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The Structure of Semantic Memory: Category-based vs. Modality-based

The Structure of Semantic Memory: Category-based vs. Modality-based Jeremy F. Gottlieb (jgottlieb@carthage.edu) Carthage College, 2001 Alford Park Drive Kenosha, WI 53140 USA Abstract that are related to each other have strong links with each other, regardless of the subsystem they are stored in. Thus, while the associations between pieces of information are still category-based, the physical layout of information in semantic memory is based on modality. Furthermore, a given input modality can only directly access semantic memory through its corresponding semantic subsystem, and only accesses the other subsystems indirectly through the links between them. Damage to the semantic system should result in selective impairments in processing semantic information associated with particular input modalities, instead of particular categories. There are a number of reports of patients demonstrating modality-specific semantic deficits, such as optic aphasia (Beauvois, 1982; Riddoch & Humphreys, 1987). In this deficit, patients are unable to name objects that are presented visually, but are not impaired at naming them from any other sensory domain, such as touch. They are also unable to point to the proper object when given its spoken name. However, these patients still exhibit semantic knowledge of the objects they were unable to name, such as being able to correctly mime the use of the objects and being able to draw objects and complex scenes upon a verbal request. Beauvois (1982) also reports similar cases of tactile and auditory aphasia. According to proponents of the MSH, the most parsimonious explanation of optic aphasia is to theorize that the connection between visual semantics and verbal semantics has been severed. Much of the rest of the evidence brought to bear in this particular debate is also relatively ambiguous. One frequently reported finding is that words are named faster than pictures, but pictures are categorized faster than words (e.g., Potter & Faulconer, 1975; Guenther, Klatzky, & Putnam, 1980; Seifert, 1997; Theios & Amrhein, 1989). However, word “naming” is really just word reading, which arguably involves simply mapping graphemes to phonemes and can bypass semantic memory entirely, as in many dual- route models of word reading (Coltheart, Curtis, Atkins, & Haller, 1993). PET scanning has showed that areas of the brain related to visual processing are more active when living things are being named, while those related to action are more active when tools are being named (Martin, Wiggs, Ungerleider, & Haxby, 1996), indicating that information in semantic memory is organized by sensory modality. However, there is no true indication that this is the result of a modality- based organization instead of a category-based one. It could simply be the case that living things are processed in one part of the brain and man-made objects in another. A source of significant debate in psychology is the issue of how information is stored in semantic memory. The two primary frameworks are the Unitary Content Hypothesis, which holds that information is stored based on categories, and the Multiple Semantics Hypothesis, which holds that information is stored based on sensory modality. In a series of three experiments, I attempt to shed some light on which of these two frameworks is the most probable explanation of a number of phenomena associated with semantic memory. The results indicate that the Multiple Semantics Hypothesis is the most likely explanation. Keywords: semantic memory Introduction A significant debate in cognitive science centers around the question of the underlying organizational structure of the information stored in semantic memory. Specifically, the question that has been argued is whether information is organized categorically (e.g., Caramazza, Hillis, Rapp & Romani, 1990; Caramazza & Shelton, 1998) or based on the sensory modality of the information (e.g., Shallice, 1987; McCarthy & Warrington, 1988; Beauvois, 1982). The prototypical theory that posits a category-based organization is the Unitary Content Hypothesis (UCH) of Caramazza et al. (1990). The UCH posits that semantic memory is an abstract, amodal storage system where information is organized categorically. Every input modality can access all of the information about a given category in semantic memory, and semantic memory provides information to every output modality. Thus, damage to semantic memory should affect semantic processing in a category-based fashion. There are several reports of patients who exhibit category- specific deficits (see Forde & Humphreys, 1999, and Caramazza & Shelton, 1998, for review). Patients suffering from these deficits are generally impaired at processing semantic information about living things vis a vis man-made objects (e.g., Warrington & Shallice, 1984; Gainotti and Silveri, 1996) or vice versa (e.g., Warrington & McCarthy The representative theory of a modality-based semantic organization is Shallice’s (1987) Multiple Semantics Hypothesis (MSH). The MSH posits that semantic memory is actually divided into different semantic subsystems based on the various input modalities. There is a visual-semantic system, a tactile-semantic system, a verbal-semantic system, etc. These systems are linked together, allowing for information to pass between them. Categories are a result of how these systems are linked together. 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