Experimental models of vascular dementia and vascular cognitive impairment: a systematic review

Vascular cognitive impairment (VCI) encompasses vascular dementia and is the second most common cause of dementing illness after Alzheimer's disease. The main causes of VCI are: cerebral small vessel disease; multi-infarct dementia; strategic infarct (i.e. located in a functionally-critical brain area); haemorrhage/microbleed; angiopathy (including cerebral amyloid angiopathy); severe hypoperfusion (e.g. cardiac arrhythmia); and hereditary vasculopathy (e.g. cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy, CADASIL). In this systematic analysis, we aimed to relate cognitive and neuropathological features of experimental models to clinical VCI. We extracted data from 107 studies covering 16 models. These included: brief global ischaemic insults (in rats, mice or gerbils); chronic global hypoperfusion (rats, mice, gerbils); chronic hypertension (in primates or stroke-prone, spontaneously-hypertensive rats); multiple ischaemic lesions because of intra-vascular emboli (in rodents, rabbits or primates); strategic ischaemic lesions (in rats or mini-pigs); generalised vasculopathies, because of mutant Notch3, hyperhomocysteinaemia, experimental diabetes mellitus or lack of cerebral vasodilator M(5) receptors (rats or mice). Most cognitive testing showed deficits in working and reference memory. The lesions observed were microinfarcts, diffuse white matter lesions, hippocampal neuronal death, focal ischaemic lesions and micro-haemorrhages. The most-used model was bilateral carotid artery occlusion in rats, leading to chronic hypoperfusion and white matter injury.

Posterior cingulate neurometabolite profiles and clinical phenotype in frontotemporal dementia

Proton magnetic resonance spectroscopy was used to compare metabolite levels from a posterior cingulate voxel in a group of patients with 2 syndromic subtypes of frontotemporal dementia (n=10) and an age and education-matched group with Alzheimer disease (n=10). Overall, frontotemporal dementia was indistinguishable from Alzheimer disease, though differences in N-acetylaspartate emerged between patients with the SD and progressive nonfluent aphasia subtypes, attributable to 2 atypical results among the latter. Such values may index cases with atrophy in posterior cortical regions presenting with progressive nonfluent aphasia.

Patterns of regional brain hypometabolism associated with knowledge of semantic features and categories in Alzheimer's disease

The study of semantic memory in patients with Alzheimer's disease (AD) has raised important questions about the representation of conceptual knowledge in the human brain. It is still unknown whether semantic memory impairments are caused by localized damage to specialized regions or by diffuse damage to distributed representations within nonspecialized brain areas. To our knowledge, there have been no direct correlations of neuroimaging of in vivo brain function in AD with performance on tasks differentially addressing visual and functional knowledge of living and nonliving concepts. We used a semantic verification task and resting 18-fluorodeoxyglucose positron emission tomography in a group of mild to moderate AD patients to investigate this issue. The four task conditions required semantic knowledge of (1) visual, (2) functional properties of living objects, and (3) visual or (4) functional properties of nonliving objects. Visual property verification of living objects was significantly correlated with left posterior fusiform gyrus metabolism (Brodmann's area [BA] 37/19). Effects of visual and functional property verification for non-living objects largely overlapped in the left anterior temporal (BA 38/20) and bilateral premotor areas (BA 6), with the visual condition extending more into left lateral precentral areas. There were no associations with functional property verification for living concepts. Our results provide strong support for anatomically separable representations of living and nonliving concepts, as well as visual feature knowledge of living objects, and against distributed accounts of semantic memory that view visual and functional features of living and nonliving objects as distributed across a common set of brain areas.

Lost in semantic space: a multi-modal, non-verbal assessment of feature knowledge in semantic dementia

A novel, non-verbal test of semantic feature knowledge is introduced, enabling subordinate knowledge of four important concept attributes--colour, sound, environmental context and motion--to be individually probed. This methodology provides more specific information than existing non-verbal semantic tests about the status of attribute knowledge relating to individual concept representations. Performance on this test of a group of 12 patients with semantic dementia (10 male, mean age: 64.4 years) correlated strongly with their scores on more conventional tests of semantic memory, such as naming and word-to-picture matching. The test's overlapping structure, in which individual concepts were probed in two, three or all four modalities, provided evidence of performance consistency on individual items between feature conditions. Group and individual analyses revealed little evidence for differential performance across the four feature conditions, though sound and colour correlated most strongly, and motion least strongly, with other semantic tasks, and patients were less accurate on the motion features of living than non-living concepts (with no such conceptual domain differences in the other conditions). The results are discussed in the context of their implications for the place of semantic dementia within the classification of progressive aphasic syndromes, and for contemporary models of semantic representation and organization.

Presymptomatic semantic impairment in a case of fronto-temporal lobar degeneration associated with the +16 mutation in MAPT

We describe a patient who came to neurological attention because of his at-risk status for the +16 exon 10 splice mutation in the tau gene (microtubule associated protein tau, MAPT), which had given rise to progressive behavioural disturbances in two of his siblings. The patient began to exhibit early signs of behavioural disturbance at around the age of symptom onset in both of his siblings. Although he did not spontaneously complain of difficulties in the domain of language, he met clinical, radiological and neuropsychological criteria for semantic dementia. On the assumption that his illness is mediated by the same pathological process as those of his siblings, we propose that this clinical picture represents the earliest changes of a semantic impairment - a phase of the illness that is often retrospectively described by patients and their relations, but has never previously been documented at first hand. Although typical of semantic dementia in many respects, the illness had several interesting and atypical features that emerged on detailed testing: first, he exhibited no insight into his difficulties; secondly, progression over a twelve-month interval was unusually slow; thirdly, he evinced a striking and consistent advantage for nonliving over living concepts; fourthly, a differential impairment of distinctive over shared knowledge did not emerge except when items that he could still name were compared with those for which he was anomic. Finally, the availability of post mortem pathological analysis from the brains of both of his affected siblings allowed us to attribute his illness to a specific pathological process which is considered unusual for patients with this clinical phenotype.

Knowledge of living, nonliving and "sensory quality" categories in semantic dementia

This article reports the findings from 3 patients with semantic dementia (SD) who were given a novel battery of 33 items from sensory quality categories (SQCs) as previously described by Borgo and Shallice (2001; 2003) and Laiacona, Capitani and Caramazza (2003). Their performance on three tasks (two naming, one word-to-picture matching) was compared with performance on similar tasks using a conventional semantic battery. At the group level, patients performed worse than age-matched controls overall, but neither group showed any differences in performance between domains (i.e., living, nonliving and SQCs). Individual patient analyses, however, showed contrasting profiles in the three patients. The results are discussed in terms of the SFT (Warrington & Shallice, 1984) and individual differences (Lambon-Ralph et al., 2003) accounts of category-specificity in SD.

A presenilin 1 R278I mutation presenting with language impairment

Presenilin (PSEN)1 mutations are responsible for many cases of autosomal dominant Alzheimer disease (AD), although the clinical spectrum has not been fully defined. The authors describe two members of a kindred with a novel PSEN1 mutation (R278I) presenting with language impairment and relative preservation of memory. Screening for PSEN1 mutations may be appropriate in cases of familial dementia even where the clinical phenotype is not typical of AD.

Semantic feature knowledge and picture naming in dementia of Alzheimer's type: a new approach

This study addresses continuing controversies concerning the nature of semantic impairment in early dementia of the Alzheimer type (DAT), and the relationship between conceptual knowledge and picture naming. A series of analyses of fine-grained feature knowledge data show that: (1) distinctive features of concepts were more vulnerable than shared; (2) the amount of attribute knowledge about a concept was associated reliably, and in a graded fashion, with the ability to name a picture of that item; (3) sensory features were differentially important in naming; and (4) the degree of disruption to different types of attribute knowledge did not vary between items from living and nonliving domains. These findings are discussed in the context of contemporary cognitive and computational models of semantic memory organisation.

The effects of very early Alzheimer's disease on the characteristics of writing by a renowned author

Iris Murdoch (I.M.) was among the most celebrated British writers of the post-war era. Her final novel, however, received a less than enthusiastic critical response on its publication in 1995. Not long afterwards, I.M. began to show signs of insidious cognitive decline, and received a diagnosis of Alzheimer's disease, which was confirmed histologically after her death in 1999. Anecdotal evidence, as well as the natural history of the condition, would suggest that the changes of Alzheimer's disease were already established in I.M. while she was writing her final work. The end product was unlikely, however, to have been influenced by the compensatory use of dictionaries or thesauri, let alone by later editorial interference. These facts present a unique opportunity to examine the effects of the early stages of Alzheimer's disease on spontaneous written output from an individual with exceptional expertise in this area. Techniques of automated textual analysis were used to obtain detailed comparisons among three of her novels: her first published work, a work written during the prime of her creative life and the final novel. Whilst there were few disparities at the levels of overall structure and syntax, measures of lexical diversity and the lexical characteristics of these three texts varied markedly and in a consistent fashion. This unique set of findings is discussed in the context of the debate as to whether syntax and semantics decline separately or in parallel in patients with Alzheimer's disease.

Dissociation of lexical syntax and semantics: evidence from focal cortical degeneration

The question of whether information relevant to meaning (semantics) and structure (syntax) relies on a common language processor or on separate subsystems has proved difficult to address definitively because of the confounds involved in comparing the two types of information. At the sentence level syntactic and semantic judgments make different cognitive demands, while at the single word level, the most commonly used syntactic distinction (between nouns and verbs) is confounded with a fundamental semantic difference (between objects and actions). The present study employs a different syntactic contrast (between count nouns and mass nouns), which is crossed with a semantic difference (between naturally occurring and man-made substances) applying to words within a circumscribed semantic field (foodstuffs). We show, first, that grammaticality judgments of a patient with semantic dementia are indistinguishable from those of a group of age-matched controls, and are similar regardless of the status of his semantic knowledge about the item. In a second experiment we use the triadic task in a group of age-matched controls to show that similarity judgments are influenced not only by meaning (natural vs. manmade), but also implicitly by syntactic information (count vs. mass). Using the same task in a patient with semantic dementia we show that the semantic influences on the syntactic dimension are unlikely to account for this pattern in normals. These data are discussed in relation to modular vs. nonmodular models of language processing, and in particular to the semantic-syntactic distinction.

Structure and Deterioration of Semantic Memory: A Neuropsychological and Computational Investigation

Wernicke (1900, as cited in G. H. Eggert, 1977) suggested that semantic knowledge arises from the interaction of perceptual representations of objects and words. The authors present a parallel distributed processing implementation of this theory, in which semantic representations emerge from mechanisms that acquire the mappings between visual representations of objects and their verbal descriptions. To test the theory, they trained the model to associate names, verbal descriptions, and visual representations of objects. When its inputs and outputs are constructed to capture aspects of structure apparent in attribute-norming experiments, the model provides an intuitive account of semantic task performance. The authors then used the model to understand the structure of impaired performance in patients with selective and progressive impairments of conceptual knowledge. Data from 4 well-known semantic tasks revealed consistent patterns that find a ready explanation in the model. The relationship between the model and related theories of semantic representation is discussed.

Cognitive dysfunction after isolated brain stem insult. An underdiagnosed cause of long term morbidity

Cognitive dysfunction adversely influences long term outcome after cerebral insult, but the potential for brain stem lesions to produce cognitive as well as physical impairments is not widely recognised. This report describes a series of seven consecutive patients referred to a neurological rehabilitation unit with lesions limited to brain stem structures, all of whom were shown to exhibit deficits in at least one domain of cognition. The practical importance of recognising cognitive dysfunction in this group of patients, and the theoretical significance of the disruption of specific cognitive domains by lesions to distributed neural circuits, are discussed.

Longitudinal profiles of semantic impairment for living and nonliving concepts in dementia of Alzheimer's type

Two types of theoretical account have been proposed to explain the phenomenon of category-specific impairment in tests of semantic memory: One stresses the importance of different cortical regions to the representation of living and nonliving categories, while the other emphasize the importance of statistical relationships among features of concepts belonging to these two broad semantic domains. Theories of the latter kind predict that the direction of a domain advantage will be determined in large part by the overall damage to the semantic system, and that the profiles of patients with progressive impairments of semantic memory are likely to include a point at which an advantage for one domain changes to an advantage for the other. The present series of three studies employed semantic test data from two separate cohorts of patients with probable dementia of Alzheimer's type (DAT) to look for evidence of such a crossover. In the first study, longitudinal test scores from a cohort of 58 patients were examined to confirm the presence of progressive semantic deterioration in this group. In the second study, Kaplan-Meier survival curves based on serial naming responses and plotted separately for items belonging to living and nonliving domains indicated that the representations of living concepts (as measured by naming) deteriorated at a consistently and significantly faster rate than those of nonliving concepts. A third study, carried out to look in detail at the performance of mildly affected patients, employed an additional cross-sectional cohort of 20 patients with mild DAT and utilized a graded naming assessment. This study also revealed no evidence for a crossover in the advantage of one domain over the other as a function of disease severity. Taken together with the model of anatomical progression in DAT based on the work of Braak and Braak (1991), these findings are interpreted as evidence for the importance of regional cerebral anatomy to the genesis of semantic domain effects in DAT.

Semantic memory is impaired in both dementia with Lewy bodies and dementia of Alzheimer's type: a comparative neuropsychological study and literature review

OBJECTIVE

To test the hypothesis that semantic impairment is present in both patients with dementia with Lewy bodies (DLB) and those with dementia of Alzheimer's type (DAT).

METHODS

A comprehensive battery of neuropsychological tasks designed to assess semantic memory, visuoperceptual function, verbal fluency, and recognition memory was given to groups of patients with DLB (n=10), DAT (n=10) matched pairwise for age and mini mental state examination (MMSE), and age matched normal controls (n=15).

RESULTS

Both DLB and DAT groups exhibited impaired performance across the range of tasks designed to assess semantic memory. Whereas patients with DAT showed equivalent comprehension of written words and picture stimuli, patients with DLB demonstrated more severe semantic deficits for pictures than words. As in previous studies, patients with DLB but not those with DAT were found to have impaired visuoperceptual functioning. Letter and category fluency were equally reduced for the patients with DLB whereas performance on letter fluency was significantly better in the DAT group. Recognition memory for faces and words was impaired in both groups.

CONCLUSIONS

Semantic impairment is not limited to patients with DAT. Patients with DLB exhibit particular problems when required to access meaning from pictures that is most likely to arise from a combination of semantic and visuoperceptual impairments.

Semantic memory is impaired in both dementia with Lewy bodies and dementia of Alzheimer's type: a comparative neuropsychological study and literature review

OBJECTIVE

To test the hypothesis that semantic impairment is present in both patients with dementia with Lewy bodies (DLB) and those with dementia of Alzheimer's type (DAT).

METHODS

A comprehensive battery of neuropsychological tasks designed to assess semantic memory, visuoperceptual function, verbal fluency, and recognition memory was given to groups of patients with DLB (n=10), DAT (n=10) matched pairwise for age and mini mental state examination (MMSE), and age matched normal controls (n=15).

RESULTS

Both DLB and DAT groups exhibited impaired performance across the range of tasks designed to assess semantic memory. Whereas patients with DAT showed equivalent comprehension of written words and picture stimuli, patients with DLB demonstrated more severe semantic deficits for pictures than words. As in previous studies, patients with DLB but not those with DAT were found to have impaired visuoperceptual functioning. Letter and category fluency were equally reduced for the patients with DLB whereas performance on letter fluency was significantly better in the DAT group. Recognition memory for faces and words was impaired in both groups.

CONCLUSIONS

Semantic impairment is not limited to patients with DAT. Patients with DLB exhibit particular problems when required to access meaning from pictures that is most likely to arise from a combination of semantic and visuoperceptual impairments.

Non-verbal semantic impairment in semantic dementia

The clinical presentation of patients with semantic dementia is dominated by anomia and poor verbal comprehension. Although a number of researchers have argued that these patients have impaired comprehension of non-verbal as well as verbal stimuli, the evidence for semantic deterioration is mainly derived from tasks that include some form of verbal input or output. Few studies have investigated semantic impairment using entirely non-verbal assessments and the few exceptions have been based on results from single cases ([3]: Breedin SD, Saffran EM, Coslett HB. Reversal of the concreteness effect in a patient with semantic dementia. Cognitive Neuropsychology 1994;11:617-660, [12]: Graham KS, Becker JT, Patterson K, Hodges JR. Lost for words: a case of primary progressive aphasia? In: Parkin A, editor. Case studies in the neuropsychology of memory, East Sussex: Lawrence Erlbaum, 1997. pp. 83-110, [21]: Lambon Ralph MA, Howard D. Gogi aphasia or semantic dementia? Simulating and assessing poor verbal comprehension in a case of progressive fluent aphasia. Cognitive Neuropsychology, (in-press). This study employed sound recognition and semantic association tasks to investigate the nature of the verbal and non-verbal comprehension deficit in 10 patients with semantic dementia. The patients were impaired on both verbal and non-verbal conditions of the assessments, and their accuracy on these tasks was directly related to their scores on a range of other tests requiring access to semantic memory. Further analyses revealed that performance was graded by concept and sound familiarity and, in addition, identified significant item consistency across the different conditions of the tasks. These results support the notion that the patients' deficits across all modalities were due to degradation within a single, central network of conceptual knowledge. There were also reliable differences between conditions. The sound-picture matching task proved to be more sensitive to semantic impairment than the word-picture matching equivalent, and the patients performed significantly better on the picture than word version of a semantic association test. We propose that these differences arise directly from the nature of the mapping between input modality and semantic memory. Words and sounds have an arbitrary relationship with meaning while pictures benefit from a degree of systematicity with conceptual knowledge about the object.

Semantic dementia: clinical, radiological and pathological perspectives

Semantic dementia (SD) is a recently described clinical syndrome characterised by an acquired progressive inability to name or comprehend common concepts, with little or no distortion of the phonological and syntactic aspects of language, and relative sparing of other aspects of cognition, such as episodic memory, nonverbal problem-solving, and perceptual and visuo-spatial skills. The cognitive locus of this syndrome appears to lie in the permanent store of long-term memory representing general world knowledge-semantic memory. The anatomical distribution of atrophy is less well-defined, and the contribution of various imaging modalities is discussed in the context of a body of 45 published and unpublished cases. We conclude that involvement of the left infero-lateral temporal cortex is the critical area in the genesis of SD. SD probably always represents a non-Alzheimer neurodegenerative process; a variety of pathological lesions may be present, and possible causes, together with debates about their correct classification, are discussed.

The differentiation of semantic dementia and frontal lobe dementia (temporal and frontal variants of frontotemporal dementia) from early Alzheimer's disease: a comparative neuropsychological study

The authors compared age-matched groups of patients with the frontal and temporal lobe variants of frontotemporal dementia (FTD; dementia of frontal type [DFT] and semantic dementia), early Alzheimer's disease (AD), and normal controls (n = 9 per group) on a comprehensive neuropsychological battery. A distinct profile emerged for each group: Those with AD showed a severe deficit in episodic memory with more subtle, but significant, impairments in semantic memory and visuospatial skills; patients with semantic dementia showed the previously documented picture of isolated, but profound, semantic memory breakdown with anomia and surface dyslexia but were indistinguishable from the AD group on a test of story recall; and the DFT group were the least impaired and showed mild deficits in episodic memory and verbal fluency but normal semantic memory. The frontal and temporal presentations of FTD are clearly separable from each other and from early AD.

The differentiation of semantic dementia and frontal lobe dementia (temporal and frontal variants of frontotemporal dementia) from early Alzheimer's disease: a comparative neuropsychological study

The authors compared age-matched groups of patients with the frontal and temporal lobe variants of frontotemporal dementia (FTD; dementia of frontal type [DFT] and semantic dementia), early Alzheimer's disease (AD), and normal controls (n = 9 per group) on a comprehensive neuropsychological battery. A distinct profile emerged for each group: Those with AD showed a severe deficit in episodic memory with more subtle, but significant, impairments in semantic memory and visuospatial skills; patients with semantic dementia showed the previously documented picture of isolated, but profound, semantic memory breakdown with anomia and surface dyslexia but were indistinguishable from the AD group on a test of story recall; and the DFT group were the least impaired and showed mild deficits in episodic memory and verbal fluency but normal semantic memory. The frontal and temporal presentations of FTD are clearly separable from each other and from early AD.

Category specific semantic loss in dementia of Alzheimer's type. Functional-anatomical correlations from cross-sectional analyses

In the context of focal brain injury, selective loss of semantic knowledge in the domain of either natural kinds or artefacts is usually considered to reflect the differential importance of temporal and frontoparietal regions to the representations of perceptual and functional attributes, respectively. It is harder to account far as a feature of a more diffuse process, and previous cross-sectional analyses of patients with dementia of Alzheimer's type (DAT) have differed over whether category effects occur. In our series of 58 patients with probable DAT, we demonstrated a significant group advantage for artefacts, and explored possible reasons for the inconsistency of this finding in other studies. A multiple single-case strategy revealed not only individuals with consistent advantages for artefacts but also individuals with consistent advantages for natural kinds. By ranking the individuals according to measures of naming performance and global intellectual ability, we showed that the strength of the group advantage for artefacts was dependent on the former but not the latter variable. The findings are discussed in the context of two competing theories of semantic breakdown in DAT. One differentiates between domains of knowledge in terms of the structure of semantic representations within a single distributed network; the other emphasizes the importance of different brain regions in the category distinction. We conclude that our findings are in keeping with the predictions of the latter hypothesis.