The depth cues are signals given from a visual stimulus (secondary cues), or from our own visual system (primary cues). These cues help us to perceive distance and depth, and can be either monocular (requiring only one eye) or binocular (requiring the use of both eyes).

             The perceptual constancies are visual stimuli which are put into three categories - brightness constancy, size constancy and shape constancy, which are thought to be innate, or learned while still very young. Perceptual constancy is the ability to understand that an object remains constant although changes occur in the retinal image.

             Brightness constancy occurs when an object is perceived to maintain a constant brightness, despite changes in lighting being present. .

             Size constancy occurs when, although the retinal image of an object may change, we still perceive it to be the same size. The brain recognises that although the retinal image is changing, the size of the object remains constant. An example is a train moving away from a station. The retinal image gets smaller as the train moves away, however we realise that the train is not actually shrinking. Instead, the brain uses size constancy and we understand that the train is moving away from us.

             Shape constancy involves recognising that although the image being cast on the retina changes, the shape of the object remains the same. When an object is viewed from different perspectives, the angle of the retinal image changes, however shape constancy compensates for that and we understand that the object remains constant.

             An example is a coin being flipped. As it spins, the retinal image suggests that the shape is changing because it changes from a circle to a half moon to a line, the automatic process of shape constancy enables us to understand that the coin remains the same shape.

             Although every person, given normal vision will sense the same image on their retina when seeing the coin flipping, their perception of the image may differ.