sensation and perception - shape.ppt © 2001 dr. laura snodgrass, ph.d.1 shape, pattern, form what...

Sensation and Perception - shape.ppt © 2001 Dr. Laura Snodgrass, Ph.D. 1

Shape, Pattern, Form

• What is needed for shape (pattern, form)?

• Facts a theory must account for

• Organization

• Four categories of theories

• The Information Processing approach

• Bottom-up and Top-down processing


What Is Needed For Shape

• Contours– discontinuity, change, edge, border

• Contours are necessary for shape but do not equal the shape– ambiguous figures

– sketches

– illusory or anomalous contours

• shape without the retinal image of the shape

• need information to indicate the boundaries


Illusory or Anomalous Contours


Facts To Be Explained

• Retinal image is ambiguous– size and shape constancy

• the retinal images changes but perception remains the same

– ambiguous figures

• retinal image remains the same but perception changes

• Segmentation of image into objects


Facts To Be Explained

• Perceptual Learning– learn new discriminations

• Figural Adaptation– figural affereffects


Organization Of The Retinal Image

• Figure/Ground segregation– separating objects from background

• Gestalt Grouping Principles– figuring out which parts of the image belong with which objects


Figure/Ground Segregation

• What part of the image becomes the figure– bounded (closed) area– smaller area– symmetric area– convex area– brighter area


Bounded and Smaller


Symmetric and Convex


Figure/Ground Qualities

• Figure– shape– bounded– near– identified– meaning– remembered

• Ground– shapeless– continuous– far– not identified– meaningless– forgotten


Gestalt Grouping Principles

• Proximity

A A A

AAA AAA A A A

• Similarity

AAAAABBBBAAAAAAA

AAAAAABBAAAAAAAA

AAAAABBBBAAAAAAA


Gestalt Grouping

• Good continuation


Gestalt Grouping

• Closure– illusory contours are based on good continuation and closure


Gestalt Grouping

• Common fate– camouflage is broken by common fate


Gestalt Grouping

• Pragnanz– simplicity and regularity

• Set and past experience– familiarity and expectations

– “Street figures” and ambiguous figures

– perception organized by your expectations


Four Categories Of Theories

• Theories differ on– what is encoded

– how the encoded information is stored and used

• Categories– analytic feature models

– spatial frequency models

– template models (prototype models)

– cognitive construction models


Analytic Feature Models

• What is encoded?– Small local features or elements

– based on Hubel and Wiesel feature detectors

• Theories within this category differ on how the features are reassembled

• Example: Selfridge’s Pandemonium Model– hierarchy of feature detectors

– parallel processing (all features detected at once)


Selfridge’ Pandemonium Model


Spatial Frequency Model

• Fourier analysis (what is encoded)– break complex image into component sine waves

– sine waves are a graph of the distribution of brightness over space (blurry bars)

• The components (blurry bars) can vary in– frequency

– amplitude

– orientation

– phase


Sine Waves

Sine wave patterns

Also called blurry bars or gratings


Sine Waves

Top pattern is low frequency, high amplitude

The next three increase in frequency, but decrease in amplitude

The first four patterns start in-phase, but quickly get out-of-phase (the peaks and valleys do not line up after the first peak)

As you add the patterns together you get a square wave (black and white stripes)


Spatial Frequency Model

• Adding the components together reproduces the original image

• Theories in this category vary in– how the components are added together

• Examples of evidence– contrast sensitivity function

– block portraits

– selective adaptation


Template Model

• What is encoded– whole objects (but not whole scene)

– model (template) stored in memory

– Prototype

• Incoming image is compared to stored representation


Cognitive Construction Models

• What is encoded:– depends upon expectations

– can encode an entire scene at a time if given proper schema

– can encode local elements if no schema is available or if incoming information conflicts with schema

• Schemas– sets of expectations

• e.g. a living room



• Image is processed in steps– make hypothesis based on schema (expectations)

– if incoming information matches the hypothesis there is rapid identification

– if incoming information does not make, must make new hypothesis and try again

– cycle through matching procedure until get a match



• Examples of evidence– ambiguous pictures

– “Street” figures

– scene perception research

– research on the effects of “set” and context


Set and Context


Information Processing Approach

• Stages of processing of form through the memory and recognition system

• Not a separate theory of form perception

• The models of shape perception– refer to processing in LTM that results in recognition

– propose different mechanisms that shape may be stored in LTM


Information Processing Approach

• Distal Stimulus– object in the environment

• Stage I Proximal Stimulus– transduction produces the retinal image

• Stage II Sensory Register– iconic storage for 1/4 sec

• Stage III Short Term Memory– factors that influence further processing

• Stage IV Long Term Memory


Sensory Register

• Sperling’s Partial Report Technique– can report 3 -5 letters before image fades

– proves that entire image is available but fades fast


Short Term Memory

• Limited capacity– 7 +/- 2 items

• Many factors influence either what gets further processing or the speed of further processing

• Any information that does not get further processing is lost


Factors That Influence Processing

• Task requirements– may not need to do a full analysis

– global discrimination

• in which of the following would it be easier to find the letter Q? Why?

– EKLXAVFTZ

– ODCBGPRU


Factors that influence processing

• Instructional Set– process only the features you are instructed to attend to, such as color,

location, form, or size



• Familiarity– familiar things are processed faster

• word superiority

– “m” presented alone

– “aemg” m presented as part of a letter string

– “game” m presented as part of a meaningful word

• the letter is detected fastest when part of a familiar word, even faster than if presented alone



• Interference– slows down processing

– features you can’t ignore

– e.g. Stroop Task - color words presented in incongruous ink colors.

• The automaticity of reading interferes with color naming.


Stroop

• Blue• Green• Red• Blue• Green• Yellow• Yellow• Red• Blue

• Yellow• Green• Red• Orange• Green• Red• Blue• Yellow• Orange



• Schemas and expectations– you are faster at processing things you expect to see

– you are faster at processing things that fit your schemas


Long Term Memory

• Incoming sensory information is compared with the representation stored in LTM

• Each of the theories of form perception makes different predictions about:– what is stored in LTM

– the mechanism of the comparison process


Direction of Processing

• Bottom-up (data driven)– the features encoded rom the

environment determine the final representation

– usually fast and automatic, relatively fixed

– e.g. most feature models and spatial frequency models

• Top-down (conceptually driven)

– the representations already stored in LTM determine what gets encoded from the environment

– slower, more flexible

– experience, expectations, schemas, and set influence encoding

– e.g. cognitive construction models


Direction of Processing

• Both direction of processing are necessary – may use one or the other in different situations

– sometimes both are needed

• e.g. ambiguous figures

– abstract art

sensation and perception - shape.ppt © 2001 dr. laura snodgrass, ph.d.1 shape, pattern, form what...

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