Visual illusions

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  • 1. Visual Illusions Rizwan
  • 2. William Hogarth 1754 - "Whoever makes a DESIGN without the knowledge of PERSPECTIVE will be liable to such Absurdities as are shown in this Frontispiece."     
  • 3. Theories of Geometrical Illusions
    • Eye-movement  perceived length
    • Perspective cues
    • Transactionalist approach
    • Adaptation-level theory
  • 4. Eye-Movement Theory
    • Line length  eye movement
    • Testable, but usually fails – initial perception, eyes are stable
    • Finding an index of eye movements a problem
  • 5. Müller-Lyer Lines
    • Eye-movement theory: Arrowheads influence extent of eye movements
  • 6. Perspective Cues
    • Pictures converted in our brain from 2-dimensional drawings to represent 3-dimensional scenes
    • Different level of explanation – does not propose a mechanism for perception
    • Well established, although some ‘loopholes’ have been found
  • 7. Müller-Lyer Lines Revisited
    • The same illusion through perspective cues
    • Oculomotor Macropsia/Micropsia
  • 8. Transactionalist Theory
    • The world is a product of perception, not a cause of it
      • Hamlet: Do you see yonder cloud that’s almost in shape of a camel?
      • Polonius: By the Mass, and ‘tis like a camel indeed.
      • Hamlet: Methinks it is like a weasel
      • Polonius: It is backed like a weasel
      • Hamlet: Or like a whale?
      • Polonius: Very like a whale
      • Hamlet (Act III, Scene II)
    • Change our way of looking  Perception will change
    Old Man
  • 9. Adaptation-Level Theory
    • Helson, 1964 – “spatial pooling”
    • Green & Stacey, 1966 applied to illusions
    • Past stimulation  current stimulation
    • “ stored norms”
    • “ Top-down processing”
    • Some flaws – Ames room
  • 10. Depth Cues on a Flat Surface
    • 1967, R.L. Gregory – all pictures are “impossible objects”
    • Conflicting depth cues in the content of the picture with the flat surface on which it is presented
  • 11. Retinal Disparity
    • No retinal disparity on a flat surface
    • As a viewer of an image, we choose to suppress the cue of retinal disparity
    Source: http://frank.mtsu.edu/~pyskip/splec6.htm
  • 12. The Acceptance of Perspective
    • We have come to accept that although we are seeing a flat surface, that the objects on it represent 3 dimensional concepts
    • Pictorial cues: interposition (occlusion), relative size, linear perspective & texture gradients
    • Ambiguous dimensional cues can lend themselves to be great visual illusions
  • 13. Depth Ambiguity
    • Because of the way everything we see is projected onto the retina, there is a great deal of ambiguity
  • 14. Wundt’s crosses
    • Hering (1879) & Wundt (1898)
    • Most ambiguous of all figures
    • Infinite number of interpretations, but perceptual system tries to settle with a ‘best’ one
  • 15. Sanford’s figure
    • Sanford, 1903
    • Although there may be an obvious ‘best’ interpretation, once can easily be persuaded to accept an alternate one!
  • 16. Of Ambiguous Figures and Depth Reversals
    • Necker cube
    • Mach Book
  • 17. Of Ambiguous Figures & Depth Reversals 2
    • Not enough information in the image to make a decision as to the “best” interpretation
    • Taken advantage of to create “impossible” figures
  • 18. The ‘freemish’ crate
    • Cochran’s photo of his ‘freemish’ crate (1966).
  • 19. How did he do that??? Any guesses?
  • 20. Viewing from a single, special perspective
    • Viewing the image from a misleading perspective
    • Viewing from another angle wrecks the effect
    • Monocular viewing required
    • Occlusion
  • 21. Misleading depth cues
    • Stage scenery – gives impression of greater depth
    • The Ames Room
  • 22. The Ames Room
  • 23. Of Giants and Dwarves?
    • Of course not!
    • But how?
  • 24. What’s going on here?
    • Adelbert Ames, Jr. (1946) – concept by Helmoltz
    • Special viewpoint – monocular
    • Floor, ceiling, some walls, & windows are trapezoidal
    • Inclined floor
    • Appears as a normal cubic room
  • 25. So how does it work?
    • Peephole removes stereopsis
    • Forms an identical image of a cubic room on your retina
    • Both corners of the room subtend the same visual angle to your eye – appear equidistant
    • Seckel & Klarke: Past experiences not relevant
  • 26. But what about the people?
    • A split between perception & expectation
    • Apparent cubic perspective overrides sense of size constancy
    • Stanford psychologist Robert Shepherd – use background & relationship to the horizon to judge size
  • 27. Retinal Size != Apparent Size
    • Distance cues: relative size of elements, separation, density, clarity, background
  • 28. But is the Ames Room necessary?
    • Seckel and Klarke: only charm
    • An apparent horizontal path is all that’s necessary
    • Richard Gregory: same effect, ambiguous background
  • 29. The Moon Illusion
    • Perceived distance, visual angle, & linear size != physical values
    • Illusion from comparison of perceived values at the horizon & at the zenith
    • Subtends .5º in the eye no matter what
    • Not atmospheric
    • Illusion disappears in a “mooning position” 
  • 30. Theories
    • Apparent distance theory – appears farther away  larger
      • Size-distance paradox
    • Distance, visual angle, & linear size illusions work together
      • Oculomotor micropsia / macropsia  visual angle
      • Distance cues  macropsia for horizon moon
  • 31. The Mystery Spot
    • Tilted house
    • No visible horizon – assumed horizon with internal reference frame of house
    • Your body is on a tilt as well – enhances effects as much as 3x
    • Application to pilots
  • 32. A new perspective on seeing
    • Many theories, none are all-encompassing yet
    • New ways to see things – become more aware of space through witnessing these illusions
    • Perspective is a powerful tool – in ‘imitating’ reality, it can also deceive
    • Seeing is believing  Perceiving is believing