Semantic memory is one of the divisions of long-term memory, which is distinct from episodic memory. In contrast to episodic memory, which mainly concerns personally experienced events, semantic memory is concerned with linguistic knowledge and knowledge about the world. Since the 1970s, neuropsychological studies on semantic memory have been dedicated to the following two questions :
   a) how the semantic memory system is related to other cognitive systems such as episodic memory ;
   b) how the semantic memory system itself is organized.
   For the former question, a number of case studies on patients with selective impairment of semantic memory, reported as “semantic amnesia ”or “semantic dementia ”, demonstrated that the semantic memory system is at least partially independent of other cognitive systems. Recent interest has focused on the relationship between semantic and episodic memory at the stage of encoding. Reports on patients with “developmental amnesia ”, with hippocampal damage sustained perinatally or during childhood, shed light on this issue.
   For the latter question, category specific semantic impairment, especially the animate/inanimate dissociation, has been the main focus of attention. Several hypotheses have been proposed to explain the existence of patients with category specific semantic impairment. There are, however, still controversies on this issue.

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   Semantic dementia (SD) due to temporal lobar atrophy, shows selective breakdown of comprehending words, faces and objects. Patterns of lobar atrophy in the temporal lobes are usually asymmetric. The relationship between left or right predominance of the atrophy and distinct cognitive dysfunctions remains unknown.
   To address this issue, we analyzed cognitive features comparing performances on the standardized neuropsychological tests (SLTA, the picture naming and word picture matching test, and WAIS-R) between two subgroups of SD. The groups represented predominantly left or right temporal lobe atrophy. Patients with predominantly left temporal lobar atrophy (SD-L), characterized by “gogi aphasia”showed more marked deterioration of verbal production and comprehension compared to patients with greater abnormality on the right (SD-R). SD-L patients also showed severe degradation of verbal IQ (VIQ) of WAIS-R, although they achieved near normal on the performance IQ (PIQ). In contrast, SD-R patients exhibited relatively worse comprehension of caricatures than the SD-L patients, whose performance on the rest of the language tests were more degraded than SD-R patients. The SD-R group showed degradation of PIQ equal to that of VIQ. These results suggest that SD-R patients suffered degraded intelligence about visual materials.
   Using newly developed completion tests for objects and for proverbs (Nakagawa, et al 1993), we compared representational deficit in visual and verbal materials across the two subgroups of SD. Although all patients with SD were impaired in both completion tests for objects and proverbs, they indicated two different patterns probably due to asymmetry in laterality of atrophy. Performance of SD-L patients in the proverb completion tests were more severely degraded compared to SD-R patients. Patients with SD-R showed more marked deterioration of performances in the object completion tests than SD-L patients. These distinct double dissociations between subgroups of SD suggest two possibilities : first, that the left temporal lobe may be more important in verbal representation than the right ; and second, the possibility that neural substrates of visual representation may be subserved by the right temporal lobe mainly.

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   Disruption in the structure of semantic memory in the early stages of this type of memory deficit was investigated in 20 patients with mild to moderate dementia of Alzheimer's type and 20 healthy normal controls. A semantic deficit for both a verbal representation (a word) and a visual representation (a line drawing) observed in the DAT group did not relate to the clinical disease severity, an attention deficit, visuospatial dysfunction, and cognitive dysfunction evaluated on the MMSE score. Moreover, a semantic deficit having a significant correspondence between the same individual items was most frequently found to exist for both representations. These results suggest that semantic memory itself (semantic contents meant by each representation) is disrupted in DAT and that the semantic memory system is amodal or single.
   Disruption of semantic memory structure was also investigated in a case (case 1) involving selective semantic memory deficits due to the left-sided temporal lobe lesions resulting from herpes simplex virus encephalitis, and a case (case 2) of semantic dementia in which remarkable atrophy was seen in both temporal lobes. Disruption of semantic memory structure observed in case 1 was similar to that found in the DAT group, suggesting that semantic memory itself is also disrupted in case 1. On the other hand, from the perspective of a single semantic memory system, case 2 involves the severe impairment of access between a representation and semantic memory rather than the disruption of semantic memory per se. Disruption of semantic memory itself may be caused by localized damages of the left temporal lobe such as those seen in case 1. In contrast, the severe impairment in access to semantic memory may be caused by degeneration in both temporal lobes such as that seen in case 2.

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[in Japanese]

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   Aphasic and dyslexic patterns were assessed in a 13-year-old patient with acquired childhood aphasia since age 12. Although her oral language improved early on for daily conversation, there remained difficulty in naming low-frequency words and repeating short sentences, as well as symptoms of dyslexia and dysgraphia. Her dyslexic patterns, at the early stage, demonstrated deep dyslexia characterized by semantic paralexias with imageability and lexicality effects in reading aloud. Even after subsequent recovery for Kana words and then Kana nonwords, a deficit for Kanji words was still observed. In the experimental task of reading aloud two-character Kanji words, she demonstrated impaired performance for words having low levels of familiarity, frequency, imageability and consistency. Although she also showed impaired reading more for ON-reading words than for KUN-reading words, imageability and consistency effects were observed in both ON- and KUN-reading words, indicating that both the indirect orthography-to-semantic-to-phonology pathway and the direct orthography-to-phonology pathway are employed in her reading aloud of Kanji words. One interpretation of these dyslexic patterns is that both the indirect and direct pathways function imperfectly, as indicated in the imageability and consistency (and also ON/KUN) effects, respectively. An alternative explanation is that her dyslexic and also aphasic symptoms might arise from phonological impairment ; this is suggested on the basis of preserved comprehension and impaired performance in non-reading phonological tasks such as nonword repetitions.

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   Phonological dyslexia (PD) is a type of acquired disorder in which patients demonstrate selective impairment in reading aloud nonwords as opposed to real words. PD has been thought, in the classical dual-route model of reading, to stem from selective impairment of the “non-lexical route.” We examined a case of PD using 1) oral reading tasks of real words, pseudohomophones (orthographic nonwords homophonic with real words) and non-homophonic nonwords ; 2) lexical decision tasks ; and 3) phonological tasks involving no reading processes. In the first tasks, the subject exhibited a pseudohomophone effect (reading performance of pseudohomophones was superior to that of non-homophonic nonwords) and a “familiarity” effect in pseudohomophones (familiarity of real words whose pronunciations correspond with those of pseudohomophones) . In the visual lexical decision tasks, a significant deficit in non-homophonic nonwords was observed. These findings are thought to be explicable only within the context of the classical interpretation of PD, and we take a skeptical view of the assumption that the reading process is completely different for real words and nonwords. In some parts of the third set of tasks, performance was remarkably low. We believe that these various results support the “phonological impairment hypothesis” behind PD, and we suggest that dissociated performance in reported cases may arise partly from differences in level of difficulty among the phonological tasks.

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   Findings from case studies and functional imaging studies have shown that the human amygdala plays an important role in the recognition of emotion, especially fear. However, there is contradictory evidence as to whether amygdala damage consistently impairs emotion recognition. This probably depends on the etiology (congenital vs. adult-onset) and extension (selective amygdala damage vs. damage to the amygdala and other brain regions) of the lesion. In addition, the difference in the role of the amygdala in recognizing facial and non-facial expressions has not been investigated fully. We studied a patient with extensive amygdala damage after encephalitis, and investigated her ability to recognize emotion from facial (moving and static) , prosodic, and written verbal stimuli. The patient had a deficit in recognizing fear from prosodic and written verbal stimuli. On the other hand, she recognized emotions normally from moving facial expressions. In addition, she recognized facial expressions of sadness, anger, and disgust more correctly from moving than from static faces. Our results suggest that adult-onset amygdala damage causes a deficit in recognizing the emotion fear from prosodic and written stimuli when the lesion is extensive. In addition, previously reported deficits of facial expression recognition other than fear are possibly due to a lack of dynamic information.

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