Look at the diagram on the left. It is called Fraser’s Spiral. When you look at it you see a spiral curving downwards like a staircase but in fact everything is not quite as it seems. If you trace your finger around the spiral how far ‘down’ do you get? The answer of course is nowhere, there is no spiral the diagram is simply concentric circles, one inside the other. The spiral doesn’t exist, you simply perceive a spiral because your senses are tricked partly due to prior expectations.
You should be able to :EXPLAIN AND UNDERSTAND GREGORY’S THEORY OF CUES AND HYPOTHESIS
When you look at the picture on the left, what do you see? Some people see a vase, others two faces. You certainly cannot see both at the same time. Our pre-conceptions of what we expect to see and our past experiences can often dictate what we perceive.
If I were to show you lots of pictures of different vases, then the picture on the left you would probably see the vase. If I were to show you faces you would probably see the faces. This type of perception is called top-down processing and refers to perception that is influenced by our knowledge or expectations. Gregory (1973) carried out large amounts of research on visual illusions and his theory explains perception in terms of hypothesis forming (making a ‘best guess’) and then testing it. Basically he argued that information received by the eye interacts with prior knowledge to create “psychological data” which we can then make sense of. Illusions occur when the hypothesis from ‘real world’ experiences are applied to non-real ‘trick’ images and then tested only to find they are inappropriate.
Take the Zollner and Poggendorf illusions below for example,
Firstly the Poggendorf illusion (left), Gregory would argue that in a 3D world it would be unlikely that the diagonal line would meet as you can see in the explanation below, therefore we perceive the line not to be continuous.
The Zollner illusion (bottom right) may fool us because we are used to living in a world of rectangles. Instead of perceiving the lines as parallelograms (which they are) we perceive them as rectangles as if we were looking at them from an angle and if the little rectangles are at an angle our brain assumes the longer lines are on an angle too.
Interestingly when this illusion is shown to Zulu tribes who live in a world largely without rectangles they are far less likely to be fooled by it.
There is also evidence to suggest that the more times you look at he illusion the less it’s impact and that you do start to ‘learn’ that you are being fooled.
The Devil’s pitchfork illusion according to Gregory works because we create a hypothesis of a fork, test it but realise it cannot exist. Each side of the illusion works but cannot integrate them together so the image ends up making no sense to us.
With the Kanizska triangle, we base conceptions due to previous experience that the triangle is covering three circles rather than the truth – that the triangle does not exist.
Support for Gregory
Perception can be ambiguous. The Necker cube and Ruben’s vase shows that an image can be perceived in two different ways and that can only mean that it is our interpretation of the image that changes rather than the image itself.
- When we perceive a picture we understand what an object is even though it may be many times smaller than it is in real life and in 2D rather than 3D. We recognise the picture below as a football even though it is smaller and in 2D.
- Perception therefore must be based on prior knowledge. The Poggendorf illusion and the Muller-Lyer illusion (below) can be explained in the same way.
CRITICISM OF GREGORY
- Illusions still fool us even though we know the trick and have seen the same illusion many times before.
- Gregory seems to be very good at explaining illusions but not very good at explaining how the real world works.
YOU SHOULD KNOW:
- THE USE OF MONOCULAR and 2D IMAGES IN DISTANCE PERCEPTION.
After your brain has perceived what the image it is looking at is, the next task is to decipher where the object is. When you are looking around you can usually instantly tell how far away something is. It is this ability that is essential for any predators in order to catch their prey. However as usual we can be tricked!
Both the pictures above are genuine and have not been doctored. The picture on the left is a photo of two identical twins who are the same height, the picture on the right, a mother (who is not a dwarf!) and daughter (who is not a giant!). The picture is of the famous Ames room. Strange things seem to happen in the Ames room. People standing on one side of the room appear huge, whereas people on the other side look tiny. People who walk from one side to the other appear to shrink or grow mysteriously dependent on which side they started from. The secret to the illusion will be discussed later.
To perceive depth we rely on two types of information : Monocular cues (information provided by just one eye and include the objects height on a plane, linear perspective, overlap, relative size, gradient of texture, aerial perspective, shadowing and relative motion. On a normal TV screen only monocular cues give the sensation of depth.) and Binocular cues (information about depth provided by both eyes)
We shall deal with each of these types of cues at a time and explain how they give us a sense of depth.
Activity 3 - MONOCULAR CUES
Using monocular cues of depth is what movie - makers use to make us believe the impossible – huge animals, monsters and aliens that interact with their normal environments. Monocular cues give us simple clues about distance:
Height on a plane: We perceive things that are higher as further away. In a room objects that are further away always appear to be on a higher plane than those closest to us.
Overlap(or superposition): We know that if one object appears to overlap another then it must be closer than the object it has overlapped.
Relative size: As long as you know the size of an object you can judge how far away it is by seeing how large it appears relative to another object you can see.
Linear Perspective: This is the idea that parallel lines look like they meet in the distance. This idea is the reason we are fooled by the Ponzo illusion (below). Aircraft have two lights on the back of them because pilots found that one light led to aircraft driving in to each other as they couldn’t judge the distance they were away from the plane in front.
The top line looks longer than the bottom line because of our knowledge of linear perspective.
Gradient of texture: Things that are far away look smoother than things that are near. A good example is a field of grass. The grass in front of you is very defined, you can see individual blades and imperfections but if you look further away the grass looks smooth.
Shadow: Shadows provide information about angles and curves and depth. Shadow is quite often used in sign writing and lettering to give an illusion of depth.
Relative motion (motion parallax): If you move your head from side to side quickly you will notice that things near to you seem to move a lot but things far away hardly move at all.
Summary of monocular cues – Your brain uses most or all of the cues we have mentioned before to give us an understanding of depth needing only one eye. Motion pictures work because they fool us into thinking that things are moving. IMAX cinemas often give us the impression we are actually moving, this is because the screen is so big it extends past or field of vision thus removing the cues that are present to tell us we are NOT moving.
Activity 4/ Plenary - Look at the picture below, what monocular cues are present to tell you about distance?
LESSON 7 – DISTANCE PERCEPTION
YOU SHOULD KNOW -
- HOW BINOCULAR CUES ARE USED IN DISTANCE PERCEPTION
- EVERYDAY EXAMPLES AND EXPLANTIONS OF DISTORTIONS OF PERCEIVED SIZE, DISTANCE AND MOVEMENT
SIZE CONSTANCY AND DEPTH PERCEPTION
Although you only need one eye to judge depth, you need two to judge depth accurately. Binocular cues give us extra detail for more accuracy
Convergence: When you look at an object close to you your eyes converge. In other words they focus at a point. If you look at an object in the distance your eyes do not converge and instead focus on ‘infinity’.
Disparity: Because your eyes are situated apart from each other, each one sees a slightly different image. This difference helps us to judge how far away something is with more accuracy.
Size Constancy
As you watch a person walking towards you the size of the image going into your brain increases, yet you do not think that person is growing. This is because your brain uses past experience to figure out the relative size of objects. This adjustment is called constancy scaling.
The picture on the left shows an example of this. The two images of the gentleman holding the book are exactly the same size. Yet when we look at the image it is the gentleman on the far right who looks out of place rather than the gentleman on the left.
Moon illusion – When the moon is low down in the sky it looks much larger than when it is high in the sky. This is because when the moon is high in the sky we have no visual cues to compare it with apart from the odd star. When the moon is low in the sky we can see it relative to buildings, trees, towns and cities – we know how big they are so we perceive the moon as being huge! If you cut out a hole in a piece of paper to mask out the surroundings and then look at the moon, the illusion doesn’t work and it looks ‘normal’ size again. This is size constancy at work!
When you look at a car driving off into the distance, the information coming into your visual system is telling you that the image is getting smaller. Yet rather than assume that this is because the car is getting smaller, we know that it is the car getting further away.
If you look at a regular object like a mug, the mug looks different from the top compared to the side and different again looking from the bottom – yet you still perceive all the images as a mug.
These two examples explain shape constancy. One piece of evidence to show this is a learnt skill is the case of S.B. .SB was a 53 year old man who regained his sight with a cataract operation. One day in the hospital he was found trying to climb out of the window to look at the little objects he could see on the floor. What SB failed to realise was that he was 4 floors up and the little objects were in fact cars on the street below!!
You should know:
- HOW FACTORS SUCH AS MOTIVATION, PERCEPTUAL SET, PREVIOUS EXPERIENCE, CONTEXT, PERCEPTUAL DEFENCE, PERCEPTUAL SENSITISATION AND EMOTION EFFECT PERCEPTION
- MOTIVATION: Gilchrist and Nesburg (1952) gave participants pictures and asked them to rate how brightly- coloured they were. Participants who had hone without food for more than 4 hours reported that the pictures with food and drink on them were significantly brighter than the others and more bright than they were when they were not hungry. This highlights the fact that our motivation effects our perception.
✓Activity
Quickly read the following list of words out loud then read the notes below
MACBETH MACARTHER MACILVEEN
MACDOWELL MACNAMARA MACMILLAN
MACETEER MACDONALDS MACHINES
MACKENSIE
look at the list you have just read. Look at the next to last word. Did you pronounce it Mac Hines? If so you were demonstrating the effects of perceptual set.
Perceptual set refers to the way in which immediate, preceding experiences can create a readiness to perceive things in a similar way. Due to this ‘perceptual set’ we become prepared or ready, to take in certain kinds of information rather than others.
CONTEXT:
Bruner and Minturn conducted a study that highlights how perceptual set works. In this study participants were given a list of numbers or letters and were asked to read them out loud. In amongst those figures, towards the end was placed an ambiguous figure that looked like this:
The participants that were given the letters perceived the figure as a letter ‘B’ whereas the people who were given the numbers perceived the figure as a number ‘13’.
Solley and Haigh (1958) asked children to draw pictures of Father Christmas in the weeks leading up to Christmas and in the weeks afterwards. As Christmas approached, Father Christmas became larger and larger and gained more and more presents, after Christmas he became smaller and the presents decreased.
McGinnies(1949) used the term to explain why recognition of emotionally arousing words are slower then less arousing words. He asked participants to read out words from a card, some words were neutral whereas some were taboo. They researchers found the taboo words were slower to recognise. This suggests we have a perceptual defence against emotionally arousing words.
Recently, critics have suggested the recognition of these words were faster and the delay was a reluctance to read them out.
However if perceptual defence does exist, it raises the intriguing possibility that we somehow look at a stimulus in order to decide whether or not to perceive it. In fact considerable evidence exists for this kind of subliminal perception — the reception of stimuli which does not involve conscious recognition. For example studies have shown that there can be significant increases in bodily responses such as galvanic skin response (GSR) and self-ratings of anxiety following subliminally presented anxiety-arousing stimuli.
The ace of hearts study by Bruner and Postman (1949)that we have already mentioned is an example of the way in which previous experience can dictate our perception..
LESSON 9 – PERCEPTION
You should know – How the factors that affect perception can be applied to everyday life.
You will – REVISE THE TOPIC.
-1. What role does perceptual set have on stereotyping?
-2. How do advertisers the influence of emotion?
-3. What factors make proof-reading so difficult?
-4. How does context effect our ability to read?
-5. What are the dangers of emotion when crimes are committed?
COGNITIVE PSYCHOLOGY – REVISION CATEGORIES
Higher level work is indicated in italics
Perception:
- Structure and function of the eye.
- Sensation v Perception.
- Types and examples of Illusions.
- Gregory’s Theory of Perception and explanations of Illusions.
- Monocular and binocular cues of distance perception.
- Examples of distortions of perceived size/movement. (moon illusion)
- Size constancy and explanations of distortions of perceived size/movement.
- Social factors influencing perception (emotion, motivation etc) and studies conducted.
- Discussion of factors affecting perception as they relate to everyday life.
