Objects and properties: a study of the breakdown of semantic memory

Validated measures of semantic knowledge and semantic control: normative data from young and older adults for more than 300 semantic judgements

Recent studies suggest that knowledge representations and control processes are the two key components underpinning semantic cognition, and are also crucial indicators of the shifting cognitive architecture of semantics in later life. Although there are many standardized assessments that provide measures of the quantity of semantic knowledge participants possess, normative data for tasks that probe semantic control processes are not yet available. Here, we present normative data from more than 200 young and older participants on a large set of stimuli in two semantic tasks, which probe controlled semantic processing (feature-matching task) and semantic knowledge (synonym judgement task). We verify the validity of our norms by replicating established age- and psycholinguistic-property-related effects on semantic cognition. Specifically, we find that older people have more detailed semantic knowledge than young people but have less effective semantic control processes. We also obtain expected effects of word frequency and inter-item competition on performance. Parametrically varied difficulty levels are defined for half of the stimuli based on participants' behavioural performance, allowing future studies to produce customized sets of experimental stimuli based on our norms. We provide all stimuli, data and code used for analysis, in the hope that they are useful to other researchers.

Using concept typicality to explore semantic representation and control in healthy ageing

Successful use of conceptual knowledge entails the assembling of semantic representations and control processes to access the subsets of knowledge relevant in each situation. Research has suggested that representation and control mechanisms interact to support categorization. Notably, depleted representations in semantic dementia and disrupted control processes in semantic aphasia impair categorization of atypical concepts. Yet, it remains unclear how knowledge accumulation and control decay in healthy ageing impact categorization. To address this question, we compared young and older adults' performance in a categorization task of items varying in concept typicality. Critically, older adults were more accurate in categorizing atypical concepts than the younger counterparts, as indicated by the interaction between group and typicality. Moreover, the elderly outperformed the younger in categorizing atypical concepts that were also less familiar. Thus, the decay in semantic control observed along ageing did not significantly hinder the categorization of atypical items. Our data suggest that, relative to young adults, older adults possess enriched conceptual knowledge, which supports retrieval of the category-related features needed for categorizing atypical and less familiar exemplars.

A Brief Drawing Task for the Differential Diagnosis of Semantic Dementia

BACKGROUND

Semantic dementia (SD) is a subtype of frontotemporal lobe degeneration characterized by semantic loss, with other cognitive functions initially preserved. SD requires differential diagnosis with Alzheimer's disease (AD) and behavioral variant frontotemporal dementia (bvFTD). Semantic knowledge can be evaluated through different tests; however, most of them depend on language.

OBJECTIVE

We describe the development of a brief drawing task that may be helpful for the differential diagnosis of SD.

METHODS

Seventy-two patients, including 32 AD, 19 bvFTD, and 21 SD were asked to draw 12 items with different age of acquisition and familiarity, belonging to four different semantic categories. We employed the drawings of healthy volunteers to build a scoring scheme.

RESULTS

Turtle, strawberry, train, and envelope were the items of each category that best discriminated between groups and were selected for the Brief drawing task. The discriminatory power of the Brief drawing task between SD versus AD and bvFTD patients, estimated through the area under the curve was 0.84 (95% CI = 0.72-0.96, p = 0.000007). In a logistic model, the Brief drawing task (p = 0.003) and VOSP "number location" subtest (p = 0.016) were significant predictors of the diagnosis of SD versus AD and bvFTD after adjustment by the main covariates. The Brief drawing task provided clinically useful qualitative information. SD drawings were characterized by loss of the distinctive features, intrusions, tendency to prototype, and answers like "I don't know what this is".

CONCLUSION

The Brief drawing task appears to reveal deficits in semantic knowledge among patients with SD that may assist in the differential diagnosis with other neurodegenerative diseases.

A snake that bites its own tail. Acquisition and loss of concepts in children and semantic dementia patients through the analysis of drawings

OBJECTIVE

The structure of the semantic network is constructed and organized during childhood development. Previous publications have hypothesized that neurodegenerative diseases would lead to a disruption of this network reversing the steps acquired in childhood. Semantic Dementia (SD) is a subtype of frontotemporal lobe degeneration in which the main symptom is a specific loss of semantic memory. We aimed to describe the sequential acquisition of concepts in 3-8 years old children evaluated through the production of drawings and, in parallel, their progressive loss in SD patients.

METHODS

104 children between 40 and 96 months categorized into tertiles according to their age, 21 SD patients categorized into tertiles according to their score on a category fluency task and 34 healthy volunteers were asked to draw 12 items with, a priori, different age of acquisition and familiarity, belonging to four different semantic categories. We employed the drawings of the healthy volunteers to build a scoring scheme. We considered that a concept was acquired in children when 50% or more of its features were present in their drawings, and it was lost in patients when more than 50% were missing.

RESULTS

Those concepts which the children were able to acquire earlier, according to our scoring scheme, tended to remain in patients with more advanced SD. While the items that children acquired later, were, in general, those that the SD patients lost at earlier disease stages.

CONCLUSION

The patterns of concept acquisition in children were the mirror image of the loss in patients with SD. Our study supports the hypothesis that the sequence of concept acquisition in childhood is reversed in SD patients.

Divergent effects of healthy ageing on semantic knowledge and control: Evidence from novel comparisons with semantically impaired patients

Effective use of semantic knowledge requires a set of conceptual representations and control processes which ensure that currently relevant aspects of this knowledge are retrieved and selected. It is well-established that levels of semantic knowledge increase across the lifespan. However, the effects of ageing on semantic control processes have not been assessed. I addressed this issue by comparing the performance profiles of young and older people on a verbal comprehension test. Two sets of variables were used to predict accuracy and RT in each group: (1) the psycholinguistic properties of words probed in each trial and (2) the performance on each trial by two groups of semantically impaired neuropsychological patients. Young people demonstrated poor performance for low-frequency and abstract words, suggesting that they had difficulty processing words with intrinsically weak semantic representations. Indeed, performance in this group was strongly predicted by the performance of patients with semantic dementia, who suffer from degradation of semantic knowledge. In contrast, older adults performed poorly on trials where the target semantic relationship was weak and distractor relationships strong - conditions which require high levels of controlled processing. Their performance was not predicted by the performance of semantic dementia patients, but was predicted by the performance of patients with semantic control deficits. These findings indicate that the effects of ageing on semantic cognition are more complex than has previously been assumed. While older people have larger stores of knowledge than young people, they appear to be less skilled at exercising control over the activation of this knowledge.

Concepts, control, and context: A connectionist account of normal and disordered semantic cognition

Semantic cognition requires conceptual representations shaped by verbal and nonverbal experience and executive control processes that regulate activation of knowledge to meet current situational demands. A complete model must also account for the representation of concrete and abstract words, of taxonomic and associative relationships, and for the role of context in shaping meaning. We present the first major attempt to assimilate all of these elements within a unified, implemented computational framework. Our model combines a hub-and-spoke architecture with a buffer that allows its state to be influenced by prior context. This hybrid structure integrates the view, from cognitive neuroscience, that concepts are grounded in sensory-motor representation with the view, from computational linguistics, that knowledge is shaped by patterns of lexical co-occurrence. The model successfully codes knowledge for abstract and concrete words, associative and taxonomic relationships, and the multiple meanings of homonyms, within a single representational space. Knowledge of abstract words is acquired through (a) their patterns of co-occurrence with other words and (b) acquired embodiment, whereby they become indirectly associated with the perceptual features of co-occurring concrete words. The model accounts for executive influences on semantics by including a controlled retrieval mechanism that provides top-down input to amplify weak semantic relationships. The representational and control elements of the model can be damaged independently, and the consequences of such damage closely replicate effects seen in neuropsychological patients with loss of semantic representation versus control processes. Thus, the model provides a wide-ranging and neurally plausible account of normal and impaired semantic cognition.

The contribution of executive control to semantic cognition: Convergent evidence from semantic aphasia and executive dysfunction

Semantic cognition, as described by the controlled semantic cognition (CSC) framework (Rogers et al., 2015, Neuropsychologia, 76, 220), involves two key components: activation of coherent, generalizable concepts within a heteromodal ‘hub’ in combination with modality‐specific features (spokes), and a constraining mechanism that manipulates and gates this knowledge to generate time‐ and task‐appropriate behaviour. Executive–semantic goal representations, largely supported by executive regions such as frontal and parietal cortex, are thought to allow the generation of non‐dominant aspects of knowledge when these are appropriate for the task or context. Semantic aphasia (SA) patients have executive–semantic deficits, and these are correlated with general executive impairment. If the CSC proposal is correct, patients with executive impairment should not only exhibit impaired semantic cognition, but should also show characteristics that align with those observed in SA. This possibility remains largely untested, as patients selected on the basis that they show executive impairment (i.e., with ‘dysexecutive syndrome’) have not been extensively tested on tasks tapping semantic control and have not been previously compared with SA cases. We explored conceptual processing in 12 patients showing symptoms consistent with dysexecutive syndrome (DYS) and 24 SA patients, using a range of multimodal semantic assessments which manipulated control demands. Patients with executive impairments, despite not being selected to show semantic impairments, nevertheless showed parallel patterns to SA cases. They showed strong effects of distractor strength, cues and miscues, and probe–target distance, plus minimal effects of word frequency on comprehension (unlike semantic dementia patients with degradation of conceptual knowledge). This supports a component process account of semantic cognition in which retrieval is shaped by control processes, and confirms that deficits in SA patients reflect difficulty controlling semantic retrieval.

The meaning of 'life' and other abstract words: Insights from neuropsychology

There are a number of long-standing theories on how the cognitive processing of abstract words, like 'life', differs from that of concrete words, like 'knife'. This review considers current perspectives on this debate, focusing particularly on insights obtained from patients with language disorders and integrating these with evidence from functional neuroimaging studies. The evidence supports three distinct and mutually compatible hypotheses. (1) Concrete and abstract words differ in their representational substrates, with concrete words depending particularly on sensory experiences and abstract words on linguistic, emotional, and magnitude-based information. Differential dependence on visual versus verbal experience is supported by the evidence for graded specialization in the anterior temporal lobes for concrete versus abstract words. In addition, concrete words have richer representations, in line with better processing of these words in most aphasic patients and, in particular, patients with semantic dementia. (2) Abstract words place greater demands on executive regulation processes because they have variable meanings that change with context. This theory explains abstract word impairments in patients with semantic-executive deficits and is supported by neuroimaging studies showing greater response to abstract words in inferior prefrontal cortex. (3) The relationships between concrete words are governed primarily by conceptual similarity, while those of abstract words depend on association to a greater degree. This theory, based primarily on interference and priming effects in aphasic patients, is the most recent to emerge and the least well understood. I present analyses indicating that patterns of lexical co-occurrence may be important in understanding these effects.

Disorders of representation and control in semantic cognition: Effects of familiarity, typicality, and specificity

We present a case-series comparison of patients with cross-modal semantic impairments consequent on either (a) bilateral anterior temporal lobe atrophy in semantic dementia (SD) or (b) left-hemisphere fronto-parietal and/or posterior temporal stroke in semantic aphasia (SA). Both groups were assessed on a new test battery designed to measure how performance is influenced by concept familiarity, typicality and specificity. In line with previous findings, performance in SD was strongly modulated by all of these factors, with better performance for more familiar items (regardless of typicality), for more typical items (regardless of familiarity) and for tasks that did not require very specific classification, consistent with the gradual degradation of conceptual knowledge in SD. The SA group showed significant impairments on all tasks but their sensitivity to familiarity, typicality and specificity was more variable and governed by task-specific effects of these factors on controlled semantic processing. The results are discussed with reference to theories about the complementary roles of representation and manipulation of semantic knowledge.

Neurocognitive insights on conceptual knowledge and its breakdown

Conceptual knowledge reflects our multi-modal 'semantic database'. As such, it brings meaning to all verbal and non-verbal stimuli, is the foundation for verbal and non-verbal expression and provides the basis for computing appropriate semantic generalizations. Multiple disciplines (e.g. philosophy, cognitive science, cognitive neuroscience and behavioural neurology) have striven to answer the questions of how concepts are formed, how they are represented in the brain and how they break down differentially in various neurological patient groups. A long-standing and prominent hypothesis is that concepts are distilled from our multi-modal verbal and non-verbal experience such that sensation in one modality (e.g. the smell of an apple) not only activates the intramodality long-term knowledge, but also reactivates the relevant intermodality information about that item (i.e. all the things you know about and can do with an apple). This multi-modal view of conceptualization fits with contemporary functional neuroimaging studies that observe systematic variation of activation across different modality-specific association regions dependent on the conceptual category or type of information. A second vein of interdisciplinary work argues, however, that even a smorgasbord of multi-modal features is insufficient to build coherent, generalizable concepts. Instead, an additional process or intermediate representation is required. Recent multidisciplinary work, which combines neuropsychology, neuroscience and computational models, offers evidence that conceptualization follows from a combination of modality-specific sources of information plus a transmodal 'hub' representational system that is supported primarily by regions within the anterior temporal lobe, bilaterally.

The degraded concept representation system in semantic dementia: damage to pan-modal hub, then visual spoke

The core clinical feature of semantic dementia is a progressive yet selective degradation of conceptual knowledge. Understanding the cognitive and neuroanatomical basis for this deficit is a key challenge for both clinical and basic science. Some researchers attribute the deficit to damage to pan-modal conceptual representations that are independent of any particular sensory-motor modality and are represented in the ventrolateral anterior temporal lobes. Others claim that damage to modality-specific visual feature representations in the occipitotemporal 'ventral stream' is responsible. In the present study, we tested the hypothesis that concept degradation in semantic dementia involves a combination of these pan-modal and modality-specific elements. We investigated factors influencing knowledge of object concepts by analysing 43 sets of picture-naming data from patients with semantic dementia. We found a strong influence of two pan-modal factors: highly familiar and typical items were named more accurately than less familiar/atypical items at all stages of the disorder. Items associated with rich sensory-motor information were also named more successfully at all stages, and this effect was present for sound/motion knowledge and tactile/action knowledge when these modalities were studied separately. However, there was no advantage for items rich in visual colour/form characteristics; instead, this factor had an increasingly negative impact in the later stages of the disorder. We propose that these results are best explained by a combination of (i) degradation of modality-independent conceptual representations, which is present throughout the disorder and is a consequence of atrophy focused on the ventrolateral anterior temporal lobes; and (ii) a later additional deficit for concepts that depend heavily on visual colour/form information, caused by the spreading of atrophy to posterior ventral temporal regions specialized for representing this information. This explanation is consistent with a graded hub-and-spoke model of conceptual knowledge, in which there is a gradual convergence of information along the temporal lobes, with visual attributes represented in the posterior cortex giving way to pan-modal representations in the anterior areas.

A semantic contribution to nonword recall? Evidence for intact phonological processes in semantic dementia

Patients with semantic dementia make numerous phonological errors in their immediate serial recall of words that they understand poorly. Previous studies have argued that these errors result from a reduction in the normal contribution made by semantics to the coherence of items in the phonological system. It is possible, however, that the errors might reflect additional subtle phonological deficits. Six patients with semantic dementia were tested on a variety of phonological processing and short-term memory tasks, in order to explore these possibilities. For the most part, the patients showed normal performance in phonological awareness and discrimination tasks and normal effects of phonological similarity and word length in immediate serial recall. The more severely impaired patients, however, showed some weakness on tests of nonword repetition and recall. Every patient showed better recall of words that were still relatively well understood, compared with words that were more semantically degraded. This difference extended to nonwords that were phonologically similar to the known and degraded words, suggesting that the patients' semantic deficits could account for their impairments in nonword recall. The recall advantage for semantically known over degraded items also extended to a nonverbal delayed picture copying task, suggesting that the patients' immediate serial recall impairments were underpinned by a central semantic deficit, and not by a separable lexical deficit.

Semantic memory is impaired in patients with unilateral anterior temporal lobe resection for temporal lobe epilepsy

Contemporary clinical and basic neuroscience studies have increasingly implicated the anterior temporal lobe regions, bilaterally, in the formation of coherent concepts. Mounting convergent evidence for the importance of the anterior temporal lobe in semantic memory is found in patients with bilateral anterior temporal lobe damage (e.g. semantic dementia), functional neuroimaging and repetitive transcranial magnetic stimulation studies. If this proposal is correct, then one might expect patients with anterior temporal lobe resection for long-standing temporal lobe epilepsy to be semantically impaired. Such patients, however, do not present clinically with striking comprehension deficits but with amnesia and variable anomia, leading some to conclude that semantic memory is intact in resection for temporal lobe epilepsy and thus casting doubt over the conclusions drawn from semantic dementia and linked basic neuroscience studies. Whilst there is a considerable neuropsychological literature on temporal lobe epilepsy, few studies have probed semantic memory directly, with mixed results, and none have undertaken the same type of systematic investigation of semantic processing that has been conducted with other patient groups. In this study, therefore, we investigated the semantic performance of 20 patients with resection for chronic temporal lobe epilepsy with a full battery of semantic assessments, including more sensitive measures of semantic processing. The results provide a bridge between the current clinical observations about resection for temporal lobe epilepsy and the expectations from semantic dementia and other neuroscience findings. Specifically, we found that on simple semantic tasks, the patients' accuracy fell in the normal range, with the exception that some patients with left resection for temporal lobe epilepsy had measurable anomia. Once the semantic assessments were made more challenging, by probing specific-level concepts, lower frequency/more abstract items or measuring reaction times on semantic tasks versus those on difficulty-matched non-semantic assessments, evidence of a semantic impairment was found in all individuals. We conclude by describing a unified, computationally inspired framework for capturing the variable degrees of semantic impairment found across different patient groups (semantic dementia, temporal lobe epilepsy, glioma and stroke) as well as semantic processing in neurologically intact participants.

The importance of multiple assessments of object knowledge in semantic dementia: the case of the familiar objects task

Semantic dementia (SD) is characterized by a dramatic loss of conceptual knowledge about the meaning of words and the identity of objects. Previous research has suggested that SD patients' knowledge is differentially influenced by the disease and may decline at different degrees depending on a patient's everyday familiarity with certain items. However, no study has examined (a) semantic knowledge deterioration and (b) the potential significance of autobiographical experience for the maintenance of object concepts in the same cohort of SD patients by using comprehensive assessments of different aspects of object knowledge across an experience-based, distributed semantic memory network. Here, we tested four SD patients and three Alzheimer's disease (AD) control patients using a range of tasks - including naming, gesture generation, and autobiographical knowledge - with personally familiar objects or perceptually similar or different object analogs. Our results showed dissociations between performance on naming relative to other assessments of object knowledge between SD and AD patients, though we did not observe a reliable familiar objects advantage. We discuss different factors that may account for these findings, as well as their implications for research on SD.

Reverse concreteness effects are not a typical feature of semantic dementia: evidence for the hub-and-spoke model of conceptual representation

The role of anterior temporal lobes (ATLs) in semantic processing is controversial. One theory, influenced by semantic dementia (SD) patients, is that this region is a pan-modal hub for all concepts. An alternative view is that atrophy in SD specifically affects knowledge for visual features. This is supported by reports of reverse concreteness effects in a few SD patients, suggesting that abstract word knowledge is spared relative to concrete words. However, it is not clear whether such effects are typical in SD, hence reliably associated with ATL damage, because most reports are of single cases and group studies have produced conflicting results. To address these contradictions, we investigated concreteness effects in 7 SD patients, using multiple tests from earlier studies in addition to new assessments. Comprehension was impaired for both word types but was better for concrete words. However, this pattern was not found uniformly across all tests and was most likely to be observed when: 1) concrete and abstract words were well matched for word frequency and 2) concrete and abstract words were selected with sufficient variation along the imageability scale. These factors account for the variability in previous studies and indicate that reverse concreteness effects are not common in SD.

Semantic diversity accounts for the "missing" word frequency effect in stroke aphasia: insights using a novel method to quantify contextual variability in meaning

Word frequency is a powerful predictor of language processing efficiency in healthy individuals and in computational models. Puzzlingly, frequency effects are often absent in stroke aphasia, challenging the assumption that word frequency influences the behavior of any computational system. To address this conundrum, we investigated divergent effects of frequency in two comprehension-impaired patient groups. Patients with semantic dementia have degraded conceptual knowledge as a consequence of anterior temporal lobe atrophy and show strong frequency effects. Patients with multimodal semantic impairments following stroke (semantic aphasia [SA]), in contrast, show little or no frequency effect. Their deficits arise from impaired control processes that bias activation toward task-relevant aspects of knowledge. We hypothesized that high-frequency words exert greater demands on cognitive control because they are more semantically diverse--they tend to appear in a broader range of linguistic contexts and have more variable meanings. Using latent semantic analysis, we developed a new measure of semantic diversity that reflected the variability of a word's meaning across different context. Frequency, but not diversity, was a significant predictor of comprehension in semantic dementia, whereas diversity was the best predictor of performance in SA. Most importantly, SA patients did show typical frequency effects but only when the influence of diversity was taken into account. These results are consistent with the view that higher-frequency words place higher demands on control processes, so that when control processes are damaged the intrinsic processing advantages associated with higher-frequency words are masked.

The organization and dissolution of semantic-conceptual knowledge: is the 'amodal hub' the only plausible model?

In recent years, the anatomical and functional bases of conceptual activity have attracted a growing interest. In particular, Patterson and Lambon-Ralph have proposed the existence, in the anterior parts of the temporal lobes, of a mechanism (the 'amodal semantic hub') supporting the interactive activation of semantic representations in all modalities and for all semantic categories. The aim of then present paper is to discuss this model, arguing against the notion of an 'amodal' semantic hub, because we maintain, in agreement with the Damasio's construct of 'higher-order convergence zone', that a continuum exists between perceptual information and conceptual representations, whereas the 'amodal' account views perceptual informations only as a channel through which abstract semantic knowledge can be activated. According to our model, semantic organization can be better explained by two orthogonal higher-order convergence systems, concerning, on one hand, the right vs. left hemisphere and, on the other hand, the ventral vs. dorsal processing pathways. This model posits that conceptual representations may be mainly based upon perceptual activities in the right hemisphere and upon verbal mediation in the left side of the brain. It also assumes that conceptual knowledge based on the convergence of highly processed visual information with other perceptual data (and mainly concerning living categories) may be bilaterally represented in the anterior parts of the temporal lobes, whereas knowledge based on the integration of visual data with action schemata (namely knowledge of actions, body parts and artefacts) may be more represented in the left fronto-temporo-parietal areas.

Personal experience and arithmetic meaning in semantic dementia

Arithmetic skills are generally claimed to be preserved in semantic dementia (SD), suggesting functional independence of arithmetic knowledge from other aspects of semantic memory. However, in a recent case series analysis we showed that arithmetic performance in SD is not entirely normal. The finding of a direct association between severity of patients' semantic disorder and arithmetic impairment pointed to a closer relationship between semantic memory and arithmetic knowledge than previously acknowledged. The present study aimed to determine whether arithmetic performance in SD is subject to the same influences as has been found in other semantic domains, namely an autobiographical effect. SD patients were assessed on their ability to perform arithmetic in personally relevant compared to non-personal contexts. Patients who were regular, current followers of the television game show Countdown performed better on a simplified version of the show's number game compared to a conventional written calculation task, whereas patients with no experience of Countdown demonstrated the opposite pattern. SD patients showed better knowledge of multiplication table 'facts' when these were embedded in a situation relevant to their daily lives compared to a non-personal arithmetic task. They also performed better on a written calculation task when problems were presented as a monetary transaction compared to a standard arithmetic problem. A reference group of patients with Alzheimer's disease (AD) showed the reverse pattern. The findings provide convincing evidence that autobiographical relevance influences SD patients' arithmetic performance. Moreover, they challenge current views on conceptual number knowledge as a unitary, abstract competence.

"Pre-semantic" cognition revisited: critical differences between semantic aphasia and semantic dementia

Patients with semantic dementia show a specific pattern of impairment on both verbal and non-verbal "pre-semantic" tasks, e.g., reading aloud, past tense generation, spelling to dictation, lexical decision, object decision, colour decision and delayed picture copying. All seven tasks are characterised by poorer performance for items that are atypical of the domain and "regularization errors" (irregular/atypical items are produced as if they were domain-typical). The emergence of this pattern across diverse tasks in the same patients indicates that semantic memory plays a key role in all of these types of "pre-semantic" processing. However, this claim remains controversial because semantically impaired patients sometimes fail to show an influence of regularity. This study demonstrates that (a) the location of brain damage and (b) the underlying nature of the semantic deficit affect the likelihood of observing the expected relationship between poor comprehension and regularity effects. We compared the effect of multimodal semantic impairment in the context of semantic dementia and stroke aphasia on the seven "pre-semantic" tasks listed above. In all of these tasks, the semantic aphasia patients were less sensitive to typicality than the semantic dementia patients, even though the two groups obtained comparable scores on semantic tests. The semantic aphasia group also made fewer regularization errors and many more unrelated and perseverative responses. We propose that these group differences reflect the different locus for the semantic impairment in the two conditions: patients with semantic dementia have degraded semantic representations, whereas semantic aphasia patients show deregulated semantic cognition with concomitant executive deficits. These findings suggest a reinterpretation of single-case studies of comprehension-impaired aphasic patients who fail to show the expected effect of regularity on "pre-semantic" tasks. Consequently, such cases do not demonstrate the independence of these tasks from semantic memory.

Neuroimaging of semantic processing in schizophrenia: a parametric priming approach

The use of fMRI and other neuroimaging techniques in the study of cognitive language processes in psychiatric and non-psychiatric conditions has led at times to discrepant findings. Many issues complicate the study of language, especially in psychiatric populations. For example, the use of subtractive designs can produce misleading results. We propose and advocate for a semantic priming parametric approach to the study of semantic processing using fMRI methodology. Implications of this parametric approach are discussed in view of current functional neuroimaging research investigating the semantic processing disturbance of schizophrenia.

Comprehension of concrete and abstract words in semantic dementia

The vast majority of brain-injured patients with semantic impairment have better comprehension of concrete than abstract words. In contrast, several patients with semantic dementia (SD), who show circumscribed atrophy of the anterior temporal lobes bilaterally, have been reported to show reverse imageability effects, that is, relative preservation of abstract knowledge. Although these reports largely concern individual patients, some researchers have recently proposed that superior comprehension of abstract concepts is a characteristic feature of SD. This would imply that the anterior temporal lobes are particularly crucial for processing sensory aspects of semantic knowledge, which are associated with concrete not abstract concepts. However, functional neuroimaging studies of healthy participants do not unequivocally predict reverse imageability effects in SD because the temporal poles sometimes show greater activation for more abstract concepts. The authors examined a case-series of 11 SD patients on a synonym judgment test that orthogonally varied the frequency and imageability of the items. All patients had higher success rates for more imageable as well as more frequent words, suggesting that (1) the anterior temporal lobes underpin semantic knowledge for both concrete and abstract concepts, (2) more imageable items--perhaps because of their richer multimodal representations--are typically more robust in the face of global semantic degradation and (3) reverse imageability effects are not a characteristic feature of SD.

Semantic memory is key to binding phonology: converging evidence from immediate serial recall in semantic dementia and healthy participants

Patients with semantic dementia (SD) make numerous phoneme migration errors when recalling lists of words they no longer fully understand, suggesting that word meaning makes a critical contribution to phoneme binding in verbal short-term memory. Healthy individuals make errors that appear similar when recalling lists of nonwords, which also lack semantic support. Although previous studies have assumed that the errors in these two groups stem from the same underlying cause, they have never been directly compared. We tackled this issue by examining immediate serial recall for SD patients and controls on "pure" word lists and "mixed" lists that contained a mixture of words and nonwords. SD patients were equally poor at pure and mixed lists and made numerous phoneme migration errors in both conditions. In contrast, controls recalled pure lists better than mixed lists and only produced phoneme migrations for mixed lists. We also examined the claim that semantic activation is critical for words in the primacy portion of the list. In fact, the effect of mixed lists was greatest for later serial positions in the control group and in the SD group recall was poorest towards the ends of lists. These results suggest that mixing nonwords with words in healthy participants closely mimics the impact of semantic degradation in SD on word list recall. The study provides converging evidence for the idea that lexical/semantic knowledge is an important source of constraint on phonological coherence, ensuring that phonemes in familiar words are bound to each other and emerge together in recall.

Deficits of knowledge versus executive control in semantic cognition: insights from cued naming

Deficits of semantic cognition in semantic dementia and in aphasia consequent on CVA (stroke) are qualitatively different. Patients with semantic dementia are characterised by progressive degradation of central semantic representations, whereas multimodal semantic deficits in stroke aphasia reflect impairment of executive processes that help to direct and control semantic activation in a task-appropriate fashion [Jefferies, E., & Lambon Ralph, M. A. (2006). Semantic impairment in stroke aphasia vs. semantic dementia: A case-series comparison. Brain 129, 2132-2147]. We explored interactions between these two aspects of semantic cognition by examining the effects of cumulative phonemic cueing on picture naming in case series of these two types of patient. The stroke aphasic patients with multimodal semantic deficits cued very readily and demonstrated near-perfect name retrieval when cumulative phonemic cues reached or exceeded the target name's uniqueness point. Therefore, knowledge of the picture names was largely intact for the aphasic patients, but they were unable to retrieve this information without cues that helped to direct activation towards the target response. Equivalent phonemic cues engendered significant but much more limited benefit to the semantic dementia patients: their naming was still severely impaired even when most of the word had been provided. In contrast to the pattern in the stroke aphasia group, successful cueing was mainly confined to the more familiar un-named pictures. We propose that this limited cueing effect in semantic dementia follows from the fact that concepts deteriorate in a graded fashion [Rogers, T. T., Lambon Ralph, M. A., Garrard, P., Bozeat, S., McClelland, J. L., & Hodges, J. R., et al. (2004). The structure and deterioration of semantic memory: A neuropsychological and computational investigation. Psychological Review 111, 205-235]. For partially degraded items, the residual conceptual knowledge may be insufficient to drive speech production to completion but these items might reach threshold when they are bolstered by cues.

The impact of semantic impairment on verbal short-term memory in stroke aphasia and semantic dementia: A comparative study

This study presents the first direct comparison of immediate serial recall in semantic dementia (SD) and transcortical sensory aphasia (TSA). Previous studies of the effect of semantic impairment on verbal short-term memory (STM) have led to important theoretical advances. However, different conclusions have been drawn from these two groups. This research aimed to explain these inconsistencies. We observed (a) qualitative differences between SD and TSA in the nature of the verbal STM impairment and (b) considerable variation within the TSA group. The SD and TSA patients all had poor semantic processing and good phonology. Reflecting this, both groups remained sensitive to phonological similarity and showed a reduced effect of lexicality in immediate serial recall. The SD patients showed normal serial position effects; in contrast, the TSA patients had poor recall of the initial list items and exhibited large recency effects on longer lists. The error patterns of the two groups differed: the SD patients made numerous phoneme migration errors whereas the TSA group were more likely to produce entire words in the wrong order, often initiating recall with terminal list items. The SD cases also showed somewhat larger effects of word frequency and imageability. We propose that these contrasting performance patterns are explicable in terms of the nature of the underlying semantic impairment. SD is associated with anterior lobe atrophy and produces degradation of semantic knowledge - this is more striking for less frequent/imageable items, accentuating the effects of these lexical/semantic variables in STM. SD patients frequently recombine the phonemes of different list items due to the reduced semantic constraint upon phonology (semantic binding: Patterson et al., 1994). In contrast, the semantic impairment in TSA follows frontal or temporoparietal lesions and is associated with poor executive control of semantic processing (deregulated semantic cognition: Jefferies and Lambon Ralph, 2006), explaining why these patients are liable to recall entire words out of serial order.

Semantic impairment in stroke aphasia versus semantic dementia: a case-series comparison

Different neuropsychological populations implicate diverse cortical regions in semantic memory: semantic dementia (SD) is characterized by atrophy of the anterior temporal lobes whilst poor comprehension in stroke aphasia is associated with prefrontal or temporal-parietal infarcts. This study employed a case-series design to compare SD and comprehension-impaired stroke aphasic patients directly on the same battery of semantic tests. Although the two groups obtained broadly equivalent scores, they showed qualitatively different semantic deficits. The SD group showed strong correlations between different semantic tasks--regardless of input/output modality--and substantial consistency when a set of items was assessed several times. They were also highly sensitive to frequency/familiarity and made coordinate and superordinate semantic errors in picture naming. These findings support the notion that amodal semantic representations degrade in SD. The stroke aphasia group also showed multimodal deficits and consistency across different input modalities, but inconsistent performance on tasks requiring different types of semantic processing. They were insensitive to familiarity/frequency--instead, tests of semantic association were influenced by the ease with which relevant semantic relationships could be identified and distractors rejected. In addition, the aphasic patients made associative semantic errors in picture naming that SD patients did not make. The aphasic patients' picture naming performance improved considerably with phonemic cues suggesting that these patients retained knowledge that could not be accessed without contextual support. We propose that semantic cognition is supported by two interacting principal components: (i) a set of amodal representations (which progressively degrade in SD) and (ii) executive processes that help to direct and control semantic activation in a task-appropriate fashion (which are dysfunctional in comprehension-impaired stroke aphasic patients).

The natural history of late-stage "pure" semantic dementia

Relatively little is known about the neuropsychological profile of late-stage semantic dementia. This article provides a detailed assessment of patient MK who, despite her very severe semantic impairments, remained cooperative to testing and, unusually, did not show additional behavioral/personality changes. Although MK's initial presentation was typical of semantic dementia (SD), her performance began to deviate from the normal pattern. She developed impairments of single word repetition and regular word reading, and began to produce phonological errors in picture naming and spontaneous speech. These deficits might suggest that late-stage SD includes an independent disorder of phonology. An alternative possibility, however, is that phonological processing cannot proceed normally in the face of profound semantic degradation. A series of experiments supported the latter explanation of MK's deficits. In picture naming, MK showed little effect of progressive phonological cueing, did not reveal an increased sensitivity to word length or phonological complexity and continued to show a high degree of item-specific consistency in both accuracy and errors: she tended to produce the same erroneous phonemes for each item. She remained sensitive to the effects of phonological similarity in immediate serial recall. Letter substitution errors in regular word reading were more common for lower frequency letters (e.g., Q, Z). These letters also produced more item errors in immediate serial recall, suggesting that a frequency-graded loss of letter knowledge, rather than separate orthographic and phonological deficits, accounted for the deficits in both of these tasks. These findings are discussed in terms of theories that posit strong interactivity between phonology and semantics.

The role of the temporal lobe semantic system in number knowledge: evidence from late-stage semantic dementia

Previous reports have demonstrated that many aspects of number knowledge remain unimpaired in semantic dementia, despite severe comprehension problems in other domains. It is argued that this advantage for numbers arises because the disease spares the parietal lobe magnitude system thought to be critical for number processing. Models of numerical cognition that favour a separation between verbal and magnitude representations of number might, however, predict a restricted impairment of the verbal number code in this condition. We obtained support for this hypothesis in a patient with late-stage semantic dementia. She was impaired at a variety of tasks tapping the verbal number code; for example, reading and writing Arabic numerals, naming and word-picture matching with dot pictures, reading aloud number words, digit span and magnitude comparison/serial ordering tasks with number words. In contrast, she demonstrated good understanding of the magnitude and serial order of numbers when tested with Arabic numerals and non-symbolic representations. These findings suggest that although the magnitude meaning of numbers is isolated from the temporal lobe semantic system, the anterior infero-temporal lobe may play a critical role in binding English number words to their non-symbolic magnitude meaning.

Surface dyslexia in semantic dementia: a comparison of the influence of consistency and regularity

This study addressed the question of exactly which aspects of spelling-sound consistency influence accuracy of reading aloud in surface dyslexic patients with semantic dementia. Oral reading data were obtained from twelve patients on three sets of words that varied in regularity (defined according to grapheme-phoneme correspondences) and consistency (defined according to the pronunciation of word body neighbours). The patients were less accurate for irregular/inconsistent words, which they commonly pronounced in line with sound-spelling regularities, as expected in surface dyslexia. They produced plausible but incorrect responses for some regular as well as many irregular words, suggesting that their reading performance was influenced by sound-spelling relationships not captured by grapheme-phoneme correspondences. On a set of items that varied consistency and regularity independently, the patients showed a large effect of regularity and a smaller but significant effect of consistency in reading aloud. In addition, there was a correlation between degree of semantic impairment and level of reading accuracy for inconsistent items. These findings are discussed in terms of two influential models of reading: the dual-route-cascaded model (Coltheart et al., 2001) and the triangle model (Plaut et al., 1996). It is argued that the triangle model provides a more straightforward account of the relationship between word comprehension and consistency effects in reading.

A category-specific advantage for numbers in verbal short-term memory: evidence from semantic dementia

This study explored possible reasons for the striking difference between digit span and word span in patients with semantic dementia. Immediate serial recall (ISR) of number and non-number words was examined in four patients. For every case, the recall of single-digit numbers was normal whereas the recall of non-number words was impaired relative to controls. This difference extended to multi-digit numbers, and remained even when frequency, imageability, word length, set size and size of semantic category were matched for the numbers and words. The advantage for number words also applied to the patients' reading performance. Previous studies have suggested that semantic memory plays a critical role in verbal short-term memory (STM) and reading: patients with semantic dementia show superior recall and reading of words that are still relatively well known compared to previously known but now semantically degraded words. Additional assessments suggested that this semantic locus was the basis of the patients' category-specific advantage for numbers. Comprehension was considerably better for number than non-number words. Number knowledge may be relatively preserved in semantic dementia because the cortical atrophy underlying the condition typically spares the areas of the parietal lobes thought to be crucial in numerical cognition but involves the inferolateral temporal-lobes known to support general conceptual knowledge.

When does word meaning affect immediate serial recall in semantic dementia?

Patients with semantic dementia can show superior immediate recall of words that they still understand relatively well, as compared with more semantically degraded words, suggesting that conceptual knowledge makes a major contribution to phonological short-term memory. However, a number of studies have failed to show such a recall difference, challenging this view. We examined the effect of several methodological factors on the recall of known and degraded words in 4 patients with semantic dementia, in order to investigate possible reasons for this discrepancy. In general, our patients did exhibit poorer recall of the degraded words and made more phonological errors on these items. In addition, set size affected the magnitude of the recall advantage for known words. This finding suggests that semantic degradation influenced the rate of learning in the immediate recall task when the same items were presented repeatedly. The methods used to select known and degraded items also impacted on the recall difference. List length, however, did not affect the advantage for known words. The coherence of items in phonological short-term memory was affected by their semantic status, but not by the length of the material to be retained. The implications of these findings for the role of semantic and phonological representations in verbal short-term memory are discussed.

The semantic category effect increases with worsening anomia in Alzheimer's type dementia

A large number of patients (n=72) with probable Alzheimer's type dementia (DAT) and mild cognitive impairment (MCI) carried out a picture naming task which comprised stimuli from biological and nonbiological categories. The results were stratified into five ranges of overall naming ability. Every group except those with scores within the range of elderly normal individuals demonstrated better nonbiological naming than biological naming, an effect which increased with worsening impairment. In general, patients diagnosed with other dementia (n=15) did not fit well within the pattern of the DAT/MCI participants, except those known to have a significant semantic impairment. A category effect favoring nonbiological items appears to be robust and produce a predictable pattern across progressive levels of impairment in AD.

Aphasic naming in Spanish: predictors and errors

Sixteen Spanish aphasic patients named drawings of objects on three occasions. Multiple regression analyses were carried out on the naming accuracy scores. For the patient group as a whole, naming was affected by visual complexity, object familiarity, age of acquisition, and word frequency. The combination of variables predicted naming accuracy in 15 of the 16 individual patients. Age of acquisition, word frequency, and object familiarity predicted performance in the greatest number of patients, while visual complexity, imageability, animacy, and length all affected performance in at least two patients. High proportions of semantic and phonological errors to particular objects were associated with objects having early learned names while high proportions of no-response errors were associated with low familiarity and low visual complexity. It is suggested that visual complexity and object familiarity affect the ease of object recognition while word frequency affects name retrieval. Age of acquisition may affect both stages, accounting for its influence in patients with a range of different patterns of disorder.

Semantic range and relevance of emotive utterances in patients with frontotemporal degeneration

Patterns of aberrant language due to lateralized frontal brain lesions have been described. The present study investigated the consequences of lateralized lesion for aspects of semantic range and relevance in speech of patients with frontotemporal degeneration. Demented patients with predominantly left (n=10) and right (n=4) brain degeneration as well as 5 healthy controls participated. Significant differences were found concerning semantic range and relevance in descriptions of emotionally loaded pictures within the demented group and between the demented group and the controls. The demented individuals' speech was less varied and relevant in contrast to the healthy individuals. The speech of the left hemispheric group was more relevant but also more stereotypic and unspecified than that of the right hemispheric group. The results are discussed in terms of impaired semantic retrieval and in relation to lateralization of lesion.