The Visual System. Light Enters the eye as electromagnetic radiation Enters the eye as...

The Visual SystemThe Visual System

LightLight Enters the eye as Enters the eye as

electromagnetic electromagnetic radiationradiation

Travels in a wave Travels in a wave that vary in that vary in amplitude amplitude (height) and (height) and wavelength wavelength (between peaks)(between peaks)

Amplitude= Amplitude= brightnessbrightness

Wavelength= Wavelength= color (hue)color (hue)

Humans see a Humans see a mixture of several mixture of several wavelengthswavelengths

Varies purityVaries purity Small portion of Small portion of

the spectrumthe spectrum Animals- what do Animals- what do

they see?they see?

Parts of the eyeParts of the eye

Parts of the EyeParts of the Eye Cornea (1) transparent Cornea (1) transparent

covering/protectioncovering/protection Pupil and Iris (2) bright Pupil and Iris (2) bright

light/contracts, dim light/contracts, dim light/relaxlight/relax

Lens (3) focuses light on Lens (3) focuses light on the retina, the retina, accommodationaccommodation

Retina (4) neural tissue Retina (4) neural tissue that absorbs lightthat absorbs light

Optic disk & Blind Spot (5)Optic disk & Blind Spot (5) Optic Nerve (6)Optic Nerve (6) Fovea (7)Fovea (7)

Blind Spot Blind Spot Activity!!Activity!!

The Retina The Retina

Millions of Millions of receptor cellsreceptor cells

10 % of light10 % of light Rods- 100 to Rods- 100 to

125 million125 million Cones- 5-6 Cones- 5-6

millionmillion Expt- What Expt- What

Color is it??Color is it??

Light and Dark AdaptationLight and Dark Adaptation

From the Eye to the BrainFrom the Eye to the Brain

Processing in the Visual Processing in the Visual CortexCortex

Feature DetectorsFeature Detectors Simple CellsSimple Cells Complex cellsComplex cells Ventral (what) Ventral (what)

pathwaypathway Dorsal (where) Dorsal (where)

pathwaypathway

Color Theory Color Theory

Psychological interpretation- Blends Psychological interpretation- Blends of three aspects or properties of lightof three aspects or properties of light

Three theories regarding colorThree theories regarding color

Trichromatic Theory-Trichromatic Theory- Young and Young and Von HelmholtzVon Helmholtz

The eye does the mixing by varying The eye does the mixing by varying the ratio of neural activity among the ratio of neural activity among these 3 typesthese 3 types

Color blindnessColor blindness– DichromatsDichromats– Monochromats Monochromats

Opponent Process Theory-Opponent Process Theory- HeringHering

Color perception depends on Color perception depends on receptors that make antagonistic receptors that make antagonistic responses to 3 pairs (red v. green) responses to 3 pairs (red v. green) (yellow v. blue) (white v. black)(yellow v. blue) (white v. black)

When excited they respond to one When excited they respond to one color and when inhibited they color and when inhibited they respond to the otherrespond to the other

Opponent Process Theory-Opponent Process Theory- HeringHering

Complementary colors (p.101)Complementary colors (p.101)

AfterimageAfterimage

Reconciling TheoryReconciling Theory

Both theories correctBoth theories correct Perception of color is processed in Perception of color is processed in

stagesstages Beginning stages- trichromaticBeginning stages- trichromatic Later stages- opponent processingLater stages- opponent processing

Form PerceptionForm Perception Perceptual SetPerceptual Set - The influence of - The influence of

prior assumptions and expectations prior assumptions and expectations on perceptual interpretationson perceptual interpretations

Perceptual SetPerceptual Set

What do you see in the center picture: a male What do you see in the center picture: a male saxophonist or a woman’s face? Glancing first saxophonist or a woman’s face? Glancing first at one of the two unambiguous versions of the at one of the two unambiguous versions of the picture is likely to influence your interpretation.picture is likely to influence your interpretation.

Feature AnalysisFeature Analysis

Bottom Up Processing & Top Down Bottom Up Processing & Top Down ProcessingProcessing

I cdnuolt blveiee taht I cluod aulaclty I cdnuolt blveiee taht I cluod aulaclty uesdnatnrd waht I was rdanieg. It uesdnatnrd waht I was rdanieg. It dseno’t mataetr in what oerdr the dseno’t mataetr in what oerdr the ltteres in a word are, the olny ltteres in a word are, the olny iproamtnt tihng is taht the frsit and iproamtnt tihng is taht the frsit and lsat ltter be in the rghit pclae.lsat ltter be in the rghit pclae.

Gestalt Principles (p105)Gestalt Principles (p105)

Figure GroundFigure Ground ProximityProximity ClosureClosure SimilaritySimilarity SimplicitySimplicity ContinuityContinuity

Phi PhenomenonPhi Phenomenon The illusion of motion when fixed lights The illusion of motion when fixed lights

are turned on and off in a sequenceare turned on and off in a sequence• Stare at the X in the middle and notice what Stare at the X in the middle and notice what

happens. Is the Green Dot Moving?happens. Is the Green Dot Moving?

• http://www.weeville.com/eyetest.htmhttp://www.weeville.com/eyetest.htm

Relationship Between Perceived Relationship Between Perceived Size and Perceived DepthSize and Perceived Depth

To perceive the To perceive the size of objects size of objects accurately we accurately we must also must also perceive their perceive their distance distance accuratelyaccurately

Many visual Many visual illusions occur illusions occur because of a lack because of a lack of sufficient depth of sufficient depth cuescues

This figure shows that image size depends upon both object size and distance

Retina Pupil

Image A B

Image A

Depth PerceptionDepth Perception

Binocular depth cuesBinocular depth cues– Retinal disparity-Retinal disparity- objects 25ft project objects 25ft project

images to different locations on the right images to different locations on the right & left retina& left retina

Monocular Depth CuesMonocular Depth Cues

Linear PerspectiveLinear Perspective Motion ParallaxMotion Parallax


Texture gradientTexture gradient InterpositionInterposition


Relative SizeRelative Size Height in a planeHeight in a plane

Size Size ConstancyConstancy

People are the People are the same size even same size even though their though their image sizes image sizes differdiffer

The depth cues The depth cues such as linear such as linear perspective and perspective and relative size relative size help judge the help judge the size accuratelysize accurately

Size Distance RelationshipSize Distance Relationship

Shape ConstancyShape Constancy The understanding that an object’s The understanding that an object’s

shape remains the same even shape remains the same even though the angle of view makes the though the angle of view makes the shape appear changedshape appear changed

Shape ConstancyShape Constancy

It is hard to tell if It is hard to tell if the figure on the the figure on the upper right is a upper right is a trapezoid or a trapezoid or a square slanted square slanted backward.backward.

If we add texture, If we add texture, the texture the texture gradient helps us gradient helps us see that it is see that it is actually a squareactually a square

Brightness ContrastBrightness Contrast Perceived lightness Perceived lightness

stays roughly constant stays roughly constant as long as the context as long as the context or surroundings stay or surroundings stay the same.the same.

When the context When the context changes you may changes you may perceive the color as perceive the color as changing.changing.

•Want to see more of these perceptions? Goto http://www.skidmore.edu/~hfoley/Perc4.htm#lightcon

Sensory Deprivation & Restored Sensory Deprivation & Restored VisionVision

Early visual experience can have a profound Early visual experience can have a profound effect on perception. Blakemore & Cooper’s effect on perception. Blakemore & Cooper’s restricted environment with kittens.restricted environment with kittens.

Do the kittens ever fully regain normal Do the kittens ever fully regain normal sensitivity to horizontal or vertical lines? NO.sensitivity to horizontal or vertical lines? NO.

The “Use it or lose it” phenomenon. The “Use it or lose it” phenomenon.

From the time their eyes first opened, and until the age of 5 months, these kittens were removed from darkness each day to spend 5 hours alone in a black-and-white striped cylinder with a clear glass floor. A stiff collar prevented the kittens from seeing anything else, even their own bodies. Afterward, these kittens had difficulty perceiving horizontal forms, compared with other kittens exposed only to horizontal forms.

Sensory Deprivation & Restored VisionSensory Deprivation & Restored Vision These experiments show that lacking These experiments show that lacking

stimulation, the cortical cells had not developed stimulation, the cortical cells had not developed normal connections making them normal connections making them functionally functionally blindblind to shape. to shape.

A sensory restriction does no permanent A sensory restriction does no permanent damage if it occurs later in life. This suggests damage if it occurs later in life. This suggests that visual experiences during infancy are a that visual experiences during infancy are a critical period for normal sensory and critical period for normal sensory and perceptual development. Experience guides perceptual development. Experience guides the organization of the brain’s neural the organization of the brain’s neural connections.connections.

If deafness or blindness is corrected as an If deafness or blindness is corrected as an infant, it awakens the pertinent brain area. infant, it awakens the pertinent brain area. Nurture sculpts what nature has endowed.Nurture sculpts what nature has endowed.

ContextContext The setting or environment in which we The setting or environment in which we

interpret sensory stimuliinterpret sensory stimuli CultureCulture can also influence how we perceive can also influence how we perceive

information.information.– People actively construct their perceptions by People actively construct their perceptions by

drawing on their prior learning and cultural drawing on their prior learning and cultural experiences.experiences.

– People living in urban and industrialized People living in urban and industrialized environments where there are more right angles and environments where there are more right angles and straight lines will be more susceptible to the Muller-straight lines will be more susceptible to the Muller-Lyer Illusion than people in non-carpentered natural Lyer Illusion than people in non-carpentered natural environments.environments.

Context & CultureContext & Culture

What is above the woman’s head? In one study, nearly all the East What is above the woman’s head? In one study, nearly all the East Africans who were questioned said the woman was balancing a Africans who were questioned said the woman was balancing a metal box or can on her head and that the family was sitting under metal box or can on her head and that the family was sitting under a tree. Westerners, for whom corners and boxlike architecture are a tree. Westerners, for whom corners and boxlike architecture are more common, were more likely to perceive the family as being more common, were more likely to perceive the family as being indoors, with the woman sitting under a window.indoors, with the woman sitting under a window.

IllusionsIllusions

When we misperceive the true When we misperceive the true characteristics of an object or characteristics of an object or image. image.

Help researchers understand how Help researchers understand how sensation and perception normally sensation and perception normally worksworks

Müller-Lyer IllusionMüller-Lyer Illusion

Perceptual psychologists Perceptual psychologists have hypothesized that have hypothesized that the top horizontal line the top horizontal line looks longer because it looks longer because it also looks farther awayalso looks farther away

Specifically, the Specifically, the inwardinward pointing arrows signify pointing arrows signify that the horizontal line is that the horizontal line is closest to you, and the closest to you, and the outwardoutward pointing arrows pointing arrows signify the opposite casesignify the opposite case


Most people think segment AB equals BC. In reality AB is much longer than BC.

Ponzo IllusionPonzo Illusion Converging lines indicate that top Converging lines indicate that top

line is farther away than bottom lineline is farther away than bottom line

• The interplay between perceived size and distance (a) The The interplay between perceived size and distance (a) The monocular cues for distance make the pursuing monster look monocular cues for distance make the pursuing monster look larger than the pursued. It isn’t. (b) This visual trick, called the larger than the pursued. It isn’t. (b) This visual trick, called the Ponzo illusion, is based on the same principle as the fleeing Ponzo illusion, is based on the same principle as the fleeing monsters. The two red bars cast identical-sized images on our monsters. The two red bars cast identical-sized images on our retinas. retinas. But experience tells us that a more distant object can But experience tells us that a more distant object can create the same-sized image as a nearer one only if it is actually create the same-sized image as a nearer one only if it is actually larger. As a result, we perceive the bar that seems farther away as larger. As a result, we perceive the bar that seems farther away as larger.larger.

Moon IllusionMoon Illusion Moon appears larger when it is on the horizon Moon appears larger when it is on the horizon

than when it is directly overhead. than when it is directly overhead. Objects on the horizon are perceived as farther Objects on the horizon are perceived as farther

away than those above usaway than those above us The moon appears to be behind those objects The moon appears to be behind those objects

on the horizon. Since it is bigger than those on the horizon. Since it is bigger than those object it is perceived as huge! (click on box object it is perceived as huge! (click on box below for explanation)below for explanation)

Click Below to View an Explanation:

Moon Illusion

Ames Room Illusion: Ames Room Illusion: Secret RevealedSecret Revealed

We perceive the room to be as we are used to, a perfect square or rectangular.

When in fact it is a trapezoid!

Poggendorff IllusionPoggendorff Illusion

Impossible FiguresImpossible Figures These grouping principles help us These grouping principles help us

construct reality but perceptual construct reality but perceptual contradictions can lead us astray contradictions can lead us astray

See how this and others like it are

done

More Impossible FiguresMore Impossible Figures

Escher’s Impossible Escher’s Impossible ScenesScenes

ChrysanthemumChrysanthemum

Is this 3-D?

Heads or Houses?

Water or Monks?

Rocks or Horses?

In or out of the picture?

http://www.rense.com/general67/street.htm

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