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Sensation & PerceptionPSYC420

Thomas E. Van Cantfort, Ph.D.

Objects & Forms

‚ When we look out into the world we are able to seethings as trees, cars, people, books, etc. A wide variety ofobjects and forms.

‚ Given the ease with which we perceive objects, we mightassume that the process of object perception is simple.Ú but the process is complex and not clearly understood.

‚ Real progress in our understanding of perceptualorganization began with a group called the Gestaltpsychologist, which was formed about 1912 by MaxWertheimer.L

Objects & Forms

‚ Gestalt psychologists rejected the idea that perceptionsare constructed from sensations in favor of the idea thatthe stimulus must be considered as a whole.L

Gestalt Psychologist

‚ That is, they rejected the structuralist idea thatperceptions are the results of addition of manyelementary 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.L

Gestalt Psychologist (continued)


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‚ The Gestalt psychologists came up with a compendiumof rules to explain good form.

‚ The Laws 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.L



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< Principle of Similarity states that we group likethings together.L


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< Prägnanz (good form) we tend to see things asbelonging together if they combine to form a “good”figure; the better the figure the more strongly theytend 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 otherforms.L


Prägnanz

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Prägnanz

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‚ We perceive objects when they form figures against theirbackgrounds

‚ This separation of figure and background, which is calledfigure-ground segregation, has been of great interest toGestalt psychologists

‚ who studied reversible figure-ground pattern.L

Figure-Ground Segregation

‚ When the vase is seen as figure, it is perceived in front ofa black background.

‚ When the faces are seen as figure, they are in front of awhite background.L

Reversible Figure-Ground


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‚ This figure can be seen as a cube floating in front of eightdisks

‚ or as a cube seen through eight holes.‚ In the first case the edges of the cube appear as subjective

contours.L


‚ 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.L


‚ The Forest has Eyes by Bev Doolittle (1985).‚ Can you find 13 faces in this picture?L


‚ Ponies by Bev Doolittle (1979)‚ We have good continuation (contours of the horses’

backs and legs)L


‚ Gestalt psychology tells us that we should pay attentionto the overall stimulus pattern.

‚ It also offers a number of laws of organization that helpgovern 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 illustratethem, the operation of some of the laws is not always asstraightforward.L


‚ There are three major theories of perceptual processingas applied to form perception;Ú Feature Integration Approach – 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 asense

‚ and then constructs our perception of the scene bycombining these features.L

Modern Theories of Form Perception

‚ Anne Treisman proposed that form perception takesplace in two or more stages.

‚ In the preattentive stage, which is automatic and rapidÚ the stimulus is decomposed into a number of basic

properties which she calls primitives – the basicphonemes 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.L

Feature Integration Approach

‚ Once primitives are extracted they are combined in thefocused attention stage.

‚ In contrast to preattentive stage, this stage is notautomatic and requires conscious attention.Ú For example, raise your hand as soon as you picked out

the T.L


‚ Let try one more‚ Raise your hand as soon as you picked out the R.L


‚ It is more difficult to find the stimulus R.L


‚ Focused attention stage is not automatic and requiresconscious attention.L


‚ Three factors have been identified in the processing atthe 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 otherstimuli.L


O OXOOXOXXOXXOXOOXXOOOXOOOXXXOXOXOOXOOXOXXXOOXXOO

Ú 1) location of stimulus is important during the focusedattention 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.L



‚ Raise your hand when you find the different colored O inthe next slideL



‚ Easier to remember the location of the target stimulus.L



‚ The Gestalt principle of proximity appears to influencefocused attention stage.L


‚ Subjects are asked if they see t’sÚ more subjects reported t’s for B than for C.Ú Note that spatial separation for B and C are the

same.L


‚ 3) Top-down and bottom-up processing.Ú The analysis of objects into parts is called bottom-up

processing because the processing starts with basicunits< 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 bythe observer’s knowledge of the world.

Ú This taking into account of meaning or familiarity iscalled 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.L


‚ In the next slide where is the fire hydrant?L


‚ More errors are made when the fire hydrant was in astrange location.

‚ Top-down processing influences our perception.L


‚ Simplified flow diagram for Treisman’s two-stageprocessing sequence.L


‚ The preattentive stage acts on stimuli by breaking themdown 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 stageÚ and by top-down processes such as the observer’s

knowledge of the names and functions of variousobjects.

‚ The result of this processing is perception of a wholeobject.L


‚ 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 recognition-

by-components that is concerned not with how attributeslike color and shape are combined but,Ú how we recognize three-dimensional objects based on

our perception of the components that make up theseobjects.

‚ The basic idea behind recognition-by-components is thatwe recognize an object by decomposing that object intobasic components, or primitives, called geons.L

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.L


‚ Biederman’s primitives, unlike Treisman’s, arevolumetric.Ú That is, three dimensional.

‚ The basic principle governing recognition-by-components 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 correctlyrecognized.L


‚ Can you recognize the object is?‚ It is difficult to recognized the object when you are

deprived of geons.L


‚ Can you recognize the object when the geons arepresent?L


‚ One way to look at the computational approach is tothink of it as a computer that is programmed to take intoaccount certain physical properties of the world.

‚ The data fed into this computer are the characteristics ofthe 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 theimage on the retina.L

Computational Approach

‚ The importance of Marr’s system is that it proposes thatwe can perceive forms based solely on an analysis of theinformation in the retinal images.

‚ Marr’s system does not rely on top-down processes thatinvolve things like the observer’s knowledge of whatspecific objects are used for or where certain objects areusually found.

‚ It is, therefore, accurate to say that Marr’s system relieson bottom-up processing.(

Computational Approach

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