Lexical knowledge degradation in schizophrenia

Semantic Memory Impairment Across the Schizophrenia Continuum: A Meta-Analysis of Category Fluency Performance

Semantic memory (SM) impairments are a core feature of schizophrenia and are present along the psychosis continuum. It is, however, unclear whether the degree of SM impairments vary along this continuum and if demographic and clinical factors affect impairment severity. This study performed meta-analyses of category fluency task performance (a task commonly used to assess SM) in 4 groups along the schizophrenia continuum: high schizotypes (HSZT), first-degree relatives (FDR), recent-onset patients (≤2 y; ROP) and chronic patients (CSZ). Electronic databases were searched for relevant studies published up to October 2019 resulting in the inclusion of 48 articles. The main analyses assessed fluency productivity scores in 2978 schizophrenia spectrum disorder patients, 340 first-degree relatives of schizophrenia spectrum disorder patients, and 3204 healthy controls. Further analyses assessed errors, mean cluster size, and switching data that were available in the CSZ group only. Results revealed significant impairments in fluency productivity were present in the FDR, ROP, and CSZ groups relative to healthy controls, but not in HSZT. In the CSZ group, significant differences relative to healthy controls were also observed in non-perseverative errors, mean cluster size, and number of switches. The findings collectively suggest that SM deficits are present at each stage of the continuum and are exacerbated post-illness onset. They also support the centrality of SM impairments in schizophrenia and most elevated risk groups. Future studies with more diverse measures of SM function are needed to replicate and extend this research.

The effect of associative strength on semantic priming in schizophrenia

The present research was designed to investigate the pattern of semantic priming in schizophrenia as a function of strength of association (or semantic distance between concepts in the semantic network). Thirty schizophrenia patients, without formal thought disorder, and twenty-nine healthy controls participated in a lexical decision task in which prime-target associative strength (strong, weak and not related) and stimulus-onset asynchrony (SOA: 250ms and 750ms) were manipulated. Patients and controls showed the same associative strength effect on RTs. In the short SOA condition priming effects were obtained for both strong and weak prime-target associative conditions. However in the long SOA priming was only significant for strongly associated pairs. This pattern of priming effects was similar in both groups, with higher priming on the short SOA and strong association conditions. Altogether results suggest that automatic semantic spreading activation is unimpaired in schizophrenia patients without formal thought disorder. These results are in line with the general evidence of impaired implicit priming observed only in patients with formal thought disorder. At the same time patients use context as controls to facilitate word processing. Finally, these findings evidence that, prime-target associative strength could moderate results in studies of semantic memory deficits in schizophrenia.

Assessing the Relationship between Semantic Processing and Thought Disorder Symptoms in Schizophrenia

Aberrant semantic processing has been linked to the etiology of formal thought disorder (TD) symptoms in schizophrenia. In this cross-sectional study, two prominent theories, overactivation and disorganized structure of semantic memory (SM), were examined in relation to TD symptoms using the continuum approach across two established semantic tasks (direct/indirect semantic priming and categorical fluency). The aim was to examine the validity of the two TD theories in relation to TD symptoms in schizophrenia. Greater direct and indirect priming, fluency productivity and category errors were expected if the data supported the overactivation theory. Reduced fluency productivity and increased category errors would be characteristic of disorganized storage. Fifty-seven schizophrenia/schizoaffective disorder patients and 48 controls completed a clinical assessment and the semantic tasks. There was significantly reduced direct priming in patients compared to controls (p<.05), while indirect priming was not significantly different; there was no association between TD and degree of priming. Patients produced more category-inappropriate words (p<.005) than controls, which was related to increasing severity of circumstantiality. The pattern of results was more indicative of a disorganized SM storage problem in this sample. This phenomenon may underlie some TD symptoms in general schizophrenia. The findings strengthen the relationship between SM deficits and TD symptoms, though this appears to differ between individual symptoms. The authors discuss the value of the continuum approach in addressing research questions in TD etiology. Given low levels of TD in this study, replication of these findings in a sample with greater TD is desirable.

Are conceptual abilities impaired in schizophrenia?

Whereas semantic processing deficits are well-documented in schizophrenia, conceptual abilities have been poorly explored. This study aims at specifically exploring conceptualization abilities in 34 adults fulfilling schizophrenia according to DSM-IV and 34 healthy controls with similar socio-demographic characteristics. The 2 groups were assessed on the WAIS-R similarities test and the concept generation test (Raoux et al., 2014) consisting of free-sorting 6 cards of pictures of animals and geometric shapes to be separated in two groups or categories based on common attributes. After each sorting, the participant is asked to explain his/her sorting. Whereas the schizophrenic patients performed significantly poorer than the control participants in the semantic knowledge and lexico-semantic tests, there was no difference neither in the WAIS-R similarities test nor in the concept generation test, which supports the hypothesis of preserved high level conceptualization abilities in schizophrenia. However, qualitative differences in performing the concept generation test were evidenced. The patients used more often mixed criteria leading them to compare two different hierarchical levels (e.g., low-level physical attributes vs. high-level semantic criteria). Furthermore, the qualitative analysis based on the explanations provided by the participants shows that the categorizations achieved by schizophrenic patients are more often based on unexpected criteria.

Semantic clustering of category fluency in schizophrenia examined with singular value decomposition

Decreased productivity on verbal fluency tasks by persons with schizophrenia has been attributed to semantic system abnormalities. Semantic structure is often assessed using multidimensional scaling (MDS) to detect normal and aberrant semantic clustering. However, MDS has limitations that may be particularly problematic for such assessments. Here, we introduce a different clustering technique, singular value decomposition (SVD), to elucidate abnormalities of the semantic system in schizophrenia. We compared 102 treated outpatients with schizophrenia to 109 healthy adults on two category-cued word fluency tasks. Patients with schizophrenia showed semantic clustering patterns that differ markedly from those of healthy adults. However, SVD revealed more detailed and critical semantic system abnormalities than previously appreciated using MDS. Patients with schizophrenia showed less coherent semantic clustering of both low- and high-frequency category exemplars than healthy adults. These results suggest the intriguing possibility that impaired automatic activation of semantic information is a key deficit in schizophrenia.

Executive control in schizophrenia in task involving semantic inhibition and working memory

Executive dysfunctions have been consistently demonstrated in patients with schizophrenia. This study aimed to investigate deficits in specific executive functioning components, namely working memory and inhibition, in schizophrenia. In study 1, a set of neurocognitive function tests was administered to 41 patients with schizophrenia and 25 healthy controls to capture specific components of executive functioning, including semantic inhibition (the Stroop-like paradigm and the Chinese Version of the Hayling Sentence Completion Test (HSC)), working memory (the spatial n-back), and response inhibition (the stop signal task (SST)). Results showed that schizophrenia patients did significantly worse than controls under both working memory and inhibition demands in the Stroop-like paradigm. In particular, patients were impaired when inhibiting a semantically associated response; and performance was correlated with negative symptoms. In study 2, we employed a modified semantic inhibitory error monitoring paradigm to examine whether patients with schizophrenia (n=11) were impaired in semantic inhibitory error monitoring or not as compared to 11 healthy controls. The results suggested that patients with schizophrenia in this study remained intact in semantic inhibition error monitoring. There was no difference in the semantic inhibitory monitoring performance between healthy controls and patients with schizophrenia. Taken together, these results suggested impaired working memory context maintenance and semantic inhibition in schizophrenia patients, and these impairments were related to clinical symptoms of schizophrenia.

Action and object naming in schizophrenia

Patients with schizophrenia demonstrate impaired action verbal fluency, but no study has examined verb-noun differences using picture naming. The present study compared object and action naming in 20 adult patients diagnosed with schizophrenia (DSM-IV-TR, Diagnostic and Statistical Manual of Mental Disorders-Fourth Edition, Text Revision; American Psychiatric Association, 2000) criteria, and 20 demographically matched healthy controls, using pictures. Overall, schizophrenic patients showed poorer naming than controls on all measures of object and action lexical semantic access and retrieval despite normal comprehension for action and object names. Results further indicated that action names were significantly more difficult to retrieve than object names in schizophrenic patients. The absence of dissociation in comprehension of action and object names but semantic errors in naming both classes suggests intact conceptual-semantic stores among middle-aged community-dwelling outpatients with schizophrenia but difficulties mapping semantics onto the lexicon. Action-naming impairments can arise from both semantic and postsemantic origins in schizophrenia. These results have implications for the neurobiology of language given the association between both schizophrenia and verb processing and frontal damage. Moreover, the issue being addressed is important for a cognitive characterization of schizophrenia and for an understanding of the representations of action and object names in the brain.

Is semantic memory impaired in schizophrenia? A systematic review and meta-analysis of 91 studies

INTRODUCTION

Semantic memory impairments in schizophrenia have been reported across a wide range of neuropsychological tests. Set against a backdrop of fairly widespread cognitive impairments, it is difficult to know whether there is a primary, or secondary, impairment of semantic memory in schizophrenia. Also, whether there is a profile of differential impairment across the range of neuropsychological tests.

METHODS

Employing a systematic search strategy, 91 papers were identified which have assessed participants with schizophrenia on a measure of semantic memory. A series of meta-analyses were then conducted which provided combined weighted means for performance on tasks of naming, word-picture matching, verbal fluency, priming, and categorisation.

RESULTS

An uneven profile of impairment is reported with large effect sizes for tests of naming and verbal fluency, medium effect sizes for word-picture matching and association and small effect sizes for categorisation and priming tests.

CONCLUSIONS

This uneven profile supports the claim that a degradation of semantic knowledge may not be adequate in explaining the semantic memory impairment in schizophrenia. This conclusion is supported by the data which report a relationship between an executive dysfunction and poor priming and fluency performance particularly. The data support a link between Formal Thought Disorder and semantic memory impairments on tests of naming and verbal fluency but on other tests evidence is equivocal.

Verbal fluency, semantics, context and symptom complexes in schizophrenia

Lexical-semantic access and retrieval was examined in 15 adults diagnosed with schizophrenia and matched controls. This study extends the literature through the inclusion of multiple examinations of lexical-semantic production within the same patient group and through correlating performance on these tasks with various positive and negative clinical symptoms. On tasks of verbal fluency, meaning generation, sentence production using contextual information and confrontation naming, participants with schizophrenia made significantly more semantic errors on naming tasks; produced fewer meanings for homophones; produced fewer items on semantic, phonological, cued and switching fluency tasks; and produced more errors on sentence production tasks when compared to healthy controls. Significant correlations were also observed between ratings of psychomotor poverty and measures of semantic production and mental inflexibility. This study has provided additional evidence for deficits in lexical-semantic retrieval which are not due to underlying semantic store degradation, do not involve phonological based retrieval, and at the level of sentence generation appear to vary as a function of the contextual constraints provided.

Semantic memory impairment in schizophrenia--deficit in storage or access of knowledge?

This study evaluates whether patients with schizophrenia have a degraded memory store for semantic knowledge. 20 patients with a chronic history of schizophrenia and evidence of cognitive impairment were selected, since the literature indicates that this subgroup is most likely to manifest a degraded semantic knowledge store. Their profile of semantic memory impairments was compared to that of a group of Alzheimer's Dementia (AD) patients (n=22), who met neuropsychological criteria for degraded semantic store. Both groups were matched for Performance IQ. 15 elderly healthy controls were also included in the study. The AD and schizophrenia groups produced substantially different profiles of semantic memory impairment. This is interpreted as indicating that the semantic impairments in this subgroup of patients with schizophrenia do not result from a degraded store. This is corroborated by an analysis of the data using other neuropsychological criteria for determining degraded store. We conclude that there is little evidence for a classic degradation of semantic knowledge in schizophrenia, and it appears that impairments result from an inability to use semantic knowledge appropriately, particularly when selection of salient semantic relations is required.

Formal thought disorder is characterised by impaired lexical access

Recent studies have proposed that difficulty with accessing the lexical-semantic memory store may underpin some of the specific linguistic problems associated with formal thought disorder (FTD). We examined the consistency of name retrieval as an indicator of the ability to access lexical-semantic knowledge in patients with and without marked FTD to see if problems are specific to the former or common to schizophrenic patients in general. A graded naming test was administered on two separate occasions 8-16 weeks apart to 48 participants in three groups: 16 schizophrenic patients with high ratings of FTD, 16 schizophrenic patients with low ratings of FTD and 16 healthy controls. We compared the groups for naming consistency across time and the relationship between naming consistency and specific symptoms of FTD. Both patient groups had impaired naming and this was significantly greater in high than low FTD patients. The high FTD patients showed a profile that differed from both low FTD patients and healthy controls insofar as their naming was inconsistent across time, characteristic of an access disorder. Specifically, the FTD symptoms of derailment, tangentality and incoherence were related to the ability to access the lexical-semantic store. In conclusion, most patients with schizophrenia show an impaired semantic memory store. Nevertheless, FTD is associated with additional lexical-semantic difficulties that are quantitatively different to those seen in patients without FTD, and which may reflect disorganized semantic access.

Are semantic deficits in schizophrenia due to problems with access or storage?

Semantic processing deficits are central to cognitive abnormalities in schizophrenia. Such semantic deficits may be related to either poor access or poor storage of semantic knowledge. 32 schizophrenia patients and 32 matched normal controls performed five semantic processing tasks that examined item-specific consistency over time, the word frequency effect and semantic priming. A subgroup of patients performed tasks on three separate occasions. It is generally assumed that a storage deficit is signalled by item-specific consistent performance, an exaggerated effect of word frequency, and the occurrence of hyperpriming; an access deficit is signalled by the absence of these effects. The data demonstrated item-specific consistency, a frequency effect and significant hyperpriming. The pattern is consistent with a storage deficit of semantic memory in schizophrenia.

Name relearning in elderly patients with schizophrenia: episodic and temporary, not semantic and permanent

INTRODUCTION

Recent reports of lexical-semantic deficits in patients with schizophrenia (Laws, Al-Uzri, & Mortimer, 2000; Laws, McKenna, & Kondel, 1998) suggest that younger patients have problems accessing intact memories and older patients show apparent "loss" of the lexical-semantic memory representations themselves.

METHODS

Picture naming for everyday items was examined in a unique series of elderly patients with schizophrenia (n = 10) with a mean illness duration of 45.5 years; and compared with that in patients with probable Alzheimer's disease (n = 18) and elderly healthy controls (n = 27). Naming consistency across time was used as an indicator of whether the schizophrenic patients had difficulty accessing representations or a loss of the representations themselves. Finally, we examined the ability of the schizophrenic patients to relearn the names of unnamed items across four weekly retraining sessions and to retain them at a one month follow-up.

RESULTS

The elderly schizophrenic patients were as anomic as patients with probable Alzheimer's disease. Consistency analysis revealed that the patients had storage deficits. Analysis of patient error types was consistent with a semantic deficit. Finally, the schizophrenic patients showed significant improvement with relearning, but this was not maintained at follow-up.

CONCLUSIONS

Elderly patients with schizophrenia show a profound and stable anomia. Although name relearning induced some significant gains in naming, these were short-term and reflect episodic rather than semantic reinstatement of representations. Implications for cognitive remediation are discussed.

A longitudinal analysis of memory in patients with schizophrenia

Memory deficits are widely reported in patients with schizophrenia, but uncertainties remain about the extent and the longitudinal course of these deficits. Twenty-eight patients with a DSM-IV diagnosis of schizophrenia were tested on multiple aspects of memory at baseline, 9- and 18-month follow-up. Measures included: digit span, the Rivermead Behavioural Memory test (RBMT) battery, the Graded Naming Test (GNT) and several computerized memory tests from the Cambridge Automated Neuropsychological Testing Battery (CANTAB). A group of healthy controls (N=17) was tested on the CANTAB battery at baseline and 9-month follow up. The patients performed significantly poorer than controls on all CANTAB measures; however, there was no difference in change between groups over a 9-month period. Within-group patient comparisons revealed that symptoms reduced significantly over the study period, but had no association with memory. Significant improvements were observed for patients on two verbal memory tasks: the GNT and digit span, but not on any other measure. Interestingly, these were the only two tests on which patients were within normal limits at baseline. This study shows that patients with schizophrenia have deficits in multiple aspects of memory which remain stable over long periods of time. In addition, patients showed a tendency to improve on memory tasks which contained a verbal component.

Storage and access procedures in schizophrenia: evidence for a two phase model of lexical impairment

Evidence has accumulated to show that schizophrenia is characterized by lexical-semantic difficulties; however, questions remain about whether schizophrenics have problems in accessing intact representations or a loss of the representations themselves. Both access and storage types of disorder have been reported and it has been speculated that this may reflect a transition from the former to latter with increasing length of illness. This study investigated whether illness duration, age or estimated premorbid IQ predict the size and accessibility of the lexical store. Fifty-six schizophrenic patients (chosen to represent a wide range of illness duration from 3-40 years) and 24 matched healthy controls were asked to name 120 pictures on two occasions. Estimates of store size and retrieval probability were derived from a two parameter stochastic Markov chain model. This revealed that even early in the course of illness, schizophrenics appear to have suffered a reduction in lexical store size and that those with longer length of illness show deficits in both their store size and their ability to retrieve names from that store.

An early marker for semantic memory impairment in patients with schizophrenia

INTRODUCTION

Semantic memory impairment is now a well-documented phenomenon in patients with schizophrenia. Nevertheless, the characteristics of this deficit and any early markers remain contentious.

METHODS

In this preliminary study, 12 schizophrenic patients underwent longitudinal assessment using a battery of semantic memory tests. Patient performance was compared to 12 matched controls. Using set criteria (derived from Warrington & Shallice, 1979), we examined whether the patients had a disorder affecting access to intact representations, or a degradation/loss of the representations themselves. The criteria were: consistency across time and modality, level of attribute information, and responsiveness to cueing. Finally, we compared patient naming for the same items across two naming tests (naming-to-description and picture naming) to determine cross-modality consistency.

RESULTS

As expected, normal controls outperformed the patients on all tests. Naming-to-description was the most significant differentiator between patients and controls. Patients were inconsistent across both time and modality, showed minimal attributional knowledge impairment, and improved significantly with cueing on two naming tests.

CONCLUSION

The profile of results indicates an access-type semantic deficit in this cohort of patients with schizophrenic. Finally, on a naming-to-description task, the patients failed to name up to 20% of items that they could name to picture. This suggests that naming-to-definition may act as an early marker of semantic memory impairment.

Lateralized Lexical Decision in Schizophrenia: Hemispheric Specialization and Interhemispheric Lexicality Priming

Reports of left-hemisphere dysfunction and abnormal interhemispheric transfer in schizophrenia are mixed. The authors used a unified paradigm, the lateralized lexical decision task, to assess hemispheric specialization in word recognition, hemispheric error monitoring, and interhemispheric transfer in male, right-handed participants with schizophrenia (n=34) compared with controls (n=20). Overall, performance and error monitoring were worse in patients. However, patients like controls showed left-hemisphere superiority for lexical processing and right-hemisphere superiority for error monitoring. Only patients showed selective-interhemispheric lexicality priming for accuracy, in which performance improved when the lexical status of target and distractor stimuli presented to each hemifield was congruent. Results suggest that schizophrenia is associated with impaired monitoring and with increased interhemispheric automatic information transfer rather than with changed hemispheric specialization for language or error monitoring.

Interhemispheric transfer and laterality effects in simple visual reaction time in schizophrenics

INTRODUCTION

There is evidence that schizophrenics have an abnormal corpus callosum and an abnormal pattern of cerebral asymmetries. We investigated whether there are corresponding functional abnormalities in interhemispheric transfer (IT) and laterality effects.

METHODS

Medicated schizophrenic patients and matched controls were tested in the Poffenberger paradigm, that is, a simple manual reaction time (RT) paradigm with laterally presented visual stimuli designed to provide a behavioural estimate of IT. By subtracting RT averaged across the uncrossed hand-hemifield conditions, from RT averaged across the crossed hand-hemifield conditions, one can obtain an estimate of IT time.

RESULTS

In schizophrenic patients the difference between crossed and uncrossed conditions was 0 because of an unusually prolonged RT in the uncrossed condition right hand/ right field. A broadly similar result has been obtained previously in the tactile modality (Ditchfield & Hemsley, 1990) and is consistent with a left hemisphere impairment. This effect was still present when the patients were retested about 2 years later.

CONCLUSIONS

These results demonstrate the existence in schizophrenic patients of a consistent slowing down of simple visuomotor responses subserved by the left hemisphere.