Academic journal article Cognitive, Affective and Behavioral Neuroscience

Distinct and Common Cortical Activations for Multimodal Semantic Categories

Academic journal article Cognitive, Affective and Behavioral Neuroscience

Distinct and Common Cortical Activations for Multimodal Semantic Categories

Article excerpt

If semantic representations are based on particular types of perceptual features, then category knowledge that arises from multimodal sensory experiences should rely on distinct and common sensory brain regions depending on the features involved. Using a similarity-based generation-and-comparison task, we found that semantic categories activated cortical areas associated with taste and smell, biological motion, and visual processing. Fruit names specifically activated medial orbitofrontal regions associated with taste and smell. Labels for body parts and clothing activated lateral temporal occipitoparietal areas associated with perceiving the human body. More generally, visually biased categories activated ventral temporal regions typically ascribed to visual object recognition, whereas functional categories activated lateral frontotemporal areas previously associated with the representation of usage properties. These results indicate that semantic categories that are distinguished by particular perceptual properties rely on distinct cortical regions, whereas semantic categories that rely on similar types of features depend on common brain areas.

Concepts are considered to be the building blocks of human higher order cognition (Margolis & Laurence, 1999). Yet, theories differ according to how these symbolic representations are instantiated within the brain. The amodal characteristics of word meaning (Landauer & Dumais, 1997) imply that knowledge is stored independently of perceptual experiences. Mounting evidence, however, suggests that concepts depend upon cortical regions typically ascribed to sensory input (Martin & Chao, 2001). This embodiment of semantic representations through perceptual mechanisms can explain how word meanings necessarily draw upon sensory experiences of the referenced objects (Barsalou, 1999). Yet, theories of semantic knowledge differ according to the level at which object categories are said to rely on particular brain regions. These differences concern how semantic information is organized: by the sensory features of items (Martin & Chao, 2001), by innate domain-based mechanisms (Caramazza & Shelton, 1998), or on the basis of a uniform distribution (Forde & Humphreys, 2002; Tyler & Moss, 2001). The resolution of this debate is critical to solving how conceptual understanding is acquired, maintained, and even hindered by the supporting neural circuitry.

The neuropsychological literature is filled with case studies of individuals who demonstrate a selective loss of semantic memory (for recent reviews, see Capitani, Laiacona, Mahon, & Caramazza, 2003; Caramazza & Shelton, 1998; Saffian & Schwartz, 1994), even though such cases are relatively rare (Coltheart, 2001 ). A distinction between natural kinds and artifacts has been the one most frequently reported among patients with semantic memory impairments (Mummery, Patterson, Hodges, & Price, 1998). Damage localized to the inferomedial temporal cortex, usually resulting from encephalitis due to the herpes simplex virus, has been associated with impaired performance for natural kinds (e.g., animals, fruits, and vegetables) in comparison with a sparing of artifact (e.g., tools, household objects, or vehicles) knowledge (Gainotti & Silver!, 1996; Hart & Gordon, 1992). The reverse pattern of category-specific deficits (i.e., artifacts impaired in comparison with preservation of natural kind knowledge), usually caused by damage to frontoparietal areas, has also been found (Hills & Caramazza, 1991; Sacchett & Humphreys, 1992; Warrington & McCarthy, 1987)-though significantly less often than selective impairments for natural kinds (Capitani et al., 2003). In sum, this double dissociation has led to the inference that semantic knowledge is organized on some level by category-specific information.

In the face of these category-specific deficits and more diffuse semantic disturbances when stimulus factors are rigorously controlled, a number of proposals have been advanced to explain the natural kind-artifact dissociation found among patients. …

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