By-Components Recognition-By-Components On the left are geons On the right are some objects created from the geons on the left. The number on the objects indicate which geons are present. Note that recognizable objects can be formed by combining just two or three geons. Also note that the relations between geons matter as illustrated by the cup and pail. Biederman s primitives, unlike Treisman s, are volumetric. Ú That is, three dimensional. The basic principle governing recognition-bycomponents is the principle of component recovery, Ú which states that if an object s geons can be identified, then the object can be rapidly and correctly recognized. Recognition-By-Components Recognition-By-Components Can you recognize the object is? It is difficult to recognized the object when you are deprived of geons. Can you recognize the object when the geons are present?

10 Computational Approach One way to look at the computational approach is to think of it as a computer that is programmed to take into account certain physical properties of the world. The data fed into this computer are the characteristics of the retinal image, Ú particularly the pattern of light and dark areas in the image. The computer calculates the existence of objects in the environment based on this data. Computer scientists and engineers are trying to develop with mathematical algorithms that can describe the image on the retina. Computational Approach The importance of Marr s system is that it proposes that we can perceive forms based solely on an analysis of the information in the retinal images. Marr s system does not rely on top-down processes that involve things like the observer s knowledge of what specific objects are used for or where certain objects are usually found. It is, therefore, accurate to say that Marr s system relies on bottom-up processing.(