Chapter 3Chapter 3Sensation and PerceptionSensation and Perception

Sensation vs. PerceptionSensation vs. Perception

• Sensation:Sensation: result of energy (light) result of energy (light) stimulating a sense organ (eye)stimulating a sense organ (eye)

• Perception:Perception: the process of creating the process of creating meaningful patterns (a word) from raw meaningful patterns (a word) from raw sensory informationsensory information

• Vision: Vision: is considered by many to be is considered by many to be our our most important most important sensesense

Sensory ThresholdsSensory Thresholds• Psychophysics: Psychophysics: study of our ability to detect study of our ability to detect

sensory stimuli (e.g., light) sensory stimuli (e.g., light)

• absolute thresholdabsolute threshold:: minimum intensity of a minimum intensity of a stimulus (light) one can detect 50% of the stimulus (light) one can detect 50% of the times it is presented times it is presented (e.g., 50 out of 100 trials)(e.g., 50 out of 100 trials)

• difference thresholddifference threshold:: minimum CHANGE in minimum CHANGE in stimulus intensity that one can detect 50% of stimulus intensity that one can detect 50% of the times it is changed the times it is changed (e.g., 50 out of 100 trials)(e.g., 50 out of 100 trials) – Also called the “Just Noticable Difference” or

“jnd”

Weber’s LawWeber’s Law• The The jndjnd for any sense is a constant for any sense is a constant

fraction or proportion of the fraction or proportion of the stimulation being judged.stimulation being judged.

• For judging weight:For judging weight:– 2 pounds must be added to a 10 lb. to detect the

change (2/10 = 20%)– to detect a change in a 100 lb. weight, 20 lbs.

must be added (20%)

Subliminal PerceptionSubliminal Perception

• definition:definition: sensing and perceiving sensing and perceiving information without being aware of itinformation without being aware of it

• can it effect us?can it effect us?::– Maybe, in minor ways (buying more popcorn)– No, not in major ways (going against our

values)– Sorry, we cannot study subliminally

Does subliminal advertising work?Does subliminal advertising work?

• The goal of using subliminal The goal of using subliminal advertising is to increase the likelihood advertising is to increase the likelihood that you will buy a particular product.that you will buy a particular product.

• Subliminal advertising Subliminal advertising CANNOTCANNOT reliably or significantly change an reliably or significantly change an individual’s behavior.individual’s behavior.

• Most advertising relies on repetition, Most advertising relies on repetition, which is different and which is different and DOESDOES work. work.

Visual SystemVisual System

Light: The Stimulus for VisionLight: The Stimulus for Vision

Light: that part of the electromagnetic spectrum we can detect with our eyes

Cross Section of the EyeCross Section of the Eye

Parts of the EyeParts of the Eye• Cornea: Cornea: clear protective layer on the clear protective layer on the

front of eyefront of eye

• Iris: Iris: colored part of eye, opens and colored part of eye, opens and closes to let in more or less lightcloses to let in more or less light

• Pupil: Pupil: opening created by iris through opening created by iris through which light passeswhich light passes

• Lens: Lens: changes shape to focus on changes shape to focus on objects at different distancesobjects at different distances

More about the LensMore about the Lens• Accommodation: Accommodation: tiny muscles change tiny muscles change

the shape of the lens to focus on far or the shape of the lens to focus on far or near objectsnear objects– far: muscles are relaxed making lens flatter– near: muscles are tensed making lens thicker

• Presbyopia: Presbyopia: normal loss of ability to normal loss of ability to accommodate that comes with ageaccommodate that comes with age

Near and Far SightednessNear and Far Sightedness• Caused by shape of eye and/or power of lensCaused by shape of eye and/or power of lens

• Farsightedness: is less commonFarsightedness: is less common– eye too short and/or lens too weak– light focuses behind retinal– correct with “convex” lens to add power

• Nearsightedness: is more commonNearsightedness: is more common– eye is too long and/or lens is too powerful– light focuses in front of retina– correct with “concave” lens to reduce power

The Retina and its PartsThe Retina and its Parts• Retina: Retina: inner layer on back of eye that inner layer on back of eye that

contains “light-sensitive” rods and conescontains “light-sensitive” rods and cones

• Optic Nerve: Optic Nerve: bundle of axons running bundle of axons running from retina to visual (occipital) cortexfrom retina to visual (occipital) cortex

• Blind Spot: Blind Spot: spot on the retina where optic spot on the retina where optic nerve exits eye, there are no receptors nerve exits eye, there are no receptors (rods or cones) there(rods or cones) there

• Fovea: Fovea: center of the retina where “acuity” center of the retina where “acuity” (ability to see fine detail) is greatest(ability to see fine detail) is greatest

Cells in the RetinaCells in the Retina

Cells in the RetinaCells in the Retina• Receptor cells: Receptor cells: rods and cones, fire in rods and cones, fire in

response to lightresponse to light

• bipolar cells: bipolar cells: carry signals from carry signals from receptors to ganglion cellsreceptors to ganglion cells

• Ganglion cells: Ganglion cells: axons of ganglion cells axons of ganglion cells form the optic nerveform the optic nerve

• Wiring: Wiring: each cone has its own bipolar each cone has its own bipolar and ganglion cell while several rods and ganglion cell while several rods share one bipolar and ganglion cellshare one bipolar and ganglion cell

Distribution of Cells in the RetinaDistribution of Cells in the Retina

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Fovea Periphery

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RodsRods• located mainly in peripherylocated mainly in periphery

• responsible for night visionresponsible for night vision

• detail not detecteddetail not detected

• see black, white, and graysee black, white, and gray

• several rods share 1 bipolar and 1 several rods share 1 bipolar and 1 ganglion cellganglion cell

• rod vision lacks detail, but, by rod vision lacks detail, but, by combining their efforts, groups of combining their efforts, groups of rods allow us to see in low lightrods allow us to see in low light

ConesCones• located mainly in fovealocated mainly in fovea

• work best in bright lightwork best in bright light

• enable us to see fine detailenable us to see fine detail

• responsible for color visionresponsible for color vision

• each cone has its own bipolar and each cone has its own bipolar and ganglion cellganglion cell

• this allows us to see detail but this allows us to see detail but bright light is neededbright light is needed

Rods see in low light but lack detailRods see in low light but lack detail

Brain sees one blurry dot

A

B

C

Cones see detail but require bright Cones see detail but require bright lightlight

Brain sees A and B

A

B

Visual Pathways to the BrainVisual Pathways to the BrainOptic Chiasm: Where optic nerves (actually 1/2 of each retina) “cross over” from left to right

Dark Adaptation Dark Adaptation (not in book)(not in book)

• dark adaptation:dark adaptation: increased sensitivity increased sensitivity of rods and cones in darknessof rods and cones in darkness

——e.g., entering a darkened roome.g., entering a darkened room

• Cones: Cones: adapt for 10 minutes bet never adapt for 10 minutes bet never become very sensitivebecome very sensitive

• Rods: Rods: continue adapting for 30 minutes continue adapting for 30 minutes and become much more sensitiveand become much more sensitive

Rods vs. Cones Rods vs. Cones (not in book)(not in book)

5 10 15 20 25 30

amount of light needed to see

Minutes in darkness

ConesRods

Dark Adaptation ExperimentDark Adaptation Experiment

• Keep left eye closed or covered for at Keep left eye closed or covered for at least 10 minutes, 15 or 20 is better.least 10 minutes, 15 or 20 is better.

• Then, close right eye and open leftThen, close right eye and open left

• Switch back and forth a few timesSwitch back and forth a few times

• Do you see a difference?Do you see a difference?

• You should if you kept your left eye You should if you kept your left eye closed long enough closed long enough

Color VisionColor Vision

3 Properties of Color3 Properties of Color• 1. hue1. hue:: the actual color (red, green, etc.) the actual color (red, green, etc.)

– determined by “wavelength” of light

• 2. saturation2. saturation:: how pure and vivid the how pure and vivid the color iscolor is– single wavelength (laser) > fully saturated light– many wavelengths (light bulb) > desaturated light

• 3. brightness3. brightness:: intensity of the light intensity of the light– low light results in loss of color– extreme brightness seems to lighten color

wavelength

intensity

Light of a Single wavelengthLight of a Single wavelength

2 Types of Color Mixing2 Types of Color Mixing

• subtractive color mixingsubtractive color mixing:: mixing mixing pigments (like paints)pigments (like paints)– more pigments = more wavelengths absorbed,

paint gets darker (until black)

• additive color mixingadditive color mixing:: mixing lights of mixing lights of differing wavelengthsdiffering wavelengths– more wavelengths = more light, surface gets

lighter (until white)

Primary ColorsPrimary Colors

• Primary Colors: Primary Colors: sets of 3 colors that sets of 3 colors that can be mixed to produce any other can be mixed to produce any other colorcolor

• For Visual System: For Visual System: set of interest is set of interest is “Red Green and Blue”“Red Green and Blue”

Complementary ColorsComplementary Colors(additive mixing only)(additive mixing only)

• Pair of colors that when mixed cancel Pair of colors that when mixed cancel out to produce “gray” and not another out to produce “gray” and not another colorcolor

• Two pair of complementary colors for Two pair of complementary colors for the visual systemthe visual system– 1. Red - Green

– 2. Blue - Yellow

Theories of Color VisionTheories of Color Vision

TrichromaticTrichromatic

andand

Opponent - ProcessOpponent - Process

Trichromatic TheoryTrichromatic Theory

• There are three types of cones: There are three types of cones: redred,, greengreen,, and and blueblue..

• The colors we see result from the The colors we see result from the combined signals from the three types combined signals from the three types of cones.of cones.

Flag Afterimage StimulusFlag Afterimage Stimulus

AfterimagesAfterimages

• Afterimage: Afterimage: visual image that visual image that remains after the light stimulus is remains after the light stimulus is removed (after flash from camera) removed (after flash from camera)

• Complementary Afterimages: Complementary Afterimages: afterimage appearing in afterimage appearing in complementary colors, create a complementary colors, create a ““problemproblem” for trichromatic theory” for trichromatic theory

Opponent-Process TheoryOpponent-Process Theory

• we have pairs of opposing color we have pairs of opposing color cells higher in the visual systemcells higher in the visual system– red-green– yellow-blue– (also black-white)

• this provides an explanation for this provides an explanation for complementary afterimagescomplementary afterimages

Explaining Complementary Explaining Complementary AfterimagesAfterimages

• white normally stimulates the red and green white normally stimulates the red and green cells equallycells equally

• exposure to green fatigues the green cell exposure to green fatigues the green cell while the red cell restswhile the red cell rests

• exposure to white NOW causes red receptor exposure to white NOW causes red receptor to respond but green receptor is “tired”to respond but green receptor is “tired”

• we see red instead of whitewe see red instead of white

Both Theories are CorrectBoth Theories are Correct

• We do have three types of cones We do have three types of cones (Trichromatic) in the retina (Trichromatic) in the retina

• We also have pairs of color cells We also have pairs of color cells (opponent process) higher in the visual (opponent process) higher in the visual systemsystem

ColorblindnessColorblindness

• trichromats: trichromats: people with normal color people with normal color visionvision

• dichromats:dichromats: can’t distinguish either can’t distinguish either– red fom green or

– yellow from blue

• monochromats:monochromats: people who are totally people who are totally colorblind, more severecolorblind, more severe

HearingHearing

3 Properties of Sound3 Properties of SoundSound travels in waves as does lightSound travels in waves as does light

• 1. Pitch: 1. Pitch: determined by “frequency,” the determined by “frequency,” the number of cycles per second of a sound number of cycles per second of a sound wavewave

• 2. Loudness:2. Loudness: determined by “amplitude” determined by “amplitude” (height) of the sound wave(height) of the sound wave

• 3. Timbre: 3. Timbre: determined by “complexity and determined by “complexity and shape” of the sound wave, gives each shape” of the sound wave, gives each sound its unique qualitysound its unique quality

Units of Measurement for SoundUnits of Measurement for Sound

• hertz (Hz):hertz (Hz): to measure pitch to measure pitch (frequency), number of cycles per (frequency), number of cycles per second of the sound wavesecond of the sound wave– a baby’s cry is at about 3,000 Hz

• decibel (db):decibel (db): to measure loudness to measure loudness (amplitude) of the sound wave(amplitude) of the sound wave– I’m speaking at about 70 db

Decibel Levels for Common SoundsDecibel Levels for Common Sounds

© Prentice Hall, 1999

The EarThe Ear

Structure of the EarStructure of the Ear

• 1. Outer Ear:1. Outer Ear:– Pinna: directs sound waves into the auditory

canal– Auditory Canal: conducts sound to the eardrum– Eardrum: thin membrane that vibrates in

response to sound, and transfers sound energy to bones of the middle ear

Structure of the EarStructure of the Ear

• 2. Middle Ear: 2. Middle Ear: three tiny bones three tiny bones “amplify sound” and transfer sound “amplify sound” and transfer sound energy to the inner earenergy to the inner ear– Hammer– Anvil– Stirrup

Structure of the EarStructure of the Ear• 3. Inner Ear: 3. Inner Ear: where sound energy is where sound energy is

registeredregistered– Cochlea: snail shaped fluid-filled structure– Oval window: thin membrane, transfers

vibrations from stirrup to fluid of cochlea– Basilar membrane: runs the length of the

cochlea– Organ of Corti: rests on basilar membrane,

contains “receptor” cells – Round window: absorbs energy and equalizes

pressure in the cochlea

Receptor Cells on the Organ of CortiReceptor Cells on the Organ of Corti

• Recetor (hair) cells have hair-like fibersRecetor (hair) cells have hair-like fibers

• Waves in fluid bend the fibers making Waves in fluid bend the fibers making the cells “fire”the cells “fire”

• Axons (afferent) from these cells form Axons (afferent) from these cells form the “auditory nerve”the “auditory nerve”

• There are also “efferent” axons that There are also “efferent” axons that adjust sensitivity of the recptor cellsadjust sensitivity of the recptor cells

Hair Cell in the Organ of CortiHair Cell in the Organ of Corti

Theories of HearingTheories of Hearing

• Loudness: the louder the sound, the Loudness: the louder the sound, the more receptors firemore receptors fire

• Pitch (frequency) 3 theoriesPitch (frequency) 3 theories– place theory: – frequency theory– volley theory (principle)

Place TheoryPlace Theory

• different frequencies cause vibrations different frequencies cause vibrations at different locations (places) along at different locations (places) along basilar membranebasilar membrane

• higher frequencies at base, lower higher frequencies at base, lower frequencies at topfrequencies at top

Frequency TheoryFrequency Theory

• The entire basilar membrane vibrates at The entire basilar membrane vibrates at the same frequency as the incoming the same frequency as the incoming soundsound

• What is the problem here? (hint, we What is the problem here? (hint, we hear frequencies close to 20,000 Hz)hear frequencies close to 20,000 Hz)

• It takes a neuron several (3) It takes a neuron several (3) milliseconds to fire and resetmilliseconds to fire and reset– at best we could hear at about 333 Hz

Volley Theory (Principle)Volley Theory (Principle)

• Receptor cells fire in groups:Receptor cells fire in groups: first one, first one, then another, then a thirdthen another, then a third

• Resetting: Resetting: the first group of cells to fire the first group of cells to fire are resetting while the second and third are resetting while the second and third groups fire and so on groups fire and so on

Which Treory is Correct?Which Treory is Correct?

All seem to be correctAll seem to be correct• Place theory: Place theory: operates at lower operates at lower

frequenciesfrequencies

• Frequency Theory: Frequency Theory: operates for the operates for the higher frequencies but utilizing the higher frequencies but utilizing the “volley principle”“volley principle”

Vestibular SenseVestibular Sense

Information about balance and body position iInformation about balance and body position i

• 3 fluid filled structures work just like like 3 fluid filled structures work just like like the cochleathe cochlea– Semi-circular canals: body rotation

– Utricle: horizontal movement

– Saccule: Vertical movement/gravity

• Motion Sickness: Motion Sickness: caused by disagreement caused by disagreement between eyes and vestibular sensebetween eyes and vestibular sense

Perceptual Constancies (visual)Perceptual Constancies (visual)

• Even though the light stimulus on the Even though the light stimulus on the retina changes, retina changes, EXPERIENCEEXPERIENCE helps us helps us perceive a “constant” environmentperceive a “constant” environment

• Size constancySize constancy

• Shape constanceShape constance

• Color constnacy Color constnacy

Perceptual Constancies (cont.)Perceptual Constancies (cont.)

• SizeSize - The farther away an object is, the - The farther away an object is, the smaller the image is on the retina. Yet smaller the image is on the retina. Yet we know it is still the same size.we know it is still the same size.

• ShapeShape - A partially open door casts a - A partially open door casts a “trapezoid” on the retina. Yet, we know “trapezoid” on the retina. Yet, we know it is a rectangle.it is a rectangle.

• ColorColor - Different lighting dramatically - Different lighting dramatically alters color. Yet, we know the color of alters color. Yet, we know the color of an object does not change.an object does not change.

Depth Perception (seeing in 3D)Depth Perception (seeing in 3D)

• monocular cuesmonocular cues:: depth cues depth cues requiring only one eye (requiring only one eye (learnedlearned))

• binocular cuesbinocular cues:: depth cues depth cues requiring both eyes (requiring both eyes (inate/inborninate/inborn))

Monocular CuesMonocular Cues

• superpositionsuperposition

• linear perspectivelinear perspective

• aerial perspectiveaerial perspective

• elevationelevation

• texture gradienttexture gradient

• shadowingshadowing

• motion parallaxmotion parallax

1. Superposition1. Superposition• One object, by partially blocking One object, by partially blocking

another, is perceived as being closer.another, is perceived as being closer.

2. Linear Perspective2. Linear Perspective

• Two parallel lines appear to converge Two parallel lines appear to converge as they move away.as they move away.

3. Aerial Perspective3. Aerial Perspective

• More distant objects have less detail More distant objects have less detail than closer objects because the than closer objects because the atmosphere bends and distorts light atmosphere bends and distorts light waveswaves

• Imagine the New York city skylineImagine the New York city skyline

4. Elevation4. Elevation

The higher an object is, the farther away The higher an object is, the farther away it appears.it appears.

5. Texture Gradient5. Texture Gradient

• Objects seen at greater distances Objects seen at greater distances appear to be smoother and less appear to be smoother and less textured.textured.

• Imagine looking out over a field of Imagine looking out over a field of crops.crops.

6. Shadowing6. Shadowing• Shadows often appear on the parts of Shadows often appear on the parts of

objects that are more distant.objects that are more distant.

7. Motion Parallax7. Motion Parallax

• Has to do with relationship between Has to do with relationship between movement and distancemovement and distance

• When riding in train or carWhen riding in train or car– near objects rush past you– distant objects seem to move with you

Binocular CuesBinocular Cues

• 1. Retinal Disparity: 1. Retinal Disparity: each retina sees a each retina sees a slightly different image because of slightly different image because of separationseparation

• 2. convergence:2. convergence: as eyes turn inward to as eyes turn inward to focus on close object, muscles send focus on close object, muscles send signals to brainsignals to brain

Perceiving MovementPerceiving Movement(Illusions of Movement)(Illusions of Movement)

• The autokinetic illusionThe autokinetic illusion

• stroboscopic motionstroboscopic motion

• the phi phenomenonthe phi phenomenon

Autokinetic IllusionAutokinetic Illusion

• A stationary spot of light in a dark A stationary spot of light in a dark room appears to move room appears to move

• Occurs because we lack of a “frame of Occurs because we lack of a “frame of reference”reference”

• Our eyes constantly make small Our eyes constantly make small involuntary movements which we are involuntary movements which we are normally unaware ofnormally unaware of

Stroboscopic MotionStroboscopic Motion

• Apparent motion resulting from seeing Apparent motion resulting from seeing gradually changing images in rapid gradually changing images in rapid succession (e.g., a movie or cartoon)succession (e.g., a movie or cartoon)

• Once a certain speed is reached, the Once a certain speed is reached, the eye cannot separate the images and eye cannot separate the images and continuous motion is perceivedcontinuous motion is perceived

Phi PhenomenonPhi Phenomenon

• Apparent movement caused by Apparent movement caused by flashing lights in sequence (e.g., movie flashing lights in sequence (e.g., movie theatre marquis)theatre marquis)

• One light flashes, then a second, and a One light flashes, then a second, and a third. With the right combination of third. With the right combination of distance and speed, we perceive one distance and speed, we perceive one moving lightmoving light