Perception |
Objectives |
By the end of this session you will be working towards being able to:
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- Define perception
- Describe visual illusions, constancies, and depth cues
- Explain visual constancies
- Describe the role of nature in perception.
- Evaluate studies of the role of nature in perception.
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What is perception? |
Perception is the process of interpreting, organising and elaborating on sensory information.
Visual illusions |
Illusions |
| In visual illusions our perception plays tricks on us, because of the way it interprets the information which the eyes receive
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Muller-Lyer Illusion |
| Both these lines are of equal length, yet we perceive the one with the outgoing fins as longer.
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Necker cube |
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| If you look at the red dot on the cube it will eventually appear to jump.
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Ponzo illusion |
| The line A appears to be longer than line B
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Visual constancies |
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Size constancy |
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| If somebody walks towards you, the image on the retina will get larger but they do not appear to grow.
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Shape constancy |
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| When you open a door the image of its shape alters.
When you open a door the image of its shape alters.
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Depth cues |
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| When we look at objects around us we know that some are near to us and some are far away. How do you think that we can judge the distance of an object from us? |
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Binocular depth cues |
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| This is using information from both eyes to perceive depth. |
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Binocular disparity |
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| Each eye looks at the world from a slightly different viewpoint and so each retina receives a slightly different image. The brain detects the difference and uses it as a cue to distance.
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Ocular convergence |
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| The nearer an object is, the more your eyes turn inward to see it. Information passes from the eye muscles to the brain helping us to gather information on distance.
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Monocular cues |
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| Monocular cues, are depth cues that we can perceive using only one eye.
We can perceive depth using only one eye because of visual cues in our environment.
We use monocular cues to show depth in two dimensional images, such as a film, a painting, a photograph or a television screen.
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Monocular cues - linear perspective
Linear perspective |
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| Parallel lines appear to converge as they recede into the distance.
This cue is used by painters and illustrators to give the impression of depth in a picture. Artists call the point in the distance where the lines converge the vanishing point
Linear perspective
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Texture gradient |
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| Textured surfaces, such as pebbles on a beach, or waves on the sea, look rougher closer up than from a distance.
We use texture gradient as a clue to distance
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Overlap |
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| In this example the objects nearer to you in the picture appear to overlap the objects that are further away. This cue is sometimes also called interposition.
The lines that make up the objects in the distance are hidden by the lines of the objects nearer to you.
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Height in the visual field |
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In this example the red boat looks as though it is nearer as it is lower in the plane than the green boat
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Monocular cues - relative size |
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| Larger objects are seen as closer.
Which of these vans do you think is nearest?
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The Nature/Nurture debate |
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| Infant (neonate) studies |
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- Researchers have found that newborns can:
- Focus on objects about 30cm away.
- Differentiate between light and dark
- Differentiate between and object and its background.
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Fantz (1961)
Presented two different patterns side by side to infants between one and 15 weeks of age, and measured how much time the baby spent looking at each of them.
Fantz concluded that:
infants preferred looking at more complex patterns.
Infants could differentiate between two patterns.
Why did Fantz conclude this?
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Fantz (1961) |
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| Showed 49 infants between four and six months of age three pictures.
Most infant ignored the face with no features altogether. The 6 month olds barely glanced at it. The children up to 31/2 months of age seemed interested in the scrambled features. The infants all seemed slightly more interested in the picture that was closest to a real face.
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Depth perception |
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The wanted to test if infants were able to judge depth soon after birth or whether this had to be learned.
They found that out of 36 humans aged between 6 and 14 months put on to the cliff. Nine refused to move at all, but 27 infants who did would crawl to their parents over the shallow side and only three would crawl over the deep side.
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Deprivation Studies |
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M. von Senden (1932) summarised the data from 65 people who had gained their sight after cataract operations; he found that:
- The showed the same abilities as sighted newborns, I.e. they had background and object, the could fixate objects and follow a moving object.
- They were unable to recognise simple objects or shapes which they knew by touch.
- They were unable to use sight to make judgements such as which of two sticks was longer.
- They did not show perceptual constancies, such as shape constancy.
- This evidence suggests that experience is crucial for the development of perception.
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Gregory and Wallace (1963) |
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- Reported a case study of S.B.
- S.B. was 52 years old when he gained his sight He too showed the kind of abilities and limitations described by von Senden.
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Nurture |
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Animal Studies
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Blakemore and Cooper (1970)
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Showed how important early visual experience is in developing normal visual skills. They reared some kittens in the dark, except for periods when the kittens were placed inside large drums that were painted on the inside.
- Some cats were in a drum with vertical black and white stripes, while the other drum had horizontal black and white stripes.
- At first the kittens could not recognize anything with edges that were different from the one they had seen inside their drums.
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Held and Hein |
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- The kitten carousel, made two kittens share the same visual experiences, although only one of them could move fairly independently.
- When their paw-to-eye co-ordination was tested the active kitten had far better abilities.
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Cross-cultural studies of perception |
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- Hudson (1960) investigated three-dimensional depth perception with various tribal groups in Africa.
- The cues in the picture (relative size, and height in the visual field) help the viewer to perceive depth
- Results showed that members of African tribes were unable to understand this picture, for example, they thought the man was about to spear the elephant.
| Perception readjustment studies |
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Stratton (1897) |
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Wore a patch over one eye so that he couldn’t see anything through it, and an inverting lens over the other eye, which made everything he looked at appear upside down and left to right. |
Kohler (1962) |
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| Kohler wore lenses which were half green and half red. He found that soon after putting them on he had adapted and colours seemed ‘normal’. When he took them off again though, he saw red where the lens had been green, and green where it had been red – his brain had been compensating for the colours.
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