jected on your retina is about the same size when it is near the horizon and when
it’s “far” from it (when it’s overhead). As the Moon orbits our planet, its actual
distance from the Earth doesn’t change significantly. Consequently, the conditions
of the Ponzo illusion are met.
What we learn from illusions is that the world appears to us the way it does
not only because it actually is the way it is. We also interpret sensory information,
transforming it into a constructed perceptual, or psychological, world. And it is
our perception of the world that determines much of our behavior.
(a) The Moon illusion is a variation of the.
(b) An object five feet away may look the same size as an object ten feet away. This is an
example of.
Answers: (a) Ponzo illusion; (b) size constancy.
Depth Perception: Living in a Three-dimensional World
One of the fascinating questions of perception is this one: Why do we perceive a
world of rounded shapes, of near and far things, ofdepthinstead of a flat world
with one surface? A second, related question is: How is this accomplished?
A given eye’s retina is basically a surface, not a cube. (Although the eye itself
is a three-dimensional “ball,” the surfaceof the retina is not.) Think of the infor-
mation on the surface of the retina as having some similarity to an oil painting
made on a flat canvas. Note that it ispossible to perceive depth in a landscape
painting made on a flat canvas.
Depth perception is made possible by various cues,signals or stimuli that pro-
vide an observer with information. Depth perception is made possible by cues
arising from binocular vision and monocular vision.
Binocular visionis vision with two eyes. The principal cue for depth per-
ception associated with binocular vision is retinal disparity.The pupils of the
eyes are about three inches apart. This gives the right eye a somewhat different
view of a scene than the one obtained with the left eye. Notice that although you
sense two images, you only perceive one. (This is another example of the differ-
ence between sensation and perception.) This is sometimes called the zipper
functionof the brain, the capacity of the visual portion of the cortex to integrate
two images into a meaningful whole. The whole image, in part because of retinal
disparity, appears to be three-dimensional.
(a) Binocular vision is vision with.
(b) The principal cue for depth perception arising from binocular vision is.
Answers: (a) two eyes; (b) retinal disparity.
Perception: Why Do Things Look the Way They Do? 65
