Depth perception is important to primates. All arboreal monkeys and apes need to be able to judge distance very precisely. For this reason we have two eyes in the front of our heads, offset by about 8cm. This permits the use of parallax (different views from each eye) and kinesthetic afferent input from ocular muscles used in ocular convergence as distance cues. But even with one eye closed, there are a number of stimulus cues which contribute to depth perception. These include those listed above. Height in plane refers to the horizon being at eye level, and near objects varying heights above or below the horizon line according to their relative proximity to the viewer. Texture gradient refers to more detailed textures on closer objects than more distant ones, while interposition refers to closer objects hiding more distant objects behind them.

Most of these monocular cues are learned. For example, people who live in an environment of predominantly “near” objects have problems judging the size of objects when in open country, where a lack of familiar size cues gives no indication to distance.