Chapter 6 Flashcards
realism
A philosophical position arguing that there is a real world to
sense.
positivism
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.
binocular
Referring to two eyes.
probability summation
The increased detection probability based on the statistical advantage of having two (or more) detectors rather than one.
binocular summation
The combination (or “summation”) of signals from both eyes in ways that make perfor- mance on many tasks better than with either eye alone.
binocular disparity
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.
stereopsis
The ability to use binocular disparity as a cue to depth.
monocular depth cue
A depth cue that is available even when the world is viewed with one eye alone.
binocular depth cue
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.
pictorial depth cue
A cue to distance or depth used by artists to depict three-dimensional depth in two-dimensional pictures.
occlusion
A cue to relative depth order in which, for example, one object obstructs the view of part of another object.
nonmetrical depth cue
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).
metrical depth cue
A depth cue that provides quantitative information about distance in the third dimension.
projective geometry
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.
relative size
A comparison of size between items without knowing the absolute size of either one.
texture gradient
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.
relative height
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.
familiar size
A depth cue based on knowledge of the typical sizes of objects, such as humans or pennies.
relative metrical depth cue
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.
absolute metrical depth cue
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).
haze or aerial perspective
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.
linear perspective
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.
vanishing point
The apparent point at which parallel lines receding in depth converge.
anamorphosis or anamorphic projection
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.
motion parallax
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.
optic flow
The pattern of apparent motion of objects in a visual scene produced by the relative motion between the observer and the scene.
accommodation
The process by which the eye changes its focus (in which the lens gets fatter as gaze is directed toward nearer objects).
convergence
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).
divergence
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).
corresponding retinal points
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.
Vieth-Müller circle
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.
horopter
The location of objects whose images lie on corresponding points. The surface of zero disparity.
diplopia
Double vision. If visible in both eyes, stimuli falling outside of Panum’s fusional area will appear diplopic.
Panum’s fusional area
The region of space, in front of and behind the horopter, within which binocular single vision is possible.
crossed disparity
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.
uncrossed disparity
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.
stereoscope
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.
free fusion
The technique of converging (crossing) or diverging the eyes in order to view a stereogram without a
stereoscope.
stereoblindness
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.”
random dot stereogram (RDS)
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.
Cyclopean
Referring to stimuli that are defined by binocular disparity alone. Named after the one-eyed Cyclops of Homer’s Odyssey.
correspondence problem
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.
uniqueness constraint
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.
continuity constraint
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.
Bayesian approach
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).
binocular rivalry
The competition between the two eyes for control of visual perception, which is evident when completely different stimuli are presented to the two eyes
stereoacuity
A measure of the smallest binocular disparity that can generate a sensation of depth.
dichoptic
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.
critical period
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.
strabismus
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.
esotropia
Strabismus in which one eye deviates inward.
exotropia
Strabismus in which one eye deviates outward.
tilt aftereffect
The perceptual illusion of tilt, produced by adaptation to a pattern of a given orientation.
suppression
In reference to vision, the inhibition of an unwanted image. Suppression occurs frequently in people with strabismus.
