Page 167 - Foundations of Cognitive Psychology : Core Readings

Perception 171

Figure 7.30
Examples of texture as a depth cue. The wheat field is a natural example of the way texture is used
as a depth cue. Notice the way wheat slants. The geometric design uses the same principles.

shadows, and relative size, but they had been unable to depict realistic scenes
that showed objects at various depths.
Your visual system’s interpretation of converging lines gives rise to the Ponzo
illusion (also shown in figure 7.29). The upper line looks longer because you
interpret the converging sides according to linear perspective as parallel lines
receding into the distance. In this context, you interpret the upper line as
though it were farther away, so you see it as longer—a farther object would
have to be longer than a nearer one for both to produce retinal images of the
same size.
Texture gradients provide depth cues because the density of a texture becomes
greater as a surface recedes in depth. The wheat field in figure 7.30 is an example
of the way texture is used as a depth cue. You can think of this as another con-
sequence of the size/distance relation. In this case, the units that make up the
texture become smaller as they recede into the distance, and your visual system
interprets this diminishing grain as greater distance in three-dimensional space.
Gibson (1966, 1979) suggested that the relationship between texture and depth
is one of the invariants available in the perceptual environment.
By now, it should be clear that there are many sources of depth information.
Under normal viewing conditions, however, information from these sources
comes together in a single, coherent three-dimensional interpretation of the
environment. You experience depth, not the different cues to depth that existed
in the proximal stimulus. In other words, your visual system uses cues like
differential motion, interposition, and relative size automatically, without your
conscious awareness, to make the complex computations that give you a per-
ception of depth in the three-dimensional environment.

Perceptual Constancies
To help you discover another important property of visual perception, we are
going to ask you to play a bit with your textbook. Put your book down on a
table, then move your head closer to it so that it’s just a few inches away. Then