Perceptual Constancies: Size Constancy, Brightness Constancy, and Shape Constancy

Perceptual Constancies: Size Constancy, Brightness Constancy, and Shape Constancy

What is perceptual constancy?

Perceptual constancy is a top-down process allowing individuals to recognize an object as unchanging in size, brightness, and shape even though there are changing stimuli, such as angle, distance, and illumination. 

 Shape Constancy

Shape constancy allows for us to recognize the shape of familiar objects despite images that show otherwise. An example of this is looking at a circular clock at different angles. In this example as you stand 90° to the wall that is holding the clock, you notice that the clock is the shape of a circle. As you move to the left, however, the shape of the clock becomes more of an ellipse. The same shape then appears if you move to the right of 90°. Despite the change in shape of the clock that we see when we look at it at different angles, we are still able recognize that the clock is unchanging in shape due to shape constancy.

Here is the example where the clock's shape differs as you change direction but it still is circular when looked upon at 90°.

Size Constancy

Due to size constancy, we can recognize that an object's size remains constant even if the distance between us and the object varies. Distance plays a role in how we perceive size by giving us clues as to how large an object actually is. An example of size constancy can be seen while bowling. When bowling we know how large the bowling ball is when we go to pick it up. After sending the bowling ball down to the pins, however,  we see that the bowling ball's size seems to shrink. There is a size-distance relationship that we experience. The distance from our bodies to the ball allow us to recognize that the ball is truly not shrinking in size but rather its distance is increasing from our bodies making it seem that the ball is decreasing in size. Illusions can distort the distance from our perspective thereby creating an image where we visualize an object that has an unrealistic size compared to its actual size.

In the picture above, the apple seems larger than the man's head. Due to size constancy and the size-distance relationship, however, we are able to interpret that the apple is a normal-size apple and the man is an average-size man. 

Lightness Constancy

Light constancy is also known as brightness constancy and allows us to perceive an object with a constant lightness despite illumination nuances. Relative luminance is the amount of light an object reflects relative to its surroundings. By using relative luminance we are able to perceive the lightness of an object. In the picture to the left, we visualize the girls' skin color and eye color to be different from each other. The background, due to relative luminance, affects how we perceive the colors of the girls' skin and eyes. The darker background reflects less than the white background making the girl in the black background to appear to have a lighter skin color that the girl in the white background. The eye color is then affected by the color of the skin that we perceive. Since we see a lighter skin color of the girl in the black background, meaning her skin color reflects more light than the darker skin color, we visualize darker eyes on her compared to the girl with "darker" skin in the white background, when in reality, the two girls are identical in every way except for which background is presented behind them.

Motion Perception

Motion Perception is what enables us to perceive motion. The brain normally computes motion based on its assumption that shrinking objects are retreating and that enlarging objects are approaching. However there are exceptions, for example, large objects such as trains, appear to move more slowly than smaller objects, such as cars, moving at the same speed. Holiday lights create the allusion of movement using the phi phenomenon.When two adjacent stationary lights blink on and off in quick succession, the single light is perceived as moving back and forth between them. All in all, our brain constructs our perceptions.

Example.
 

Monocular Cues

Monocular cues are depth clues available to either eye alone.  Linear perspective and Interposition are the two common cues associated with monocular cues.

Linear perspective are parallel lines, such as railroad tracks, that appear to get closer together or converge with distance. Linear perspective can contribute to rail-crossing accidents by leading people to overestimate a train's distance.

Example.
















As you can see, the father you look in the distance, the closer the road seems to converge.

Interposition occurs when one object partially blocks our view of another, so we perceive it to be closer.

Example:

The green triangle appears to be closer, and the red triangle appears father away, but in reality they are both the same distance from you as measured between your eyes and the screen. This happens because the green triangle partially blocked our view of the red one.

Rules of Perceptual Organization (closure, continuity, similarity)

Closure

Our brains fill in empty gaps in order to complete a whole object. This usually helps us to construct reality, but sometimes it only confuses us.

These four objects, although unrelated, looks like they form the corners of a rectangle.

Continuity

Our brains take discontinued patterns and make us perceive them as smooth and continuous.

Our brains see these lines as a squiggly line and two or three straight lines when in reality it's two half circles, a quarter circle, and three straight lines. 

Similarity

Our brains group objects with similar features together.

Our brains group together the similar objects so we perceive the triangles as a pattern on a five sided die and the rectangles as a diamond shape.