The Perception of Distance, Depth
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.
(ii) Convergence:
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.
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