Introduction

Semantic dementia (SD; also known as progressive fluent aphasia) is regarded as a part of the spectrum of non-Alzheimer dementias that produce selective atrophy of the anterior temporal and/or orbitomedial frontal lobes; these conditions are referred to collectively as either the frontotemporal dementias (FTD) or frontotemporal lobar degeneration (FTLD) (Neary,1994; Neary et al., 1998). Although previously thought to be rare, FTD in fact has about the same prevalence as Alzheimer's disease (AD) below the age of 65 (Ratnavalli et al., 2002). Three clinical presentations of FTD are commonly described: a behavioral variant (bv-FTD), and two language variants, SD and progressive non-fluent aphasia (PNFA) (Hodges and Miller, 2001a,b). Since the mid 1990s, research on SD has produced a great deal of information about the clinical and neuropsychological features, progression, anatomy and neuropathology of the condition, which we attempt to review and synthesize here.

Early history

Although the term “semantic dementia” is recent (Snowden et al., 1989), the syndrome has been recognized under different labels for over a century. Between 1892 and 1904, Arnold Pick (1892, 1904) reported a series of remarkable cases characterized by progressive amnesic aphasia and changes in behavior; at autopsy these patients had marked atrophy of the left temporal lobe. Pick was perhaps the first neuroscientist to draw attention to the fact that progressive brain atrophy may lead to focal symptoms.

Patterson and Erzinçlioǧlu discuss drawing as one technique for investigating the nature of semantic memory and its disorders. Semantic dementia (SD) is a neurodegenerative condition characterized by gradual degradation of central conceptual knowledge. Given that patients with SD have (a) well-preserved perceptual and motor skills (which means that they can easily copy drawings with the model present) and (b) reasonable preservation of at least some aspects of short-term and episodic memory, the technique of delayed copy drawing can be employed. A line drawing of a familiar object is presented for the patient to study; it is then removed and the patient does something else for 10 or 15 seconds, and is finally asked to draw what she or he was looking at. Because literal visual memory is fragile even in the normal cognitive system, the delayed-copy task evokes and relies on a combination of short-term visual memory and long-term conceptual knowledge. Thus normal individuals, in delayed copy drawing of a rhinoceros, draw it with horns and ‘armoured’ skin not only because they remember seeing these specific features but also because their conceptual knowledge of rhinos demands these features in the reproduction. The degraded knowledge in SD patients means that they cannot recognize this thing as a rhinoceros; they only know it is some sort of animal. The impoverished drawings that they produce a mere 10 seconds after studying the targets provide an informative window on the quality of their conceptual knowledge.

Introduction

The human faculty of language is a breathtaking skill. It allows us to communicate observations, thoughts, wishes, intentions, emotions, etc., to another person in the same room (by speaking), to a person in the next room (by shouting), to someone thousands of kilometers away (by speaking on the telephone or sending a fax), and even to future generations (by writing stories, poems, books, or scientific articles). Language is characterized by almost infinite variation and creativity. Every person alive today (with the exception of pre-verbal infants and people with severely impaired language skills) probably utters a number of sentences every day that he or she has never produced before. What other form of behavior could compete with this for degree of novelty and originality?

Language is typically considered to involve a set of interacting, but somewhat separate, domains of ability or knowledge. These include the sound structure of the language (phonology); word meanings (semantics); the ways in which individual morphemes combine to create complex words (morphology); the ways in which morphologically simple or complex words combine to create phrases and sentences (syntax); and finally, at least in the relatively brief time since a substantial proportion of the world's population has become literate, knowledge of how words are written in the speaker's language (orthography).

How and where does the brain represent and process this complex set of abilities? Because language is unique to humans, we can only learn about this topic by studying humans.

Introduction

In discussing the neuropsychological profile of frontotemporal dementia (FTD) there are a number of particular problems. Firstly, there is the problem of the “gold standard” for diagnosis. Very few neuropsychological studies have included cases with a pathological verification of the diagnosis and, as there is inexact correspondence between the clinical and pathological diagnosis of FTD (see Chapter 7), there are problems of contamination by cases with underlying Alzheimer's disease (AD) pathology (Forman et al., 2006). A brief summary, based upon current clinico-pathological studies, suggests that this is not such a great problem in the case of either behavioural variant frontotemporal dementia (bv-FTD) or semantic dementia (SD), both of which seem rarely to be secondary to AD pathology; but in progressive non-fluent aphasia (PNFA) up to a third of cases have AD, at least in the Cambridge series (Knibb et al., 2006). Some groups have attempted to circumvent this problem by using the pattern of brain atrophy, or hypometabolism on single-photon emission computed tomography (SPECT), as the defining feature and have compared cases classified on this basis (Miller et al., 1991), although again the correspondence between these patterns and pathology is uncertain.

Secondly, there is the somewhat artificial division between the variants of FTD. For instance, many patients with SD have prominent behavioural features, and at least mild semantic deficits are sometimes present in bv-FTD. Thirdly, the problem of the boundary between psychiatric disease, personality disorders and earlystage bv-FTD is a major issue when discussing tests sensitive to bv-FTD. As outlined in Chapter 3, some patients diagnosed as suffering from bv-FTD show no progression over a number of years and there is growing evidence that they may have a behavioural phenocopy of bv-FTD which represents either a functional disruption of frontotemporal networks or a long-standing personality disorder with a degree of late-life decompensation. Thus, unless there is independent evidence of frontal abnormality on imaging or of clear-cut progression, the insensitivity of some tests of frontal lobe function might reflect the fact that groups of FTD patients are contaminated by patients with non-organic disorders.

With these provisos in mind we will discuss neuropsychological findings in FTD and attempt to cover both clinically and theoretically orientated studies.

In his Vie d'Henri Brulard, the French novelist Henri Stendhal articulated the difficulties faced by an autobiographer endeavouring to recapture his life of thirty years earlier: ‘I make many discoveries … They are like great fragments of fresco on a wall, which, long forgotten, reappear suddenly, and by the side of these well-preserved fragments there are … great gaps where there's nothing to be seen but the bricks of the wall. The plaster on which the fresco had been painted has fallen and the fresco has gone forever.’ The richness of human life depends on our ability to remember the past, yet – like Stendhal – we are painfully aware of our memory's selectivity and vulnerability. Whether we are trying to recall particular details of our own lives, or to construct historical narratives describing broader cultural changes, we must all confront the gaps and distortions inherent in recapturing the past. This perplexing faculty, so central to our existence, exerts a universal fascination: as individuals, we wish to learn more about how our own memory functions; as members of society, we are concerned to appreciate the multiple ways in which our history is preserved. What we remember is intimately linked to how we remember, but innumerable approaches have been devised to explore that complex web of connections. The eight chapters in this volume, all by leading experts in their fields, transcend this diversity to address together the relationships between individual experience and collective memory.