perceiving objects and scenes

Object perception bad

Object perception is very hard

Our best computer algorithms are still quite bad at it

What makes it hard:

Stimulus on retina is ambiguous

Objects can be hidden or blurred

Objects look different from different viewpoints and in different poses

Stimulus on retina is ambiguous

All lines fom the same retinal image; 1D retinal image is ambiguous; 2D retinal images are also ambiguous in that multiple stimuli can give rise to the same 2D retinal image

Objects can be partially occluded or blurred

Machine would have difficulty recognizing object because they are partially occluded

Objects look different in different poses and from different viewpoints

Machines find it hard to recognise objects when they appear in unexpected poses or are viewed from unexpected angles

How do humans successfully perceive objects and scenes

Structuralism and Gestaltism

Structuralism

Distinguishes between sensations and perceptions.

Sensations: elementary processes occur in response to stimulation

Perceptions: conscious awareness of objects and scenes

Structuralism claims that sensations combine to form perceptions

In other words, according to structuralism, conscious awareness is the sum of these elementary sensations..and contains nothing that was not already present in these elementary sensations

Gestaltism

Contradicts structuralism

Claims that conscious awareness is more than the sum of the elementary sensations.

In other words, conscious awareness can have characteristics not present in any of the elementary sensations

Evidence for Gestaltism

Apparent motion; illusory contours

Apparent motion

Sees two stationary dots flashed in succession.

Although each of the dots is stationary, the observer perceives motion

(Stop motion disney animation)

Illusory contours

Conscious awareness has a characteristic not present in the elementary sensations

Illusory contours are seen in locations where there are no physical contours

The conscious awareness of the illusory contour is constructed - there is no physical contour at these locations

Gestalt principles of grouping and segregation

Humans are able to perceive objects and scenes because of perceptual organisation. In other words, humans are able to make sense of visual image because they can perceptually organism it into the constituent objects.

Grouping - parts of an image are perceptually bound together to form perceptual whole (eg perception of an object)

Segregation - parts of scene are perceptually separated to form separate wholes (eg perception of separate objects)

Together, grouping and separation allow a scene to perceptually organized into its constituent objects thereby allowing observers to make sense of the scene.

Gestalt principles of grouping

Original gestalt principles:

• good continuation

• Pregnant

• Similarity

• Proximity

• Common fate

Two additional ones (added later)

• common region

• Uniform connectedness

Good continuation (gestalt principle)

Aligned (or nearly aligned) contours are grouped together to form a single object

Pragnanz (Gestalt principle)

German for “good figure” Also known as “principle of good figure” or “principle of simplicity”

Groupings occur to make the resultant figure as simple as possible

Random splotches make a image of panda

Similarity

Similar objects are grouped together:

Eg. A) All dots are same colour so unclear whether things are organised vertically or horizontal; B) colour similarity groups the dots into columns

Proximity

Closer the dots are, the more likely they are grouped together

Grouping by proximity forms horizontal rows

Common fate

Things that are moving in the same way are grouped together

Common region

Elements that are within the same region of space tend to group together

Uniform connectedness

Connected regions with the same visual characteristics tend to group together

What are the three main difficulties of object perception?

• The stimulus on the retina is ambiguous

• Objects can be hidden or blurred

• Objects look different from different viewpoints and in different poses

Describe two bits of evidence for Gestaltism

Apparent motion; illusory contours

How does Gestaltism claim that perceptual organisation is achieved?

By grouping and segregation: grouping - images are perceptually bound together to form a perceptual whole; segregation - parts of scene are perceptually separate to form separate wholes

Name four of the Gestalt principles of grouping

Good continuation, Pragnaz, Similarity, Proximity, Common fate, Common region, Uniform connectedness

Principles of segregation

Objects are normally perceived as “figures” and the background is typically perceived as “ground”

Figural properties

More likely to be seen as figure if:

• They are in front of the rest of the image

• They are at the bottom of the image

• They are convex

• They are recognisable

Rubin vase is ambiguous - can be perceived as either a vase or two faces, there not clear what the figure is

Figural properties - bottom of the image

Lower areas more likely to be seen as figures

Figural properties - convexity

Perceive convex region as figure - however if you see mutiple regions, each with the same colour, you are more likely to perceive those regions as figure

Gist perception

When scenes are flashed rapidly in front of an observer, may not be able to identify all the objects in the scene. But get overall impression of what the scene is about.

Longer the stimulus presented, the more detailed and accurate the description

Summary of lecture

• The problem

• Competitions solutions

• Principles of grouping

• Principles of segregation

• Gist perception

Please list four figural cues

• In front of the rest of the image

• They are at the bottom of the image

• They are convex

• They are recognisable

What is the gist of a scene?

Scenes flashed rapidly, viewer gets an overall impression of what the scene is about

How long do you need to get a rudimentary gist?

27 ms - couldn’t see much, picture mostly dark with some square things, maybe furniture

40 Ms - looked like indoor shot. Saw what looked like a large framed object on a white background