Chapter 4 Summary
1. A series of extrastriate visual areas continue the work of visual processing. Emerging from V1 (primary visual cortex) are two broad streams of processing: one going into the temporal lobe and the other into the parietal lobe. The
pathway seems specifically concerned with what a stimulus might be. This chapter follows that pathway. (The
where pathway will be considered in later chapters.)
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2. After early visual processes extract basic features from the visual input, it is the job of
vision to organize these features into the regions, surfaces, and objects that can, in turn, serve as input to object recognition and scene-understanding processes.
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3. Perceptual “committees” serve as an important metaphor in this chapter. The idea is that many
processes are working on the input at the same time. Different processes may come to different conclusions about the presence of an edge or the relationship between two elements in the input. Under most circumstances, we see the single conclusion that the committees settle upon. Bayesian models are one way to formalize this process of finding the most likely explanation for input.
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4. Multiple processes seek to carve the input into regions and to define the
of those regions, and many rules are involved in this parsing of the image. For example, image elements are likely to group together if they are
in color or shape, if they are near each other, or if they are connected. Many of these grouping principles were first articulated by members of the
school.
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5. Other, related processes seek to determine whether a region is part of a foreground figure (like this black O) or part of the background (like the white area around the O). These rules of grouping and figure-ground assignment are driven by an implicit understanding of the
of the world. Thus, events that are very
to happen by chance (e.g., two contours parallel to each other) are taken to have meaning. (Those parallel contours are likely to be part of the same figure.)
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6. The processes that divide visual input into objects and background have to deal with many complexities. Among these are the fact that parts of objects may be hidden behind other objects (
) and the fact that objects themselves have a structure. Is your nose an object or a part of a larger whole? What about glasses or hair or a wig?
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7. In addition to perceiving the shape of objects and their parts, we are also very adept at categorizing the material that an object seems to be made of—glass, stone, cloth, and so on. We use material perception to estimate physical properties. What would it feel like? Can it be grasped like a bottle or would it slip through our fingers like sand?
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8.
models of object recognition hold that an object in the world is recognized when its image fits a particular representation in the brain in the way that a key fits a lock. It has always been hard to see how naïve template models could work, because of the astronomical number of templates required: we might need one “lock” for every object in every orientation in every position in the visual field.
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9.
models propose that objects are recognized by the relationship of parts. Thus, an H could be defined as two parallel lines with a horizontal line joining them between their centers. A cat would be more difficult, but similar in principle. In their pure form, such models are viewpoint
. The orientation of the H doesn’t matter. Object recognition, however, is often viewpoint-
, suggesting that the correct model lies between the extremes of naïve template matching and pure structural description.
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10. Faces are an interesting special case in which viewpoint is very important. Upright faces are much
to recognize than inverted faces. Moreover, some regions of the brain seem to be specifically interested in faces. They lie near regions in the
lobes that are important for the recognition of other sorts of objects.
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11. Recent physiological work showed very specific responses to very specific objects (e.g., the actress Jennifer Aniston) in the human temporal lobe. Other work showed that the first, rough acts of object recognition take place so fast that they must be accomplished by the first,
sweep of activity from the retina to the higher processing centers of the visual system. However, routine perception of objects requires
from higher visual areas to those lying earlier in the pathway.
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