The Visual System

Light Enters the eye as electromagnetic radiation Enters the eye as electromagnetic radiation Travels in a wave that vary in amplitude (height) and wavelength (between peaks) Travels in a wave that vary in amplitude (height) and wavelength (between peaks) Amplitude= brightness Amplitude= brightness Wavelength= color (hue) Wavelength= color (hue)

Humans see a mixture of several wavelengths Humans see a mixture of several wavelengths Varies purity Varies purity Small portion of the spectrum Small portion of the spectrum Animals- what do they see? Animals- what do they see?

Parts of the eye

Parts of the Eye Cornea (1) transparent covering/protection Cornea (1) transparent covering/protection Pupil and Iris (2) bright light/contracts, dim light/relax Pupil and Iris (2) bright light/contracts, dim light/relax Lens (3) focuses light on the retina, accommodation Lens (3) focuses light on the retina, accommodation Retina (4) neural tissue that absorbs light Retina (4) neural tissue that absorbs light Optic disk & Blind Spot (5) Optic disk & Blind Spot (5) Optic Nerve (6) Optic Nerve (6) Fovea (7) Fovea (7) Blind Spot Activity!! Blind Spot Activity!!

The Retina Millions of receptor cells Millions of receptor cells 10 % of light 10 % of light Rods- 100 to 125 million Rods- 100 to 125 million Cones- 5-6 million Cones- 5-6 million Expt- What Color is it?? Expt- What Color is it??

Light and Dark Adaptation

Afterimage

Form Perception Perceptual Set - The influence of prior assumptions and expectations on perceptual interpretations 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. 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 Bottom Up Processing & Top Down Processing I cdnuolt blveiee taht I cluod aulaclty uesdnatnrd waht I was rdanieg. It dseno’t mataetr in what oerdr the ltteres in a word are, the olny iproamtnt tihng is taht the frsit and lsat ltter be in the rghit pclae.

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

Phi Phenomenon The illusion of motion when fixed lights are turned on and off in a sequence 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? Stare at the X in the middle and notice what happens. Is the Green Dot Moving?

Depth Perception Binocular depth cues Binocular depth cues –Retinal disparity- objects 25ft project images to different locations on the right & left retina

Monocular Depth Cues Linear Perspective Linear Perspective Motion Parallax Motion Parallax

Monocular Depth Cues Texture gradient Texture gradient Interposition Interposition

Monocular Depth Cues Relative Size Relative Size Height in a plane Height in a plane

Size Constancy People are the same size even though their image sizes differ 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 The depth cues such as linear perspective and relative size help judge the size accurately

Size Distance Relationship

Shape Constancy The understanding that an object’s shape remains the same even though the angle of view makes the shape appear changed The 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. 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 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. 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. When the context changes you may perceive the color as changing. Want to see more of these perceptions? Goto

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. 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. When we misperceive the true characteristics of an object or image. Help researchers understand how sensation and perception normally works 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 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 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

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

Müller-Lyer Illusion

Ponzo Illusion Converging lines indicate that top line is farther away than bottom line 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. 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. 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 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) 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 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

Escher’s Impossible Scenes

Chrysanthemum Is this 3-D?

Heads or Houses? Water or Monks?

Rocks or Horses? In or out of the picture?

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