Perceiving the World

What are perceptual constancies, and what is their role in perception?

What basic principles do we use to group sensations into meaningful patterns?

How is it possible to see depth and judge distance? What effect does learning have on perception? How are perceptions altered by attention, motives,

values, and expectations? How reliable are eyewitness reports? Is extrasensory perception possible?

I was in a supermarket when a girl suddenly came running around the corner. She looked back and screamed, “Stop! Stop! You’re killing him! You’re killing my father!” Naturally I was interested! As I quickly retraced her path, I was greeted by a grisly scene. A man was stretched out on the floor with another man on top of him. The guy on top was huge. At 6 feet 6 inches tall and 300 pounds, he looked only half human. He had his victim by the throat an was beating his head against the floor. There was blood everywhere. I decided to do the right thing. I ran.

• What happened next?

By the time the store manager and I returned to the “scene of the crime”, the police were just arriving. It took quite a while to sort things out, but here is what happened:

The “guy on the bottom” had passed out and hit his head. That caused the cut (actually quite minor) which explained the “blood everywhere.” “The guy on top” saw the first man fall and was trying to prevent him from further injuring himself, He was also loosening the man’s collar.

If I had never returned, I would have sworn in court that I had just seen a murder. The girl’s words completely dictated my own perceptions. This perhaps is understandable. But what I will never forget is the shock I felt when I met the “murderer”- the man I had see a few moments before, in broad daylight, as a huge, vicious, horrible-looking creature. The man was not a stranger. He was a neighbor of mine. I had seen him dozens of times before. I know him by name. He is a rather small man.

Sensation involves the actual stimulus (seeing the oval shaper of a bowl held at an angle),

Whereas perception involves the brain’s interpretation of that stimulus (seeing a bowl at thousands of angles over the years)

Perception: • How we assemble sensations into meaningful patterns

Size Constancy: • Perceived size of an object remains constant, DESPITE

changes in its retinal image size Native Perception:

• A perceptual experience based on innate processes Empirical Perception:

• A perception strongly influenced by prior experience Shape Constancy:

• The perceived shape of an object unaffected by changes in its retinal image

Brightness Constancy: • Apparent brightness of an object stays the same under

changing lighting conditions

Card trick

Figure-Ground Organization: Inborn; part of a stimulus stands out as a figure (object) against a plainer background (ground)

Reversible Figure: Figure and ground that can be reversed

The first step in perceiving an image is determining the figure and ground.

Do you see the arrow?

Nearness: • Stimuli that are near each other tend to be

grouped together Similarity:

• Stimuli that are similar in size, shape, color, or form tend to be grouped together

Continuation, or Continuity: • Perceptions tend toward simplicity and

continuity

Closure: • Tendency to complete a figure so that it has

a consistent overall form Contiguity:

• Nearness in time and space; perception that one thing has caused another

Common Region: • Stimuli that are found within a common

area tend to be seen as a group

© E.R. Degginger/Animals Animals

Depth Perception: • Ability to see three-dimensional space and to

accurately judge distances Visual Cliff:

• Apparatus that looks like the edge of an elevated platform or cliff

Depth Cues:• Features that supply information about distance

and space Monocular Depth Cue:

• Depth cue that can be sensed with one eye Binocular Depth Cue:

• Depth cue that can be sensed with two eyes

Accommodation: • Bending of the lens of the eye to focus on nearby

objects Convergence:

• Binocular cue; when you look at something 50 feet or closer, your eyes must turn in (converge) to focus the object

Retinal Disparity:• Discrepancy in the images that reach the right and

left eyes Stereotopic Vision:

• Three-dimensional sight

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Features found in paintings, drawings, and photographs that supply information about space, depth, and distance

Linear Perspective: (7-14a)• Based on apparent convergence of parallel lines in environment

Relative size: (7-14b)• Depict 2 objects of the same size at different distances, the

more distant objects smaller

Height in the picture plane (7-14b)• Objects are placed higher (closer to the horizon line) in a

drawing tend to be perceived as more distant

Light and shadow (7-14c) (7-16)• Most objects lighted in ways that create clear patterns of light and

shadow

Overlap (Interposition): (7-14d)• When one object partially blocks another

Texture Gradients: (7-14e)• Texture changes can contribute to depth perception; coarse texture

implies closeness, fine texture implies distance

Aerial perspective: • Smog, fog, dust, and haze add to the apparent distance of an object

Relative Motion (Motion Parallax): (7-17)• Nearby objects move a lot as your head moves; distant objects

move slightly

Sensation and Perception: Visual Illusions

Video

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Figure

7-

20

Moon Illusion: • Apparent change in size that occurs as the

moon moves from the horizon (large moon) to overhead (small moon)

Apparent-Distance Hypothesis: • Horizon seems more distant than the night

sky

Handout (Ambiguous figures)• Personal experience• Perceptual abilities• Personal needs

Homework

Change in the brain that alters how we process sensory information

Perceptual Habits: • Ingrained patterns of organization and attention

Other-Race Effect: • Tendency to be better at recognizing faces from one’s own racial group

than faces from other racial or ethnic groups

Active Movement: • Self-generated action; accelerates perceptual adaptation

Context: • Information surrounding a stimulus; affects perception (Fig. 7-25)

Frames of Reference: • Internal standards for judging stimuli

Video

An Ames room (named for the man who designed it) • Is a lopsided space that appears square

when viewed from a certain point (fig 7-23)

Illusion: • Length, position, motion, curvature, or direction is constantly

misjudged Hallucination:

• When people perceive objects or events that have no external basis in reality

Stroboscopic Movement: • Illusory motion perceived when objects are shown in rapidly

changing positions Muller-Lyer Illusion: (Fig. 27a)

• Two equal-length lines topped with inward or outward pointing V’s appear to be of different length; based on experience with edges and corners of rooms and buildings

Handout AB-BC

Airplane

If two objects make images of the same size, the more distant object must be larger, this is known as Size-distance invariance

Same concept explains the Muller-Lyer illusion. If the V-tipped line looks father away than the arrowhead-tipped line, then you must compensate by seeing the V-tipped line as longer

Selective Attention: • Refers to the fact that we give some

messages priority Divided Attention:

• Divide your mental effort among tasks, each of which requires more or less attention

Orientation Response: • Bodily changes that prepare an organism to

receive information from a particular stimulus Ex: Pupils enlarge, breathing stops briefly

How many passes

Bottom-Up Processing: • Analyzing information starting at the bottom (small

units) and going upward to form a complete perception

Top-Down Processing: • Pre-existing knowledge that is used to rapidly

organize features into a meaningful whole Perceptual Set:

• Past experiences, motives, contexts, or suggestions that prepare us to perceive in a certain way

Flash-X

Stress: • High levels impair accuracy

Weapon Focus:• Presence of a weapon impairs eyewitness’ accuracy

Exposure Time: • Less time an eyewitness has to observe an event, the less

s/he will perceive and remember it Accuracy-Confidence:

• Confidence is not a good predictor of his/her accuracy Cross-Racial Perceptions:

• Eyewitnesses are better at identifying members of their own race than of other races

Post-Event Information: • Testimony reflects not only what was actually seen but also

information obtained later on Color Perception:

• Judgments of color made under monochromatic light are very unreliable

Unconscious Transference: • A culprit who is identified may have been seen in another

situation or context Alcohol Intoxication:

• Impairs later ability to recall events Attitudes and Expectations:

• May affect eyewitness’ perception of events

Reality Testing: • Obtaining additional information to check

your perceptions Habituate:

• Tend to respond less to predictable and unchanging stimuli

Dishabituation: • Reversal of habituation

Bomber on roof

Video

Parapsychology: Study of ESP and other psi phenomena (events that seem to defy accepted scientific laws)• Clairvoyance:

Purported ability to perceive events unaffected by distance or physical barriers

• Telepathy: Purported ability to read minds

• Precognition: Purported ability to accurately predict the future

(“Minority Report” and the “Pre-Cogs,” like Agatha)• Psychokinesis (Mind Over Matter):

Purported ability to influence physical objects by willpower

Zener Cards: Deck of 25 cards, each having one of five symbols

Run of Luck: • Statistically unusual outcome that could occur by chance

alone (e.g., getting five heads in a row, two jackpots within six pulls of a slot machine)

Stage ESP: • Simulation of ESP for entertainment purposes

Conclusion:• Existence of ESP has NOT been scientifically demonstrated;

positive results are usually inconclusive and easily criticized In sum:

• Be skeptical! If it seems too good to be true, it probably is!

Test

(a) (b c)

Telepathy – mind reading Clairvoyance – perceiving remote

events Precognition – Knowing things before

they happen Telekinesis (psychokinesis) – moving

objects with one’s mind (not technically ESP)