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.