The Visual System

Light Enters the eye as

electromagnetic radiation

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

Amplitude= brightness

Wavelength= color (hue)

Humans see a mixture of several wavelengths

Varies purity Small portion of

the spectrum Animals- what do

they see?

Parts of the eye

Parts of the Eye Cornea (1) transparent

covering/protection Pupil and Iris (2) bright

light/contracts, dim light/relax Lens (3) focuses light on the

retina, accommodation Retina (4) neural tissue that

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

Blind Spot Activity!!

The Retina Millions of receptor

cells 10 % of light Rods- 100 to 125

million Cones- 5-6 million Expt- What Color is

it??

Light and Dark Adaptation

Light and Dark Adaptation

Afterimage

Form Perception Perceptual Set - The influence of

prior assumptions and expectations on perceptual interpretations

Perceptual Set

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

Feature Analysis Bottom Up Processing & Top Down

Processing

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Gestalt Principles (p105) Figure Ground Proximity Closure Similarity Simplicity Continuity

Phi Phenomenon The illusion of motion when fixed lights

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

happens. Is the Green Dot Moving?

• http://www.weeville.com/eyetest.htm

Depth Perception Binocular depth cues

– Retinal disparity- objects 25ft project images to different locations on the right & left retina

Monocular Depth Cues Linear

Perspective Motion Parallax

Monocular Depth Cues Texture gradient Interposition

Monocular Depth Cues Relative Size Height in a plane

Size Constancy

People are the same size even though their image sizes differ

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

Size Distance Relationship

Size Distance Relationship

Size Distance Relationship

Shape ConstancyThe understanding that an object’s

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

Shape Constancy

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

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

Brightness Contrast Perceived lightness

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

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

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

Context & Culture

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

Illusions

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

Help researchers understand how sensation and perception normally works

Müller-Lyer Illusion

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

Specifically, the inward pointing arrows signify that the horizontal line is closest to you, and the outward pointing arrows signify the opposite case

Müller-Lyer Illusion

Müller-Lyer Illusion

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

Müller-Lyer Illusion

Müller-Lyer Illusion

Müller-Lyer Illusion

Ponzo Illusion Converging lines indicate that top

line is farther away than bottom line

• The interplay between perceived size and distance (a) The monocular cues for distance make the pursuing monster look larger than the pursued. It isn’t. (b) This visual trick, called the Ponzo illusion, is based on the same principle as the fleeing monsters. The two red bars cast identical-sized images on our retinas. But experience tells us that a more distant object can 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.

Moon Illusion Moon appears larger when it is on the horizon

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

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

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

Click Below to View an Explanation:

Moon Illusion

Ames Room Illusion: Secret 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 Illusion

Impossible Figures These grouping principles help us

construct reality but perceptual contradictions can lead us astray

See how this and others like it are done

More Impossible Figures

More Impossible Figures

Escher’s Impossible Scenes

Chrysanthemum

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