Perceptual constancy is the ability to recognize that objects remain consistent and unchanged even when their appearance varies due to changes in sensory input. There are four main types of perceptual constancy: size constancy, shape constancy, color constancy, and brightness constancy.

Size constancy is the recognition that an object remains the same size, even when its image on the retina changes. For instance, a bus is perceived to be large enough to carry people, even if it looks tiny from a distance. This understanding allows for accurate perception of objects regardless of their distance from the observer.

Shape constancy refers to the ability to perceive an object as having a constant shape, even when its orientation changes. For example, a door is seen as a rectangle, whether it is open or closed, even though its appearance changes. This occurs because the brain rapidly learns to recognize different views of the same object, maintaining a stable perception of its shape.

Color constancy is the recognition that an object retains its color despite changes in lighting. For example, a green apple appears green whether viewed in the bright noon sunlight or under grocery store lighting. This perception is achieved through the brain's calculations of the light reflected by the object in comparison to surrounding objects. The brain effectively adjusts for the varying lighting conditions, allowing for consistent color perception.

Brightness constancy, or lightness constancy, is the ability to see an object's brightness as constant, even when illumination changes. This depends on relative luminance, which is the amount of light an object reflects compared to its surroundings. For instance, white paper reflects 90 percent of the light that falls on it, while black paper reflects only 10 percent. Despite these differences, both are perceived as having consistent brightness.

These types of perceptual constancy allow for a stable and consistent perception of the world, enabling accurate recognition and interaction with objects despite changes in sensory input.