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PubMed2004

Neural systems for word meaning modulated by semantic ambiguity.

One important issue in neuroimaging research on language is how the brain processes and represents lexical semantics. Past studies with various paradigms reveal that the left inferior prefrontal and mid-superior temporal regions play a crucial role in semantic processing. Those studies, however, typically utilize words having a precise and dominant meaning as stimuli and have not manipulated lexico-semantic ambiguity, a key feature of human language, as an experimental variable. Here, we used a word generation paradigm to examine whether neuroanatomical networks for meaning are modulated by lexical ambiguity. We found that, compared with semantically precise words, semantically ambiguous words were mediated by strong brain activations in the left dorsal-lateral frontal areas, the anterior cingulate, and the right inferior parietal lobe. Semantically precise words, instead, were associated with the left inferior prefrontal and mid-superior temporal sites. These findings indicate that semantic analysis of written words is a dynamic process involving coordination of widely distributed neural subsystems, which are weighted by semantic ambiguity.

Adult↗

PubMed2004

The neural areas that control the retrieval and selection of semantics.

Semantic retrieval consistently activates left inferior frontal regions, yet lesions to these areas do not typically result in semantic deficits. This discrepancy has led to the hypothesis that left prefrontal areas are primarily involved in executive processes while semantic information is retrieved from temporal cortices. We investigated semantic executive processing by changing, over trials, the semantic association to a stimulus. Using fMRI and PET, we demonstrate that changes in semantic association increased activation in temporal as well as frontal areas. The similar effects in temporal and frontal areas suggest that semantic executive processes are not confined to the left inferior frontal cortex but might be distributed throughout the semantic system.

Adult↗

PubMed2006

The nature of lexico-semantic processing deficits in mild cognitive impairment.

Lexico-semantic impairments in Alzheimer disease (AD) have been attributed to abnormalities in both intentional and automatic access to semantic memory. However, the order in which these impairments appear during the course of the disease is unclear. We sought to answer this question by documenting lexico-semantic impairments in 61 subjects with mild cognitive impairment (MCI), a pre-AD stage, and by comparing them to those of 39 AD and 60 normal elderly (NE) subjects. All subjects were tested with intentional access tasks (picture naming and semantic probes), automatic access tasks (lexical decision and priming), and executive function tasks (Stroop and Stroop-Picture naming). Results indicated that the MCI group was only impaired on tasks of intentional access relative to the AD group who was impaired on both types of tasks. Because most MCI subjects eventually develop AD, these results suggest that intentional access to semantic memory is impaired before automatic access. Further, impairments on the Stroop-Picture naming task but not on the Stroop task, suggest that lexico-semantic impairments in the MCI group may be related to inhibition deficits during semantic search. Findings are discussed in light of executive dysfunctions within the framework of semantic memory theories.

Aged↗

PubMed2006

Repetition suppression and semantic enhancement: an investigation of the neural correlates of priming.

The priming of a stimulus by another has become an important tool for exploring the neural underpinnings of conceptual representations. However, priming effects can derive from many different types of relationships and it is important to distinguish between them in order to be able to develop theoretical accounts of the representation of conceptual knowledge. While it is well known that repetition priming (the repeated presentation of the same stimulus) is associated with a reduced neural response, called repetition suppression (RS), the neural correlates of semantic priming (when two stimuli are related in meaning but not identical) are not so well established. We compared the neural correlates of repetition and semantic priming using written words, independently manipulating form and meaning. In an fMRI study, subjects saw single words and made a concrete-abstract decision. Two consecutive words were identical (town-town) or varied along a continuum of semantic relatedness, from highly related (cord-string) to unrelated (face-sail). We found distinct patterns of activation for repetition and semantic priming. Repetition priming was associated with RS in LIFG, bilateral parahippocampal gyrus and R fusiform gyrus. We also observed increased activation for word repetition in the RMFG and RMTG/STG, which may reflect recognition of item's earlier presentation. There was no evidence of suppression for semantic relatedness. Semantic priming was associated with enhanced activation in multiple bilateral fronto-temporal areas, i.e. semantic enhancement. The results suggest that repetition and semantic priming in visual word recognition depend on distinct cognitive processes and neural substrates.

Adult↗

PubMed2006

Children with reading disorder show modality independent brain abnormalities during semantic tasks.

Neuroimaging studies have suggested that left inferior frontal gyrus, left inferior parietal lobule and left middle temporal gyrus are critical for semantic processing in normal children. The goal of the present functional magnetic resonance imaging (fMRI) study was to determine whether these regions are systematically related to semantic processing in children (9- to 15-year-old) diagnosed with reading disorders (RD). Semantic judgments required participants to indicate whether two words were related in meaning. The strength of semantic association varied continuously from higher association pairs (e.g., king-queen) to lower association pairs (e.g. net-ship). We found that the correlation between association strength and activation was significantly weaker for RD children compared to controls in left middle temporal gyrus and left inferior parietal lobule for both the auditory and the visual modalities and in left inferior frontal gyrus for the visual modality. These results suggest that the RD children have abnormalities in semantic search/retrieval in the inferior frontal gyrus, integration of semantic information in the inferior parietal lobule and semantic lexical representations in the middle temporal gyrus. These deficits appear to be general to the semantic system and independent of modality.

Adolescent↗

PubMed2004

Hemispheric asymmetries of hypometabolism associated with semantic memory impairment in Alzheimer's disease: a study using positron emission tomography with fluorodeoxyglucose-F18.

Considerable disagreement exists about the neuroanatomical basis of conceptual-semantic impairments observed in a subgroup of patients with Alzheimer's disease (AD) at mild to moderate stages of the disease. Several studies of groups of patients have shown correlations between focal hypometabolism or hypoperfusion in left hemispheric areas and measures of verbal semantic memory impairment in AD patients. The question remains, however, whether left hemispheric hypometabolism is sufficient to produce such impairment in the single case and whether nonverbal semantic knowledge is also affected. We used positron emission tomography (PET) with fluorodeoxyglucose-F18 (FDG), statistical parametric mapping (SPM), and tests of verbal and nonverbal semantic memory in 11 AD patients with a mean score on the Mini-Mental State Examination of 22.6 (+/-2.8). Naming impairment was significantly associated with left hemispheric asymmetry of hypometabolism on a single-case basis. Our correlation analysis showed that metabolism in left anterior temporal, posterior inferior temporal, inferior parietal and medial occipital areas (Brodmann areas: 21/38, 37, 40 and 19) correlated with both verbal and nonverbal semantic performance. We conclude that left hemispheric synaptic dysfunction, as measured by regional glucose hypometabolism, was sufficient to produce semantic impairments in our patients. The majority of areas affected in our patients with semantic impairments were involved in multimodal or supramodal (verbal and nonverbal) semantic knowledge.

Aged↗

PubMed2004

Can disorganisation of semantic memory account for the abnormalities of thought in schizophrenia-a controlled experimental study.

BACKGROUND: Disorganisation of semantic memory could provide a cognitive explanation for the disturbances of thinking and reasoning in schizophrenia. In this study, we directly test this explanation by identifying patients with disorganised semantic categories and then examine how they use their knowledge about these same categories in an inductive reasoning task. METHOD: Experiment 1 utilised a semantic category-sorting task to identify patients with disorganisation of semantic memory. In Experiment 2, the patients with disorganised categories carried out a category-based inductive reasoning task. Accurate performance on this task requires access to well-organised semantic knowledge about the objects and categories used in Experiment 1. RESULTS: Patients with disorganised semantic categories in Experiment 1 did not demonstrate any difficulties or unusual responses when reasoning about the same categories in Experiment 2. CONCLUSION: Disorganisation of semantic memory may not be the primary cause of disturbed reasoning or thought in schizophrenia. Patients with schizophrenia tend to generate ad hoc categories, which are unsuited to the current context. Impaired performance on semantic memory tasks can arise from a misunderstanding of social context.

Adult↗

PubMed2006

Structured cueing on a semantic fluency task differentiates patients with temporal versus frontal lobe seizure onset.

Patients with frontal lobe dysfunction (e.g., Huntington's disease) reportedly benefit more from cueing on measures of semantic fluency than do patients with damage to temporal lobe structures (e.g., Alzheimer's disease). This differential benefit from cueing suggests that different neurocognitive functions are impaired in these two groups. Patients with frontal lobe dysfunction are presumed to have difficulty with the executive aspects of this generative fluency task, whereas patients with temporal lobe impairment are limited by deficits in semantic memory. We studied the performance of patients with complex partial seizures of frontal or temporal lobe onset, as determined by video/EEG monitoring, on standard and cued measures of semantic fluency administered in a counterbalanced sequence across groups. These groups did not differ significantly in terms of age, education, gender, age at seizure onset, total number of antiepileptic drugs, or IQ, and all patients subsequently underwent surgery for intractable epilepsy. Patients with frontal lobe dysfunction (FL group) performed significantly worse than patients with temporal lobe impairment (TL group) on the standard semantic fluency paradigm (TL group: M=18.4, SD=4.7; FL group: M=11.1, SD=5.3), t(27)=-3.75, P<0.001. Nevertheless, results of an ANCOVA demonstrated that the FL group showed significantly greater performance improvement than the TL group when provided with a cued semantic fluency format, even after controlling for baseline differences in ability on the standard semantic fluency task (TL group: M=0.45, SD=3.8; FL M=9.4, SD=5.1), F(1,29)=12.37, P=0.002. These findings support previous research suggesting that frontal and temporal structures contribute uniquely to semantic generative fluency and suggest that using a combination of standard and cued semantic fluency tasks may help confirm localization of seizure onset in partial epilepsy by localizing the associated cognitive dysfunction.

Adult↗

PubMed2000

The role of semantics in reading and spelling: evidence for the 'summation hypothesis'.

This study documents a patient, SA, with an impairment of semantic memory arising as a result of Semantic Dementia (Pick's disease). The patient is impaired at deriving semantic knowledge from both words and pictures. However, his ability to derive semantic knowledge of countries is relatively spared compared to concrete nouns and famous people. The presence of a semantic deficit was used to investigate the role of semantics in reading and spelling. Several novel cueing/priming paradigms are reported which suggest that SA is able to use partial semantic knowledge to constrain his reading and spelling. These results are broadly consistent with the 'summation hypothesis' [27] and suggest that normal reading and spelling may take place by integrating both semantic information and knowledge of direct orthography-phonology correspondences.

Atrophy↗

PubMed2001

Functional differences in the semantic processing of concrete and abstract words.

There is considerable debate as to whether the semantic system is a unitary one in which meanings are available in a peculiar, perceptual-free format, or whether it is functionally segregated into anatomically discrete, modality-specific but semantic regions. In the former case, concrete and abstract words should not differ in the amount of activation of semantic areas. Neuroimaging studies in this field are, however, far from conclusive, and one reason for this may be that the degree of imageability of the stimuli - probably a crucial variable - has not been considered. Recognition Potential (RP) reflects semantic processing and appears to originate in basal extrastriate regions involved in semantic processing. In this study, we compared the RP of concrete and abstract words that actually differ in their degree of imageability. Results indicate that the semantic processing areas in which the RP originates display a higher activation for concrete (more imageable) material, but that abstract material also evokes a notably larger RP component compared with pseudowords or unpronounceable letter strings. Accordingly, the study appears to suggest that there is no full functional segregation of the semantic systems. Rather, our data support the existence of a semantic system that is specialised in concrete, imageable material, and that is also activated, though to a lower extent, by abstract material.

Adolescent↗

PubMed1998

Identification without manipulation: a study of the relations between object use and semantic memory.

The role of semantic knowledge in object utilisation is a matter of debate. It is usually presumed that access to semantic knowledge is a necessary condition for manipulation, but a few reports challenged this view. The existence of a direct, pre-semantic route from vision to action has been proposed. We report the case of a patient with a disorder of object use in everyday life, in the context of probable Alzheimer's disease. This patient was also impaired when manipulating single objects. He showed a striking dissociation between impairment in object use and preserved capacity to perform symbolic and meaningless gestures. To elucidate the nature of the disorder, and to clarify the relations between semantic knowledge and object use, we systematically assessed his capacity to recognise, name, access semantic knowledge, and use 15 common objects. We found no general semantic impairment for the objects that were not correctly manipulated, and, more importantly, no difference between the semantic knowledge of objects correctly manipulated and objects incorrectly manipulated. These data, although not incompatible with the hypothesis of a direct route for action, are better accommodated by the idea of a distributed semantic memory, where different types of knowledge are represented, as proposed by Allport (Allport, D. A. Current perspectives in dysphasia, pp. 32-60. Churchill Livingstone, Edinburgh, 1985).

Alzheimer Disease↗

PubMed2002

Semantic involvement in the lexical and sentence processing of Japanese kanji.

This study examined how skilled Japanese readers activate semantic information when reading kanji compound words at both the lexical and sentence levels. Experiment 1 used a lexical decision task for two-kanji compound words and nonwords. When nonwords were composed of kanji that were semantically similar to the kanji of real words, reaction times were longer and error rates were higher than when nonwords had kanji that were not semantically similar. Experiment 2 used a proofreading task (detection of kanji miscombinations) for the same two-kanji compound words and nonwords at the sentence level. In this task, semantically similar nonwords were detected faster than dissimilar nonwords, but error rates were much higher for the semantically similar nonwords. Experiment 3 used a semantic decision task for sentences with the same two-kanji compound words and nonwords. It took longer to detect semantically similar nonwords than dissimilar nonwords. This indicates that semantic involvement in the processing of Japanese kanji produces different effects, depending on whether this processing is done at the lexical or sentence level, which in turn is related to where the reader's attention lies.

Adult↗

PubMed2003

Compound fracture: the role of semantic transparency and morphological headedness.

This paper explores the role of semantic transparency in the representation and processing of English compounds. We focus on the question of whether semantic transparency is best viewed as a property of the entire multimorphemic string or as a property of constituent morphemes. Accordingly, we investigated the processing of English compound nouns that were categorized in terms of the semantic transparency of each of their constituents. Fully transparent such as bedroom are those in which the meanings of each of the constituents are transparently represented in the meaning of the compound as a whole. These compounds were contrasted with compounds such as strawberry, in which only the second constituent is semantically transparent, jailbird, in which only the first constituent is transparent, and hogwash, in which neither constituent is semantically transparent. We propose that significant insights can be achieved through such analysis of the transparency of individual morphemes. The two experiments that we report present evidence that both semantically transparent compounds and semantically opaque compounds show morphological constituency. The semantic transparency of the morphological head (the second constituent in a morphologically right-headed language such as English) was found to play a significant role in overall lexical decision latencies, in patterns of decomposition, and in the effects of stimulus repetition within the experiment.

Humans↗

PubMed2003

Functional imaging of the semantic system: retrieval of sensory-experienced and verbally learned knowledge.

This paper considers how functional neuro-imaging can be used to investigate the organization of the semantic system and the limitations associated with this technique. The majority of the functional imaging studies of the semantic system have looked for divisions by varying stimulus category. These studies have led to divergent results and no clear anatomical hypotheses have emerged to account for the dissociations seen in behavioral studies. Only a few functional imaging studies have used task as a variable to differentiate the neural correlates of semantic features more directly. We extend these findings by presenting a new study that contrasts tasks that differentially weight sensory (color and taste) and verbally learned (origin) semantic features. Irrespective of the type of semantic feature retrieved, a common semantic system was activated as demonstrated in many previous studies. In addition, the retrieval of verbally learned, but not sensory-experienced, features enhanced activation in medial and lateral posterior parietal areas. We attribute these "verbally learned" effects to differences in retrieval strategy and conclude that evidence for segregation of semantic features at an anatomical level remains weak. We believe that functional imaging has the potential to increase our understanding of the neuronal infrastructure that sustains semantic processing but progress may require multiple experiments until a consistent explanatory framework emerges.

Adult↗

PubMed2003

An event-related potential investigation of the relationship between semantic and perceptual levels of representation.

The present study was conducted to investigate relationships between semantic and perceptual levels of representation. A picture-word repetition paradigm was used in which we manipulated the semantic relationship between pictures and words. Experiment 1 involved two types of trials, one with words that had the same meaning as pictures and one with words that were unrelated to pictures. In Experiment 2 we replaced words that were identical in meaning with words that were semantically associated to pictures. In both experiments, visually presented probe stimuli were used to determine the presence of perceptual effects within the visual system, originating from the semantic interaction between words and pictures. In both experiments, conditions with unrelated picture-word pairs generated a search process following the N400 which included activity overlying the visual cortex. Probe stimuli were found to attenuate the amplitude of the search related negativity. The latency of the interaction, which was significant at the time of the N1 response to the probe, suggested that the attempt to find a relationship between the picture and the word involved processing within extrastriate visual areas. UVF probes provided stronger attenuation, possibly because the UVF has direct transmission to the ventral processing stream which is believed to be involved in visual semantic processing. Semantic interactions between matching picture-word pairs in Experiment 1 were found to have an effect on the ERPs to probes presented at the same location as pictures. Probes presented under these conditions showed a stronger P2 over frontal areas followed by a more negative P3 over occipital areas. Although we had expected beforehand to find earlier effects in the latency of the probes' P1 and N1 responses, this result is consistent with the idea that retinotopic levels of object representation are linked with the semantic level of object description. Unlike Experiment 1, same location probes presented in associated picture-word conditions of Experiment 2 did not result in any specific ERP effects on the P2 and P3 components. This suggest that semantic interactions between pictures and words do not automatically propagate to the perceptual level, unless there is direct reference from the word to the visual representation of the object.

Adolescent↗

PubMed2001

Electrophysiological evidence of a semantic system commonly accessed by animals and tools categories.

Some theoretical perspectives propose a semantic system in which categories are represented in different brain regions. Others assume that distinctions are based rather on differences in the demands placed by different categories on shared processing systems. In this study semantic categorization processes were investigated using the recognition potential (RP), an event-related brain response that reflects semantic processing, peaks at around 250 ms after stimulus onset and originates in areas subserving perceptual-semantic analyses. Results indicate that the RP shows some degree of sensitivity to categorization processes, but that categories assumed to differ markedly in their processing demands share, to a large extent, a common neural generator. This provides support for the non-categorical view on the organization of the semantic system, though introducing subtle variations, and suggesting the existence of a semantic subsystem specializing in the processing of perceptual-semantic features regardless of the semantic category involved.

Adolescent↗

PubMed2002

Differential task effects on semantic and syntactic processes as revealed by ERPs.

Two experiments investigated the time-course of semantic and syntactic processes in auditory language comprehension as well as their possible functional dependencies, using event-related brain potentials (ERPs). Participants listened to sentences which were either correct, semantically incorrect, syntactically incorrect, or both semantically and syntactically incorrect. In experiment 1, participants judged the overall correctness of these sentences. The semantic violation elicited an N400 whereas the syntactic phrase structure violation elicited an early anterior negativity followed by a P600. Sentences in which the critical element violated both semantic and syntactic constraints elicited the same pattern of ERPs as the syntactic violation alone, not evoking an N400. In experiment 2, participants judged the same sentences for semantic coherence, required to ignore syntactic violations. Again, an early anterior negativity was elicited for those sentences containing phrase-structure errors. In contrast to experiment 1, however, combined violations elicited both an early negativity and an N400. Together, the results suggest that the N400 associated with semantic aspects of sentence comprehension reflects controlled processes whereas initial parsing processes associated with the early anterior negativity are independent of semantic constraints and task requirements.

Adult↗

PubMed2005

Atypical antipsychotic drugs and organization of long-term semantic memory: multidimensional scaling and cluster analyses of category fluency performance in schizophrenia.

Organization of semantic memory, one of the domains of cognitive function, is impaired in patients with schizophrenia, and is predictive of functional outcomes. The Category Fluency Task (CFT) has been used to evaluate organization of long-term semantic memory by means of visualizing semantic associations in the form of 'cognitive map' and cluster structures. While atypical antipsychotic drugs (AAPDs) have been shown to ameliorate overall cognitive deficits, little is known about the efficacy of AAPDs for improving higher cognitive functions, such as semantic memory organization. The purpose of the present study was to determine if treatment with olanzapine or ziprasidone has beneficial influence on organization of semantic memory, as revealed by analysis of data from the CFT, in patients with schizophrenia. A retrospective analysis of an open-label trial was conducted for 33 patients with schizophrenia who were treated with either olanzapine or ziprasidone. Nineteen subjects were unmedicated at baseline. The CFT and Letter Fluency Task, as well as the Brief Psychiatric Rating Scale (BPRS) and Quality of Life Scale (QLS), were administered at baseline and 6 wk of the treatment. Semantic structures were obtained by multidimensional scaling analysis and hierarchical cluster analysis of verbal outputs from the CFT. At baseline, no meaningful dimension or cluster was observed in the semantic structure; however, knowledge-based dimensions (wild vs. domestic) appeared after treatment with olanzapine or ziprasidone. Cluster structures also became organized, especially after treatment with olanzapine. Scores of QLS, but not those of BPRS, improved during treatment with the AAPDs. These results suggest a facilitative influence of AAPDs on higher cognitive functions, such as organization of semantic memory, in patients with schizophrenia.

Adult↗