Chapter 6

Question 1

realism

Answer 1

A philosophical position arguing that there is a real world to
sense.

Question 2

positivism

Answer 2

A philosophical position arguing that all we really have to go on is the evidence of the senses, so the world might be nothing more than an elaborate hallucination.

Question 3

binocular

Answer 3

Referring to two eyes.

Question 4

probability summation

Answer 4

The increased detection probability based on the statistical advantage of having two (or more) detectors rather than one.

Question 5

binocular summation

Answer 5

The combination (or “summation”) of signals from both eyes in ways that make perfor- mance on many tasks better than with either eye alone.

Question 6

binocular disparity

Answer 6

The differences between the two retinal images of the same scene. Disparity is the basis for stereopsis, a vivid perception of the three-dimensionality of the world that is not available with monocular vision.

Question 7

stereopsis

Answer 7

The ability to use binocular disparity as a cue to depth.

Question 8

monocular depth cue

Answer 8

A depth cue that is available even when the world is viewed with one eye alone.

Question 9

binocular depth cue

Answer 9

A depth cue that relies on information from both eyes. Stereopsis is the primary example in humans, but convergence and the ability of two eyes to see more of an object than one eye sees are also bin- ocular depth cues.

Question 10

pictorial depth cue

Answer 10

A cue to distance or depth used by artists to depict three-dimensional depth in two-dimensional pictures.

Question 11

occlusion

Answer 11

A cue to relative depth order in which, for example, one object obstructs the view of part of another object.

Question 12

nonmetrical depth cue

Answer 12

A depth cue that provides information about the depth order (relative depth) but not
depth magnitude (e.g., his nose is in front of his face).

Question 13

metrical depth cue

Answer 13

A depth cue that provides quantitative information about distance in the third dimension.

Question 14

projective geometry

Answer 14

For purposes of studying perception of the three- dimensional world, the geometry that describes the transformations that occur when the three-dimensional world is projected onto a two-dimensional surface. For example, parallel lines do not converge in the real world, but they do in the two-dimensional projection of that world.

Question 15

relative size

Answer 15

A comparison of size between items without knowing the absolute size of either one.

Question 16

texture gradient

Answer 16

A depth cue based on the geometric fact that items of the same size form smaller images when they are farther away. An array of items that change in size smoothly across the image will appear to form a surface tilted in depth.

Question 17

relative height

Answer 17

As a depth cue, the observation that objects at different distances from the viewer on the ground plane will form images at different heights in the retinal image. Objects farther away will be seen as higher in the image.

Question 18

familiar size

Answer 18

A depth cue based on knowledge of the typical sizes of objects, such as humans or pennies.

Question 19

relative metrical depth cue

Answer 19

A depth cue that could specify, for example, that object A is twice as far away as object B without providing information about the absolute distance to either A or B.

Question 20

absolute metrical depth cue

Answer 20

A depth cue that provides quantifiable information about distance in the third dimension (e.g., his nose sticks out 4 centimeters in front of his face).

Question 21

haze or aerial perspective

Answer 21

A depth cue based on the implicit understanding that light is scattered by the atmosphere. More light is scattered when we look through more atmosphere. Thus, more distant objects are subject to more scatter and appear fainter, bluer, and less distinct.

Question 22

linear perspective

Answer 22

A depth cue based on the fact that lines that are parallel in the three-dimensional world will appear to converge in a two-dimensional image.

Question 23

vanishing point

Answer 23

The apparent point at which parallel lines receding in depth converge.

Question 24

anamorphosis or anamorphic projection

Answer 24

Use of the rules of linear
perspective to create a two-dimensional image so distorted that it looks correct only when viewed from a special angle or with a mirror that counters the distortion.

Question 25

motion parallax

Answer 25

An important depth cue that is based on head movement.
The geometric information obtained from an eye in two different positions at two different times is similar to the information from two eyes in different positions in the head at the same time.

Question 26

optic flow

Answer 26

The pattern of apparent motion of objects in a visual scene produced by the relative motion between the observer and the scene.

Question 27

accommodation

Answer 27

The process by which the eye changes its focus (in which the lens gets fatter as gaze is directed toward nearer objects).

Question 28

convergence

Answer 28

The ability of the two eyes to turn inward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typically on the fovea of each eye). Convergence reduces the disparity of that feature to zero (or nearly zero).

Question 29

divergence

Answer 29

The ability of the two eyes to turn outward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typically on the fovea of each eye). Divergence reduces the disparity of that feature to zero (or nearly zero).

Question 30

corresponding retinal points

Answer 30

Two monocular images of an object in the world are said to fall on corresponding points if those points are the same distance from the fovea in both eyes. The two foveas are also corresponding points.

Question 31

Vieth-Müller circle

Answer 31

The location of objects whose images fall on geometrically corresponding points in the two retinas. If life were simple, this circle would be the horopter, but life is not simple.

Question 32

horopter

Answer 32

The location of objects whose images lie on corresponding points. The surface of zero disparity.

Question 33

diplopia

Answer 33

Double vision. If visible in both eyes, stimuli falling outside of Panum’s fusional area will appear diplopic.

Question 34

Panum’s fusional area

Answer 34

The region of space, in front of and behind the horopter, within which binocular single vision is possible.

Question 35

crossed disparity

Answer 35

The sign of disparity created by objects in front of the plane of fixation (the horopter). The term crossed is used because images of objects located in front of the horop- ter appear to be displaced to the left
in the right eye and to the right in the left eye.

Question 36

uncrossed disparity

Answer 36

The sign of disparity created by objects behind the plane of fixation (the horopter). The term uncrossed is used because images of objects located behind the horopter will appear to be displaced to the right in the right eye and to the left in the left eye.

Question 37

stereoscope

Answer 37

A device for simultaneously presenting one image to one eye and another image to the other eye. Stereoscopes can be used to present dichoptic stimuli for stereopsis and binocular rivalry.

Question 38

free fusion

Answer 38

The technique of converging (crossing) or diverging the eyes in order to view a stereogram without a
stereoscope.

Question 39

stereoblindness

Answer 39

An inability to make use of binocular disparity as a depth cue. This term is typically used to describe individuals with vision in both eyes. Someone who has lost one (or both) eyes is not typically described as “stereoblind.”

Question 40

random dot stereogram (RDS)

Answer 40

A stereogram made of a large number (often in the thousands) of randomly placed dots. Random dot stereograms contain no monocular cues to depth. Stimuli visible stereoscopically in random dot stereograms are Cyclopean stimuli.

Question 41

Cyclopean

Answer 41

Referring to stimuli that are defined by binocular disparity alone. Named after the one-eyed Cyclops of Homer’s Odyssey.

Question 42

correspondence problem

Answer 42

In reference to binocular vision, the problem of figuring out which bit of the image in the left eye should be matched with which bit in the right eye. The problem is particularly vexing when the images consist of thousands of similar features, like dots in random dot stereograms.

Question 43

uniqueness constraint

Answer 43

In reference to stereopsis, the observation that a feature in the world is represented exactly once in each retinal image. This constraint simplifies the correspondence problem.

Question 44

continuity constraint

Answer 44

In reference to stereopsis, the observation that, except at the edges of objects, neighboring points in the world lie at similar distances from the viewer. This is one of several constraints that have been proposed as helpful in solving the correspondence problem.

Question 45

Bayesian approach

Answer 45

A way of formalizing the idea that our perception is
a combination of the current stimulus and our knowledge about the conditions of the world—what is and is not likely to occur. The Bayesian approach is stated mathematically as Bayes’ the- orem—P(A|O) = P(A) × P(O|A)/P(O). This theorem enables us to calculate the probability (P) that the world is in a particular state (A) given a particular observation (O).

Question 46

binocular rivalry

Answer 46

The competition between the two eyes for control of visual perception, which is evident when completely different stimuli are presented to the two eyes

Question 47

stereoacuity

Answer 47

A measure of the smallest binocular disparity that can generate a sensation of depth.

Question 48

dichoptic

Answer 48

Referring to the presentation of two different stimuli, one to each eye. Different from binocular presentation, which could involve both eyes looking at a single stimulus.

Question 49

critical period

Answer 49

A period of time during development when the organism is particularly susceptible to developmental change. There are critical periods in the development of binocular vision, human language, and so on.

Question 50

strabismus

Answer 50

A misalignment of the two eyes such that a single object in space is imaged on the fovea of one eye and on a nonfoveal area of the other (turned) eye.

Question 51

esotropia

Answer 51

Strabismus in which one eye deviates inward.

Question 52

exotropia

Answer 52

Strabismus in which one eye deviates outward.

Question 53

tilt aftereffect

Answer 53

The perceptual illusion of tilt, produced by adaptation to a pattern of a given orientation.

Question 54

suppression

Answer 54

In reference to vision, the inhibition of an unwanted image. Suppression occurs frequently in people with strabismus.