Understanding Psychology (10th Ed)

depth perception
The ability to view 
the world in three dimensions and to 
perceive distance.
Railroad tracks that seem to join 
together in the distance are an example 
of linear perspective.
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130 Chapter 
4 
Sensation and Perception
is, the more quickly it will appear to move. In contrast, objects beyond the tree will 
seem to move at a slower speed, but in the same direction as you are. Your brain is 
able to use these cues to calculate the relative distances of the tree and other objects. 
Similarly, experience has taught us that if two objects are the same size, the one 
that makes a smaller image on the retina is farther away than is the one that provides 
a larger image—an example of the monocular cue of relative size . But it’s not just size 
of an object that provides information about distance; the quality of the image on 
the retina helps us judge distance. The monocular cue of texture gradient provides 
information about distance, because the details of things that are far away are less 
distinct (Proffi tt, 2006). 
Finally, anyone who has ever seen railroad tracks that seem to come together in 
the distance knows that distant objects appear to be closer together than are nearer 
ones, a phenomenon called linear perspective. People use linear perspective as a mon-
ocular cue in estimating distance, allowing the two-dimensional image on the retina 
to record the three-dimensional world (Bruce, Green, & Georgeson, 1997; Bruggeman, 
Yonas, & Konczak, 2007; Dobbins et al., 1998; Shimono & Wade, 2002).
Perceptual Constancy
Consider what happens as you fi nish a conversation with a friend and he begins to 
walk away from you. As you watch him walk down the street, the image on your 
retina becomes smaller and smaller. Do you wonder why he is shrinking? 
Of course not. Despite the very real change in the size of the retinal image, 
because of perceptual constancy you factor into your thinking the knowledge that 
your friend is moving farther away from you.