Cognitive Science 126/ Psych 126
Section quiz – week of 3/20/00
1. Give an example that shows that our perceptual system uses size constancy when
perceiving objects.
Constancy is the tendency to perceive the unchanging properties of external objects
rather than the more transient properties of their projected retinal images. Size
constancy is the ability to correctly perceive the constant size of an object despite its
variable retinal size due to differences in distance.
The two monsters are actually the same size on the page, but if this were actually in
depth, then the top monster would be farther and actually larger (and thus a threat to
the smaller monster!). The perceived size is based on distance info + retinal size info.
An elephant at a distance seems large despite the fact that it may take up the same
amount of area on the retina as a small cat at a much closer distance.
2. Why might this size constancy be useful to us?
Size constancy is important because it allows us to perceive a constant property of a
external object at a distance. Imagine if the monster above only appeared to be
threateningly large when it was dangerously close. If you were estimating size based
only on retinal size then when it was far away (and you still had time to flee), you
would perceive it as small and it would only appear as a large threat when it came
upon you. Presumably this would be bad for you as an organism trying to survive!
3. Describe what the size-distance relation is (h = 2d * tan (a/2)) and how it relates to
Emmert’s Law.
The size-distance relation is a simple concept that relates the size of an image on the
retina to its distance on the retina. A good illustration of the formula in action can be
found in Figure 7.1.1. Basically, the calculation involves some trigonometry that
allows you to calculate the length of one side of a right triangle given an angle (1/2
the visual angle of the stimulus on the retina) and the distance from the observer to
the object.
Emmert's Law states that the perceived size of a constant retinal image is proportional
to its perceived distance (which may differ from its actual distance). The Moon
Illusion is a good example of how a retinal image of the same size can appear to be
different sizes based on differences in the perceived distance of the object. When the
moon is near the horizon it appears to be farther. If it were indeed farther than the
moon high in the sky, it would need to be larger to cast a retinal image of the same
size of the one high in the sky. In order to cast the same size retinal image at a farther
distance, the moon would have to be larger. You can work this out with the sizedistance relation.
4. Give an example of an illusion of a)shape and b)orientation:
Shape Illusions:
The Shepard Illusion - Figure 7.2.7
Ames Room - http://www.illusionworks.com/html/ames_room.html
Orientation Illusions:
Zllner Illusion - Figure 7.3.5
Café Wall Illusion - Interactive Demo http://www.illusionworks.com/html/cafe_wall.html
Tilted Room - Figure 7.3.3
5. Where do we get information about orientation with respect to gravity?
Information about orientation with respect to gravity primarily comes from the
proprioceptive system. The vestibular system is the main organ of balance. It is found
in the middle ear and is composed of the semicircular canals and the utricle and
saccule. These fluid-filled sacs contain heavy particles that influence the bending of
special cell hairs. The amount of bending of these hairs gives information about the
orientation of the head in the environment.
6. Distinguish between the afferent theory and the efferent copy theory of position
constancy during eye movements.
The afferent theory states that feedback information about the tension of eye muscles
is sent back to the visual system in order to provide information about the positions of
the eyes and eye movements. The efferent theory states that the visual system gets a
copy of the commands that are sent to the eye to perform eye movements. These
commands can be used to determine the positions and movements of the eyes.
How might one test these theories empirically?
7. What is visual capture?
Visual capture is a phenomenon in which vision dominates other sensory modalities
when they are in conflict about the properties of objects. This seems to give some
demonstration to how adaptive the whole system is to managing conflicting inputs.
8. What is global precedence? How does it seem to be related to cerebral hemispheres?
Describe how a person with left or right hemisphere damage might perceive the
following drawing:
Y
Y
EY
E
S E
S
Y S
Y
E Y E
S
E S
Y
S Y
E
YE
S
ES
YESYESYES
Y
Y
E
E
S
S
Y
Y
E
E
S
S
Y
Y
YESYESYES
Global precedence is the notion that in hierarchical patterns like those shown above,
the global percept (in this case "no") is perceived first. Because of this, it is predicted
that (1) Responses to the global configuration (does the global configuration show
"yes" or "no"?) should be faster than that to the local configuration. (2) There should
be global-to-local interference. That is, when asked to identify the local patterns,
there should be interference from the global pattern if it is inconsistent. (3) No localto-global interference.
Neuropsychological evidence has made some dissociation between the hemisphere
for global and local processing. Patients with right hemisphere damage seem to have
some difficulty with perceiving the global aspects of stimuli like the one above. If
asked to draw the above figures they would probably reproduce many of the "YES"
words. However, these "yes" words would probably not be organized into the global
"N" and "O". Left-hemisphere patients seem to have the opposite problem. Such
patients have a difficult time identifying the local information in the stimulus. They
may only be able to report the global pattern in the stimulus. Examples of drawing
made by the two types of patients can be found in Figure 7.6.11.