INTRODUCTION

Semantic memory is a term for our repository of general knowledge about the world, our representation or ‘map’ of external reality that invests its referents with meaning. The concept of semantic memory can be traced to Tulving's (1972) bipartite model of long-term memory, in which declarative knowledge could be divided into episodic and semantic memories (Chapters 3 and 12). Whereas episodic memory refers to a system of storing contextually specific episodes or events, semantic memory includes general knowledge about the meaning of concepts, words and objects. According to Tulving (1995), ‘semantic memory makes possible the acquisition and retention of factual information in the broadest sense; the structured representation of this information, semantic knowledge, models the world’ (p. 841).

The goal of this chapter is to review how semantic memory is affected by various neurodegenerative diseases. We first review evidence that helps elucidate what neurocognitive systems subserve semantic memory. We then examine how semantic memory is affected when these neurocognitive systems are compromised by neurodegenerative disease, with emphasis on Alzheimer's disease (AD) as the prototypical cortical degenerative process. Along the way, we note the inferences that can be drawn about the structure and organization of semantic memory from its disorders in neurodegenerative diseases.

NEURAL SUBSTRATES OF SEMANTIC MEMORY

Before examining what is known about semantic memory in the dementia syndromes that result from neurodegenerative diseases, we will briefly review the neural systems that seem to be essential to semantic memory. The functional neuroanatomy of semantic memory is complex, but clues to its regional localization and organization come from lesion studies and from functional neuroimaging studies with normals.