Visual perception: theories and illusions

Theories and illusions 

• Information encoded by the retina is complex and incomplete
• The visual system analyses different features and combines them into a meaningful. unified whole. How does it do this?
•  The Binding Problem

Gestalt psychology

Gestalt psychologists - Koffka, Kohler, Wertheimer (early 20th century)

Theory of perceptual organisation - certain features of the sensory input are grouped together to form a whole, or Gestalt.

Governed by innate principles: 

• Laws of proximity, similarity, good continuation, closure
• Law of Pragnanz

Law of Prägnanz

•  A general principle that underpins all human perception.
• Of all the possible ways of interpreting the visual field and the objects within it, the simplest and most encompassing will be selected.
• In the previous examples the individual components, or parts, are grouped together to make a unified whole because it is the simplest interpretation

Gestaltism in the real world

Strengths:

• Describes perceptual organisation well.

• Principles have stood the test of time (though alternative laws have been suggested).

Weaknesses: • More descriptive than explanatory.

• Underestimated the importance of knowledge.

 

Constructivism

• Roots in philosophical empiricism
• Hermann von Helmholtz - perception as unconscious inference
• The modern constructivist approach
• Theoretical and experimental work conducted by, for example, Richard Gregory, Irvin Rock, and Arien Mack
• Shared assumptions:
• Perception is an indirect process of construction, guided by knowledge from experience.
•  There is an interaction of external and internal sources of information (e.g. expectations, motivations, emotions).
• Hypotheses and expectations can lead to errors producing visual illusions.

1. Ambiguous drawings

• demonstrate the phenomenon of expectancy
• Rat man
• Depednding on whether the participants had been exposed to line drawings of animals or faces in a previous series of slides, they reported seeing either a. a rat or b. a man wearing glasses. This is an example of how knowledge and recent experience influences interpretation of the data

2. Size constancy

• Our perception of an object's size remains constant despite variations in retinal size, i.e. sensory input
• The sensory input does not provide information about the true size of the object in space
• Objects of different heights at different distances can subtend the same degree of visual angle on the retina
• Objects of the same height at different distances subtend different degrees of visual angle on the retina
• Size must therefore be inferred from cues to distance

The perception of distance and depth

• The perception of distance in a three dimensional world is not straightforward as retina codes input in just two dimensions
• Constructivists argue that we learn to infer distance on the basis of multiple cues that have become associated with each other through experience and learning i.e. Knowledge of the dimension is acquired by associating different visual and kinesthetic cues

Binocular disparity (binocular cue)

• Your eyes are about 7cm apart and this distance guarantees that the two eyes will have slightly different views of the world whenever nearby objects are at different distances
• Judging depth binocularly is known as stereopsis

Vergence (binocular/oculomotor cue)

• The eyes converge or move together to look at nearby objects and diverge or move apart to look at far away objects

Accomodation

• Involves information provided by muscles that control lens shape. There is a change in the shape of lens in your eye as you focus on objects at different distances (thin lens for distant objects; thick lens for nearby objects).

Autostereogram

• a 2D image and yet it can be viewed as a 3D image when there is a dissociation of vergence and accomodation. Look past the image and then try to focus without allowing the eyes to converge on the image (which they will naturally do). Viewing the 3D image requires independent control over extraocular and ciliary muscles

Linear perspective

• Parallel lines appear to meet in the distance

Shading 

• monocular cue
• A cue provided by the pattern of light and shadows - can tell you which surface of an object is closer to you. Observers typically assume that lighting is from overhead

Relative and familiar size

• monocular cue
• relative size - an object's size relative to other subjects can helpful in telling us which of two objects is closer
• Familiar size - past experience with objects

Interposition 

• One object overlaps or partly occludes another. We judge the partly covered object to be farther away than the object that is completely visible

Evidence for the constructivist approach from visual illusion

• Illusions - are systematic visual discrepancies from simple measurements with rules, photometers, clocks and so on (Gregory, 1997)
• Cognitive illusions are due to a misapplication of knowledge (Gregory, 1997)

Shading cues and faces

• We typically assume that light comes from above
• However, we also assume that faces are convex (pushed out towards us) - thus, when presented with hollow-mask (concave) faces, we perceive them as convex, even when shading cues contradict this assumption
• Shows the importance of learned experience and context on perception

The Muller-Lyer illusion

• Because of our experience, the brain infers that the line must be: bigger when configured with angles-out (far away) smaller when configured with angles-in (near)

The effect of culture

• Susceptibility to the Muller-Lyer and other such illusions is not innate but culture dependent
• Pedersen and Wheeler (1983)
• Studied responses to the Muller-Lyer illusion among two groups of Native Americans (Navajos)
• The group who lived in rectangular house tended to see the illusion
• The group who lived in round houses tended NOT to see the illusion