Chapter 6 Study Questions
1. What is the idea behind positivism?
Answer: Positivism is a philosophical position arguing that all you really have to go on is the evidence of your senses, so the world might be nothing more than an elaborate hallucination.
2. What is the advantage of binocular summation?
Answer: The advantage of binocular summation is that detecting a stimulus can be done with two eyes, as opposed to just one, and so this yields more information about the stimulus.
3. Explain the difference between a monocular depth cue and a binocular depth cue.
Answer: A monocular depth cue is available when the world is viewed with only one eye. A binocular depth cue requires information from both eyes.
Monocular Cues to Three-Dimensional Space
4. Name three monocular depth cues.
Answer: Any three of the following: occlusion, relative size, familiar size, relative height, texture gradients, linear perspective, aerial perspective, motion parallax, accommodation, or convergence.
5. What is the difference between metrical and nonmetrical depth cues?
Answer: A metrical depth cue provides precise, quantitative information about distance from the observer, while a nonmetrical depth cue only provides information about relative depth ordering, not depth magnitude.
6. Explain what a texture gradient is.
Answer: A texture gradient is a depth cue based on the geometric fact that items of the same size form smaller images when they are farther away. Thus, an array of items that change in size across the image will appear to form a surface in depth. The texture gradient depth cue is a combination of the relative height and relative size depth cues.
7. What kind of information does aerial perspective provide about the stimulus?
Answer: Aerial perspective is a depth cue that is based on the implicit understanding that light is scattered by the atmosphere. More light is scattered when you look through more atmosphere. Thus, more distant objects are subject to more scatter and appear fainter, bluer, and less distinct. Aerial perspective provides information about the relative distance of objects from the observer.
8. What is a vanishing point?
Answer: A vanishing point is the apparent point at which parallel lines receding in depth converge. Vanishing points are often used in paintings and drawings to add a sense of realism and depth.
9. What is a pictorial depth cue?
Answer: A pictorial depth cue is a cue to distance or depth used by artists to depict three-dimensional depth in two-dimensional pictures.
10. What kind of movement does motion parallax depend on? Explain.
Answer: Motion parallax depends on either object movement or head movement. During either type of motion, closer objects move faster across the visual field than farther objects, allowing one to determine the depth of objects relative to each other.
11. How are convergence and divergence important to depth perception?
Answer: Convergence refers to turning your eyes inward while divergence refers to turning your eyes outward. These two types of eye movements are important because they allow the observer 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). They both reduce the disparity of that feature to zero, or nearly zero.
Binocular Vision and Stereopsis
12. Explain the concept of corresponding retinal points.
Answer: Corresponding retinal points are points on the retina of each eye where the monocular retinal images of a single object are formed at the same distance from the fovea in each eye. The two foveas are also corresponding points.
13. What is the horopter?
Answer: The horopter is the set of points in the environment that have the same disparity as the object currently fixated (disparity 0).
14. What is the difference between crossed disparity and uncrossed disparity?
Answer: Crossed disparity is the sign of disparity created by objects in front of the plane of fixation (the horopter). Images of objects that are located in front of the horopter will appear to be displaced to the left in the right eye, and to the right in the left eye. Uncrossed disparity is the sign of disparity created by objects behind the plane of fixation. Images of objects that are located behind the horopter will appear to be displaced to the right in the right eye, and to the left in the left eye.
15. What is a stereoscope?
Answer: A stereoscope is a device for presenting one image to one eye and another image to the other eye. Once these two images are fused by the observer, they create a single three-dimensional image with a strong impression of depth.
16. When is free fusion used?
Answer: Free fusion is a technique of converging or diverging the eyes in order to view a stereogram without a stereoscope.
17. What does stereoblindness often result from?
Answer: Stereoblindness, or the inability to make use of binocular disparity as a depth cue often results from a childhood visual disorder such as a strabismus, in which the two eyes are misaligned.
18. What is a random dot stereogram?
Answer: A random dot stereogram is a stereogram made of a large number of randomly placed dots. The random dot stereogram contains no monocular cues to depth.
19. When does one view cyclopean stimuli?
Answer: One views cyclopean stimuli when looking at random dot stereograms. These are stimuli that are defined by binocular disparity alone.
20. Define the correspondence problem.
Answer: The correspondence problem is 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 correspondence problem must be solved by the visual system before stereo depth can be perceived.
21. Name two ways of solving the correspondence problem.
Answer: Any two of the following: 1) Blurring the image to remove high spatial frequencies, so that there are fewer dots to analyze; 2) using the uniqueness constraint in which a feature in the world is represented exactly once in each retinal image; or 3) using the continuity constraint which assumes that neighboring points in the world lie at similar distances from the viewer, except at the edges of objects.
Combining Depth Cues
22. In what sense does the Bayesian approach take into account past experience? Explain.
Answer: The Bayesian approach states that prior knowledge influences one’s estimates of the probability of a current event. In the case of vision, since the retinal images formed on the two retinas could be a result of an infinite number of scenes, this approach helps to narrow down the possible choices to the ones that are the most likely, based on past experiences.
23. Why do we sometimes see optical illusions?
Answer: The images that fall on our retinas are ambiguous since there are a number of spatial layouts that could give rise to the same images. Therefore, our visual systems assess the depth cues available and make a guess as to the true spatial layout. In some cases, the guesses our visual systems make are wrong, leading to optical illusions.
24. What is binocular rivalry?
Answer: Binocular rivalry is the competition between the two eyes for control of visual perception, which is evident when completely different stimuli are presented to the two eyes.
Development of Binocular Vision and Stereopsis
25. What is stereoacuity?
Answer: Stereoacuity is a measure of how accurate an observer’s stereo vision is, defined as the smallest binocular disparity that the observer can detect.
26. What is a critical period and why is it important for stereo vision?
Answer: A critical period is a duration of time during development in which an organism is particularly susceptible to developmental change. In terms of stereo vision, if an observer does not receive the correct stimulation during their critical period they will not be able to see in stereo later in life. Disorders such as strabismus (a misalignment of the two eyes) can cause a lack of the correct stimulation during the critical period and lead to stereoblindness.
27. What is strabismus?
Answer: Strabismus refers to the misalignment of the two eyes such that a single object in space is imaged on the fovea of one eye and a nonfoveal area of the other (turned) eye.
28. When does suppression occur in vision?
Answer: Suppression is the inhibition of an unwanted image and typically occurs in people with strabismus. In those cases, the turned eye which has an object projecting to a nonfoveal area is suppressed so as not to interfere with the other eye in which the object of interest falls on the fovea.