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

The role of the left anterior temporal lobe in language processing revisited: Evidence from an individual with ATL resection

Various hypotheses about the role of the anterior temporal lobe (ATL) in language processing have been proposed. One hypothesis is that it binds the semantic/conceptual properties of words, functioning as a hub for linking modality-specific conceptual properties of objects. This hypothesis predicts that damage to ATL would give rise to impaired conceptual knowledge of all categories. A related school of hypotheses assumes that the left ATL is critical for lexical retrieval, with different sub-regions potentially important for different categories of items. We examined these hypotheses by studying a case of surgical resection of left ATL due to a low-grade glioma (LGG). Thorough language assessments performed four months after the operation revealed the following profile: the patient showed intact conceptual knowledge for all categories of items tested using both accuracy and response latency measures; he suffered from name retrieval deficits for proper names (people and place names) and artifacts (including tools), but showed no name retrieval difficulties for animate things. This pattern of results challenges both target hypotheses about the role of ATL in language processing tested here.

Where is the semantic system? A critical review and meta-analysis of 120 functional neuroimaging studies

Semantic memory refers to knowledge about people, objects, actions, relations, self, and culture acquired through experience. The neural systems that store and retrieve this information have been studied for many years, but a consensus regarding their identity has not been reached. Using strict inclusion criteria, we analyzed 120 functional neuroimaging studies focusing on semantic processing. Reliable areas of activation in these studies were identified using the activation likelihood estimate (ALE) technique. These activations formed a distinct, left-lateralized network comprised of 7 regions: posterior inferior parietal lobe, middle temporal gyrus, fusiform and parahippocampal gyri, dorsomedial prefrontal cortex, inferior frontal gyrus, ventromedial prefrontal cortex, and posterior cingulate gyrus. Secondary analyses showed specific subregions of this network associated with knowledge of actions, manipulable artifacts, abstract concepts, and concrete concepts. The cortical regions involved in semantic processing can be grouped into 3 broad categories: posterior multimodal and heteromodal association cortex, heteromodal prefrontal cortex, and medial limbic regions. The expansion of these regions in the human relative to the nonhuman primate brain may explain uniquely human capacities to use language productively, plan, solve problems, and create cultural and technological artifacts, all of which depend on the fluid and efficient retrieval and manipulation of semantic knowledge.

Different impairments of semantic cognition in semantic dementia and semantic aphasia: evidence from the non-verbal domain

Disorders of semantic cognition in different neuropsychological conditions result from diverse areas of brain damage and may have different underlying causes. This study used a comparative case-series design to examine the hypothesis that relatively circumscribed bilateral atrophy of the anterior temporal lobe in semantic dementia (SD) produces a gradual degradation of core semantic representations, whilst a deficit of cognitive control produces multi-modal semantic impairment in a subset of patients with stroke aphasia following damage involving the left prefrontal cortex or regions in and around the temporoparietal area; this condition, which transcends traditional aphasia classifications, is referred to as 'semantic aphasia' (SA). There have been very few direct comparisons of these patient groups to date and these previous studies have focussed on verbal comprehension. This study used a battery of object-use tasks to extend this line of enquiry into the non-verbal domain for the first time. A group of seven SA patients were identified who failed both word and picture versions of a semantic association task. These patients were compared with eight SD cases. Both groups showed significant deficits in object use but these impairments were qualitatively different. Item familiarity correlated with performance on object-use tasks for the SD group, consistent with the view that core semantic representations are degrading in this condition. In contrast, the SA participants were insensitive to the familiarity of the objects. Further, while the SD patients performed consistently across tasks that tapped different aspects of knowledge and object use for the same items, the performance of the SA participants reflected the control requirements of the tasks. Single object use was relatively preserved in SA but performance on complex mechanical puzzles was substantially impaired. Similarly, the SA patients were able to complete straightforward item matching tasks, such as word-picture matching, but performed more poorly on associative picture-matching tasks, even when the tests involved the same items. The two groups of patients also showed a different pattern of errors in object use. SA patients made substantial numbers of erroneous intrusions in their demonstrations, such as inappropriate object movements. In contrast, response omissions were more common in SD. This study provides converging evidence for qualitatively different impairments of semantic cognition in SD and SA, and uniquely demonstrates this pattern in a non-verbal expressive domain-object use.

The neural correlates of verbal and nonverbal semantic processing deficits in neurodegenerative disease

OBJECTIVE

To investigate the neural correlates of verbal and nonverbal semantic processing in neurodegenerative disease.

BACKGROUND

Semantic memory is often impaired in neurodegenerative disease. Neuropsychologic and functional neuroimaging studies suggest that the semantic processing of verbal and nonverbal stimuli may depend on partially distinct brain networks.

METHODS

We examined this possibility using voxel-based morphometry to correlate performance on verbal and nonverbal versions of a semantic association task with regional gray matter atrophy in 144 individuals with a variety of neurodegenerative diseases.

RESULTS

Results showed that, regardless of stimulus type, semantic processing correlated with atrophy in both temporal lobes. In addition, material-specific correlations were found in left temporal regions for verbal stimuli and the right fusiform gyrus for nonverbal stimuli.

CONCLUSIONS

These results provide evidence for a differential role of the left and right hemispheres in the extraction of semantic information from verbal and pictorial representations. Areas in right inferior temporal lobe may be necessary to access structural descriptions of visually presented objects.

The contribution of the parietal lobes to speaking and writing

The left parietal lobe has been proposed as a major language area. However, parietal cortical function is more usually considered in terms of the control of actions, contributing both to attention and cross-modal integration of external and reafferent sensory cues. We used positron emission tomography to study normal subjects while they overtly generated narratives, both spoken and written. The purpose was to identify the parietal contribution to the modality-specific sensorimotor control of communication, separate from amodal linguistic and memory processes involved in generating a narrative. The majority of left and right parietal activity was associated with the execution of writing under visual and somatosensory control irrespective of whether the output was a narrative or repetitive reproduction of a single grapheme. In contrast, action-related parietal activity during speech production was confined to primary somatosensory cortex. The only parietal area with a pattern of activity compatible with an amodal central role in communication was the ventral part of the left angular gyrus (AG). The results of this study indicate that the cognitive processing of language within the parietal lobe is confined to the AG and that the major contribution of parietal cortex to communication is in the sensorimotor control of writing.

Neurology of anomia in the semantic variant of primary progressive aphasia

The semantic variant of primary progressive aphasia (PPA) is characterized by the combination of word comprehension deficits, fluent aphasia and a particularly severe anomia. In this study, two novel tasks were used to explore the factors contributing to the anomia. The single most common factor was a blurring of distinctions among members of a semantic category, leading to errors of overgeneralization in word-object matching tasks as well as in word definitions and object descriptions. This factor was more pronounced for natural kinds than artifacts. In patients with the more severe anomias, conceptual maps were more extensively disrupted so that inter-category distinctions were as impaired as intra-category distinctions. Many objects that could not be named aloud could be matched to the correct word in patients with mild but not severe anomia, reflecting a gradual intensification of the semantic factor as the naming disorder becomes more severe. Accurate object descriptions were more frequent than accurate word definitions and all patients experienced prominent word comprehension deficits that interfered with everyday activities but no consequential impairment of object usage or face recognition. Magnetic resonance imaging revealed three characteristics: greater atrophy of the left hemisphere; atrophy of anterior components of the perisylvian language network in the superior and middle temporal gyri; and atrophy of anterior components of the face and object recognition network in the inferior and medial temporal lobes. The left sided asymmetry and perisylvian extension of the atrophy explains the more profound impairment of word than object usage and provides the anatomical basis for distinguishing the semantic variant of primary progressive aphasia from the partially overlapping group of patients that fulfil the widely accepted diagnostic criteria for semantic dementia.

Lexical and semantic fluency discrepancy scores in aMCI and early Alzheimer's disease

Episodic memory is compromised in amnestic mild cognitive impairment (aMCI), but lesser deficits in other cognitive domains are also commonly observed and may be helpful in identifying this group. The relative difference in performance on lexical and semantic fluency tasks may be a sensitive and specific measure in aMCI and early Alzheimer's disease (AD). We compared four groups of participants, 35 early AD, 47 aMCI, 24 healthy controls, and 18 depressive out-patient controls, on semantic and lexical fluency as well as other neuropsychological tests. Early AD and aMCI patients showed a distinct pattern of semantic impairment in the two fluency measures compared with the healthy and depressive controls. The findings implicate early failure of the semantic memory system in aMCI and AD and suggest that consideration of the discrepancy in performance on semantic and lexical fluency measures may help in the early identification of AD.

Is the left uncinate fasciculus essential for language? A cerebral stimulation study

Despite a better understanding of the anatomy of the uncinate fasciculus (UF), its function remains poorly known. Our aim was to study the exact role of UF in language, and the possible existence of parallel distributed language networks within the "ventral stream", underlaid by distinct subcortical tracts--namely the inferior occipito-temporal fasciculus (IOF) and UF.We report a series of 13 patients operated on awake for a glioma involving the left anterior temporal lobe or the orbitofrontal area. We used intraoperative electrostimulation, to perform accurate and reliable anatomofunctional correlations both at cortical and subcortical levels. Using postoperative MRI, we correlated these functional findings with the anatomical locations of the sites where language disturbances were elicited by stimulation.Intraoperative cortical stimulation found perilesional language sites in all cases. Subcortically, semantic paraphasia were induced in the 13 patients by stimulating the IOF, and phonological paraphasia were generated in 6 patients by stimulating the arcuate fasciculus. Interestingly, subcortical stimulation never elicited any language disturbances when performed at the level of the UF. Moreover, after a transient postoperative language deficit, all patients recovered, despite the removal of at least one part of the UF, as confirmed by control MRI.We suggest that UF is not systematically essential for language. It can be explained by the fact that the "semantic ventral stream" might be constituted by at least two parallel pathways, i. e. a direct pathway underlaid by the IOF, crucial for language semantics, and an indirect pathway subserved by UF, which can be functionally compensated. However, we have to underline the fact not all language functions can be probed during surgery, and that more sensitive tasks have now to be added.

The role of the anterior temporal lobes in the comprehension of concrete and abstract words: rTMS evidence

Conceptual knowledge allows us to bring meaning to our world. Studies of semantic dementia (SD) patients and some functional neuroimaging studies indicate that the anterior temporal lobes, bilaterally, are a core neural substrate for the formation of conceptual representations. The majority of SD patients (who have circumscribed atrophy of the anterior temporal lobes) have better comprehension of concrete than abstract words. However, this finding remains controversial, as some individual SD patients have exhibited reverse imageability effects, i.e., relative preservation of abstract knowledge. This would imply that the anterior temporal lobes are particularly crucial for processing sensory aspects of semantic knowledge, which are an important part of concrete but not abstract concepts. To adjudicate on this debate, we used offline, low-frequency, repetitive transcranial magnetic stimulation to disrupt neural processing temporarily in the left or right temporal poles (TPs). We examined this effect using a synonym judgement task, comprising high, medium and low imageability items, which we have previously employed with a case-series of SD patients. The time required to make semantic decisions was slowed considerably, particularly for low imageability items, consistent with the pattern we observed in SD. These results confirm that both TPs make a critical contribution to semantic processing, even for abstract concepts that do not have strong sensory representations.

The influence of multiple primes on bottom-up and top-down regulation during meaning retrieval: evidence for 2 distinct neural networks

Meaning retrieval of a word can proceed fast and effortlessly or can be characterized by a controlled search for candidate lexical items and a subsequent selection process. In the current study, we facilitated meaning retrieval by increasing the number of words that were related to the final target word in a triplet (e.g., lion-stripes-tiger). To induce higher search and selection demands, we presented ambiguous words as targets (i.e., homonyms like ball) in half of the trials. Hereby, the dominant (game), low-frequent (dance), or both meanings of the homonym were primed. Participants performed a relatedness judgment during functional magnetic resonance imaging. Activation in a bilateral network (angular gyrus, rostromedial prefrontal cortex) increased linearly with multiple related primes, whereas the posterior left inferior prefrontal cortex (pLIPC) showed the reverse activation pattern for unambiguous trials. When homonyms served as targets, pLIPC responded strongest when both meanings or low-frequent concepts were addressed. Additional anterior left inferior prefrontal cortex activation was observed for the latter trials only. The data support an interaction between 2 distinct cerebral networks that can be linked to automatic bottom-up support and top-down control during meaning retrieval. They further imply a functional specialization of the LIPC along an anterior-posterior dimension.

Neologistic jargon aphasia and agraphia in primary progressive aphasia

The terms 'jargon aphasia' and 'jargon agraphia' describe the production of incomprehensible language containing frequent phonological, semantic or neologistic errors in speech and writing, respectively. Here we describe two patients with primary progressive aphasia (PPA) who produced neologistic jargon either in speech or writing. We suggest that involvement of the posterior superior temporal-inferior parietal region may lead to a disconnection between stored lexical representations and language output pathways leading to aberrant activation of phonemes in neologistic jargon. Parietal lobe involvement is relatively unusual in PPA, perhaps accounting for the comparative rarity of jargon early in the course of these diseases.

Social conceptual impairments in frontotemporal lobar degeneration with right anterior temporal hypometabolism

Inappropriate social behaviours are early and distinctive symptoms of the temporal and frontal variants of frontotemporal lobar degeneration (FTLD). Knowledge of social behaviour is essential for appropriate social conduct. It is unknown, however, in what way this knowledge is degraded in FTLD. In a recent functional MRI study, we have identified a right-lateralized superior anterior temporal lobe (aTL) region showing selective activation for 'social concepts' (i.e. concepts describing social behaviour: e.g. 'polite', 'stingy') as compared with concepts describing less socially relevant animal behaviour ('animal function concepts': e.g. 'trainable', 'nutritious'). In a further fMRI study, superior aTL activation was independent of the context of actions and feelings associated with these social concepts. Here, we investigated whether the right superior sector of the aTL is necessary for context-independent knowledge of social concepts. We assessed neuronal glucose uptake using 18-fluoro-deoxy-glucose-positron emission tomography (FDG-PET) and a novel semantic discrimination task which probed knowledge of social and animal function concepts in patients with FTLD (n = 29) and corticobasal syndrome (n = 18). FTLD and corticobasal syndrome groups performed equally poorly on animal function concepts but FTLD patients showed more pronounced impairments on social concepts than corticobasal syndrome patients. FTLD patients with right superior aTL hypometabolism, as determined on individual ROI analyses, were significantly more impaired on social concepts than on animal function concepts. FTLD patients with selective impairments for social concepts, as determined on individual neuropsychological profiles, showed higher levels of inappropriate social behaviours ('disinhibition') and demonstrated more pronounced hypometabolism in the right superior aTL, the left temporal pole and the right lateral orbitofrontal and dorsomedial prefrontal cortex as compared with FTLD patients showing selective impairments of animal function concepts. Combining both FTLD subgroup analyses, based on anatomical and neuropsychological criteria, by using inclusive masks, revealed the right superior aTL as associated with selective impairments of social concepts in both analyses. These results corroborate the hypothesis that the right aTL is necessary for representing conceptual social knowledge. Further, we provide first evidence for the potential importance of conceptual social knowledge impairments as contributing to behavioural symptoms of FTLD.

The association between semantic dementia and surface dyslexia in Japanese

One theory about reading suggests that producing the correct pronunciations of written words, particularly less familiar words with an atypical spelling-sound relationship, relies in part on knowledge of the word's meaning. This hypothesis has been supported by reports of surface dyslexia in large case-series studies of English-speaking/reading patients with semantic dementia (SD), but would have increased credibility if it applied to other languages and writing systems as well. The hypothesis predicts that, of the two systems used to write Japanese, SD patients should be unimpaired at oral reading of kana because of its invariant relationship between orthography and phonology. By contrast, oral reading of kanji should be impaired in a graded fashion depending on the consistency characteristics of the kanji target words, with worst performance on words whose component characters take 'minority' (atypical) pronunciations, especially if the words are of lower frequency. Errors in kanji reading should primarily reflect assignment of more typical readings to the component characters in these atypical words. In the largest-ever-reported case series of Japanese patients with semantic dementia, we tested and confirmed this hypothesis.

Using lexical familiarity judgments to assess verbally mediated intelligence in aphasia

In this study, a task using forced-choice lexical familiarity judgments of irregular versus archaic words (a newly developed measure called the Lexical Orthographic Familiarity Test; LOFT) was compared to a standardized oral word-reading measure (the Wechsler Test of Adult Reading; WTAR) in a group of 35 aphasic adults and a comparison group of 125 community dwelling, nonbrain damaged adults. When compared to the comparison group, aphasics had significantly lower scores on the WTAR but not the LOFT. Although both the WTAR and LOFT were significantly correlated with education in the nonbrain-damaged group, only the LOFT was correlated with education and also with the Barona full scale IQ index in the aphasic group. Lastly, WTAR performance showed a significantly greater relationship to the severity of language disorder in the aphasic group than did the LOFT. These results have both theoretical and clinical implications for the assessment of language-disordered adults, as they indicate that patients with aphasia may retain aspects of verbally mediated intelligence, and that the LOFT may provide a better estimate of premorbid functioning in aphasia than other currently available measures.

Semantic access dysphasia resulting from left temporal lobe tumours

Unlike semantic degradation disorders, the mechanisms and the anatomical underpinnings of semantic access disorders are still unclear. We report the results of a case series study on the effects of temporal lobe gliomas on semantic access abilities of a group of 20 patients. Patients were tested 1–2 days before and 4–6 days after the removal of the tumour. Their semantic access skills were assessed with two spoken word-to-picture matching tasks, which aimed to separately control for rate of presentation, consistency and serial position effects (Experiment 1) and for word frequency and semantic distance effects (Experiment 2). These variables have been held to be critical in characterizing access in contrast to degraded-store semantic deficits, with access deficits characterized by inconsistency of response, better performance with slower presentation rates and with semantically distant stimuli, in the absence of frequency effects. Degradation deficits show the opposite pattern. Our results showed that low-grade slowly growing tumours tend not to produce signs of access problems. However, high-grade tumours especially within the left hemisphere consistently produce strong semantic deficits of a clear access type: response inconsistency and strong semantic distance effects in the absence of word frequency effects were detected. However, effects of presentation rate and serial position were very weak, suggesting non-refractory behaviour in the tumour patients tested. This evidence, together with the results of lesion overlapping, suggests the presence of a type of non-refractory semantic access deficit. We suggest that this deficit could be caused by the disconnection of posterior temporal lexical input areas from semantic system.

Where (in the brain) do semantic errors come from?

BACKGROUND

Semantic errors result from the disruption of access either to semantics or to lexical representations. One way to determine the origins of these errors is to evaluate comprehension of words that elicit semantic errors in naming. We hypothesized that in acute stroke there are different brain regions where dysfunction results in semantic errors in both naming and comprehension versus those with semantic errors in oral naming alone.

METHODS

A consecutive series of 196 patients with acute left hemispheric stroke who met inclusion criteria were evaluated with oral naming and spoken word/picture verification tasks and magnetic resonance imaging within 48 h of stroke onset. We evaluated the relationship between tissue dysfunction in 10 pre-specified Brodmann's areas (BA) and the production of coordinate semantic errors resulting from (1) semantic deficits or (2) lexical access deficits.

RESULTS

Semantic errors arising from semantic deficits were most associated with tissue dysfunction/infarct of left BA 22. Semantic errors resulting from lexical access deficits were associated with hypoperfusion/infarct of left BA 37.

CONCLUSION

Our study shows that semantic errors arising from damage to distinct cognitive processes reflect dysfunction of different brain regions.

Covert processing of words and pictures in nonsemantic variants of primary progressive aphasia

The inability to name objects (anomia) is one of the most common findings in the neurologic examination of primary progressive aphasia (PPA). In the semantic variant of PPA, the anomia is profound and reflects a combination of object naming and word comprehension deficits. In contrast, nonsemantic variants of PPA display a more selective impairment of object naming, without corresponding impairments of word comprehension. The aim of the present study was to explore the nature of the anomia in nonsemantic variants of PPA with a sensitive chronometric test of covert word/picture association. We tested priming effects in 12 patients with nonsemantic variant of PPA and 18 controls. Stimuli consisted of written words and line pictures of concrete objects. Within-format (word-word and picture-picture) and cross-format (word-picture and picture-word) priming effects were assessed by measuring the shortening of response times to the second versus initial presentation of corresponding stimulus pairs. In addition to the expected impairment of picture-to-word priming, a condition simulating object naming, the nonsemantic PPA patients also showed unexpected impairments of word-to-picture and word-to-word priming. Picture-to-picture priming was preserved, demonstrating the selectivity of the deficit for lexical processing. These findings show that the information processing bottleneck in patients with nonsemantic variants of PPA is not confined to the stage of lexical access but that it also extends into the prior levels of lexical semantics. The boundaries between the semantic and nonsemantic variants are therefore far from rigid.

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.

Treatment for anomia in semantic dementia

Anomia is a striking and consistent clinical feature of semantic dementia (SD), a progressive aphasia syndrome associated with focal cortical atrophy of the anterior temporal lobes. Word retrieval deficits in patients with SD have been attributed to the loss of conceptual knowledge, resulting in an impairment referred to as semantic anomia. Whereas an abundance of research has been dedicated to treatment for anomia in individuals with focal brain damage due to stroke, considerably less work has been done regarding treatment for patients with progressive language decline. The purpose of this article is to review the available literature concerning the nature and treatment of anomia in individuals with SD. Several studies have shown that new lexical learning remains possible in these patients. However, newly learned information is likely to be constrained by the learning context, and increased reliance on perceptual and autobiographical contextual information may be necessary to provide critical support for new vocabulary acquisition. There is also evidence suggesting that treatment may slow the progression of anomia over time, even affording some protective benefit to lexical items that are not yet lost. However, treatment efforts are likely to be most beneficial at early stages of the disease, when residual semantic knowledge as well as relatively spared episodic memory may support new learning.

Generalization and differentiation in semantic memory: insights from semantic dementia

According to many theories, semantic representations reflect the parallel activation of information coded across a distributed set of modality-specific association brain cortices. This view is challenged by the neurodegenerative condition known as semantic dementia (SD), in which relatively circumscribed, bilateral atrophy of the anterior temporal lobes results in selective degradation of core semantic knowledge, affecting all types of concept, irrespective of the modality of testing. Research on SD suggests a major revision in our understanding of the neural basis of semantic memory. Specifically, it is proposed that the anterior temporal lobes form amodal semantic representations through the distillation of the multimodal information that is projected to this region from the modality-specific association cortices. Although cross-indexing of modality-specific information could be achieved by a web of direct connections between pairs of these regions, amodal semantic representations enable semantic generalization and inference on the basis of conceptual structure rather than modality-specific features. As expected from this hypothesis, SD is characterized by impaired semantic generalization, both clinically and in formal assessment. The article describes a comprehensive array of under- and overgeneralization errors by patients with SD when engaged in receptive and expressive verbal and nonverbal tasks and everyday behaviors.

Lexical retrieval and semantic knowledge in patients with left inferior temporal lobe lesions

BACKGROUND: It has been proposed that anomia following left inferior temporal lobe lesions may have two different underlying mechanisms with distinct neural substrates. Specifically, naming impairment following damage to more posterior regions (BA 37) has been considered to result from a disconnection between preserved semantic knowledge and phonological word forms (pure anomia), whereas anomia following damage to anterior temporal regions (BAs 38, 20/21) has been attributed to the degradation of semantic representations (semantic anomia). However, the integrity of semantic knowledge in patients with pure anomia has not been demonstrated convincingly, nor were lesions in these cases necessarily confined to BA 37. Furthermore, evidence of semantic anomia often comes from individuals with bilateral temporal lobe damage, so it is unclear whether unilateral temporal lobe lesions are sufficient to produce significant semantic impairment. AIMS: The main goals of this study were to determine whether anomia following unilateral left inferior temporal lobe damage reflected a loss of semantic knowledge or a post-semantic deficit in lexical retrieval and to identify the neuroanatomical correlates of the naming impairment. METHODS #ENTITYSTARTX00026; PROCEDURES: Eight individuals who underwent left anterior temporal lobectomy (L ATL) and eight individuals who sustained left posterior cerebral artery strokes (L PCA) completed a battery of language measures that assessed lexical retrieval and semantic processing, and 16 age- and education-matched controls also completed this battery. High-resolution structural brain scans were collected to conduct lesion analyses. OUTCOMES #ENTITYSTARTX00026; RESULTS: Performance of L ATL and L PCA patients was strikingly similar, with both groups demonstrating naming performance ranging from moderately impaired to unimpaired. Anomia in both groups occurred in the context of mild deficits to semantic knowledge, which manifested primarily as greater difficulty in naming living things than nonliving things and greater difficulty in processing visual/perceptual as opposed to functional/associative semantic attributes. Lesion analyses indicated that both patient groups sustained damage to anterior inferior temporal lobe regions implicated in semantic processing. CONCLUSIONS: These results contribute to a better understanding of the cognitive mechanism of naming impairment in patients with temporal lobe damage and support the notion that pure anomia and semantic anomia represent two endpoints along a continuum of semantic impairment. Unilateral left temporal lobe lesions in our patients resulted in relatively mild semantic deficits that were apparent primarily in lexical production tasks, whereas severe semantic impairment likely requires bilateral temporal lobe damage.

Longitudinal studies of semantic dementia: the relationship between structural and functional changes over time

The pattern of brain atrophy in semantic dementia and its associated cognitive effects have attracted a considerable body of research, but the nature of core impairments remains disputed. A key issue is whether the disease encompasses one neurocognitive network (semantics) or two (language and semantics). In order to address these conflicting perspectives, we conducted a longitudinal investigation of two semantic dementia patients, in which behavioural performance across a range of measures of language and semantic performance was assessed and interpreted in the context of annually acquired MRI scans. Our results indicated a core semantic impairment in early stages of the disease, associated with atrophy of the inferior, anterior temporal cortex. Linguistic impairments emerged later, and were contingent on atrophy having spread into areas widely believed to subserve core language processes (left posterior perisylvian, inferior frontal and insular cortex). We claim, therefore, that phonological, syntactic and morphological processing deficits in semantic dementia reflect damage to core language areas. Further, we propose that much of the current controversy over the nature of deficits in semantic dementia reflect a tendency in the literature to adopt a static perspective on what is a progressive disease. An approach in which the relationship between progressive neural changes and behavioural change over time is carefully mapped, offers a more constraining data-set from which to draw inferences about the relationship between language, semantics and the brain.

Performance in specific language tasks correlates with regional volume changes in progressive aphasia

BACKGROUND

Patterns of language impairment have long been used clinically to localize brain damage in stroke patients. The same approach might be useful in the differential diagnosis of progressive aphasia owing to neurodegenerative disease.

OBJECTIVE

To investigate whether scores on 4 widely used language tasks correlate with regional gray matter loss in 51 patients with progressive language impairment owing to neurodegenerative disease.

METHOD

Scores in the Boston Naming Test and in the "repetition" "sequential commands" and the "language fluency," subtests of the Western Aphasia Battery were correlated with voxel-wise gray matter volumes using voxel-based morphometry.

RESULTS

Significant positive correlations were found between each language task and regional brain volumes: (1) naming and the bilateral temporal lobes; (2) sentence repetition and the left posterior portion of the superior temporal gyrus; (3) sentence comprehension and the left dorsal middle and inferior frontal gyri; and (4) fluency of language production and the left ventral middle and inferior frontal gyri.

DISCUSSION

Performance on specific language tasks corresponds to regional anatomic damage in aphasia owing to neurodegenerative disorders. These language tests might be useful in the differential diagnosis of primary progressive aphasia variants that have been previously associated with damage to corresponding anatomic regions.

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 Influence of refractoriness upon comprehension of non-verbal auditory stimuli

An investigation of non-verbal auditory comprehension in two patients with global aphasia following stroke is reported. The primary aim of the investigation was to establish whether refractory access disorders can affect non-verbal input modalities. All previous reports of refractoriness, a cognitive syndrome characterized by response inconsistency, sensitivity to temporal factors and insensitivity to item frequency, have involved comprehension tasks which have a verbal component. Two main experiments are described. The first consists of a novel sound-to-picture and sound-to-word matching task in which comprehension of environmental sounds is probed under conditions of semantic relatedness and semantic unrelatedness. In addition to the two stroke patients, the performance of a group of 10 control patients with non-vascular pathology is reported, along with evidence of semantic relatedness effects in sound comprehension. The second experiment examines environmental sound comprehension within a repetitive probing paradigm which affords assessment of the effects of semantic relatedness, response consistency and presentation rate. It is demonstrated that the two stroke patients show a significant increase in error rate across multiple probes of the same set of sound stimuli, indicating the presence of refractoriness within this non-verbal domain. The implications of the results are discussed with reference to our current understanding of the mechanisms of refractoriness.

Refractory access disorders and the organization of concrete and abstract semantics: do they differ?

Patients with 'refractory semantic access deficits' demonstrate several unique features that make them important sources of insight into the organization of semantic representations. Here we attempt to replicate several novel findings from single-case studies reported in the literature. Patient UM-103 displays the cardinal features of a 'refractory semantic access deficit' and showed many of the same effects of semantic relatedness reported in the literature. However, when probing concrete and abstract words, this patient revealed very different patterns of performance compared to two previously reported patients. We discuss the implications of our data for models of semantic organization of abstract and concrete words.

Word-finding difficulty: a clinical analysis of the progressive aphasias

The patient with word-finding difficulty presents a common and challenging clinical problem. The complaint of 'word-finding difficulty' covers a wide range of clinical phenomena and may signify any of a number of distinct pathophysiological processes. Although it occurs in a variety of clinical contexts, word-finding difficulty generally presents a diagnostic conundrum when it occurs as a leading or apparently isolated symptom, most often as the harbinger of degenerative disease: the progressive aphasias. Recent advances in the neurobiology of the focal, language-based dementias have transformed our understanding of these processes and the ways in which they breakdown in different diseases, but translation of this knowledge to the bedside is far from straightforward. Speech and language disturbances in the dementias present unique diagnostic and conceptual problems that are not fully captured by classical models derived from the study of vascular and other acute focal brain lesions. This has led to a reformulation of our understanding of how language is organized in the brain. In this review we seek to provide the clinical neurologist with a practical and theoretical bridge between the patient presenting with word-finding difficulty in the clinic and the evidence of the brain sciences. We delineate key illustrative speech and language syndromes in the degenerative dementias, compare these syndromes with the syndromes of acute brain damage, and indicate how the clinical syndromes relate to emerging neurolinguistic, neuroanatomical and neurobiological insights. We propose a conceptual framework for the analysis of word-finding difficulty, in order both better to define the patient's complaint and its differential diagnosis for the clinician and to identify unresolved issues as a stimulus to future work.

Mimicking aphasic semantic errors in normal speech production: evidence from a novel experimental paradigm

Semantic errors are commonly found in semantic dementia (SD) and some forms of stroke aphasia and provide insights into semantic processing and speech production. Low error rates are found in standard picture naming tasks in normal controls. In order to increase error rates and thus provide an experimental model of aphasic performance, this study utilised a novel method- tempo picture naming. Experiment 1 showed that, compared to standard deadline naming tasks, participants made more errors on the tempo picture naming tasks. Further, RTs were longer and more errors were produced to living items than non-living items a pattern seen in both semantic dementia and semantically-impaired stroke aphasic patients. Experiment 2 showed that providing the initial phoneme as a cue enhanced performance whereas providing an incorrect phonemic cue further reduced performance. These results support the contention that the tempo picture naming paradigm reduces the time allowed for controlled semantic processing causing increased error rates. This experimental procedure would, therefore, appear to mimic the performance of aphasic patients with multi-modal semantic impairment that results from poor semantic control rather than the degradation of semantic representations observed in semantic dementia [Jefferies, E. A., & Lambon Ralph, M. A. (2006). Semantic impairment in stoke aphasia vs. semantic dementia: A case-series comparison. Brain, 129, 2132-2147]. Further implications for theories of semantic cognition and models of speech processing are discussed.

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.

Where do you know what you know? The representation of semantic knowledge in the human brain

Mr M, a patient with semantic dementia--a neurodegenerative disease that is characterized by the gradual deterioration of semantic memory--was being driven through the countryside to visit a friend and was able to remind his wife where to turn along the not-recently-travelled route. Then, pointing at the sheep in the field, he asked her "What are those things?" Prior to the onset of symptoms in his late 40s, this man had normal semantic memory. What has gone wrong in his brain to produce this dramatic and selective erosion of conceptual knowledge?

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.

A common system for the comprehension and production of narrative speech

Humans devote much time to the exchange of memories within the context of shared general and personal semantic knowledge. Our hypothesis was that functional imaging in normal subjects would demonstrate the convergence of speech comprehension and production on high-order heteromodal and amodal cortical areas implicated in declarative memory functions. Activity independent of speech phase (that is, comprehension and production) was most evident in the left and right lateral anterior temporal cortex. Significant activity was also observed in the posterior cortex, ventral to the angular gyri. The left and right hippocampus and adjacent inferior temporal cortex were active during speech comprehension, compatible with mnemonic encoding of narrative information, but activity was significantly less during the overt memory retrieval associated with speech production. Therefore, although clinical studies suggest that hippocampal function is necessary for the retrieval as well as the encoding of memories, the former appears to depend on much less net synaptic activity. In contrast, the retrosplenial/posterior cingulate cortex and the parahippocampal area, which are closely associated anatomically with the hippocampus, were equally active during both speech comprehension and production. The results demonstrate why a severe and persistent inability both to understand and produce meaningful speech in the absence of an impairment to process linguistic forms is usually only observed after bilateral, and particularly anterior, destruction of the temporal lobes, and emphasize the importance of retrosplenial/posterior cingulate cortex, an area known to be affected early in the course of Alzheimer's disease, in the processing of memories during communication.

Social concepts are represented in the superior anterior temporal cortex

Social concepts such as "tactless" or "honorable" enable us to describe our own as well as others' social behaviors. The prevailing view is that this abstract social semantic knowledge is mainly subserved by the same medial prefrontal regions that are considered essential for mental state attribution and self-reflection. Nevertheless, neurodegeneration of the anterior temporal cortex typically leads to impairments of social behavior as well as general conceptual knowledge. By using functional MRI, we demonstrate that the anterior temporal lobe represents abstract social semantic knowledge in agreement with this patient evidence. The bilateral superior anterior temporal lobes (Brodmann's area 38) are selectively activated when participants judge the meaning relatedness of social concepts (e.g., honor-brave) as compared with concepts describing general animal functions (e.g., nutritious-useful). Remarkably, only activity in the superior anterior temporal cortex, but not the medial prefrontal cortex, correlates with the richness of detail with which social concepts describe social behavior. Furthermore, this anterior temporal lobe activation is independent of emotional valence, whereas medial prefrontal regions show enhanced activation for positive social concepts. Our results demonstrate that the superior anterior temporal cortex plays a key role in social cognition by providing abstract conceptual knowledge of social behaviors. We further speculate that these abstract conceptual representations can be associated with different contexts of social actions and emotions through integration with frontolimbic circuits to enable flexible evaluations of social behavior.

Lesion correlates of conversational speech production deficits

We assess brain areas involved in speech production using a recently developed lesion-symptom mapping method (voxel-based lesion-symptom mapping, VLSM) with 50 aphasic patients with left-hemisphere lesions. Conversational speech was collected through a standardized biographical interview, and used to determine mean length of utterance in morphemes (MLU), type token ratio (TTR) and overall tokens spoken for each patient. These metrics are used as indicators of grammatical complexity, semantic variation, and amount of speech, respectively. VLSM analysis revealed that damage to the anterior insula was predictive of low scores on MLU and tokens, consistent with prior findings of the role of this region in speech production [Dronkers, N. F. (1996). A new brain region for coordinating speech articulation. Nature, 384(6605), 159-161]. Additionally, the inferior frontal gyrus, sensorimotor and anterior temporal areas were also associated with lower scores on both of these measures. Overall, token and MLU maps were highly similar, suggesting an overlap between grammatical language networks and overall fluency. TTR maps also shared some portions of this network, but damage to posterior temporal regions also reduced scores on this measure. These results represent the first voxel-based lesion analysis of speech production performance in aphasic patients.

Neural regions essential for distinct cognitive processes underlying picture naming

We hypothesized that distinct cognitive processes underlying oral and written picture naming depend on intact function of different, but overlapping, regions of the left hemisphere cortex, such that the distribution of tissue dysfunction in various areas can predict the component of the naming process that is disrupted. To test this hypothesis, we evaluated 116 individuals within 24 h of acute ischaemic stroke using a battery of oral and written naming and other lexical tests, and with magnetic resonance diffusion and perfusion imaging to identify the areas of tissue dysfunction. Discriminant function analysis, using the degree of hypoperfusion in various Brodmann's areas--BA 22 (including Wernicke's area), BA 44 (part of Broca's area), BA 45 (part of Broca's area), BA 21 (inferior temporal cortex), BA 37 (posterior, inferior temporal/fusiform gyrus), BA 38 (anterior temporal cortex) and BA 39 (angular gyrus)--as discriminant variables, classified patients on the basis of the primary component of the naming process that was impaired (defined as visual, semantics, modality-independent lexical access, phonological word form, orthographic word form and motor speech by the pattern of performance and types of errors across lexical tasks). Additionally, linear regression analysis demonstrated that the areas contributing the most information to the identification of patients with particular levels of impairment in the naming process were largely consistent with evidence for the roles of these regions from functional imaging. This study provides evidence that the level of impairment in the naming process reflects the distribution of tissue dysfunction in particular regions of the left anterior, inferior and posterior middle/superior temporal cortex, posterior inferior frontal and inferior parietal cortex. While occipital cortex is also critical for picture naming, it is likely that bilateral occipital damage is necessary to disrupt visual recognition. These findings provide new evidence that a network of brain regions supports naming, but separate components of this network are differentially required for distinct cognitive processes or representations underlying the complex task of naming pictures.

Refractory effects in stroke aphasia: A consequence of poor semantic control

This study examined the full range of effects associated with "semantic access impairment" - namely, refractory variables (semantic relatedness, speed of presentation and item repetition), inconsistency, the absence of frequency effects and facilitation by cues - in a series of stroke patients with multimodal semantically impairment. By investigating all of these factors in a group of patients who were not specifically selected to show "access" effects, we were able to establish (1) whether this pattern is a common consequence of infarcts that produce semantic impairment and (2) if these symptoms co-occur. All of the patients showed effects of cueing and an absence of frequency effects in comprehension. Patients whose brain damage included the left inferior prefrontal cortex (LIPC) also showed marked effects of refractory variables; in contrast, two patients with temporal-parietal but not frontal lesions were less sensitive to these variables. Parallel results were obtained for cyclical naming and word-picture matching tasks suggesting that the LIPC plays a role in semantic selection as well as lexical retrieval. Rapid presentation and item repetition is likely to have increased the selection demands in both of these tasks in a similar fashion. Unlike patients with classical "semantic access impairment", our semantically impaired stroke patients showed significant test-retest consistency, indicating that their difficulties did not result from an unpredictable failure of semantic access--instead, their deficits were interpreted as arising from failures of semantic control.