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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.
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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
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??
Processing in the Visual Cortex • Feature Detectors- neurons respond to specific features of a visual stimulus (lines, edges, movement) • Simple Cells • Complex cells • Hubel and Wiesel’s research
Processing in the Visual Cortex • Ventral (what) pathway • Temporal lobe • object identification • Dorsal (where) pathway • Motion • Parietal lobe
Color Theory • Psychological interpretation- Blends of three aspects or properties of light • Three theories regarding color
Trichromatic Theory- Young and Von Helmholtz • The eye does the mixing by varying the ratio of neural activity among these 3 types • Color blindness • Dichromats • Monochromats
Opponent Process Theory- Hering • Color perception depends on receptors that make anatagonistic responses to 3 pairs (red v. green) (yellow v. blue) (white v. black) • When excited they respond to one color and when inhibited they respond to the other
Opponent Process Theory- Hering • Complementary colors (p.101)
Reconciling Theory • Both theories correct • Perception of color is processed in stages • Beginning stages- trichromatic • Later stages- opponent processing
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 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 • 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
Retina Pupil B A Image A Image Relationship Between Perceived Size and Perceived Depth • To perceive the size of objects accurately we must also perceive their distance accurately • Thus, many visual illusions occur simply because a particular image lacks sufficient depth cues This figure shows that image size depends upon both object size and distance
Depth Perception • Binocular depth cues • Retinal disparity- objects 25ft project images to different locations on the right & left retina
Motion Parallax Monocular Depth Cues • Linear Perspective
Interposition Monocular Depth Cues • Texture gradient
Height in a plane Monocular Depth Cues • Relative Size
Size Constancy • People are the same size even though their image sizes differ • The depth cues such as linear perspective and relative size help the visual system judge the size accurately
Shape Constancy • 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. • 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. • Although the interior squares are in fact identical, we perceive the one as lighter or darker because of the contrast with its surroundings. • Want to see more of these perceptions? Goto http://www.skidmore.edu/~hfoley/Perc4.htm#lightcon
Sensory Deprivation & Restored Vision • Early visual experience can have a profound effect on perception. • Do the kittens ever fully regain normal sensitivity to horizontal or vertical lines? NO. 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 Vision • These experiments show that lacking stimulation, the cortical cells had not developed normal connections making them functionally blind to shape. • A sensory restriction does no permanent damage if it occurs later in life. This suggests that visual experiences during infancy are a critical period for normal sensory and perceptual development. Experience guides the organization of the brain’s neural connections. • If deafness or blindness is corrected as an infant, it awakens the pertinent brain area. Nurture sculpts what nature has endowed.
Context • The setting or environment in which we interpret sensory stimuli • Culture can also influence how we perceive information. • People actively construct their perceptions by drawing on their prior learning and cultural experiences. • People living in urban and industrialized environments where there are more right angles and straight lines will be more susceptible to the Muller-Lyer Illusion than people in non-carpentered natural environments.
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 Most people think segment AB equals BC. In reality AB is much longer than BC.
Müller-Lyer Illusion: Click below to view a video of how this works: Muller-Lyer Illusion Explanation
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
