Visual illusions

William Hogarth 1754 - "Whoever makes a DESIGN without the knowledge of PERSPECTIVE will be liable to such Absurdities as are shown in this Frontispiece."

Theories of Geometrical Illusions Eye-movement  perceived length Perspective cues Transactionalist approach Adaptation-level theory

Eye-Movement Theory Line length  eye movement Testable, but usually fails – initial perception, eyes are stable Finding an index of eye movements a problem

Müller-Lyer Lines Eye-movement theory: Arrowheads influence extent of eye movements

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

Müller-Lyer Lines Revisited The same illusion through perspective cues Oculomotor Macropsia/Micropsia

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

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

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

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

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

Depth Ambiguity Because of the way everything we see is projected onto the retina, there is a great deal of ambiguity

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

Sanford’s figure Sanford, 1903 Although there may be an obvious ‘best’ interpretation, once can easily be persuaded to accept an alternate one!

Of Ambiguous Figures and Depth Reversals Necker cube Mach Book

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

The ‘freemish’ crate Cochran’s photo of his ‘freemish’ crate (1966).

How did he do that??? Any guesses?

Viewing from a single, special perspective Viewing the image from a misleading perspective Viewing from another angle wrecks the effect Monocular viewing required Occlusion

Misleading depth cues Stage scenery – gives impression of greater depth The Ames Room

Of Giants and Dwarves? Of course not! But how?

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

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

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

Retinal Size != Apparent Size Distance cues: relative size of elements, separation, density, clarity, background

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

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” 

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

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

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

Visual illusions

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Visual illusions