Matthew A. Lambon Ralph

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.

Which neuropsychiatric and behavioural features distinguish frontal and temporal variants of frontotemporal dementia from Alzheimer's disease?

OBJECTIVES To investigate the prevalence of changes in mood, personality, and behaviour in frontotemporal dementia (FTD) and Alzheimers disease (AD) and hence, which features reliably distinguish between them. To establish whether the frontal and temporal variants of FTD are characterised by different behavioural changes. METHODS A questionnaire was designed to assess a wide range of neuropsychiatric changes; it incorporated features reported in previous studies of FTD and components of the neuropsychiatric inventory.1 This was completed by 37 carers of patients with Alzheimers disease (AD) and 33 patients with frontotemporal dementia (FTD), comprising 20 with temporal variant FTD (tv FTD) or semantic dementia and 13 with frontal variant FTD (fv FTD). An exploratory principal components factor analysis and discriminant function analysis was applied. RESULTS Factor analysis showed four robust and meaningful symptom clusters: factor 1—stereotypic and eating behaviour; factor 2—executive dysfunction and self care; factor 3—mood changes; factor 4—loss of social awareness. Only stereotypic and altered eating behaviour and loss of social awareness reliably differentiated AD from FTD with no effect of disease severity. By contrast, executive dysfunction, poor self care, and restlessness showed a significant effect of disease severity only, with the more impaired patients scoring more highly. Changes in mood were found to be equally prevalent in the three patient groups. Analysis of individual symptoms showed increased rates of mental rigidity and depression in the patients with semantic dementia compared with those with fv FTD. Conversely, the latter group showed greater disinhibition. Discriminant function analysis correctly classified 71.4% overall and 86.5% of the patients with AD. CONCLUSIONS This questionnaire disclosed striking differences between patients with FTD and AD, but only stereotypic behaviour, changes in eating preference, disinhibition, and features of poor social awareness reliably separated the groups. The patients with fv FTD and semantic dementia were behaviourally very similar, reflecting the involvement of a common network, the ventral frontal lobe, temporal pole, and amygdala. Dysexecutive symptoms and poor self care were found to be affected by the severity of the disease, reflecting perhaps spread to dorsolateral prefrontal areas relatively late in the course of both FTD and AD. This questionnaire may be of value in the diagnosis and the monitoring of therapies.

Age of acquisition effects in adult lexical processing reflect loss of plasticity in maturing systems : Insights from connectionist networks

Early learned words are recognized and produced faster than later learned words. The authors showed that such age of acquisition effects are a natural property of connectionist models trained by back-propagation when patterns are introduced at different points into training and learning of early and late patterns is cumulative and interleaved. Analysis of hidden unit activations indicated that the age of acquisition effect reflects a gradual reduction in network plasticity and a consequent failure to differentiate late items as effectively as early ones. Further simulations examined the effects of vocabulary size, learning rate, sparseness of coding, use of a modified learning algorithm, loss of early items, acquisition of very late items, and lesioning the network. The relationship between age of acquisition and word frequency was explored, including analyses of how the relative influence of these factors is modulated by introducing weight decay.

The Ventral and Inferolateral Aspects of the Anterior Temporal Lobe Are Crucial in Semantic Memory: Evidence from a Novel Direct Comparison of Distortion-Corrected fMRI, rTMS, and Semantic Dementia

Although there is an emerging consensus that the anterior temporal lobes (ATLs) are involved in semantic memory, it is currently unclear which specific parts of this region are implicated in semantic representation. Answers to this question are difficult to glean from the existing literature for 3 reasons: 1) lesions of relevant patient groups tend to encompass the whole ATL region; 2) while local effects of repetitive transcranial magnetic stimulation (rTMS) are spatially more specific, only the lateral aspects of the ATL are available to stimulation; and 3) until recently, functional magnetic resonance imaging (fMRI) studies were hindered by technical limitations such as signal distortion and dropout due to magnetic inhomogeneities and also, in some cases, by methodological factors, including a restricted field of view and the choice of baseline contrast for subtraction analysis. By utilizing the same semantic task across semantic dementia, rTMS, and distortion-corrected fMRI in normal participants, we directly compared the results across the 3 methods for the first time. The findings were highly convergent and indicated that crucial regions within the ATL for semantic representation include the anterior inferior temporal gyrus, anterior fusiform gyrus, and the anterior superior temporal sulcus.

Coherent concepts are computed in the anterior temporal lobes

In his Philosophical Investigations, Wittgenstein famously noted that the formation of semantic representations requires more than a simple combination of verbal and nonverbal features to generate conceptually based similarities and differences. Classical and contemporary neuroscience has tended to focus upon how different neocortical regions contribute to conceptualization through the summation of modality-specific information. The additional yet critical step of computing coherent concepts has received little attention. Some computational models of semantic memory are able to generate such concepts by the addition of modality-invariant information coded in a multidimensional semantic space. By studying patients with semantic dementia, we demonstrate that this aspect of semantic memory becomes compromised following atrophy of the anterior temporal lobes and, as a result, the patients become increasingly influenced by superficial rather than conceptual similarities.

The Neural Organization of Semantic Control: TMS Evidence for a Distributed Network in Left Inferior Frontal and Posterior Middle Temporal Gyrus

Assigning meaning to words, sounds, and objects requires stored conceptual knowledge plus executive mechanisms that shape semantic retrieval according to the task or context. Despite the essential role of control in semantic cognition, its neural basis remains unclear. Neuroimaging and patient research has emphasized the importance of left inferior frontal gyrus (IFG)—however, impaired semantic control can also follow left temporoparietal lesions, suggesting that this function may be underpinned by a large-scale cortical network. We used repetitive transcranial magnetic stimulation in healthy volunteers to disrupt processing within 2 potential sites in this network—IFG and posterior middle temporal cortex. Stimulation of both sites selectively disrupted executively demanding semantic judgments: semantic decisions based on strong automatic associations were unaffected. Performance was also unchanged in nonsemantic tasks—irrespective of their executive demands—and following stimulation of a control site. These results reveal that an extended network of prefrontal and posterior temporal regions underpins semantic control.

Anterior temporal lobes mediate semantic representation: Mimicking semantic dementia by using rTMS in normal participants.

Studies of semantic dementia and PET neuroimaging investigations suggest that the anterior temporal lobes (ATL) are a critical substrate for semantic representation. In stark contrast, classical neurological models of comprehension do not include ATL, and likewise functional MRI studies often fail to show activations in the ATL, reinforcing the classical view. Using a novel application of low-frequency, repetitive transcranial magnetic stimulation (rTMS) over the ATL, we demonstrate that the behavioral pattern of semantic dementia can be mirrored in neurologically intact participants: Specifically, we show that temporary disruption to neural processing in the ATL produces a selective semantic impairment leading to significant slowing in both picture naming and word comprehension but not to other equally demanding, nonsemantic cognitive tasks.

Prototypicality, distinctiveness, and intercorrelation: Analyses of the semantic attributes of living and nonliving concepts.

Many cognitive psychological, computational, and neuropsychological approaches to the organisation of semantic memory have incorporated the idea that concepts are, at least partly, represented in terms of their fine-grained features. We asked 20 normal volunteers to provide properties of 64 concrete items, drawn from living and nonliving categories, by completing simple sentence stems (e.g., an owl is __, has __, can__). At a later date, the same participants rated the same concepts for prototypicality and familiarity. The features generated were classified as to type of knowledge (sensory, functional, or encyclopaedic), and also quantified with regard to both dominance (the number of participants specifying that property for that concept) and distinctiveness (the proportion of exemplars within a conceptual category of which that feature was considered characteristic). The results demonstrate that rated prototypicality is related to both the familiarity of the concept and its distance from the average of the exemplars within the same category (the category centroid). The feature database was also used to replicate, resolve, and extend a variety of previous observations on the structure of semantic representations. Specifically, the results of our analyses (1) resolve two conflicting claims regarding the relative ratio of sensory to other kinds of attributes in living vs. nonliving concepts; (2) offer new information regarding the types of features−across different domains−that distinguish concepts from their category coordinates; and (3) corroborate some previous claims of higher intercorrelations between features of living things than those of artefacts.

SD-Squared: On the Association Between Semantic Dementia and Surface Dyslexia

Within the connectionist triangle model of reading aloud, interaction between semantic and phonological representations occurs for all words but is particularly important for correct pronunciation of lower frequency exception words. This framework therefore predicts that (a) semantic dementia, which compromises semantic knowledge, should be accompanied by surface dyslexia, a frequency-modulated deficit in exception word reading, and (b) there should be a significant relationship between the severity of semantic degradation and the severity of surface dyslexia. The authors evaluated these claims with reference to 100 observations of reading data from 51 cases of semantic dementia. Surface dyslexia was rampant, and a simple composite semantic measure accounted for half of the variance in low-frequency exception word reading. Although in 3 cases initial testing revealed a moderate semantic impairment but normal exception word reading, all of these became surface dyslexic as their semantic knowledge deteriorated further. The connectionist account attributes such cases to premorbid individual variation in semantic reliance for accurate exception word reading. These results provide a striking demonstration of the association between semantic dementia and surface dyslexia, a phenomenon that the authors have dubbed SD-squared.

Conceptual Knowledge Is Underpinned by the Temporal Pole Bilaterally: Convergent Evidence from rTMS

Conceptual knowledge provides the basis on which we bring meaning to our world. Studies of semantic dementia patients and some functional neuroimaging studies indicate that the anterior temporal lobes, bilaterally, are the core neural substrate for the formation of semantic representations. This hypothesis remains controversial, however, as traditional neurological models of comprehension do not posit a role for these regions. To adjudicate on this debate, we conducted 2 novel experiments that used off-line, low-frequency, repetitive transcranial magnetic stimulation to disrupt neural processing temporarily in the left or right temporal poles (TPs). The time required to make semantic decisions was slowed considerably, yet specifically, by this procedure. The results confirm that both TPs form a critical substrate within the neural network that supports conceptual knowledge.