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In many ways, the human eye is like a camera, both have a lens and focus light onto a light sensitive source and both can alter the amount of light that enters it. However the eye is far more complicated and active than we perhaps sometimes give it credit for! How many times have you seen a home-movie when the cameraman starts to run or jump? The picture shakes and makes the viewer feel sick! However our eyes are constantly moving due to the movement of our head yet we do not experience this same feeling. The difference lies in the fact that a camera passively collects data, whereas the eye as part of the nervous system interacts with the light and tries to make sense of the information it collects.

Sensation: the physical stimulation of the sensory receptors. They are passive and respond only to stimulation. Perception: the process of interpreting and understanding sensory information. This is an active process. What is perception? Perception is all about examining the ways you organise your senses to form perceptions of the world around you. Perception is basically the act of interpreting the information, which reaches us through our senses. When we look at something, we don’t just ‘see’ it – but make sense of it according to a number of different influences including our pre conceptions and past experiences.

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If you look at the Necker cube for any length of time, the cube seems to turn itself inside out. At first we see the cube facing one way but if we keep looking it reverses itself, not only that but this happens whether we want it to or not. This highlights an important part of perception, the idea that no matter what it is we look at we unconsciously guess or form a hypothesis of what we can see. Most of the time you do not even notice it but with the Necker cube your brain realises there are two distinct hypotheses of what it is looking at – both equally likely and the brain simply has no way of deciding which is the most right. So the brain alternates between the two and constantly flips the image. -Despite the fact that all you are looking at is 12 lines on a piece of paper. Sensation is merely the receiving of information to the brain; perception is your brains attempt to make sense of this information and to process it accordingly – making sense of your sensations!

Illusions of ambiguity: If you refer back to the Necker cube from last lesson you will be aware that you can interpret an image on the retina in more than one way, the illusion is therefore ambiguous and is an example of depth reversal. We cannot see both at the same time and are unable to control when the change occurs. Illusions of distortion This is when an illusion appears to be larger or a different shape than it actually is. An everyday example is when the moon rises behind buildings (see later).

Illusions of Paradox This is when a two dimensional object cannot be ‘built’ as a three dimensional object. Illusions of fiction This illusion takes place when we can ‘see’ something that is not there. Often, when we are studying how perception works, it is useful to study how it can go wrong and visual illusions are an excellent example of this. 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.


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.