Review
The neurobiology of semantic memory

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Semantic memory includes all acquired knowledge about the world and is the basis for nearly all human activity, yet its neurobiological foundation is only now becoming clear. Recent neuroimaging studies demonstrate two striking results: the participation of modality-specific sensory, motor, and emotion systems in language comprehension, and the existence of large brain regions that participate in comprehension tasks but are not modality-specific. These latter regions, which include the inferior parietal lobe and much of the temporal lobe, lie at convergences of multiple perceptual processing streams. These convergences enable increasingly abstract, supramodal representations of perceptual experience that support a variety of conceptual functions including object recognition, social cognition, language, and the remarkable human capacity to remember the past and imagine the future.

Section snippets

The centrality of semantic memory in human behavior

Human brains acquire and use concepts with such apparent ease that the neurobiology of this complex process seems almost to have been taken for granted. Although philosophers have puzzled for centuries over the nature of concepts [1], semantic memory (see Glossary) became a topic of formal study in cognitive science only relatively recently [2]. This history is remarkable, given that semantic memory is one of our most defining human traits, encompassing all the declarative knowledge we acquire

Central issues in semantic processing

A major issue in the study of semantic memory concerns the nature of concept representations. Efforts in the last century to develop artificial intelligence focused on knowledge representation in the form of abstract symbols [10]. This approach led to powerful new techniques for information representation and manipulation (e.g., semantic nets, feature lists, ontologies, schemata). Recent advances in this area used machine learning techniques together with massive verbal inputs to create a

Evidence for modality-specific simulation in comprehension

The idea that sensory and motor experiences form the basis of conceptual knowledge has a long history in philosophy, psychology, and neuroscience 1, 3, 12, 13. In recent years, this proposal has gained new steam under the rubric of ‘embodied’ or ‘situated’ cognition, supported by numerous neuroimaging and behavioral studies. Some of the imaging studies showing modality-specific activations during language processing are summarized in Figure 1. A number of these address action concepts and show

Evidence for high level convergence zones

In addition to modality-specific simulations, we propose that the brain uses abstract, supramodal representations during conceptual tasks. One compelling argument for this view is that the human brain possesses large areas of cortex that are situated between modal sensory-motor systems and thus appear to function as information ‘convergence zones’ [14]. These heteromodal areas include the inferior parietal cortex (angular and supramarginal gyri), large parts of the middle and inferior temporal

Embodied abstraction in conceptual representation

Figure 3 illustrates several prominent theories that differ in the proposed level of separation between conceptual and perceptual representations. Models based on disembodied, symbolic conceptual representations 9, 10 are often criticized on the grounds that such symbols are ultimately devoid of content [54]. From an empirical standpoint, the extensive evidence for involvement of modality-specific sensory, action, and emotion systems during language comprehension is also inconsistent with such

A neuroanatomical model of semantic processing

Figure 4 outlines a neuroanatomical model of semantic memory consistent with a broad range of available data. Modality-specific representations (yellow areas in Figure 4), located near corresponding sensory, motor, and emotion networks, develop as a result of experience with entities and events in the external and internal environment. These representations code recurring spatial and temporal configurations of lower-level modal representations. Although depicted as somewhat modular, we view

Social cognition, declarative memory retrieval, prospection, and the default mode

The network of brain regions we associate here with semantic processing has also been linked with more specific functions. Nearly all parts of the network have been implicated in aspects of social cognition, including theory-of-mind (processing of knowledge pertaining to mental states of other people), emotion processing, and knowledge of social concepts 36, 37, 63, 64, 65, 66, 67. Much of the network has been implicated in retrieval of episodic and particularly autobiographical memories 68, 69

Concluding remarks

This review proposes a large-scale brain model of semantic memory organization in the human brain based on synthesis of a large body of empirical imaging data with a modified embodiment theory of knowledge representation. In contrast to strong versions of embodiment theory, the data show that large areas of heteromodal cortex participate in semantic memory processes. The multimodal convergence of information toward these brain areas enables progressive abstraction of conceptual knowledge from

Acknowledgements

Supported by NIH grants R01 NS33576 and R01 DC010783. Thanks to Lisa Conant, Will Graves, Colin Humphries, Tim Rogers, and Mark Seidenberg for helpful discussions.

Glossary

Embodied cognition
in cognitive neuroscience, the general theory that perceptual and motor systems support conceptual knowledge, that is, that understanding or retrieving a concept involves some degree of sensory or motor simulation of the concept. A related term, situated cognition, refers to a more general perspective that emphasizes a central role of perception and action in cognition, rather than memory and memory retrieval.
Heteromodal cortex
cortex that receives highly processed, multimodal

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