In October 2000, a man known by the initials JB went for a neurological assessment because he was having difficulty understanding the meaning of words, even though he still performed well on many other perceptual and cognitive tasks. Over the next few years, his color language deteriorated dramatically; he had great difficulty naming colors and could not even match objects with their typical colors (e.g., strawberry and red, banana and yellow). Yet he could still classify colors normally, sorting color patches into groups of green, yellow, red, and blue. JB retained an intact concept of colors despite the decline of his language ability—a finding that suggests that we need to look at factors in addition to language in order to understand concepts (Haslam et al., 2007).

Concept refers to a mental representation that groups or categorizes shared features of related objects, events, or other stimuli. The brain organizes our concepts about the world, classifying them into categories based on shared similarities. Our category for dog may be something like “small, four-footed animal with fur that wags its tail and barks.” Our category for chair may be something like “sturdy, flat-bottomed thing you can sit on.” We form these categories in large part by noticing similarities among objects and events that we experience in everyday life. Concepts are fundamental to our ability to think and make sense of the world.

What is your definition of dog? Can you come up with a rule of “dogship” that includes all dogs and excludes all nondogs? Most people can’t, but they still use the term dog intelligently, easily classifying objects as dogs or non-dogs. Three theories seek to explain how people perform these acts of categorization.

Family Resemblance Theory

Eleanor Rosch developed a theory of concepts based on family resemblance, that is, features that appear to be characteristic of category members but may not be possessed by every member (Rosch, 1973, 1975; Rosch & Mervis, 1975; Wittgenstein, 1953/1999). For example, you and your brother may have your mother’s eyes, although you and your sister may have your father’s high cheekbones. There is a strong family resemblance between you, your parents, and your siblings despite the fact that there is no single defining feature that you all have in common. Similarly, many members of the bird category have feathers and wings, so these are the characteristic features. Anything that has these features is likely to be classified as a bird because of this “family resemblance” to other members of the bird category (see FIGURE 9.4).

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Prototype Theory

Building on the idea of family resemblance, Rosch also proposed that categories are organized around a prototype, the “best” or “most typical” member of a category. A prototype possesses most (or all) of the most characteristic features of the category. For North Americans, the prototype of the bird category would be something like a wren: a small animal with feathers and wings that flies through the air, lays eggs, and migrates (see FIGURE 9.5). People make category judgments by comparing new instances to the category’s prototype. According to prototype theory, if your prototypical bird is a robin, then a canary would be considered a better example of a bird than would an ostrich because a canary has more features in common with a robin than an ostrich does.

Exemplar Theory

In contrast to prototype theory, exemplar theory holds that we make category judgments by comparing a new instance with stored memories for other instances of the category (Medin & Schaffer, 1978). Imagine that you’re out walking in the woods, and from the corner of your eye, you spot a four-legged animal that might be a wolf but reminds you of your cousin’s German shepherd. You figure it must be a dog and continue to enjoy your walk rather than fleeing in panic. You probably categorized this new animal as a dog because it bore a striking resemblance to other dogs you’ve encountered; in other words, it was a good example (or an exemplar) of the category dog. Exemplar theory does a better job than prototype theory in accounting for certain aspects of categorization, especially in that we recall not only what a prototypical dog looks like but also what specific dogs look like. FIGURE 9.6 illustrates the difference between prototype theory and exemplar theory.

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Researchers using neuroimaging techniques have concluded that we use both prototypes and exemplars when forming concepts and categories. The visual cortex is involved in forming prototypes, whereas the prefrontal cortex and basal ganglia are involved in learning exemplars (Ashby & Ell, 2001; Ashby & O’Brien, 2005). This evidence suggests that exemplar-based learning involves analysis and decision making (prefrontal cortex), whereas prototype formation is a more holistic process involving image processing (visual cortex).

Some of the most striking evidence linking concepts with the brain comes from patients with brain damage. One such patient could not recognize a variety of human-made objects or retrieve any information about them, but his knowledge of living things and foods was perfectly normal (Warrington & McCarthy, 1983). Other patients exhibit the reverse pattern: They can recognize information about human-made objects, but their ability to recognize information about living things and foods is severely impaired (Martin & Caramazza, 2003; Warrington & Shallice, 1984). Such unusual cases became the basis for a syndrome called category-specific deficit, a neurological syndrome characterized by an inability to recognize objects that belong to a particular category, although the ability to recognize objects outside the category is undisturbed.

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The type of category-specific deficit suffered depends on where the brain is damaged. Deficits usually result when an individual suffers damage to areas in the left hemisphere of the cerebral cortex (Mahon & Caramazza, 2009). Damage to the front part of the left temporal lobe results in difficulty identifying humans; damage to the lower left temporal lobe results in difficulty identifying animals; and damage to the region where the temporal lobe meets the occipital and parietal lobes impairs the ability to retrieve names of tools (Damasio et al., 1996). Similarly, when healthy people undertake the same task, imaging studies have demonstrated that the same regions of the brain are more active during naming of tools than animals and vice versa, as shown in FIGURE 9.7 (Martin, 2007; Martin & Chao, 2001).

How do particular brain regions develop category preferences for objects such as tools or animals? In one fMRI study, blind and sighted individuals each heard a series of words, including some words that referred to animals and others that referred to tools. Category-preferential regions showed highly similar patterns of activity in the blind and sighted individuals (Mahon et al., 2009). In both groups, for example, regions in the visual cortex and temporal lobe responded to animals and tools in much the same manner as shown in FIGURE 9.7. These results provide compelling evidence that category-specific organization of visual regions does not depend on an individual’s visual experience. The simplest explanation may be that category-specific brain organization is innately determined (Bedny & Saxe, 2012; Mahon et al., 2009).