Perceptual Constancy

Perceptual constancy is the ability of animals and humans to perceive a sensory stimulus as constant in terms of its size, shape, and color despite the observable changes in that stimulus.

For instance, while standing on a roadside, we see a bus coming near us. At first, when the bus is at a farther distance from us, it looks small to us. However, as it approaches near us, it seems to be getting big and bigger. This is because the image of an object is formed in different sizes on our retina depending on changing distance from the object (bus in this case). However, our brain perceives the size of the bus in its standard size. We do not tend to think that the bus was in reality smaller (in its size) when it was farther away from us; and not that it in reality becomes bigger in size as it gets nearer to us. Our brain perceives the bus in its standard size irrespective of observable changes in its size and shape due to distance from the bus or angle of looking at the bus.

Depending on the distance from an object, position, or angle of viewing an object, amount of light available at the place, our eyes may see the object slightly differently. In other words, the images of the object formed on the retina are slightly different based on distance, angle of viewing or amount of available light. However, due to our previous experience (or familiarity) with the object, our brain has its standard size, shape and color and thus, we perceive the image as constant in terms of its size, shape and color (in accordance with its preconceived standard size, shape and color) despite we see the object from far or near, or from any angle, or in any amount of light.

Perceptual constancy is an important ability because it helps us to perceive the world as a constant and organized world. Without perceptual constancy, we would perceive the world as a disorganized and confusing place, where the colors, shapes and sizes of objects are constantly changing without having any stability in them. Perceptual constancy makes the world an organized place.

   TYPES OF PERCEPTUAL CONSTANCY

There are three types of perceptual constancy: 1) color constancy, 2) shape constancy, and 3) size constancy, as explained below.

   Color constancy

Color constancy is the ability to perceive color of an object as constant despite the changing amount of available light. We see color of an object because when light falls on the surface of an object, it reflects back and form an image on the retina of our eyes. Light can have different wavelengths depending on its intensity. Due to changing intensity of light, differing wavelengths are reflected back which may slightly change the color of the object as in its image form on the retina of our eyes. For instance, we look at a yellow mango in daylight, we see it as bright yellow but if we look at it in the evening (in relatively dim light), we may see it as relatively dark yellow (near to red color). However, in both cases, we perceive it as a constant yellow though the changing amount of light may change the intensity of its color slightly. This is because of perceptual constancy.

Another example is the color of the walls in our room. In the daytime they look white, however, in the evening or nighttime (in the little amount of light), they may look a bit darker but still, we perceive them as white walls. The color constancy is also sometimes referred to as brightness constancy. Varying amount of brightness may slightly change the intensity of color of an object as in its image formed on our retina, but we still perceive it in its standard color. 

Thanks to the color constancy that the colors of objects are perceived constant and organized, even in the differing amount of illumination because it helps us not to get confused about this colorful world. 

   Shape constancy

Shape constancy refers to the phenomenon of perceiving the shape of an object as constant despite the changing angles to look at the object. When we look at an object from different angles, the images of the object formed on the retina are different. To understand this, take an example of looking at a round plate lying a table. When we look at it directly from above, it forms a perfectly circular image on our retina. If we change our position to look at it from one side, it forms a relatively elliptical (or oval) image on our retina. However, in both positions, we perceive the shape of the plate as circular due to the shape constancy.

Shape constancy applies to all objects with different shapes e.g., cube, cylindrical, triangular, rectangular, or other shapes with different dimensions.

   Size constancy

Size constancy refers to the perception of the size of an object as constant despite the object being viewed from a near or far distance. As noted earlier, the object lying near to us form a bigger image on our retina whereas the object lying at a far distance from us form a smaller image on our retina. However, we perceive it as constant in terms of its size (in accordance with its standard size). For instance, a person is looking at a tree from a farther distance. He perceives the tree as a normal tree rather than a small plant even though the tree may look like a small plant. It is because of size constancy.