Visual Perception Principles
Depth Perception Principles
Cues (signals) that help us to determine how near or far an object is from another object or
ourselves.
There are two types of depth cues: primary and secondary.
Primary depth cues are internal signals provided by our body to assist in judging depth and
distance.
Secondary depth cues are external signals provided by the environment that assist in
judging depth and distance.
Primary depth cues are: retinal disparity, convergence and accommodation.
Retinal disparity: the blending together of two slightly different images received by each
retina.
Convergence: the inward turning of the eyes to focus an object on the retina.
Accommodation: the ability of the eye to change its focus from distant to near objects, by
changing the shape of the lens.
Secondary depth cues are: linear perspective, interposition, texture gradient, relative size
and height in the visual field. Secondary depth cues are also known as pictorial depth cues.
Linear perspective: two parallel lines that converge together provide a cue for depth.
Interposition: objects that overlap one another provide a cue for depth.
Texture gradient: the amount of detail present in surfaces provide a cue for depth.
Relative size: the size of an image cast onto the retina provide a cue for depth.
Height in the visual field: an object’s location in relation to the horizon line provides a cue
for depth.
 Student Activity Manual: Activity 2.7 (page 52)
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Perceptual Constancies
Constancies help us to perceive our world as remaining stable and unchanging, even though
the images projected onto our retina are quite the contrary.
There are five constancies:
1. Size constancy
2. Shape constancy
3. Orientation constancy
4. Brightness constancy
5. Colour constancy
Examples of size (stick figure – different sizes), shape (different angles of car) and
orientation (tv screen)
Size constancy – even though an object’s size changes on the retina, the object’s actual size
is interpreted to remain stable and unchanging
Shape constancy – even though an object’s shape changes on the retina, the object’s actual
shape is interpreted as remaining stable and unvarying.
Orientation constancy – the perceptual constant whereby we perceive an object’s true
orientation despite changes in the orientation of the object on our retina.
Visual stimuli are interpreted using two different processes. Bottom-up processing (visual
perception principles) and top-down processing (perceptual set).
Bottom-up processing – processing information by starting with the individual elements of a
visual stimulus and gradually building up a final representation and interpretation of the
stimulus. (Visual perception principles: Gestalt, depth cues, constancies)
Top-down processing – using psychological factors such as pre-existing knowledge and
expectations to interpret and assign meaning to a visual stimulus. (perceptual set)
Perceptual Set
A mental predisposition to interpret stimuli in an expected and predictable way.
Factors affecting perceptual set include: past experience, context, culture, motivation and
emotional state.
Examples
1. Past experience – Old lady / Young lady, spelling
2. Context – A B C or 12 13 14
3. Culture – Aborigines/Europeans, American Indians
4. Motivation – running out of petrol, sign says ‘food ahead’, but expect it to say ‘fuel ahead’
5. Emotional state - upset/angry, expect those around you to be talking about you
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Visual Illusions
An object that is interpreted in a way that is not consistent with the object’s actual reality.
Write a description of how each of these illusions work:
Muller-Lyer Illusion: two parallel lines of equal length. One is capped with feather ends and
the other is capped with arrowheads. The feather end looks longer than the arrowhead.
‘Carpentered-world hypothesis’ states that we associate the feather end as an inside corner
of a room and the arrowheads as the outside edge of a building. This creates depth and we
assume that the inside corner is further away, so the line must be longer.
Apparent-Distance theory states that the feather end is further away and therefore must be
longer.
Ames Room Illusion: a trapezoidal shaped room with a sloping roof. The room appears to be
normal in size and shape when it is viewed through a peep hole, however size constancy and
binocular depth cues cannot be used. Therefore, when a person walks from one side of the
room to another, they appear to grow or shrink.
Ponzo Illusion: two horizontal lines of equal length drawn within converging parallel lines.
The line in the narrower section appears to be longer than the other line. Apparent-distance
hypothesis helps to explain it, as we perceive the converging parallel lines as creating depth
(linear perspective), as well as the horizon (height in the visual field). This means that we
interpret the top line as being further away than the bottom line. Therefore, the top line is
mis-interpreted as being longer because it has to be further away.
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Culture:
Evidence that Indians have a different manner of looking at the world can be found in the contrast
between the ways in which Indian and non-Indian artists depict the same events. That difference is
not necessarily a matter of 'error' or simply a variation in imagery. It represents an entirely individual
way of seeing the world. For instance, in a sixteenth-century anonymous engraving of a famous scene
from the white man's history an artist depicted a sailing vessel anchored offshore with a landing party
of elegantly dressed gentlemen disembarking while regal, Europeanized Indians look on - one carrying
a 'peace pipe' expressly for this festive occasion.
The drawing by an Indian, on the other hand, records a totally different scene: Indians gasping in
amazement as a floating island, covered with tall defoliated trees and odd creatures with hairy faces,
approaches.
When I showed the two pictures to white people they said in effect: 'Well, of course you realize that
what those Indians thought they saw was not really there. They were unfamiliar with what was
happening to them and so they misunderstood their experience.' In other words, there were no
defoliated trees, no floating island, but a ship with a party of explorers.
Indians, looking at the same pictures, pause with perplexity, and then say, 'Well, after all, a ship is a
floating island, and what really are the masts of a ship but the trunks of tall trees?' In other words,
what the Indians saw was real in terms of their own experience.
Activity 3.26 – question 6
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