The Neural Basis For Semantic Memory

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The strong association between disorders of the temporal lobes and semantic memory impairments emphasizes the critical role of temporal neocortex in semantic memory. There is converging evidence to implicate the anterior temporal lobes in particular, and the inferior temporal gyri specifically. In semantic dementia, brain imaging shows atrophy of the temporal pole (e.g. Mummery et al., 2000) and particularly involvement of the inferior temporal gyrus (Breedin et al., 1994; Cardebat et al., 1996; Mummery et al., 2000). Autopsy examination (Snowden et al., 1992; Snowden et al., 1996b) has shown atrophy of the inferior and middle temporal gyri with notable sparing of the superior temporal gyri, including Wernicke's area, and medial temporal structures. In herpes simplex encephalitis patients, there is evidence of damage to the inferior temporal neocortex (Pietrini et al., 1988; Sartori & Job, 1988; Silveri & Gainotti, 1988; Sartori et al., 1993a). In a patient with semantic impairment following radionecrosis (Kapur et al., 1994), damage was reported in the anterior inferior temporal cortex. The medial temporal lobes were relatively spared. Similarly, a semantic disorder resulting from a ruptured aneurysm of the left posterior communicating artery (Sartori et al., 1994), was also attributed to damage to the inferior temporal gyrus.

It is undoubtedly the case that both left and right hemispheres contribute to semantic memory function, and that the most severe semantic deficits occur in patients with bilateral disease. Some authors have placed greater emphasis on the importance of the left temporal lobe than the right (Mummery et al., 2000), on the grounds that many patients with semantic dementia show an asymmetrical distribution of atrophy affecting the left side more than the right. However, in our experience, patients with an equally severe semantic memory deficit may exhibit more marked right than left temporal lobe atrophy. Patients with semantic dementia, whose presenting symptom is in naming and word comprehension, typically show more marked left than right temporal atrophy. Conversely, patients in whom face recognition deficits predominate show more marked atrophy in the right temporal lobe. The association between face recognition impairments and the right temporal lobe has also been demonstrated by others (Evans et al., 1995). A patient described by McCarthy et al. (1996) with right temporal lobe damage was particularly impaired in his recognition of pictures of famous buildings and landmarks, suggesting that hemispheric differences may contribute to category effects. The precise role of the two hemispheres remains to be explored.

Notwithstanding the central importance of temporal neocortex, it would be incorrect to assume that semantic memory is localized to this single brain region. The literature on category-specific impairments draws attention to the contribution of diverse brain regions. Those who consider that knowledge is organized by category would argue that specific cortical regions are dedicated to particular domains or categories of knowledge. By contrast, the functional-sensory account would suggest that concepts are represented in a distributed fashion across cortical sites, each representing information about specific attributes. The temporal ventral pathways are particularly important for the processing of visual-sensory information and, according to the functional-sensory account, for the representation of sensory knowledge. By contrast, the frontoparietal lobes are assumed to contribute more to knowledge of functional and abstract linguistic properties. Manipulation knowledge is linked to premotor areas, and topographical knowledge with the parietal lobes. Support for this distributed account has come from functional imaging studies using PET (Martin et al., 1996; Chao et al., 1999). The brain regions active during object identification were found to depend upon the intrinsic properties of the object presented. Thus, for example, naming animals activated occipital regions involved in early visual processing, in addition to the temporal lobes, whereas naming tools activated the left premotor area, involved in object manipulation.

These neocortical sites appear particularly important for knowledge about abstract properties of words and objects. However, the notion that personal experiential memories may support some semantic information, and that event scripts provide the basic mechanism for concept acquisition, suggest a putative role in semantic knowledge, both for the medial temporal lobes, important for event memory, and the frontal neocortex. Goal-directed, purposeful actions are known to be dependent on the frontal lobes, and goal-directed activities break down in patients with frontal lobe lesions (Buxbaum et al., 1997; Godbout & Bouchard, 1999; Schwartz et al., 1995). Patients with semantic dementia, by contrast, frequently have preserved frontal lobe function and are able to establish goals. Aspects of semantic information embedded within a goal-driven action script, subserved by frontal lobe functions, might be preserved, while abstracted knowledge is lost.

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