When information from the outside world reaches us, either by touch, sight, smell taste or hearing, we make sense of what that information is in order to identify it.
Perception is the process of interpreting, organising and elaborating on sensory information.
The sensory information for vision is light waves, which enter the eye, strike the retina at the back of the eye and are then changed into electrical impulses which are transmitted to the brain.
The image that our eyes see is two dimensional (flat) it is also upside down,
So although we don’t feel like seeing involves much effort, actually a huge amount of work needs to be done by the brain in order for perception to take place.
In visual perception the brain is making sense of all the information that comes into the eyes.
Visual illusions are “figures which appear to be other than they really are”.
An illusion is a perceptual mistake. They occur when we misinterpret an image or have trouble seeing it for what it really is.
Illusions occur because our brains make guesses about what we are looking at. This is usually a very rapid process which happens without us noticing.
When we look at something our brains use the knowledge that they have stored over a lifetime to help us decide exactly what it is we are seeing.
The ability to use past learning greatly increases the power of the perceptual system and helps to explain why perception is usually so accurate and fast. Sometimes however we encounter unusual situations in which past learning is not appropriate and this leads to error.
If we see it from the outside it is obviously convex (projecting outwards). If we rotate it slowly so we are looking at the inside, the concave inside will suddenly spring out so it looks convex again. This is because we know faces are convex not concave. So we perceive the image as it should be seen not as it actually is.
Because of the use of past experience we often see what we expect to see, even if this is wrong.
In this picture the word a is written twice, but most people don’t notice because in our previous experience sentences usually make sense.
Types of illusions
Some illusions share certain themes and they can be roughly grouped into four major categories. (although some can fall into more than one category)
An ambiguous figure is a figure that can be interpreted in more than one way, so the brain is not sure what to make of it.
Because there are two or more likely explanations of the image, when the brain tries to guess what it is seeing, it cannot decide which guess is best.
Which of your illusions are ambiguous figures?
The Necker Cube : There is uncertainty as to whether the marked face is at the front or the back. If we stare at it for a while it will flip from one perspective to the other. This is because our brains first try one possibility find it works and then the other and fin it works as well and so cannot decide which alternative is best. The Old/young woman and the duck/rabbit : Again there are two equally sensible options to see so the brain cannot decide.
We use information about distance to help us decide how big or small something is.
Sometimes this information can lead us to make the wrong decisions, but our minds are so set on these rules that they find it hard not to use them, even when you know the reality.
They are both exactly the same length but the information around the lines makes the brain think otherwise. Again they are both the same size, but we see them as different because of the lines around the monsters giving us information about distance. Which monster is bigger? Which line is longer?
The Ames Room
The Ames Room
Fictions The Kanizsa triangle : Looking at this picture we have the impression that there are two triangles, one that is drawn with a black line and one that is white. The white one appears on top of the line one. There appear to be three circles covered partially by the corners of the white triangle. But there actually no triangle or circles! We create the triangles in our minds. They are not really there. They are fictional. This is because our brain decides that it is more likely there are triangles than these strangely arranged shapes, so that is what we see. These illusions cause us to see things that are not true.
Paradox A paradox is something that may appear ridiculous but is actually true. These illusions are also called impossible figures because they are impossible. If you tried to make these into real (3D) objects you would not be able to. For example this drawing by Escher shows an impossible building look carefully at the water!
The impossible Trident : How many prongs does the trident (fork) have? This trident does not work, it would be impossible to create in 3D. The Penrose Triangle : The same is the case for this triangle, it can be seen on paper but cannot be made into a real object. L’Egistential Elephant : This elephant would have problems walking!!!
Some other illusions just for fun!!!! All these squares are straight edged!!!
The Hermann Grid Illusion
As we found out before the image that falls on our retina is two-dimensional but we perceive the world as three-dimensional. How?
This seems to rely on using certain cues (hints in the environment) which give us information on depth and distance.
There two main types of depth cue:
Binocular cues (dependent on using both eyes)
Monocular cues (Available to each eye acting independently)
Binocular depth cues
Binocular disparity : Our eyes are around 6-7cm apart
this means that each eye receives slightly different images.
Test this out………………..
Activity : Hold a pen straight out in front of you, and close one eye. Now close the other. Did it change positions?
Activity: Hold up two pens with their points facing each other, one in each hand, at nearly arm’s length. Try to touch the points together
First- with one eye closed
Secondly- with both eyes open
How important is binocular vision?
The brain uses the information about the differences in position of an object in the two images to give the impression of depth. The greater the difference the nearer the object is.
Binocular depth cues
Ocular convergence: The nearer an object the more the eyes turn inwards (converge) in order to see that object. Information passes from the eye muscles to the brain, telling the brain how much the eyes have turned helping us to perceive how close or far away an object is.
Activity: raise your finger out in front of you. Being it slowly towards your nose. What happens to your eyes?
How do we know that the person in the red circle is further away than the person in the blue circle?
Height in the visual field : Objects that are a long way away are seen as being closer to the horizon. Therefore if one object appears to be lower than the other, we perceive it as closer.
Linear perspective : One of the most powerful monocular cues. This is where parallel lines pointing directly away from us seem to get closer together as they recede into the distance.
Texture Gradient : Most objects have a texture and the texture becomes finer as it gets further away.
Relative size : If there is a range of similar objects, it is assumed that smaller ones are further away.
Shading : If you look at an object in a picture and there is no shadow then that object appears flat.
Motion Parallax: If we are moving, objects closer to us appear to move faster than objects further away. Example
Overlap (or imposition) : Where one object overlaps or hides part of another the complete object is closer.
As we move about, and things we look at move about, the information we receive is constantly changing.
But, what we see does not seem to change.
For example, our friends don't change colour when we go from a dark inside to a brighter outside do they? And objects don’t get bigger as we get closer to them.
This is because our perceptual abilities adjust to these changes.
There are 3 main types of constancies
Objects appear to stay the same size even though the size of the images they make in our eyes change as the object gets nearer or further away.
As people move away from us they make a smaller image in our eyes, yet we do not see them as shrinking, just getting further away. This is because your perception has made adjustments to the information.
E.g. In this picture we do not see the man further up the picture as smaller just further away. When we compare it with the second picture we see the size difference.
The perspective lines in the corridor indicate distance so we ‘scale up’ the image to compensate for the distance.
Imagine that you are looking at a closed door from your seat.
The door will appear rectangular.
Now open the door 45 degrees.
The door still appears the same shape as when you looked at it straight on. This is true even though the image of the door on your retina has changed a lot.
Walk from the window (where it is bright) to the other side of the room (where it is darker). Our clothes change colour, but we don’t really notice this.
The perceived colour of objects remains the same even when lighting conditions change.
We keep the colours constant.
Imagine if we saw things changing colours all the time?