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Chapter 5: Perceiving Objects and Scenes

Figure 5-1 p96

Figure 5-2 p96

Why Is It So Difficult to Design a Perceiving Machine?

•  The stimulus on the receptors is ambiguous. –  Inverse projection problem: An image on

the retina can be caused by an infinite number of objects.

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Figure 5-4 p97 Figure 5-7 p98

Figure 5-5 p98 Figure 5-6 p98

A  par&cular  image  on  the  re&na  can  be  created  by  an  infinite  number  of  objects,  among  them  the  two  rectangles  and  the  &lted  trapezoid  shown  here.  This  is  why  we  say  that  the  image  on  the  re&na  is  ambiguous.    

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Why Is It So Difficult to Design a Perceiving Machine?

•  Objects can be hidden.

Figure 5-9 p99

Objects can be blurred.

People are also able to recognize objects that are not in sharp focus.

Why Is It So Difficult to Design a Perceiving Machine? - continued

•  Objects look different from different viewpoints – Viewpoint invariance: the ability to

recognize an object regardless of the viewpoint

– This is a difficult task for computers to perform

Figure 5-10 p99

Although humans continue to perceive the chair as the same chair viewed from different angles, this isn’t so obvious to a computer

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Perceptual Organization

– How do humans overcome these complexities? •  Answered by perceptual organization

Perceptual Organization

•  Incoming stimuli in the environment become perceptually grouped to create our perception of objects. – Grouping: visual info are “put together” into

objects. – Segregation: separating object from another.

Figure 5-11 p100

Perceptual Organization

•  Structuralism: Approach established by Wundt (late 1800s) – States that perceptions are created by combining elements called sensations.

According to structuralism, a number of sensations (represented by the dots) add up to create our perception of the face. Another principle of structuralism is that the combination of sensations to form perceptions is aided by the observer’s past experience.

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Perceptual Organization

•  Structuralism could not explain apparent movement. Apparent movement can’t be explained by sensations, because there is nothing in the dark space between the flashing images.

Figure 5-14 p101

(a) One light flashes, followed by (b) a short period of darkness, followed by (c) another light flashing in a different position. The resulting perception, symbolized in (d), is a light moving from left to right. Movement is seen between the two lights even though there is only darkness in the space between them.

Figure 5-15 p101

The letters and numbers that appear to be moving smoothly across the screen are created by hundreds of small lights that are blinking on and off.

Other message is: the whole is different than the sum of its parts

Perceptual Organization

•  Illusory contours- contours that appear real but have physical edge

The illusory contours clearly visible in (b) and (c) cannot be caused by sensations, because there is only white there. Sensations can’t explain illusory contours, because there aren’t any sensations along the contours.

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Gestalt Organizing Principles

•  Gestalt psychologists proposed that perception depends on a number of organizing principles, which determine how elements in a scene become grouped together.

•  Good continuation - connected points resulting in straight or smooth curves belong together

Figure 5-17 p102

Example of Good continuation

Figure 5-18 p102

Example of Good continuation Gestalt Organizing Principles

– Pragnanz - every stimulus is seen as simply as possible

This is usually perceived as five circles, not as the nine shapes in (b). So, also called the principle of simplicity

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Gestalt Organizing Principles

– Similarity - similar things are grouped together

This law causes circles of the same color to be grouped together.

Figure 5-21 p103

Waves,  by  Wilma  Hurskainen

Gestalt Organizing Principles - continued

•  Proximity - things that are near to each other are grouped together

Gestalt Organizing Principles - continued

•  Common fate - things moving in same direction are grouped together. – E.g. when you see hundreds of birds all

flying together, you tend to see the flock as a unit, and if some birds start flying in another direction, this creates a new unit.

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Gestalt Organizing Principles - continued

•  Common region - elements in the same region tend to be grouped together

This occurs because each oval is seen as a separate region of space.

Gestalt Organizing Principles - continued

•  Uniform connectedness - connected region of visual properties are perceived as single unit

Figure 5-24 p104

Objects (the men’s legs) are partially hidden by another object (the gray boards). In this example, the men’s legs continue in a straight line and are the same color above and below the boards, so it is highly likely that they continue behind the boards.

Perceptual Segregation

•  Figure-ground segregation - determining what part of environment is the figure so that it “stands out” from the background

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Figure 5-26 p105

(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.

Perceptual Segregation - continued

•  What causes us to perceive one area as figure and the other as ground?

– Elements located in the lower part of displays are perceived as figure.

Figure 5-27 p105

They flashed stimuli like the ones on top for 150 milliseconds. And then they determined which area was seen as figure, the red area or the green one. Result: there is no left–right preference for determining figure, but there is a definite preference for seeing objects lower in the display as figure

Figure 5-28 p106

the lower part of the scene is figure and the sky is ground.

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Subjective Factors That Determine Which are is Figure

•  Gestalt psychologists believed that experience and meaning play a role in perceptual organization.

•  Experiments showed that figure-ground can affected by meaningfulness of a stimuli.

Figure 5-30 p107

(a)  “W” on top of “M.” (b) When combined, a new

pattern emerges, overriding the meaningful letters.

Figure 5-31 p107

figure–ground formation can be affected by the meaningfulness of a stimulus

(a) The black area is more likely to be seen as figure because it is meaningful; (b) this effect doesn’t occur when meaningfulness is decreased by turning the picture upside down.

Figure 5-32 p108

At first glance, this scene appears to contain mainly trees, rocks, and water. But on closer inspection, you can see some faces in the trees

Once you perceive a particular grouping of rocks as a face, it is often difficult not to perceive them in this way.

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Perceiving Scenes and Objects in Scenes

•  A scene contains: – background elements. – objects organized in meaningful ways with each

other and the background. •  Difference between objects and scenes

– A scene is acted within – An object is acted upon

•  E.g.  if we are walking down the street and mail a letter, we would be acting upon the mailbox (an object) and acting within the street (the scene).

Perceiving Scenes and Objects in Scenes - continued

•  You can identify most scenes after viewing them for only a fraction of a second. This general description of the type of scene is called the gist of a scene. (not be able to identify specific objects.)

•  Research on perceiving gists of scenes – Potter showed that people can do this when a

picture is only presented for 1/4 second – Fei-Fei presented pictures for 27 ms to 500 ms

and asked observers to write a description of what they saw. He used masking to show that the overall gist is perceived first followed by details.

Figure 5-33 p109

Potter first presented either a description or picture, and then rapidly presented 16 pictures for 250 ms each. The observer’s task was to indicate whether the target picture/description had been presented.

When the target picture was only specified by a written description, such as “girl clapping,” observers achieved an accuracy of almost 90 percent. On the other hand, with pictures, her observers could do this with almost 100 percent accuracy even when the pictures were flashed for only 250 ms

Masking

– Fei-Fei –  the perception of a visual stimulus continues for

about 250 ms (1/4 second) after the stimulus is extinguished.

– SO, when a picture is presented for 100 ms, it will be perceived as lasting about 350 ms. But the persistence of vision can be eliminated by presenting a visual masking stimulus, usually a random pattern that covers the original stimulus, so if a picture is flashed for 100 ms followed immediately by a masking stimulus, the picture is visible for just 100 ms.

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Masking

– Fei-Fei (results)

The overall gist of the scene is perceived first, followed by perception of details and smaller objects within the scene. See VL. (5.10)

Perceiving Scenes and Objects in Scenes - continued

•  What enables observers to perceive the gist of a scene so rapidly?

•  We used info called ‘Global image features of scenes’ – Degree of naturalness (Natural scenes, such

as the ocean and forest)

Perceiving Scenes and Objects in Scenes - continued

– Degree of openness (Open scenes, such as the ocean, often have a visible horizon line and contain few objects. The forest has a lo degree of openness)

– Degree of roughness (Smooth scenes (low roughness) like the ocean contain fewer small elements. Scenes with high roughness like the forest contain many small elements and are more complex.)

Perceiving Scenes and Objects in Scenes - continued

– Degree of expansion (The convergence of parallel lines, like what you see when you look down railroad tracks that appear to vanish in the distance. Street example)

– Color (Some scenes have characteristic colors, like the ocean scene (blue) and the forest (green and brown)

•  Such features are holistic and perceived rapidly

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Regularities in the Environment: Information for Perceiving

•  We easily use our knowledge of regularities in the environment to help us perceive, even though we may not be able to identify the specific information we are using. See VL.

•  Physical regularities - regularly occurring physical properties – Oblique effect - people perceive horizontals

and vertical more easily than other orientations

– Uniform connectedness - objects are defined by areas of the same color or texture

Figure 5-36 p111

When we pick a point on the scene (such as A) and move slightly away from that point (to B), it is likely that the two points are on the same object if the color is the same. If, however, we move to C, the color changes, which means it is likely we are looking at a different object.

Regularities in the Environment: Information for Perceiving – continued

•  Physical regularities - regularly occurring physical properties – Homogenous colors. and nearby objects

have different colors – Light-from-above heuristic - light in natural

environment comes from above us

Figure 5-37 p111

Light coming from above will illuminate the top of a shape that is jutting out.

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Figure 5-38 p112

Why does (a) look like indentations in the sand and (b) look like mounds of sand?

Regularities in the Environment: Information for Perceiving - continued

•  Semantic regularities. Semantics refers to the meaning of a scene

•  Palmer experiment – Observers saw a context scene flashed

briefly, followed by a target picture. – Results showed that:

• Targets congruent with the context were identified 80% of the time .

• Targets that were incongruent were only identified 40% of the time.

Figure 5-39 p113 Figure 5-40 p113

“the multiple personalities of a blob”

See rotating masks at VL.