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The Visual System

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Goto http://www.skidmore.edu/~hfoley/Perc4.htm#lightcon Sensory Deprivation & Restored Vision Early visual experience can have ... Illusions When we misperceive the ... – PowerPoint PPT presentation

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Title: The Visual System


1
The Visual System
2
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)

3
  • Humans see a mixture of several wavelengths
  • Varies purity
  • Small portion of the spectrum
  • Animals- what do they see?

4
Parts of the eye
5
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!!

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

7
Light and Dark Adaptation
8
Light and Dark Adaptation
9
From the Eye to the Brain
10
Processing in the Visual Cortex
  • Feature Detectors
  • Simple Cells
  • Complex cells
  • Ventral (what) pathway
  • Dorsal (where) pathway

11
Color Theory
  • Psychological interpretation- Blends of three
    aspects or properties of light
  • Three theories regarding color

12
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

13
Opponent Process Theory- Hering
  • Color perception depends on receptors that make
    antagonistic 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

14
Opponent Process Theory- Hering
  • Complementary colors (p.101)

15
Afterimage
16
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17
Reconciling Theory
  • Both theories correct
  • Perception of color is processed in stages
  • Beginning stages- trichromatic
  • Later stages- opponent processing

18
Form Perception
  • Perceptual Set - The influence of prior
    assumptions and expectations on perceptual
    interpretations

19
Perceptual Set
  • What do you see in the center picture a male
    saxophonist or a womans face? Glancing first at
    one of the two unambiguous versions of the
    picture is likely to influence your
    interpretation.

20
Feature Analysis
  • Bottom Up Processing Top Down Processing
  • I cdnuolt blveiee taht I cluod aulaclty
    uesdnatnrd waht I was rdanieg. It dsenot 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.

21
Gestalt Principles (p105)
  • Figure Ground
  • Proximity
  • Closure
  • Similarity
  • Simplicity
  • Continuity

22
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

23
Relationship Between Perceived Size and
Perceived Depth
  • To perceive the size of objects accurately we
    must also perceive their distance accurately
  • Many visual illusions occur because of a lack of
    sufficient depth cues

This figure shows that image size depends upon
both object size and distance
24
Depth Perception
  • Binocular depth cues
  • Retinal disparity- objects 25ft project images to
    different locations on the right left retina

25
Monocular Depth Cues
  • Linear Perspective
  • Motion Parallax

26
Monocular Depth Cues
  • Texture gradient
  • Interposition

27
Monocular Depth Cues
  • Relative Size
  • Height in a plane

28
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

29
Size Distance Relationship
30
Size Distance Relationship
31
Size Distance Relationship
32
Shape Constancy
  • The understanding that an objects shape remains
    the same even though the angle of view makes the
    shape appear changed

33
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

34
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.htmlightcon

35
Sensory Deprivation Restored Vision
  • Early visual experience can have a profound
    effect on perception. Blakemore Coopers
    restricted environment with kittens.
  • Do the kittens ever fully regain normal
    sensitivity to horizontal or vertical lines? NO.
  • 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.
36
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 brains neural connections.
  • If deafness or blindness is corrected as an
    infant, it awakens the pertinent brain area.
    Nurture sculpts what nature has endowed.

37
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.

38
Context Culture
  • What is above the womans 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.

39
Illusions
  • When we misperceive the true characteristics of
    an object or image.
  • Help researchers understand how sensation and
    perception normally works

40
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

41
Müller-Lyer Illusion
42
Müller-Lyer Illusion
Most people think segment AB equals BC. In
reality AB is much longer than BC.
43
Müller-Lyer Illusion
44
Müller-Lyer Illusion
45
Müller-Lyer Illusion
46
Ponzo Illusion
  • Converging lines indicate that top line is
    farther away than bottom line

47
  • The interplay between perceived size and distance
    (a) The monocular cues for distance make the
    pursuing monster look larger than the pursued. It
    isnt. (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.

48
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
49
Ames Room Illusion Secret Revealed
When in fact it is a trapezoid!
We perceive the room to be as we are used to, a
perfect square or rectangular.
50
Poggendorff Illusion
51
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
52
More Impossible Figures
53
More Impossible Figures
54
Eschers Impossible Scenes
55
Chrysanthemum
Is this 3-D?
56
Water or Monks?
Heads or Houses?
57
Rocks or Horses?
In or out of the picture?
58
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http//www.rense.com/general67/street.htm
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