ReviewSemantic memory and the brain: structure and processes
Introduction
The domain of semantic memory consists of stored information about the features and attributes that define concepts and the processes that allow us to efficiently retrieve, act upon and produce this information in the service of thought and language. Before the advent of functional brain imaging, our knowledge of the neural bases of semantic memory was dependent on studies of patients with brain injury or disease. These investigations identified at least two brain regions, particularly in the left hemisphere, that play a critical role in semantic memory. Patients with damage to the left prefrontal cortex (LPC) often have difficulty retrieving words in response to specific cues (e.g. words beginning with a specific letter, the names of objects belonging to a specific semantic category), even in the absence of a frank aphasia [1]. This suggests that the LPC plays a general, albeit crucial, role in retrieving lexical and semantic information.
Patients with damage to the temporal lobes often have difficulty naming objects and retrieving information about object-specific characteristics 2., 3., 4.). This suggests that object-specific information may be stored, at least in part, in the temporal lobes.
Consistent with the clinical literature, early functional brain imaging studies of semantic processing revealed activity in broad expanses of the left prefrontal, parietal and posterior temporal lobes, commonly including ventral and lateral regions of temporal cortex [5]. Results from recent studies have begun to reveal specific functions and processes subserved by smaller regions within each of these broadly defined areas. In this review, we will highlight studies that have shed new light both on the neural structures involved in representing object-specific features and attributes, and on the processes involved in retrieving and manipulating this information.
Section snippets
The structure of semantic representations
An old idea in behavioral neurology 6., 7. is that object concepts are defined by sensory and motor attributes and features acquired during experience. Within this framework, many have suggested that object concepts may be represented in the brain as distributed networks of sensory, motor and/or more abstract functional information 8., 9., 10., 11., 12., 13., 14..
Investigations of word-generation have provided one body of evidence suggesting that information about different object features may
Distributed representations of object categories
Another body of evidence that object concepts may be represented by distributed feature networks comes from studies contrasting patterns of neural activity associated with performing the same type of task (e.g. naming) with different categories of objects. A common feature of all concrete objects is their physical form. Evidence is accumulating that suggests that all object categories elicit distinct patterns of neural activity in regions that mediate perception of object form (the ventral
Ventral occipitotemporal cortex and the representation of object form
A number of investigators have found that distinct regions of ventral temporal cortex show differential responses to different object categories. In a series of studies [27•], we found greater activity in the lateral region of the fusiform gyrus for animal than for tool stimuli. This finding was consistent among different tasks, including viewing, naming, and matching pictures, and answering written questions about object features. In contrast, the medial fusiform gyrus was more active for
Lateral temporal cortex and the representation of object motion
A number of laboratories, using a variety of paradigms with pictures and words, have reported that tools elicit greater activity in the left posterior middle temporal gyrus than animals and other object categories 27•., 37., 39•., 49••.. Moreover, the active region is just anterior to area MT (middle temporal) and slightly posterior to, or overlapping with, the region active in the verb-generation studies discussed above (see [23] for a review). Damage to this region has been reported to
Ventral premotor cortex and the representation of use-associated motor movements
If activations associated with different object categories reflect stored information about object properties, one would expect tools to elicit activity in motor-related regions. Several laboratories have reported this association. Specifically, greater activation of left ventral premotor cortex has been found for naming tools relative to naming animals 37., 56., 57. (see Fig. 1), viewing pictures of tools compared with viewing pictures of animals, faces and houses [57], and generating action
Representation of the subordinate level and unique objects
A common characteristic of the studies described above was that object categories were represented by items named at the basic level (i.e. house, face, chair, dog, hammer) rather than at the subordinate or unique-object level (e.g. the White House, Marilyn Monroe, rocking chair, collie, sledgehammer). The most commonly studied category of unique entities has been famous faces. These investigations typically have observed activity in the anterior middle temporal gyrus 63., 64•., 65. and temporal
Working with semantic representations
As noted previously, performing semantic tasks commonly activates a wide expanse of left lateral prefrontal cortex. More recently, evidence has accumulated suggesting that an anterior and inferior prefrontal region (roughly equivalent to Brodmann's Area BA 47 and the inferior aspect of BA 45) may be involved selectively in semantic processing. Specifically, as suggested by Gabrieli and co-workers, and Wagner 70., 71., 72•., this region may serve as a ‘semantic working memory system’ responsible
Conclusions
Distributed networks of discrete cortical regions are active during object processing. The distribution of these regions varies as a function of semantic category. The same regions are active, at least in part, when objects from a category are recognized, named, imagined, and when reading and answering questions about them.
Critical questions for future research will be to clarify the precise role of these regions in object semantics and how are they influenced by experience. In addition, it has
Acknowledgements
This work was supported by the National Institute of Mental Health Intramural Research Program.
References and recommended reading
Papers of particular interest, published within the annual period of review,have been highlighted as:
•of special interest
••of outstanding interest
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