Semantic Working Memory System
(Martin & Chao, Semantic Memory and the Brain)
(Current Opinion in Neurobiology 2001, 11:194-201)
(Paraphrase, Martin & Chao, Semantic memory and the brain: structure and processes)
The domain of semantic memory consists of stored information about the features and attributes that define concepts and 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 basis of semantic memory was dependent on the studies of patients with brain injury or disease. These investigations identified at least two brain regions, particularly on 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, and names of objects belonging to specific semantic category), even in the absence of a frank aphasia. This suggested 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 in naming objects and retrieving information about specific object specific characteristics. This suggests that object-specific information may be stored, at least in part, in the temporal lobes.
Results from recent studies have begun to reveal specific functions and processes subserved by smaller regions within each of the broadly defined areas.
Structure of Semantic Representations
An old idea in behavioral neurology is that object concepts are defined by sensory and motor attributes and features acquired during experience. Researchers have suggested that object concepts may be represented in the brain in distributed networks of sensory, motor and/or more abstract functional information.
Investigations of word generation have provided evidence suggesting that information about different object features may be stored in different regions of the cortex. Studies have shown that the ventral and lateral regions of posterior temporal cortex can be differentially engaged depending on the type of information retrieved.
Activation of the fusiform gyrus has been observed when subjects retrieve color word associates. Activation in a similar region has been reported during the spontaneous generation of color imagery and auditory color word synthesis and in normal individuals during color imagery.
Object specific features may be stored within the same neural systems that are active during perception.
Distributed Representations of Object Categories
Object concepts may be represented by distributed feature networks.
A common feature of all concrete objects is that physical form. All object categories either sit distinct patterns of neural activity in regions that mediate perception of logic form (the ventral occipitotemporal cortex).
Information about how objects move through space and patterns of motor movements associated with their use are other features that could aid object recognition and identification. This would be especially true for categories of man-made objects such as tools that have a more variable mapping between their name and their visual form than a category such as four-legged animals.
Naming and identifying objects with motion related attributes activate areas close to the regions that mediate perception of object motion (the posterior region of the lateral temporal lobe), with different patterns of activity associated with biological and man-made objects.
Naming manipulable man-made objects selectively activates areas close to regions active during object manipulation.
Ventral occipitotemporal cortex and the representation of object form
Distinct regions of ventral temporal cortex show different show differential responses to different object categories.
Ventral occipitotemporal cortex and the representation of object form
Distinct regions of ventral temporal cortex show differential responses to different object categories. Activity associated with each object category was not limited to one region, but a rather involved a relatively large expanse of ventral cortex. Rather than a single peak, this activity was best described by a complex pattern of peaks and valleys distributed over much of the ventral temporal and occipital cortex. The results indicate that representation of different object categories are distributed and overlapping.
The data suggest that ventral occipitotemporal cortex may be best viewed not as a mosaic of discrete category specific areas, but rather as a lumpy feature-space, representing stored information about features of object form shared by members of a category.
Lateral temporal cortex and the representation of object motion
A number of researchers have reported that 'tools' elicited greater activity in the left posterior middle temporal gyrus than 'animals' and other object categories. The active region is just anterior to the area MT (middle temporal) and slightly posterior to, or overlapping with, the region active in verb-generation studies. Damage to this region has been reported to selectively impair tool recognition and naming. In contrast, naming animals and viewing faces elicits greater activity in the superior temporal sulcus. Ventral premotor cortex and the representation of use-associated motor movements
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.
Greater activation of left ventral premotor cortex has been found for naming 'tools' relative to naming 'animals', viewing pictures of tools compared with viewing pictures of animals, faces and houses, and generating action words to tools.
As with studies of object form and object color, mental imagery (e.g. imaging manipulating objects with the right hand) results in ventral premotor activation.
The ventral premotor activation noted in human neuroimaging studies may reflect action planning.
Representation of the subordinate level and unique objects
The most commonly studied category of unique entities has been famous faces. These investigations typically have observed activity in the anterior middle temporal gyrus and temporal pole.
Damasio has asserted that the anterior region of the temporal lobes are critical for retrieving information about unique entities.
It can be assumed that naming unique entities and making subordinate level distinctions requires access to more information than basic level identification.
We may hypothesize that the temporal lobe object representation system is organized hierarchically, with increasing convergence and integration of information occurring along its posterior to anterior axis.
Working with semantic representations
Performing semantic tasks commonly activates a wide expanse of the left lateral prefrontal cortex. Evidence has accumulated suggesting that an anterior and inferior prefrontal region (a roughly equivalent to Brodmann's area BA 47 and inferior aspect of BA 45) maybe involves selectively in semantic processing. This region may serve as a semantic working memory system responsible for retrieving, maintaining, monitoring and manipulating semantic representation stored elsewhere.
The left inferior prefrontal cortex (LIPC) is involved in the selection among competing semantic features (color action object associates) stored in temporal cortex.
The location of selection-related LIPC activity is actually in the dorsal aspect of BA 44, placing it posterior and superior to the semantic working memory region. This suggests that the process of selecting among competing alternatives may occur in a different cortical region than other working memory processes.
The situation is complicated by the fact that phonological processing also activates dorsal BA 44 (as well as the more inferior portion of BA 44). BA 44 maybe fractionated functionally into two regions, with the inferior portion involved in phonological processing -- especially transforming orthographic to phonologic representations for speech production -- and the superior portion engaged in selection processes.
The polar region of the left temporal lobe may be contribute to the overall functioning of a semantic working memory system. Normal functioning of the polar region of the left temporal lobe may provide top-down modulatory input necessary for successful retrieval of semantic representations stored in posterior regions.
The role of different regions of left prefrontal cortex in semantic memory processes and, more generally, in lexical retrieval and production, remain to be specified. In particular, the idea that the polar region of the left temporal lobe is part of a semantic working memory network awaits further study.
(end of paraphrase)
Sematic dementia, which involves loss of factual memory rather than loss of personal memory, destroys the cortical area of the temporal lobe first, where semantic memories are thought to be stored. (Carter; Mapping the Mind, 176)
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