Motion parallax is a monocular cue for relative depth perception and plays a very important role in successful navigation (Rogers and Graham 1979; Ono and Wade 2005). It is also an essential contributor to three-dimensional vision. When objects are stationed at different distances from the moving observer, objects closer to the eye appear to move faster than objects that are further away. In other words, motion parallax refers to the variance in image motion between objects at different depths. One of the reasons for this perception is that, the further away we look, the wider the visual field. Closer objects have to travel a shorter distance before they disappear from the visual field. Objects that are far away have longer distances to travel before they disappear from the visual field. Hence, the variance in the perception that objects that are further away are moving slower and the ones that are closer as moving faster.

The other important cues needed for depth perception include pictorial depth cues and binocular disparity cues (Howard and Rogers 1995; Howard 2002; Julesz 1964, 1971). All the cues together create a precise understanding of relative depth between objects and help us plan our movements in the world (Foster et al. 2011). They aid in basic functions like driving a car, art/photography, etc. Some animals use motion parallax more in order to compensate for the absence of other depth perception cues like binocular disparity (Kral 2003). The neural mechanisms of motion parallax are not very well understood, but the functional importance was known for centuries. (Wheatstone who developed stereoscope in 1839 and Phillips de La Hire who was a French artist and mathematician in 1694.) Recently, motion parallax of images has been popular in imaging applications and video communications. The concept of motion parallax has also been applied to virtual reality and most recently to laproscopic surgery (Su et al. 2016).

We have a better understanding of neural mechanisms in animals as opposed to humans. Nadler et al. (2008) showed that the middle temporal lobe in Macque monkeys could be a potential neural substrate for computing depth from motion parallax. Depth perception from motion parallax also depends on the relationship between retinal motion velocity and viewing distance (Ono and Ujike 1994; Ono et al. 1986). Motion parallax creates a relative image motion on the retina, and it can be ambiguous without the extraretinal inputs. The extraretinal smooth pursuit eye movements provide the necessary inputs for depth perception with motion parallax (Nawrot 2003; Holmin et al. 2015).

This combination of image motion processing and the pursuit eye movements are affected by age and are poorly understood. Though some studies show that age did not have much of an effect on motion parallax (Norman et al. 2009), some other studies found that aging affects MP depth thresholds (Holmin and Nawrot 2016). When tested, older adults seemed to have less accurate pursuit eye movements than younger adults. As our aging population increases significantly, it is important to better understand the functional deficits resulting from the changes in depth perception and focus on how the perception of depth from MP in older adults could be improved.

The common disorders that affect depth perception include ocular conditions such as amblyopia, optic nerve hypoplasia, and strabismus, but little is known about the effect of these disorders in motion parallax. Further research is required to understand the neural mechanism of motion parallax and how various neurological and psychiatric disorders affect it.