Similarity

Have you ever noticed how you automatically group things that are similar together without even meaning to group them? Your brain does this because of a rule of perception called similarity. Similarity is the grouping of similar things together even if they are not exactly the same.  

An example of similarity is that you realize that all of the items in the picture below are books even though they are all varying in size and shape; your brain recognizes that they are similar and groups them as such.
 

Closure

Have you ever seen an image or object that is not complete but your brain automatically fills in the parts of the image that are missing and you just assume that those missing parts are there even though they aren't really? Your brain is programmed to perceive those missing parts as being there because of something called closure. Closure is the filling in of gaps present in what you are sensing, usually through vision and assuming that filling in those gaps will complete the object.

An example of closure would be if you were reading and one of the letters comprising a word was partially or completely omitted; your brain would automatically complete the letter to make the word readable.

Being able to read the words on the picture below is the skill of closure. Your brain subconciously fills in the missing letters to make sense of the phrase.

Figure-ground perception

Have you ever been in a crowd trying to find a friend or in a noisy room trying to listen for one person? Your ability to pick out that one face or one sound comes from something called figure-ground perception. The particular person you are trying to see or hear is the figure and the background distraction is the crowd.

An example of this that you would be able to relate to real life would be a "Where's Waldo?" You are searching for a specific figure, Waldo, in seas of people. 

Shape Constancy

Shape constancy states that objects regarded from different angles will produce different shapes on the retina. However, the viewer is able to recognize that the shape of the object in reality remains constant.  
For example, if one views a soda can from different angles, the angles will produce different shapes onto the observer’s retinas. If one looks at the can from the side, it possesses a cylindrical shape. However, if viewed from above the can, it will possess a circular shape. Through the integration of the different perspectives, the viewer is able to understand that the top of the cylinder is circular due to shape constancy.

Another example is demonstrated through the following photographs. In the first picture of Puck the dog, the viewer can see that he is wearing a pumpkin costume and has floppy ears, but cannot see his face. In the second picture, the viewer can see Puck’s face and a portion of the pumpkin costume, though not all of it. Even though the pictures are taken from different angles and produce different visions to the viewer’s eyes, he/she is able to identify that the shape of Puck remains consistent. 

Brightness Constancy

Brightness constancy is the concept of one perceiving objects as being a constant color even as the light reflecting off of the object changes. In other words, we perceive an object as having a constant lightness even while its illumination varies.

An example of this can be demonstrated through a brick wall. When one views the bricks during an hour of the day in which the sun directly descends on it, they appear to be the color red. However, when one views the same bricks several hours later when there is no longer any direct sunlight, he/she will still perceive the bricks as red even though their true color is then grey.

The pictures below were taken at two separate visits to the Coliseum. One picture was captured during the day, in which the landmark was exposed directly to sunlight, and one was taken at dusk, in which it was no longer receiving direct sunlight. When visiting the Coliseum on the two separate occasions, I perceived the colors to be the same. However, in accordance with the difference of illumination, its color is varied. 

Size Constancy

Size constancy is the concept that objects closer to our eyes will produce larger images on the human retina; however, it also states that humans will take distance into account with interpreting size. If one is familiar with the typical size of an object, he/she will consider the constant size and realize that it does not grow or shrink as it moves closer or farther away.

This concept can be demonstrated while standing on the top floor of a skyscraper. When looking down at the city street from this floor, the civilians and cars passing below look like tiny ants, so small that we could easily pick them up. In actuality, however, the viewer recognizes that he/she is observing from afar and objects did not literally shrink. The figures only appear smaller because they are being observed from a distance.  

In this photo example in which I am riding a donkey in Santorini, Greece, the donkey and I look much larger that the cruise ship in the background. However, this is only because the donkey and I were closer to the camera when the picture was taken. In actuality, the cruise ship could hold approximately 10,000 people.

Proximity

Do you ever find yourself sorting similar objects that are near each other into smaller groups? You do this because of something called proximity. It's easier to processes information in smaller groups rather than in one large one.

All the pens in the picture below are similar to one another and they are all lying next to each other but your brain organizes them as 3 groups of 3 rather than 9 pens because of proximity.

Continuity

Have you ever noticed how smooth and flowing your brain precieves patterns rather than choppy and disjointed? This is caused by something called continuity. Your brain would rather look at a pattern that is connected and joined as one because it is easier to follow rather than a pattern that is broken up into pieces.

Your brain would view the paper below as being one piece because of continuity. Even though there is space between the paper and they are all separate pieces of paper, your brain views them as a continuation of one another because that is easier for it to process.

Motion Perception and Phi Phenomenon

I am glad that we can perceive motion, or else life would be much more difficult! Though we are not perfect at perceiving motion, our brains can "see" motion because it concludes that enlarging objects are approaching us and shrinking objects are retreating. However, larger objects appear to be moving more slowly than smaller objects going at the same speed. This can sometimes be a problem. Also, our eyes do not see continual motion but instead fast still motion pictures, which our brain reconstructs to seem continuous (strobe).

An example of motion perception is staring at a moving object, such as a waterfall. After staring at a waterfall for a minute or two, look at a building or something else standing still and it should look like the object is moving upward. This happens because our brain has gotten used to constructing the motion, so the other object will appear to be in motion now too.

Another illusion dealing with motion perception is the phi phenomenon. The phi phenomenon is an illusion of movement created when two or more adjacent lights blink on and off in quick succession. This reinforces the idea that our brain reconstructs still images and makes it appear as if they are moving.

Monocular Cue: Texture Gradient

Another important monocular cue (depth cues available to either eye alone) is texture gradient. The closer an object is to us, the easier it is to see the texture/details of an object. When an object is further away, it is more indistinct and the texture is harder to see. Also, when objects are further away they seem more densely packed and smaller, which was shown with the previous monocular cue, relative size. Closer up, an object is more distinct and detailed. You can notice things up close that you could not see far away.

Texture gradient is easily shown in real life. Looking at a wall 30 feet away, it will look smooth and plain. However, by decreasing the distance between yourself and the wall, you start to notice the details on the wall such as the checkered texture or the extra paint splatter. When you are right in front of the wall, you can see things you could not notice even ten feet away! This increase is gradual as you get closer to the wall, not all of a sudden.

Another example of texture gradient is the picture below. Look at the picture and walk away from the picture. The further away you are, the harder it becomes to see the details. Close up, the picture looks almost 3-D, but by walking away it does not. The black is easier to see from far away, looking like a completely different image. If you walk far enough away, you can only see black and a little pink! This monocular cue allows us to see things in more detail the closer we are to the object, which is very helpful in every day life.

http://www.cmap.polytechnique.fr/~maureen/TextGrad.html

Monocular Cue: Relative Size

Unlike binocular cues, monocular cues are depth cues available to either eye alone. When we are judging how far away something is, binocular cues do not really help us. Monocular cues, on the other hand, can help us to determine how far away something actually is.

An important monocular cue is relative size. If two objects are fairly similar in size, our eyes perceive that the larger image is closer to us than the other image. Our brain interprets something smaller as something further away, not as a tiny object.

We have all experienced this, as we have stood on top of a skyscraper. Looking down from the top of the building, we can see people below. However, we know that these people are just further away from us, not that they are tinier people than we generally see on a daily basis. This is because of relative size, as we know that objects larger are generally closer to us than the objects that seem smaller.

In the two images below, the foot is the same size, but we are further away from the foot in the second example. So the foot looks smaller, but really it is the same size. It is just our distance from the foot that has increased.
 

Binocular Cue: Retinal Disparity

Depth cues that depend on the use of two eyes are called binocular cues. As generally known, two eyes are better than one, making the task easier with two eyes. This is the case for binocular cues. Our eyes see unique images, though each eye only sees something slightly different.

Retinal disparity is an important binocular cue, as it perceives depth, specifically the relative distance of objects. The brain processes the images our eyes see and computes the difference. The greater the perceived difference between the two objects, the closer the object is. This makes sense because from further away, both eyes are looking at the big picture (meaning that their images are more likely to be similar). The closer we are to the object, the more miniscule details each eye can pick up on.

There is an easy example to show retinal disparity and all you need is a pencil/ pen with a clip! Hold the pen about 18 inches in front of your face, with the clip barely showing on the right side. Close your left eye, and you should be able to see the clip still. Now change which eye is open, so your right eye is closed and your left eye is open. Is it harder to see the clip now? Move the pen closer to your face, and try the same thing again. The difference between the two images should be even greater now, and with your left eye open you might not see the clip at all! The first image is from 18 inches away with your left eye closed. The second image is from the same distance with your right eye closed. The next two images are left and right closed respectively, but the pencil is closer to our eyes and the difference is more noticeable.

 

Though it may just seem like a magic trick, your eyes are not fooling you. Retinal disparity is important to understand, so that you can fully understand binocular cues as well.