Psychology2016

Sensation and Perception 125

BINOCULAR CUES As the name suggests, these cues require the use of two eyes.

1. Convergence: Another muscular cue, convergence, refers to the rotation of the
   two eyes in their sockets to focus on a single object. If the object is close, the con-
   vergence is pretty great (almost as great as crossing the eyes). If the object is far,
   the convergence is much less. Hold your finger up in front of your nose, and then
   move it away and back again. That feeling you get in the muscles of your eyes is
   convergence. (See Figure 3.22, left.)


2. Binocular disparity: Binocular disparity is a scientific way of saying that because
   the eyes are a few inches apart, they don’t see exactly the same image. The brain
   interprets the images on the retina to determine distance from the eyes. If the two
   images are very different, the object must be pretty close. If they are almost identi-
   cal, the object is far enough away to make the retinal disparity very small. You can
   demonstrate this cue for yourself by holding an object in front of your nose. Close
   one eye, note where the object is, and then open that eye and close the other. There
   should be quite a difference in views. But if you do the same thing with an object
   that is across the room, the image doesn’t seem to “jump” or move nearly as much,
   if at all. (See Figure 3.22, right.)
   In spite of all the cues for perception that exist, even the most sophisticated
   perceiver can still fail to perceive the world as it actually is, as the next section
   demonstrates.

Perceptual Illusions

3.16 Identify some common visual illusions and the factors that influence our
perception of them.

You’ve mentioned the word illusion several times. Exactly what

are illusions, and why is it so easy to be fooled by them?

An illusion is a perception that does not correspond to reality: People think they see some-
thing when the reality is quite different. Another way of thinking of illusions is as visual

Figure 3.22 Binocular Cues to Depth Perception
(Left) Convergence is a depth cue that involves the muscles of the eyes. When objects are far away, the eye muscles are more relaxed; when objects
are close, the eye muscles move together, or converge. (Right) Binocular disparity. Because your eyes are separated by several centimeters, each eye
sees a slightly different image of the object in front of you. In A, the object is far enough away that the difference is small. In B, while the object is closer,
there is a greater difference between what each eye sees. The brain interprets this difference as the distance of the object.

Convergence

A

B > AB > A

B

Binocular disparity

B A

Interactive

convergence

Dinocular deRth RerceRtion cue the

rotation of the two eyes in their sock-

ets to focus on a single oDLect result-

ing in greater convergence for closer

oDLects and lesser convergence if

oDLects are distant.

binocular disparity

Dinocular deRth RerceRtion cue the

difference in images Detween the

two eyes which is greater for oDLects

that are close and smaller for distant

oDLects.