Depth Perception

Problem

2D images fall onour eyes

but we perceivea 3D world!

Solution

Depth PerceptionMonocular Cues

&Binocular Cues

Binocular Cues

Clues to relative distance of differentobjects

Retinal DisparityDiff between the twoDiff between the two

images images Closer the object = the

larger the disparity

ConvergenceDegree to which eyesDegree to which eyes

face inwardface inwardCloser the object = the greater convergence

Binocular CuesRetinal/Binocular disparity: Images from the two eyes differ. Try looking at your two index fingers

when pointing them towards each other half an inch apart and about 5 inches directly in front of your eyes. You will see a “finger sausage” as shown in

the inset.http://www.michaelbach.de/ot/sze_Frankfurter/index.html

http://www.netaxs.com/~mhmyers/rds-ex.htmlStereograms

Perceptual ConstancyObjects remain unchanged as their

image on the retina changes• Shape Constancy

• Size Constancy – looking at the same size objects close and far away

– Close Object –> projects larger image on our retina.

– Far Object –> projects smaller image on our retina.

• Example: as a person walks away from you their retinal image decreases in size. However, we know that they are just getting farther away not shrinking in size.

Relative SizeRelative Size

InterpositionInterposition

If two objects are similar in size, we perceive the one that casts a smaller

image to be farther away

If one object partially blocks our view of another object, we

perceive it as closer

Monocular Cues

Relative ClarityRelative Clarity

Relative BrightnessRelative Brightness

We perceive hazy objects to be farther

away than those objects that appear

sharp and clear

Nearby objects reflect more light to our eyes,

so dimmer objects appear to be farther away and brighter

objects seem closer

Monocular Cues

Texture GradientTexture Gradient

Linear PerspectiveLinear Perspective

Indistinct (fine) texture signals an

increasing distance, while coarse texture

signals a close distance.

Monocular Cues

Parallel lines appear to converge in the distance. The more the lines converge, the greater their

perceived distance.

Relative HeightRelative Height

Relative MotionRelative Motion

We perceive objects that are higher in our field of vision to be farther away

than those that are lower.

Monocular Cues

Objects closer to (in front of )a fixation point move faster and in opposing

direction to those objects that are farther away from (behind) a fixation point, moving slower and in the

same direction.

What Cues Do You See?

Optical Illusion: PonzoThe distant monster (below, left) and the top red bar

(below, right) appear bigger because of size constancy and the monocular distance cues of linear perspective

and texture gradient.Linear Perspective - parallel lines appear to converge in the distance. The more the lines converge, the greater their perceived distanceTexture Gradient – indistinct (fine) texture signals an increasing distance, while coarse texture signals a close distance Size Constancy -

If 2 objects cast identical retinal

images (i.e., they are the same

size), the object appearing further away is perceived as larger and so

the brain perceptually enlarges the

image

Optical Illusion: Horizontal- Vertical Illusion

The vertical line appears longer than the equally long

horizontal line because of size constancy and the

monocular distance cue of relative height

Relative height – Objects at the top of our visual field

appear to be more distant.

Size constancy - If 2 objects cast identical retinal images

(i.e., they are the same size), the object appearing further away is perceived as larger and so the brain perceptually lengthens the

vertical line.

Optical Illusion: Muller-Lyer • (a) Outward pointing arrows

signify that the top horizontal line is farther and thus looks longer; while (b) inward pointing arrows signify that the horizontal line is closer and thus looks shorter.

• The illusion is not cross-cultural. Non-Western subjects, and particularly subjects whose day-to-day surroundings are usually not rectangular (few buildings, doors, walls) are much less likely to be affected by it

Motion Perception: Phi Phenomenon

Phi Phenomenon: When lights flash at a certain speed they tend to present illusions of motion. Neon signs use this principle to create motion

perception.

Stroboscopic Motion – rapid sequencing of visual images

Motion Perception: Stroboscopic

Perceptual Organization

Max Wertheimer(1880-1943)

““The whole is different thanThe whole is different thanthe sum of its parts.”the sum of its parts.”

Gestalt PsychologyDescribed principles by which

we organize sensationsinto perceptions.

We combine bottom-up info with top-down knowledge to

createcreate perception

http://www.thepsychfiles.com/2007/10/episode-31-lemon-slices-and-a-new-face-on-mars-gestalt-principles-at-work/

Principles of Gestalt Psychology

Proximity (group nearby figures together)

Similarity (group figures that

are similar)

Figure & Ground(organization of the visual

field into objects (figures) that stand out from their

surroundings (ground))

Principles of Gestalt PsychologyContinuity

(perceive continuous patterns)

Closure (fill in gaps)

Connectedness (spots, lines and areas are seen as unit when connected)

Proximity• Perceiving objects close to

each other as forming a group.• Looking at the picture, since

the horizontal rows of circles are closer together than the vertical columns, we perceive two vertical lines. Since the first two columns and the last two columns have less space between them than the center two columns, we perceive two groups of two columns.

Closure• The principle of closure applies

when we tend to see complete figures even when part of the information is missing.

• Closing up or completing figures that are not, in fact, complete

• We see three black circles covered by a white triangle, even through it could just as easily be three incomplete circles joined together.

• Our minds react to patterns that are familiar, even though we often receive incomplete information. It is speculated this is a survival instinct, allowing us to complete the form of a predator even with incomplete information.

Continuation

• Perceiving smoothly flowing or continuous forms rather than disrupted or discontinuous ones.

• As seen in the example, we perceive the figure as two crossed lines instead of 4 lines meeting at the center.

• Even though the circle is not joined together, we still perceive a circle due to the principle of closure

Similarity• The principle of similarity

states that things which share visual characteristics such as shape, size, color, texture, value or orientation will be seen as belonging together.

• In the example, the two filled lines gives our eyes the impression of two horizontal lines, even though all the circles are equidistant from each other.

• In the example, the larger circles appear to belong together because of the similarity in size.

Observer Characteristics: Experience / Culture

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To an East African, the woman sitting is balancing a metal box on her head, while the family is

sitting under a tree.

The predominating norms, behavior patterns, artifacts, institutions, and attitudes of a group of people. In addition, past learning shapes perception.

Observer Characteristics: ExpectationsPreconceptions about what we are supposed to

perceive may also influence perception by causing us to delete, insert, transpose, or otherwise

modify what we see. • Perceptual familiarization/generalization - strong tendency

to see what we expect to see even if our expectation conflicts with external reality.

• Perceptual set - a readiness to perceive a stimulus in a particular way.

• Schemas -mental framework that organizes and interprets information

• Context Effects - immediate context of a figure influences one’s perception of it.

Expectations: Context Effects

Olny srmat poelpe can.

I cdnuolt blveiee that I cluod

aulaclty uesdnatnrd waht I was

rdanieg. The

phaonmneal pweor of the hmuan

mnid, aoccdrnig to rscheearch at

Cmabridgde

Uinervtisy, it deosn’t mttaer in

what oredr the ltteers in a word

are, the

olny iprmoatnt tihng is that the frist

and lsat ltteer be in the rghit

pclae

The rset can be a taotl mses and

you can still raed it wouthit a

porbelm.

This is bcuseae the huamn mnid

deos not raed ervey lteter by istlef,

but

the word as a wlohe. Amzanig

huh? Yaeh and I awlyas tghuhot

slpeling was ipmorantt!

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We interpret what we see based on the context in which we see it (first and last letter of word).

Expectations: Context EffectsDear Mother and Dad:

Since I left for college I have been remiss in writing and I am sorry for my thoughtlessness in not having written before. I will bring you up to date now, but before you read on, please sit down. You are not to read any further unless you are sitting down, okay?

Well, then, I am getting along pretty well now. The skull fracture and the concussion I got when I jumped out the window of my dormitory when it caught on fire shortly after my arrival here is pretty well healed now. I only spent two weeks in the hospital and now I can see almost normally and only get those sick headaches once a day. Fortunately, the fire in the dormitory, and my jump, was witnessed by an attendant at the gas station near the dorm, and he was the one who called the Fire Department and the ambulance. He also visited me in the hospital and since I had nowhere to live because of the burntout dormitory, he was kind enough to invite me to share his apartment with him. It’s really a basement room, but it’s kind of cute. He is a very fine boy and we have fallen deeply in love and are planning to get mar­ried. We haven’t got the exact date yet, but it will be before my pregnancy begins to show.

Yes, Mother and Dad, I am pregnant. I know how much you are looking forward to being grandparents and I know you will welcome the baby and give it the same love and devotion and tender care you gave me when I was a child. The reason for the delay in our marriage is that my boyfriend has a minor infection which prevents us from passing our pre­marital blood tests and I carelessly caught it from him. I know that you will welcome him into our family with open arms. He is kind and, although not well educated, he is ambi­tious. Although he is of a different race and religion than ours, I know your often expressed tolerance will not permit you to be bothered by that.

Now that I have brought you up to date, I want to tell you that there was no dormitory fire, I did not have a concussion or skull fracture, I was not in the hospital, I am not pregnant, I am not engaged, I am not infected, and there is no boyfriend. However, I am getting a “D” in American History, and an “F” in Chemistry and I want you to see those marks in their proper perspective.

Your loving daughter, Sharon­

Expectations: Perceptual Familiarization/Generalization

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Expectations: Schemas• Provide punctuation that will make the following

words meaningful:

• TIME FLIES I CANT THEYRE TOO FAST• The apostrophes come easily, but the rest is difficult.

TIME FLIES. I CAN’T. THEY’RE TOO FAST. It still does not seem to make sense because we’re too familiar with the slogan, “Time flies” in which “time” is a noun and “flies” is a verb. Tell students to read “time” as the verb and “flies” as the noun. It now makes perfect sense.

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30

(a)Lochness monster or a tree trunk;

(b) Flying saucers or clouds?

Expectations: Perceptual Set

Expectations: Perceptual Set

• Sine Wave Speech– http://www.lifesci.sussex.ac.uk/home/

Chris_Darwin/SWS/– http://www.mrc-cbu.cam.ac.uk/people/

matt.davis/sine-wave-speech/

• Backmasking– http://jeffmilner.com/backmasking.htm– http://www.youtube.com/watch?

v=Qwiba6_LYP0

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Observer Characteristics: Cognitive Style

• COGNITIVE STYLE: Way of processing the stimuli within the environment, which affects how we see the world. Pg.­454­H:3-4­Demo­

– Field-dependent approach = perceive the environment as a whole and do not clearly distinguish in their minds the shape, color, size, or other qualities of individual items. No details. “Levelers”

– Field-independent approach = perceive the elements of the environment as separate and distinct from one another and to draw each element as standing out from the background. A lot of details. “Sharpeners”

Observer Characteristics

• VALUES: What we deem as important will shape our perceptions.

• PERSONALITY: Person’s unique pattern of thoughts, feelings, and behaviors affect perception.

• MOTIVATION: Our desires and needs strongly shape our perceptions. People in need are more likely to perceive something that they think will satisfy that need.