Five senses; an incurably abstract intellect; a haphazardly selective memory; a set of preconceptions and assumptions so numerous that I can never examine more than a minority of them—never become even conscious of them all. How much of total reality can such an apparatus let through?

—C. S. Lewis

Nothing, at first view, may seem more unbounded than the thought of man.

—David Hume

Centuries of philosophers, psychologists, and, most recently, neuroscientists have become fascinated with questions of what we know and how we know it, and their attempts to find answers have taken many forms. This chapter approaches these questions using the new tools, and hopefully new insights, of the first generations of cognitive neuroscientists, who have searched for knowledge about knowledge in images of the brain.

Introduction

Linguists use the term “semantics” to refer to the meaning of a word or phrase. Thus, Endel Tulving borrowed the word “semantic” to refer to a memory system for “words and other verbal symbols, their meaning and referents, about relations among them, and about rules, formulas, and algorithms” for manipulating them (Tulving, 1972, 386). Today, most psychologists conceive of a broader meaning than “meaning” when they use the term “semantic memory.” In the previous edition of this Handbook, Alex Martin defined semantic memory as “a broad domain of cognition composed of knowledge acquired about the world, including facts, concepts, and beliefs” (2001, 153). Following in this tradition, we use the term “semantic memory” to refer to world knowledge, not just word knowledge.

The study of world knowledge has its origins in philosophy, although the terminology has changed across the years: Where Locke wrote of “ideas,” psychologists today would substitute the word “concepts” to refer to “those expressed by the words ‘whiteness, hardness, sweetness, thinking, motion, man, elephant, army, drunkenness,’ and others.” And while the British empiricists asked about “human understanding,” cognitive neuroscientists today speak of “semantic memory” to refer to our shared knowledge of the world. Despite the new vocabulary, many of the central themes remain. Consider the question of the relation between knowledge and experience: Locke argued “Whence comes [the mind] by that vast store, which the busy and boundless fancy of man has painted on it with an almost endless variety? … To this I answer, in one word, From experience.” Three centuries later, the British psychologist Alan Allport, continuing in the tradition of his empiricist compatriots, argued that the sensorimotor systems used to experience the world are also used to represent meaning: “The essential idea is that the same neural elements that are involved in coding the sensory attributes of a (possibly unknown) object presented to eye or hand or ear also make up the elements of the auto-associated activity-patterns that represent familiar object-concepts in ‘semantic memory’ ” (1985, 53). Allport's theory, which is reminiscent of a description of concepts offered by Freud in his 1891 monograph on aphasia, was derived from a consideration of patterns of impairments to semantic memory following brain damage. In this chapter, we examine evidence from neuroimaging studies for an isomorphism between the architecture of semantic memory and the architecture of our sensorimotor systems. These neuroimaging studies provide new insights about the relation between knowledge and experience, using methods that Locke may never have imagined possible.

We have organized the review into three main divisions that reflect parallel (but interacting) lines of inquiry into the neural bases of semantic memory.