Humans and animals tend to perceive familiar objects as having a constant size, shape, and color regardless of their distance, placement, and ambiance. In other words, perceptual transformations of an object don’t influence its identity. This phenomenon is known as perceptual constancy, object constancy, or constancy phenomenon.

The retinal image of different objects changes when an object or observer moves. However, an observer’s perception of an object remains the same despite alterations in the stimulus.

For example, when we move a plate close to or away from an observer, they’re still able to perceive the plate as the same, despite changes in distance, retinal image, and the angle of perspective. Differences in location, orientation, and the ambiance of the plate don’t snatch away your ability to recognize that it’s the same plate. It happened due to the phenomenon of perceptual constancy.

Let’s take another example to understand the topic a bit more. When a bus is approaching a stop, it produces a much smaller retinal image. The image becomes larger and larger as the bus draws nearer the stand. But thanks to the phenomenon of perceptual constancy, you don’t conclude that the bus has grown in size. Instead, you’re fully aware that it’s the same bus and of the same size.

Artists are trained to understand the way we perceive the world. Due to the training, they’re able to draw visual illusions on the canvas. These visual illusions include distance, size, shape, and perspective. For instance, to draw a far-off object, an artist draws the object higher and smaller on the canvas. On the other hand, to create an illusion that an object is not far-off, an artist draws the object larger and up close. It’s because adults are easily able to make sense of their environment and understand visual illusions.

Size Constancy and Shape Constancy in Infants

There’s a heated debate about whether infants see real objects or retinal images of objects. For example, it’s not clear whether an infant perceives an object, say a toy, as the same when they see it from two different locations. We don’t know for sure whether the infant would recognize objective characteristics of the toy and perceive it to be the same toy, or would the infant go for two different-sized images and perceive the toy as two different toys. If the infant perceives two different-sized images as the same toy, we say that infants have size constancy. If it’s the other way around, we conclude that infants lack size constancy.

Besides size constancy, there’s a notion of shape constancy. It’s the ability to perceive an object to be the same despite changes in its orientation or the angle of perspective. For instance, if an infant sees an object from the front, and then sees it from an angle of 45°, would the infant perceive the object as being the same? If they did, we would say that infants have shape constancy. Or would the infant respond to retinal images of the same object and treat the object as two different objects.

At first, researchers thought that infants don’t get size and shape constancy until they’re a few months of age. But in 1966, Bower conducted a series of experiments to show that infants have both size and shape constancy. Since Bower, several other researchers have shown that infants can make sense of their environment just like adults and don’t respond to retinal images of the same object. Some researchers have also found evidence of shape constancy in newborns.

Perceptual Constancy – Final Thoughts

From the discussion so far, we can conclude that infants don’t rely solely on retinal images to make sense of their environment and understand size and shape constancy. The question is, how do infants learn to correlate retinal images of objects with their actual shape and size? According to some researchers, the answer to this question lies in appreciating relational information.

For example, a book appears brighter when placed outdoors. But it’s not as bright relative to other outdoor objects. Similarly, a distant object doesn’t appear smaller when we appreciate the object’s relative distance. A close-up object can be two times as large, but at the same time, it’s two times as close as a distant object.