Semantic Memory, Language, and Neural Representation

Introduction to Semantic and Declarative Memory

Semantic memory is part of a larger division of memory known as declarative memory, which refers to items in memory that can be consciously retrieved or recalled, such as factual information, memories of events, and other types of knowledge (Tulving, 1972). Semantic memory is the memory of meanings and concept-based knowledge that can be consciously recollected — such as facts about the world, word meanings, and other related information — whereas the other component of declarative memory, episodic memory, refers to the memory of biographical and event-related information (Tulving, 1972). Semantic memory functions as a storehouse of knowledge that can be consciously retrieved and applied when needed in specific situations, and it comprises a large portion of what we learn about the world and our relations within it (Tulving, 1972). Semantic memories are language-based.

Human language is a distinctly different form of communication from that used by other species. Other species do possess communication systems of a sort, but these are not equivalent to human language. For example, certain monkeys have distinct warning calls for different types of threats, but they do not combine these calls to express entirely novel ideas. Human language is a system of verbal (or gestural) communication that allows for a virtually limitless number of ideas to be expressed by combining a finite set of verbal or gestural elements (Wargo, 2008). Thus, the function of human language is not only to enable communication, but also to support the expression and understanding of complex ideas.

Human Language and Its Functions

The Russian linguist Roman Jakobson described six basic functions of language: (1) referential, which describes a situation, mental state, or object; (2) expressive or emotive, concerned with self-expression; (3) conative, addressing the receiver; (4) poetic, focusing on the message for its own sake; (5) phatic, language for the sake of interacting with others; and (6) metalingual or reflexive, language used to describe itself (Jakobson, 1963).

According to Bock and Levelt, language production consists of four stages: (1) conceptualization — thinking and deciding what one wishes to communicate or express; (2) formulation of this desire into a linguistic plan; (3) execution of the plan by the muscles used for speech production; and (4) monitoring of speech production to ensure it matches what was intended to be expressed.

The Wernicke-Geschwind model of language production was based on observations of language impairments in brain-damaged individuals and illustrates how this process works in practice. Suppose one is engaged in a conversation. The auditory signals from the other person's speech are received by the primary auditory cortex and relayed to Wernicke's area in the left temporal lobe, where they are comprehended. Wernicke's area then generates a neural representation of the intended reply and transmits it via a tract of nerve fibers known as the left arcuate fasciculus to the posterior portion of the frontal lobe. There, the neural representation is activated in Broca's area, which in turn activates an articulation program that is transmitted to the appropriate neurons in the primary motor cortex controlling the muscles of facial articulation. The primary motor cortex sends this message to those muscles and the response is articulated (Poeppel & Hickok, 2004).

Stages of Language Production and the Wernicke-Geschwind Model

While the Wernicke-Geschwind model has been popular for many years, advances in neuroimaging have revealed that multiple areas of the brain are activated during language production — not only the areas identified by this model. Moreover, patients with certain types of aphasia can exhibit variable damage in the brain that does not fall specifically within the language production and reception areas the model designates (Poeppel & Hickok, 2004). Nonetheless, this model of language comprehension and expression remains widely referenced.

One interesting proposition arising from discussions of the Wernicke-Geschwind model is the notion of mentalese. Psycholinguists have proposed that some form of mentalese — a mental representation system distinct from language but translatable into linguistic form in the brain — may exist. However, there is little evidence or consensus regarding the properties of this form of pre-linguistic mental representation (Dudai, 2007; Poeppel & Hickok, 2004).

Certainly, some form of neural representation for language must exist. The stages of language production are similar to the serial method theories of the acquisition of declarative memories, especially semantic memory. Because semantic memories must somehow be represented in a formal neural code, and since semantic memories are a form of declarative memory — meaning they can be explicitly stated in language — it would follow that semantic memories are stored in the brain in a manner analogous to linguistic codes and language representations.

According to Dudai (2007), the serial model for semantic memory begins with attending to some to-be-remembered piece of information (this model also received initial support from the study of patients with brain damage). After attending to the information, one must encode it — a process typically associated with the hippocampus in the left temporal lobe. Consolidation and encoding are often achieved through some form of rehearsal. Following sufficient encoding, the information is stored in areas of the association cortex in some form of neural code. When one wishes to recall the memory, it must be retrieved from its storage site in the brain and then translated into a language code. The encoding-storage/consolidation-retrieval model thus parallels the Wernicke-Geschwind model of language production. Exactly what the neural code is and how it is represented in the brain remains an open question.

Bock, J.K. & Levelt, W.J.M. (1994). Language production: Grammatical encoding. In Gernsbacher, M.A. (ed.) Handbook of psycholinguistics (pp. 945–84). New York: Academic Press.