Deciphering Object Location: A Visual Perception Process

Categories: Light Perception Science Scientific method Vision

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Introduction

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 famous Ames room

The picture is of the famous Ames room.

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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.

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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.

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.

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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

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.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 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!!