We look at objects and try to judge distances to make necessary adjustments. Many of the objects have also their third dimension, which is to be perceived simultaneously with distance generally. To include the third dimension of objects we may name this area perception as depth perception, which means perception of distance of the object from the observer and that of its depth (third dimension). As we look at parked car we perceive it as a three dimensional object of a specific size and shape located at certain distance. It is not necessary for the object to be perceived in all the intermediate positions to be seen as moving. The eyes never follow a moving object with sufficient precision for keeping it in full view all the times. Perception is a process, as we know, that fills in the blanks.
How can the human retina - a curved two dimensional surface enable us to perceive a world of three dimensional objects? The wonderful manner in which this complicated process of perception takes place can be understood to an extent in the light of the following explanation. The retina is able to register images only in terms, left - right and up - down. Yet we have the perception as having an extra dimension of depth.
The Binocular Cues for Depth Perception
So far we have discussed the monocular cues of depth perception. One eyed people, it is said, under most of the conditions, have quite adequate depth perception. As most of us look at the world with both eyes we thus add simultaneously the binocular cues from depth perception. There are two outstanding binocular cues.
(i) Retinal Disparity:
Retinal disparity is the difference in the images falling on the retinas of the two eyes. The geometrical diagram of the situation can help explain the cue when the eyes view an object. The fovea in centre of the retina is much more sensitive than the rest. While Looking at an object, we fixate our eyes; point them in a manner of speaking so that the image of the object falls mostly on each fovea. But since the two eyes are at a distance, of 65 mm from each other, separately situated, they get slightly different view of the object. The two images will, obviously, be not exactly the similar to each other. The correspondence between distance and the amount of disparity is the reason why retinal disparity can be used as a cue for depth perception.
This is a cue from the sensory receptors in the muscles that turn and point the two eyes together. For objects of farther away than about 20 meters, the lines of sight of the two eyes are essentially parallel. For nearer and nearer objects however, the eyes turn more and more toward each other. That is to say, they converge. The greater the convergence, the greater the tension in the muscles turning the eyes. This indicates that there is a relationship between muscle tension and the distance of an object. The sensory input from the tension receptors in the muscles may give us a cue for depth perception. There is also a controversy over the role of these sensory inputs is a cue to depth.