Semantic dementia: clinical, radiological and pathological perspectives

The pathological basis of semantic dementia

Semantic dementia is a syndrome of progressive deterioration in semantic memory (knowledge of objects, people, concepts and words). It falls within the clinical spectrum of frontotemporal dementia but its pathology is yet to be studied systematically. This study included 18 consecutive post mortem cases meeting clinical criteria for semantic dementia. Clinic records and diagnostic histopathology were available for all cases; structural neuroimaging, neuropsychology and semi-quantitative histopathology/immunohistochemistry data were analysed where possible. The pathological diagnosis in a clear majority of cases was frontotemporal degeneration with ubiquitin inclusions (n = 13). Eleven of these cases had characteristic motor neuron disease-type inclusions in the dentate gyrus and cerebral cortex. Ubiquitin inclusions were found only in the inferior olivary nucleus in the other two, one of which was the only case to show degeneration of motor tracts and also to have shown evidence of motor neuron disease during life. None of the patients had motor symptoms or signs at presentation. A family history of motor neuron disease was documented in one case. Pick body-positive Pick's disease appeared three times. Two cases had Alzheimer's disease and significant coincidental Alzheimer-type pathology was also found in one of the ubiquitin inclusion cases. One of the Alzheimer's disease patients had changes in white matter signal on scanning, whereas all other scans showed cerebral atrophy only. Semi-quantitative assessment of regional neuronal loss found that anterior and inferior temporal regions bore the brunt of disease across all histopathological subtypes, usually on the left side, implicating this region in semantic processing.

Neural correlates of semantic and behavioural deficits in frontotemporal dementia

Patients with frontotemporal dementia (FTD) can present with the clinical syndrome of semantic dementia due to a progressive loss of semantic knowledge or a neuropsychiatric syndrome characterised by aberrant social behaviours although frequently both co-exist. It has been assumed that the former is underpinned by damage to the temporal lobes and the latter, predominantly, by damage to the frontal lobes. Using the technique of voxel-based morphometry, we studied a group of FTD cases (n = 18) with a range of cognitive and neuropsychiatric features to correlate loss of semantic knowledge (as measured by the sum of two semantic tests) and aberrant behaviour (as measured by the neuropsychiatric inventory, NPI) with regional loss of grey matter volume. Semantic breakdown correlated with extensive loss of grey matter volume throughout the left anterior temporal lobe and less significantly with right temporal pole and subcallosal gyrus. Aberrant behaviour correlated with loss of grey matter volume in the dorso-mesial frontal lobe--paracingulate region, Brodmann areas 6/8/9--more so on the right. The frontal paracingulate correlation suggests that damage to this region may significantly contribute to the genesis of the behavioural syndrome seen in FTD.

Seeing John Malkovich: the neural substrates of person categorization

Neuroimaging data have implicated regions of the ventral temporal cortex (e.g., fusiform gyrus) as functionally important in face recognition. Recent evidence, however, suggests that these regions are not face-specific, but rather reflect subordinate-level categorical processing underpinned by perceptual expertise. Moreover, when people possess expertise for a particular class of stimuli (e.g., faces), subordinate-level identification is thought to be an automatic process. To investigate the neural substrates of person construal, we used functional magnetic resonance imaging (fMRI) to contrast brain activity while participants judged faces at different levels of semantic specificity (i.e., identity vs. occupation). The results revealed that participants were quicker to access identity than occupational knowledge. In addition, greater activity was observed in bilateral regions of the fusiform gyrus on identity than occupation trials. Taken together, these findings support the viewpoint that person construal is characterized by the ability to access subordinate-level semantic information about people, a capacity that is underpinned by neural activity in discrete regions of the ventral temporal cortex.

The human perirhinal cortex and semantic memory

Studies in macaque monkeys indicate that the perirhinal cortex in the temporal lobe participates in object memory. This function may be analogous to aspects of human semantic memory (knowledge of objects, concepts, faces and words). To date, the status of perirhinal cortex has not specifically been investigated in patients with semantic deficits as seen in semantic dementia, the temporal lobe variant of frontotemporal dementia. High-resolution three-dimensional magnetic resonance imaging was performed in subjects with semantic dementia and Alzheimer's disease (characterized in its early stages by selective episodic memory impairment) and in healthy age-matched controls. Hippocampal, perirhinal, temporopolar and entorhinal cortex volumes were measured by outlining areas on successive scan slices according to recognized landmarks. The entorhinal and hippocampal regions were further subdivided into anterior and posterior parts. In keeping with the hypothesized contribution of the perirhinal cortex to semantic memory function, we found greater involvement of this region, together with the temporopolar and anterior entorhinal cortices, in semantic dementia than in either Alzheimer's disease patients or control subjects. Performance on a range of semantic tests also correlated with perirhinal volume. Bilateral reduction in hippocampal volume compared with controls was seen in Alzheimer's disease. In conclusion, atrophy of the human perirhinal cortex, and of directly connected areas, was associated with semantic memory impairment but not episodic memory impairment, as predicted from the primate work.

Evidence-Based and Intuition-Based Self-Knowledge: An fMRI Study

Behavioral and neuropsychological studies have suggested multiple self-knowledge systems may exist (i.e., evidence-based and intuition-based self-knowledge); however, little is known about the nature of intuition-based self-knowledge. In a functional magnetic resonance imaging study, the neural correlates of intuition-based and evidence-based self-knowledge were investigated. Participants with high and low experience in different domains (soccer and acting) made self-descriptiveness judgments about words from each domain while being scanned. High-experience domain judgments produced activation in a network of neural structures called the X-system, involved in automatic social cognition, whereas low-experience domain judgments produced activations in a network called the C-system, involved in effortful social cognition and propositional thought. The affective and slow-changing nature of intuition-based self-knowledge is discussed.

Young onset dementia

Young onset dementia is a challenging clinical problem with potentially devastating medical and social consequences. The differential diagnosis is wide, and includes a number of rare sporadic and hereditary diseases. However, accurate diagnosis is often possible, and all patients should be thoroughly investigated to identify treatable processes. This review presents an approach to the diagnosis, investigation, and management of patients with young onset dementia, with particular reference to common and treatable causes.

Dorsal and ventral streams: a framework for understanding aspects of the functional anatomy of language

Despite intensive work on language-brain relations, and a fairly impressive accumulation of knowledge over the last several decades, there has been little progress in developing large-scale models of the functional anatomy of language that integrate neuropsychological, neuroimaging, and psycholinguistic data. Drawing on relatively recent developments in the cortical organization of vision, and on data from a variety of sources, we propose a new framework for understanding aspects of the functional anatomy of language which moves towards remedying this situation. The framework posits that early cortical stages of speech perception involve auditory fields in the superior temporal gyrus bilaterally (although asymmetrically). This cortical processing system then diverges into two broad processing streams, a ventral stream, which is involved in mapping sound onto meaning, and a dorsal stream, which is involved in mapping sound onto articulatory-based representations. The ventral stream projects ventro-laterally toward inferior posterior temporal cortex (posterior middle temporal gyrus) which serves as an interface between sound-based representations of speech in the superior temporal gyrus (again bilaterally) and widely distributed conceptual representations. The dorsal stream projects dorso-posteriorly involving a region in the posterior Sylvian fissure at the parietal-temporal boundary (area Spt), and ultimately projecting to frontal regions. This network provides a mechanism for the development and maintenance of "parity" between auditory and motor representations of speech. Although the proposed dorsal stream represents a very tight connection between processes involved in speech perception and speech production, it does not appear to be a critical component of the speech perception process under normal (ecologically natural) listening conditions, that is, when speech input is mapped onto a conceptual representation. We also propose some degree of bi-directionality in both the dorsal and ventral pathways. We discuss some recent empirical tests of this framework that utilize a range of methods. We also show how damage to different components of this framework can account for the major symptom clusters of the fluent aphasias, and discuss some recent evidence concerning how sentence-level processing might be integrated into the framework.

Effects of chronic epilepsy on declarative memory systems

Memory is systematically affected by temporal lobe epilepsy. Since surgery is a promising alternative to pharmacological treatment the questions which memory system is affected and what the long-term prognosis of memory is are more relevant than ever. We address these issues by cross-sectional and longitudinal analysis of memory performance in large series of patients with temporal lobe epilepsy (TLE). The findings indicate that episodic memory rather than semantic memory is impaired in TLE, in particular in TLE with mesial temporal pathology. With the exception that mesial functions appear increasingly affected by chronic non-mesial TLE, memory decline in TLE is not different from that observed in healthy control subjects. However, since patients perform poorer than controls at any age, normal senescence brings patients to mnesic disability at a younger age. Semantic memory seems unaffected by this process but early cortical lesions appear to interfere with knowledge acquisition. Longitudinal data come to a different conclusion regarding the contribution of epilepsy/seizures to memory decline. Conservative treatment is associated with significant decline in figural memory and 37% of the patients experience some memory decline in the long run. Surgery partly anticipates the decline observed with conservative treatment, but losses are most marked after left temporal lobe surgery. After surgery, quite stable memory or even late recovery from surgery is indicated. Leaving aside the surgical intervention, the data provide evidence that the longitudinal memory outcome in TLE is determined by seizure control, seizure severity, mental reserve capacities, and the retest interval. Thus early and efficient seizure control and the prevention of any cerebral damage from the beginning of epilepsy are demanded.

The neuropsychological assessment of dementia

It has become standard practice to base the diagnosis of dementia on the combination of neuropsychological and non-behavioral findings. The present article provides a short, clinically oriented synopsis of the targets, investigational procedures, and difficulties of the modern neuropsychological approach to the diagnosis of dementia. Over the years, neuropsychology has developed assessment tools to evaluate the cognitive and behavioral abnormalities of many dementias. Validated tests of memory, language, executive, and other cognitive functions are used to screen for dementia and identifying certain dementia profiles. Behavioral assessment procedures are available for non-cognitive neurodegenerative alterations. At present, problems arise mainly with the behavioral heterogeneity of certain dementia syndromes. Especially problematic are discrimination of age-associated or mild cognitive impairments from incipient dementia and the impact of psychiatric symptoms on cognitive functions. It is concluded that neuropsychology offers a valuable contribution to the diagnosis and differential diagnosis of dementia.

Episodic memory: insights from semantic dementia

Semantic dementia, also known as the temporal lobe variant of fronto-temporal dementia, results in a progressive yet relatively pure loss of semantic knowledge about words, objects and people, and is associated with asymmetric, focal atrophy of the antero-lateral temporal lobes. Semantic dementia provides a unique opportunity to study the organization of long-term memory particularly since initial observations suggested sparing of episodic memory. Recent studies reveal, however, a more complex but theoretically revealing pattern. On tests of autobiographical memory, patients with semantic dementia show a 'reverse step function' with sparing of recall of events from the most recent 2 to 5 years but impairment on more distant life periods. Anterograde recognition memory for visual materials is extremely well preserved, except in the most deteriorated cases, although performance is heavily reliant upon perceptual information about the studied stimuli, particularly for items that are no longer known by the subjects. On tests of verbal anterograde memory such as word learning, performance is typically poor even for words which are 'known' to the patients. A source discrimination experiment, designed to evaluate familiarity and recollection-based anterograde memory processes, found that patients with semantic dementia showed good item detection, although recollection of source was sometimes impaired. Semantic knowledge about studied items and measures of item detection and source discrimination were largely independent. The implications of these findings for models of long-term memory are discussed. The results support the concept that episodic memory, or at least the recall of temporally specific autobiographical experiences, draws upon a number of separable memory processes, some of which can function independently of semantic knowledge.