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

Dissociations and associations of performance in syntactic comprehension in aphasia and their implications for the nature of aphasic deficits

Sixty-one pwa were tested on syntactic comprehension in three tasks: sentence-picture matching, sentence-picture matching with auditory moving window presentation, and object manipulation. There were significant correlations of performances on sentences across tasks. First factors on which all sentence types loaded in unrotated factor analyses accounted for most of the variance in each task. Dissociations in performance between sentence types that differed minimally in their syntactic structures were not consistent across tasks. These results replicate previous results with smaller samples and provide important validation of basic aspects of aphasic performance in this area of language processing. They point to the role of a reduction in processing resources and of the interaction of task demands and parsing and interpretive abilities in the genesis of patient performance.

Competitive mechanisms in sentence processing: common and distinct production and reading comprehension networks linked to the prefrontal cortex

Despite much interest in language production and comprehension mechanisms, little is known about the relationship between the two. Previous research suggests that linguistic knowledge is shared across these tasks and that the left inferior frontal gyrus (LIFG) may be commonly recruited. However, it remains unclear the extent to which production and comprehension share competition mechanisms. Here we investigate this issue and specifically examine competition in determining the event roles in a sentence (agent or affected participant). We used both behavioral and fMRI methods and compared the reading and production of high- and low-competition sentences, specifically targeting LIFG. We found that activity in pars opercularis (PO), independently identified by a competition-driven localizer, was modulated by competition in both tasks. Psychophysiological interaction analyses seeded in PO revealed task-specific networks: In comprehension, PO only interacted with the posterior temporal lobe, whereas in production, it interacted with a large network including hippocampal, posterior temporal, medial frontal and subcortical structures. Production and comprehension therefore recruit partially distinct functional networks but share competitive processes within fronto-temporal regions. We argue that these common regions store long-term linguistic associations and compute their higher-order contingencies, but competition in production ignites a larger neural network implementing planning, as required by task demands.

Brain regions that represent amodal conceptual knowledge

To what extent do the brain regions implicated in semantic processing contribute to the representation of amodal conceptual content rather than modality-specific mechanisms or mechanisms of semantic access and manipulation? Here, we propose that a brain region can be considered to represent amodal conceptual object knowledge if it is supramodal and plays a role in distinguishing among the conceptual representations of different objects. In an fMRI study, human participants made category typicality judgments about pictured objects or their names drawn from five different categories. Crossmodal multivariate pattern analysis revealed a network of six left-lateralized regions largely outside of category-selective visual cortex that showed a supramodal representation of object categories. These were located in the posterior middle/inferior temporal gyrus (pMTG/ITG), angular gyrus, ventral temporal cortex, posterior cingulate/precuneus (PC), and lateral and dorsomedial prefrontal cortex. Representational similarity analysis within these regions determined that the similarity between category-specific patterns of neural activity in the pMTG/ITG and the PC was consistent with the semantic similarity between these categories. This finding supports the PC and pMTG/ITG as candidate regions for the amodal representation of the conceptual properties of objects.

"Penguins don't fly": An investigation into the effect of typicality on picture naming in people with aphasia

BACKGROUND

PREVIOUS RESEARCH HAS HIGHLIGHTED PSYCHOLINGUISTIC VARIABLES INFLUENCING NAMING ABILITY FOR INDIVIDUALS WITH APHASIA, INCLUDING: familiarity, frequency, age of acquisition, imageability, operativity, and length (Nickels & Howard, 1995) and a potential link between typicality and generalisation to untreated items in intervention (Kiran, Sandberg, & Sebastian, 2011). However, the effect of concept typicality (the extent to which an item can be considered a prototype of a category) on naming in aphasia warrants further examination.

AIMS

To investigate first whether typicality can be reliably rated across a range of natural semantic categories and second whether, and if so in which direction, typicality influences naming performance for people with aphasia. To provide quantitative and qualitative information on typicality for a set of stimuli for use in future research.

METHODS & PROCEDURES

Typicality ratings were obtained and the results compared with those in the existing literature. The influence of typicality on picture naming was investigated employing both matched sets (high and low typicality matched for other psycholinguistic variables) and logistic regression analyses for the group and individual participants with aphasia (n = 20).

OUTCOMES & RESULTS

Typicality rating correlated strongly with ratings obtained in previous research (Rosch, 1975: r = .798, N = 35, p < .001; Uyeda & Mandler, 1980: r = .844, N = 47, p < .001). Typicality was a significant predictor of picture naming for the group and some individuals, with generally better performance for typical items. This was demonstrated in both matched sets and regression analyses. However, other psycholinguistic variables proved more strongly related to naming success, particularly age of acquisition.

CONCLUSIONS

Typicality can be rated reliably and should be considered alongside other psycholinguistic variables when investigating word retrieval and intervention in aphasia. Further research is necessary to accurately model the direction of typicality effects found in word retrieval. Finally, the differing nature, size, and internal structure of categories require further exploration when investigating typicality effects.

Mapping the connectivity underlying multimodal (verbal and non-verbal) semantic processing: a brain electrostimulation study

Accessing the meaning of words, objects, people and facts is a human ability, made possible thanks to semantic processing. Although studies concerning its cortical organization are proficient, the subcortical connectivity underlying this semantic network received less attention. We used intraoperative direct electrostimulation, which mimics a transient virtual lesion during brain surgery for glioma in eight awaken patients, to map the anatomical white matter substrate subserving the semantic system. Patients performed a picture naming task and a non-verbal semantic association test during the electrical mapping. Direct electrostimulation of the inferior fronto-occipital fascicle, a poorly known ventral association pathway which runs throughout the brain, induced in all cases semantic disturbances. These transient disorders were highly reproducible, and concerned verbal as well as non-verbal output. Our results highlight for the first time the essential role of the left inferior fronto-occipital fascicle in multimodal (and not only in verbal) semantic processing. On the basis of these original findings, and in the lights of phylogenetic considerations regarding this fascicle, we suggest its possible implication in the monitoring of the human level of consciousness related to semantic memory, namely noetic consciousness.

Effects of phonological and semantic deficits on facilitative and inhibitory consequences of item repetition in spoken word comprehension

Repeating a word can have both facilitative and inhibitory effects on subsequent processing. The present study investigated these dynamics by examining the facilitative and inhibitory consequences of different kinds of item repetition in two individuals with aphasia and a group of neurologically intact control participants. The two individuals with aphasia were matched on overall aphasia severity, but had deficits at different levels of processing: one with a phonological deficit and spared semantic processing, the other with a semantic deficit and spared phonological processing. Participants completed a spoken word-to-picture matching task in which they had to pick which of four object images matched the spoken word. The trials were grouped into pairs such that exactly two objects from the first trial in a pair were present on screen during the second trial in the pair. When the second trial's target was the same as the first trial's target, compared to control participants, both participants with aphasia exhibited equally larger repetition priming effects. When the second trial's target was one of the new items, the participant with a phonological deficit exhibited a significantly more negative effect (i.e., second trial response slower than first trial response) than the control participants and the participant with a semantic deficit. Simulations of a computational model confirmed that this pattern of results could arise from (1) normal residual activation being functionally more significant when overall lexical processing is slower and (2) residual phonological activation of the previous trial's target having a particularly strong inhibitory effect specifically when phonological processing is impaired because the task was phonologically-driven (the spoken input specified the target). These results provide new insights into perseveration errors and lexical access deficits in aphasia.

A new lexical card-sorting task for studying fronto-striatal contribution to processing language rules

The role of fronto-striatal regions in processing different language rules such as semantic and (grapho) phonological ones is still under debate. We have recently developed a lexical analog of the Wisconsin card sorting task which measures set-shifting abilities where the visual rules color, number, shape were replaced by three language ones: semantic, rhyme and syllable onset (attack). In the present study we aimed to compare fronto-striatal activations between the different lexical rules that are required for matching the test words to the response ones. Using functional magnetic resonance imaging (fMRI), fourteen healthy, native French-speaking participants were scanned. The results showed that some regions within the brain language network are differentially involved in semantic and phonological processes. Semantic decisions activated significantly the ventrolateral prefrontal cortex, the dorsolateral prefrontal cortex, the fusiform gyrus, the ventral temporal lobe and the caudate nucleus, while phonological decisions produced significant activation in posterior Broca's area (area 44), the temporoparietal junction and motor cortical regions. These findings provide critical support for the existence of a ventral subcortical semantic pathway and a more dorsal phonological stream as proposed by Duffau, Leroy, and Gatignol (2008). Furthermore, we propose that the strong involvement of area 47/12 of the ventrolateral prefrontal cortex and caudate nucleus observed in semantic processing, is not specific to language, but to the fact that a category or a rule has to be retrieved amongst competing ones in memory, similarly to what is observed when planning a set-shift in the original (non-lexical) version of the Wisconsin card sorting task.

Is there a semantic system for abstract words?

Two views on the semantics of concrete words are that their core mental representations are feature-based or are reconstructions of sensory experience. We argue that neither of these approaches is capable of representing the semantics of abstract words, which involve the representation of possibly hypothetical physical and mental states, the binding of entities within a structure, and the possible use of embedding (or recursion) in such structures. Brain based evidence in the form of dissociations between deficits related to concrete and abstract semantics corroborates the hypothesis. Neuroimaging evidence suggests that left lateral inferior frontal cortex supports those processes responsible for the representation of abstract words.

Neuropsychological evidence for the functional role of the uncinate fasciculus in semantic control

Understanding a word requires mapping sounds to a word-form and then identifying its correct meaning, which in some cases necessitates the recruitment of cognitive control processes to direct the activation of semantic knowledge in a task appropriate manner (i.e., semantic control). Neuroimaging and neuropsychological studies identify a fronto-temporal network important for word comprehension. However, little is known about the connectional architecture subserving controlled retrieval and selection of semantic knowledge during word comprehension. We used diffusion tensor imaging (DTI) and resting-state functional magnetic resonance imaging (rs-fMRI) in aphasic individuals with varying degrees of word comprehension deficits to examine the role of three white matter pathways within this network: the uncinate fasciculus (UF), inferior longitudinal fasciculus (ILF), and inferior fronto-occipital fasciculus (IFOF). Neuroimaging data from a group of age-matched controls were also collected in order to establish that the patient group had decreased structural and functional connectivity profiles. We obtained behavioral data from aphasic participants on two measures of single word comprehension that involve semantic control, and assessed pathway functional significance by correlating patients' performance with indices of pathway structural integrity and the functional connectivity profiles of regions they connect. Both the structural integrity of the UF and the functional connectivity strength of regions it connects predicted patients' performance. This result suggests the semantic control impairment in word comprehension resulted from poor neural communication between regions the UF connects. Inspections of other subcortical and cortical structures revealed no relationship with patients' performance. We conclude that the UF mediates semantic control during word comprehension by connecting regions specialized for cognitive control with those storing word meanings. These findings also support a relationship between structural and functional connectivity measures, as the rs-fMRI results provide converging evidence with those obtained using DTI.

Words and objects at the tip of the left temporal lobe in primary progressive aphasia

Eleven of 69 prospectively enrolled primary progressive aphasics were selected for this study because of peak atrophy sites located predominantly or exclusively within the anterior left temporal lobe. Cortical volumes in these areas were reduced to less than half of control values, whereas average volume elsewhere in the left hemisphere deviated from control values by only 8%. Failure to name objects emerged as the most consistent and severe deficit. Naming errors were attributed to pure retrieval failure if the object could not be named even when the denoting word was understood, the object recognized and the two accurately matched. Surprisingly many of the naming errors reflected pure retrieval failures, without discernible semantic or associative component. The remaining set of errors had associative components. These errors reflected the inability to define the word denoting the object more often than the inability to define the nature of the pictured object. In a separate task where the same object had to be linked to verbal or non-verbal associations, performance was abnormal only in the verbal format. Excessive taxonomic interference was observed for picture-word, but not picture-picture, matching tasks. This excessive interference reflected a blurring of intra- rather than inter-category distinctions as if the acuity of word-object associations had been diminished so that correspondences were easier to recognize at generic than specific levels. These dissociations between verbal and non-verbal markers of object knowledge indicate that the reduced neural mass at peak atrophy sites of the left temporal tip, accounting for half or more of the presumed premorbid volume, was unlikely to have contained domain-independent semantic representations of the type that would be expected in a strictly amodal hub. A more likely arrangement entails two highly interactive routes--a strongly left lateralized temporosylvian language network for verbal concepts, and a presumably more bilateral or right-sided inferotemporal/fusiform object recognition network, which remained relatively spared because peak atrophy sites were concentrated on the left. The current results also suggest that the left anterior temporal neocortex should be inserted into the language network where it is likely to play a major role in selecting verbal labels for objects and mediating the progression of word comprehension from generic to specific levels of precision.

Conceptual object representations in human anterior temporal cortex

Interaction with everyday objects requires the representation of conceptual object properties, such as where and how an object is used. What are the neural mechanisms that support this knowledge? While research on semantic dementia has provided evidence for a critical role of the anterior temporal lobes (ATLs) in object knowledge, fMRI studies using univariate analysis have primarily implicated regions outside the ATL. In the present human fMRI study we used multivoxel pattern analysis to test whether activity patterns in ATLs carry information about conceptual object properties. Participants viewed objects that differed on two dimensions: where the object is typically found (in the kitchen or the garage) and how the object is commonly used (with a rotate or a squeeze movement). Anatomical region-of-interest analyses covering the ventral visual stream revealed that information about the location and action dimensions increased from posterior to anterior ventral temporal cortex, peaking in the temporal pole. Whole-brain multivoxel searchlight analysis confirmed these results, revealing highly significant and regionally specific information about the location and action dimensions in the anterior temporal lobes bilaterally. In contrast to conceptual object properties, perceptual and low-level visual properties of the objects were reflected in activity patterns in posterior lateral occipitotemporal cortex and occipital cortex, respectively. These results provide fMRI evidence that object representations in the anterior temporal lobes are abstracted away from perceptual properties, categorizing objects in semantically meaningful groups to support conceptual object knowledge.

Refractory semantic access dysphasia resulting from resection of a left frontal glioma

The existence of semantic access disorders is now well established, however the precise cognitive and anatomical underpinnings are still debated. Here we describe the case of a patient that became aphasic after the resection of a left frontal glioma. Detailed lesion reconstruction indicates that the lesion was mostly restricted to the left dorsal and ventral prefrontal cortices and the underlying white matter, but sparing temporal lobes. Critically, the patient showed all the signs of refractory semantic access dysphasia, supporting the association between this syndrome and damage to left prefrontal areas likely to subserve retrieval and selection mechanisms for verbal material.

Fundamental deficits of auditory perception in Wernicke's aphasia

OBJECTIVE

This work investigates the nature of the comprehension impairment in Wernicke's aphasia (WA), by examining the relationship between deficits in auditory processing of fundamental, non-verbal acoustic stimuli and auditory comprehension. WA, a condition resulting in severely disrupted auditory comprehension, primarily occurs following a cerebrovascular accident (CVA) to the left temporo-parietal cortex. Whilst damage to posterior superior temporal areas is associated with auditory linguistic comprehension impairments, functional-imaging indicates that these areas may not be specific to speech processing but part of a network for generic auditory analysis.

METHODS

We examined analysis of basic acoustic stimuli in WA participants (n = 10) using auditory stimuli reflective of theories of cortical auditory processing and of speech cues. Auditory spectral, temporal and spectro-temporal analysis was assessed using pure-tone frequency discrimination, frequency modulation (FM) detection and the detection of dynamic modulation (DM) in "moving ripple" stimuli. All tasks used criterion-free, adaptive measures of threshold to ensure reliable results at the individual level.

RESULTS

Participants with WA showed normal frequency discrimination but significant impairments in FM and DM detection, relative to age- and hearing-matched controls at the group level (n = 10). At the individual level, there was considerable variation in performance, and thresholds for both FM and DM detection correlated significantly with auditory comprehension abilities in the WA participants.

CONCLUSION

These results demonstrate the co-occurrence of a deficit in fundamental auditory processing of temporal and spectro-temporal non-verbal stimuli in WA, which may have a causal contribution to the auditory language comprehension impairment. Results are discussed in the context of traditional neuropsychology and current models of cortical auditory processing.

Non-linguistic learning and aphasia: evidence from a paired associate and feedback-based task

Though aphasia is primarily characterized by impairments in the comprehension and/or expression of language, research has shown that patients with aphasia also show deficits in cognitive-linguistic domains such as attention, executive function, concept knowledge and memory. Research in aphasia suggests that cognitive impairments can impact the online construction of language, new verbal learning, and transactional success. In our research, we extend this hypothesis to suggest that general cognitive deficits influence progress with therapy. The aim of our study is to explore learning, a cognitive process that is integral to relearning language, yet underexplored in the field of aphasia rehabilitation. We examine non-linguistic category learning in patients with aphasia (n=19) and in healthy controls (n=12), comparing feedback and non-feedback based instruction. Participants complete two computer-based learning tasks that require them to categorize novel animals based on the percentage of features shared with one of two prototypes. As hypothesized, healthy controls showed successful category learning following both methods of instruction. In contrast, only 60% of our patient population demonstrated successful non-linguistic category learning. Patient performance was not predictable by standardized measures of cognitive ability. Results suggest that general learning is affected in aphasia and is a unique, important factor to consider in the field of aphasia rehabilitation.

A specific cognitive deficit within semantic cognition across a multi-generational family

We report a study of eight members of a single family (aged 8-72 years), who all show a specific deficit in linking semantic knowledge to language. All affected members of the family had high levels of overall intelligence; however, they had profound difficulties in prose and sentence recall, listening comprehension and naming. The behavioural deficit was remarkably consistent across affected family members. Structural neuroimaging data revealed grey matter abnormalities in the left infero-temporal cortex and fusiform gyri: brain areas that have been associated with integrative semantics. This family demonstrates, to our knowledge, the first example of a heritable, highly specific abnormality affecting the interface between language and cognition in humans and has important implications for our understanding of the genetic basis of cognition.

Please get to the point! A cortical correlate of linguistic informativeness

The production of informative messages is an effortful endeavor that relies on the interaction between microlinguistic (i.e., lexical and grammatical) and macrolinguistic (i.e., pragmatic and discourse) levels of processing. Although the neural correlates of microlinguistic processing have been extensively studied, investigation of the ability to organize the macrolinguistic aspects of message production is scanty. In this article, we show that repetitive TMS of the dorsal portion of the anterior left, but not right, inferior frontal gyrus reduces the levels of lexical informativeness and global coherence of narratives produced by healthy individuals. Interestingly, levels of productivity and microlinguistic processing were unaffected by the stimulation. These results suggest that the dorsal aspect of the anterior left inferior frontal gyrus is an epicenter of a wider neural network subserving the selection of contextually appropriate semantic representations.

Convergent connectivity and graded specialization in the rostral human temporal lobe as revealed by diffusion-weighted imaging probabilistic tractography

In recent years, multiple independent neuroscience investigations have implicated critical roles for the rostral temporal lobe in auditory and visual perception, language, and semantic memory. Although arising in the context of different cognitive functions, most of these suggest that there is a gradual convergence of sensory information in the temporal lobe that culminates in modality- and perceptually invariant representations at the most rostral aspect. Currently, however, too little is known regarding connectivity within the human temporal lobe to be sure of exactly how and where convergence occurs; existing hypotheses are primarily derived on the basis of cross-species generalizations from invasive nonhuman primate studies, the validity of which is unclear, especially where language function is concerned. In this study, we map the connectivity of the human rostral temporal lobe in vivo for the first time using diffusion-weighted imaging probabilistic tractography. The results indicate that convergence of sensory information in the temporal lobe is in fact a graded process that occurs along both its longitudinal and lateral axes and culminates in the most rostral limits. We highlight the consistency of our results with those of prior functional neuroimaging, computational modeling, and patient studies. By going beyond simple fasciculus reconstruction, we systematically explored the connectivity of specific temporal lobe areas to frontal and parietal language regions. In contrast to the graded within-temporal lobe connectivity, this intertemporal connectivity was found to dissociate across caudal, mid, and rostral subregions. Furthermore, we identified a basal rostral temporal region with very limited connectivity to areas outside the temporal lobe, which aligns with recent evidence that this subregion underpins the extraction of modality- and context-invariant semantic representations.

Functional Heterogeneity within the Default Network during Semantic Processing and Speech Production

This fMRI study investigated the functional heterogeneity of the core nodes of the default mode network (DMN) during language processing. The core nodes of the DMN were defined as task-induced deactivations over multiple tasks in 94 healthy subjects. We used a factorial design that manipulated different tasks (semantic matching or speech production) and stimuli (familiar words and objects or unfamiliar stimuli), alternating with periods of fixation/rest. Our findings revealed several consistent effects in the DMN, namely less deactivations in the left inferior parietal lobule during semantic than perceptual matching in parallel with greater deactivations during semantic matching in anterior subdivisions of the posterior cingulate cortex (PCC) and the ventromedial prefrontal cortex (MPFC). This suggests that, when the brain is engaged in effortful semantic tasks, a part of the DMN in the left angular gyrus was less deactivated as five other nodes of the DMN were more deactivated. These five DMN areas, where deactivation was greater for semantic than perceptual matching, were further differentiated because deactivation was greater in (i) posterior ventral MPFC for speech production relative to semantic matching, (ii) posterior precuneus and PCC for perceptual processing relative to speech production, and (iii) right inferior parietal cortex for pictures of objects relative to written words during both naming and semantic decisions. Our results thus highlight that task difficulty alone cannot fully explain the functional variability in task-induced deactivations. Together these results emphasize that core nodes within the DMN are functionally heterogeneous and differentially sensitive to the type of language processing.

Both the Middle Temporal Gyrus and the Ventral Anterior Temporal Area Are Crucial for Multimodal Semantic Processing: Distortion-corrected fMRI Evidence for a Double Gradient of Information Convergence in the Temporal Lobes

Most contemporary theories of semantic memory assume that concepts are formed from the distillation of information arising in distinct sensory and verbal modalities. The neural basis of this distillation or convergence of information was the focus of this study. Specifically, we explored two commonly posed hypotheses: (a) that the human middle temporal gyrus (MTG) provides a crucial semantic interface given the fact that it interposes auditory and visual processing streams and (b) that the anterior temporal region—especially its ventral surface (vATL)—provides a critical region for the multimodal integration of information. By utilizing distortion-corrected fMRI and an established semantic association assessment (commonly used in neuropsychological investigations), we compared the activation patterns observed for both the verbal and nonverbal versions of the same task. The results are consistent with the two hypotheses simultaneously: Both MTG and vATL are activated in common for word and picture semantic processing. Additional planned, ROI analyses show that this result follows from two principal axes of convergence in the temporal lobe: both lateral (toward MTG) and longitudinal (toward the anterior temporal lobe).

Behavioral patterns and lesion sites associated with impaired processing of lexical and conceptual knowledge of actions

To further investigate the neural substrates of lexical and conceptual knowledge of actions, we administered a battery of six tasks to 226 brain-damaged patients with widely distributed lesions in the left and right cerebral hemispheres. The tasks probed lexical and conceptual knowledge of actions in a variety of verbal and non-verbal ways, including naming, word-picture matching, attribute judgments involving both words and pictures, and associative comparisons involving both words and pictures. Of the 226 patients who were studied, 61 failed one or more of the six tasks, with four patients being impaired on the entire battery, and varied numbers of patients being impaired on varied combinations of tasks. Overall, the 61 patients manifested a complex array of associations and dissociations across the six tasks. The lesion sites of 147 of the 226 patients were also investigated, using formal methods for lesion-deficit statistical mapping and power analysis of lesion overlap maps. Significant effects for all six tasks were found in the following left-hemisphere regions: the inferior frontal gyrus; the ventral precentral gyrus, extending superiorly into what are likely to be hand-related primary motor and premotor areas; and the anterior insula. In addition, significant effects for 4-5 tasks were found in not only the regions just mentioned, but also in several other left-hemisphere areas: the ventral postcentral gyrus; the supramarginal gyrus; and the posterior middle temporal gyrus. These results converge with previous research on the neural underpinnings of action words and concepts. However, the current study goes considerably beyond most previous investigations by providing extensive behavioral and lesion data for an unusually large and diverse sample of brain-damaged patients, and by incorporating multiple measures of verb comprehension. Regarding theoretical implications, the study provides new support for the Embodied Cognition Framework, which maintains that conceptual knowledge is grounded in sensorimotor systems.

Damage to temporo-parietal cortex decreases incidental activation of thematic relations during spoken word comprehension

Both taxonomic and thematic semantic relations have been studied extensively in behavioral studies and there is an emerging consensus that the anterior temporal lobe plays a particularly important role in the representation and processing of taxonomic relations, but the neural basis of thematic semantics is less clear. We used eye tracking to examine incidental activation of taxonomic and thematic relations during spoken word comprehension in participants with aphasia. Three groups of participants were tested: neurologically intact control participants (N=14), individuals with aphasia resulting from lesions in left hemisphere BA 39 and surrounding temporo-parietal cortex regions (N=7), and individuals with the same degree of aphasia severity and semantic impairment and anterior left hemisphere lesions (primarily inferior frontal gyrus and anterior temporal lobe) that spared BA 39 (N=6). The posterior lesion group showed reduced and delayed activation of thematic relations, but not taxonomic relations. In contrast, the anterior lesion group exhibited longer-lasting activation of taxonomic relations and did not differ from control participants in terms of activation of thematic relations. These results suggest that taxonomic and thematic semantic knowledge are functionally and neuroanatomically distinct, with the temporo-parietal cortex playing a particularly important role in thematic semantics.

Naming impairment in Alzheimer's disease is associated with left anterior temporal lobe atrophy

There is considerable debate about the neuroanatomic localization of semantic memory, the knowledge of culturally shared elements such as objects, concepts, and people. Two recent meta-analyses of functional imaging studies (fMRI and PET) sought to identify cortical regions involved in semantic processing. Binder and colleagues (Binder et al., 2009) identified several regions of interest, widely distributed throughout the frontal, parietal, and temporal cortices. In contrast, Lambon Ralph and colleagues (2010) focused on the anterior temporal lobe, and found that when the potential for signal loss is accounted for (due, for example, to distortion artifact or field of view restriction), significant regional activation is detected. We set out to determine whether the anterior temporal lobe plays a significant role in picture naming, a task which relies on semantic memory. We examined a relatively large sample of patients with early Alzheimer's disease (N=145), a multifocal disease process typically characterized in the early stages by problems with episodic memory and executive function. Hypothesis-driven analyses based on regions of interest derived from the meta-analyses as well as exploratory analyses across the entire cerebral cortex demonstrated a highly specific correlation between cortical thinning of the left anterior temporal lobe and impaired naming performance. These findings lend further support to theories that include a prominent role for the anterior temporal lobe in tasks that rely on semantic memory.

Effects of word frequency and modality on sentence comprehension impairments in people with aphasia

PURPOSE

It is well known that people with aphasia have sentence comprehension impairments. The present study investigated whether lexical factors contribute to sentence comprehension impairments in both the auditory and written modalities using online measures of sentence processing.

METHOD

People with aphasia and non brain-damaged controls participated in the experiment (n = 8 per group). Twenty-one sentence pairs containing high- and low-frequency words were presented in self-paced listening and reading tasks. The sentences were syntactically simple and differed only in the critical words. The dependent variables were response times for critical segments of the sentence and accuracy on the comprehension questions.

RESULTS

The results showed that word frequency influences performance on measures of sentence comprehension in people with aphasia. The accuracy data on the comprehension questions suggested that people with aphasia have more difficulty understanding sentences containing low-frequency words in the written compared to auditory modality. Both group and single-case analyses of the response time data also indicated that people with aphasia experience more difficulty with reading than listening.

CONCLUSION

Sentence comprehension in people with aphasia is influenced by word frequency and presentation modality.

Staging of the cognitive decline in Alzheimer's disease: insights from a detailed neuropsychological investigation of mild cognitive impairment and mild Alzheimer's disease

OBJECTIVE

The decline of episodic memory in Alzheimer's disease (AD) is well established, but the exact appearance and staging of deficits in other cognitive domains is sometimes contentious. The current investigation attempted to elucidate the appearance of additional cognitive deficits in the non-episodic domains and to understand these deficits with respect to the known pathological staging of AD.

METHODS

A cross-sectional investigation compared cognitively normal age-matched controls with patients with mild AD and mild cognitive impairment (MCI) using a detailed neuropsychological assessment.

RESULTS

The systematic investigation of cognitive performance across the major cognitive domains demonstrates that the appearance of additional cognitive deficits in MCI and AD can be predicted, with impaired semantic cognition performance pre-empting the appearance of attention/executive dysfunction and visuospatial deficits in the majority of patients with MCI.

CONCLUSIONS

This progressive pattern of cognitive deficits fits with the known pathological staging of AD, and the data further highlight the relative rarity of pure amnestic MCI. These results indicate that any neuropsychological test battery used to assess patients with MCI should include language and semantic memory tests in addition to typical episodic memory tests, as changes within this domain might be a sensitive indication of incipient AD.

Prefrontal cortex rTMS enhances action naming in progressive non-fluent aphasia

BACKGROUND AND PURPOSE

Progressive non-fluent aphasia (PNFA) is a neurodegenerative disorder that is characterized by non-fluent speech with naming impairment and grammatical errors. It has been recently demonstrated that repetitive transcranial magnetic stimulation (rTMS) over the dorsolateral prefrontal cortex (DLPFC) improves action naming in healthy subjects and in subjects with Alzheimer's disease.

PURPOSE

To investigate whether the modulation of DLPFC circuits by rTMS modifies naming performance in patients with PNFA.

METHODS

Ten patients with a diagnosis of PNFA were enrolled. High-frequency rTMS was applied to the left and right DLPFC and the sham (i.e. placebo) condition during object and action naming. A subgroup of patients with semantic dementia was enrolled as a comparison group.

RESULTS

A repeated-measure anova with stimulus site (sham, left and right rTMS) showed significant effects. Action-naming performances during stimulation of both the left and right DLPFC were better than during placebo stimulation. No facilitating effect of rTMS to the DLPFC on object naming was observed. In patients with a diagnosis of semantic dementia, no effect of stimulation was reported.

CONCLUSIONS

Our study demonstrated that rTMS improved action naming in subjects with PNFA, possibly due to the modulation of DLPFC pathways and a facilitation effect on lexical retrieval processes. Future studies on the potential of a rehabilitative protocol using rTMS applied to the DLPFC in this orphan disorder are required.

Relations between Short-term Memory Deficits, Semantic Processing, and Executive Function

BACKGROUND: Previous research has suggested separable short-term memory (STM) buffers for the maintenance of phonological and lexical-semantic information, as some patients with aphasia show better ability to retain semantic than phonological information and others show the reverse. Recently, researchers have proposed that deficits to the maintenance of semantic information in STM are related to executive control abilities. AIMS: The present study investigated the relationship of executive function abilities with semantic and phonological short-term memory (STM) and semantic processing in such patients, as some previous research has suggested that semantic STM deficits and semantic processing abilities are critically related to specific or general executive function deficits. METHOD AND PROCEDURES: 20 patients with aphasia and STM deficits were tested on measures of short-term retention, semantic processing, and both complex and simple executive function tasks. OUTCOME AND RESULTS: In correlational analyses, we found no relation between semantic STM and performance on simple or complex executive function tasks. In contrast, phonological STM was related to executive function performance in tasks that had a verbal component, suggesting that performance in some executive function tasks depends on maintaining or rehearsing phonological codes. Although semantic STM was not related to executive function ability, performance on semantic processing tasks was related to executive function, perhaps due to similar executive task requirements in both semantic processing and executive function tasks. CONCLUSIONS: Implications for treatment and interpretations of executive deficits are discussed.

The function of the anterior temporal lobe: a review of the empirical evidence

Recent work on the anterior temporal lobe (ATL) has lead to substantively different theoretical branches, of its putative functions, that have in some part developed independently of one another. The ATL has dense connectivity with a number of sensory modalities. This has resulted in empirical evidence that supports different functionality dependent upon the variables under investigation. The main bodies of evidence have implicated the ATL as a domain-general semantic hub, whilst other evidence points to a domain-specific role in social or 'person-related' processing. A third body of evidence suggests that the ATLs underlie processing of unique entities. Primarily, research of the ATL has been based on lesion studies and from clinical populations such as semantic dementia or temporal lobe epilepsy patients. Although important, this neuropsychological evidence has a number of confounds, therefore techniques such as functional neuroimaging on healthy participants and the relatively novel use of non-invasive brain stimulation may be more useful to isolate specific variables that can discriminate between these different theories concerning 'normal' function. This review focuses on these latter types of studies and considers the empirical evidence for each perspective. The overall literature is integrated in an attempt to formulate a unifying theory and the functional sub-regions within the ATL are explored. It is concluded that a holistic integration of the theories is feasible in that the ATLs could process domain-general semantic knowledge but with a bias towards social information or stimuli that is personally relevant. Thus, it may be the importance of social/emotional information that gives it priority of processing in the ATL not an inherent property of the structure itself.

The differential contributions of pFC and temporo-parietal cortex to multimodal semantic control: exploring refractory effects in semantic aphasia

Aphasic patients with multimodal semantic impairment following pFC or temporo-parietal (TP) cortex damage (semantic aphasia [SA]) have deficits characterized by poor control of semantic activation/retrieval, as opposed to loss of semantic knowledge per se. In line with this, SA patients show "refractory effects"; that is, declining accuracy in cyclical word-picture matching tasks when semantically related sets are presented rapidly and repeatedly. This is argued to follow a build-up of competition between targets and distractors. However, the link between poor semantic control and refractory effects is still controversial for two reasons. (1) Some theories propose that refractory effects are specific to verbal or auditory tasks, yet SA patients show poor control over semantic processing in both word and picture semantic tasks. (2) SA can result from lesions to either the left pFC or TP cortex, yet previous work suggests that refractory effects are specifically linked to the left inferior frontal cortex. For the first time, verbal, visual, and nonverbal auditory refractory effects were explored in nine SA patients who had pFC (pFC+) or TP cortex (TP-only) lesions. In all modalities, patient accuracy declined significantly over repetitions. This refractory effect at the group level was driven by pFC+ patients and was not shown by individuals with TP-only lesions. These findings support the theory that SA patients have reduced control over multimodal semantic retrieval and, additionally, suggest there may be functional specialization within the posterior versus pFC elements of the semantic control network.

Executive semantic processing is underpinned by a large-scale neural network: revealing the contribution of left prefrontal, posterior temporal, and parietal cortex to controlled retrieval and selection using TMS

To understand the meanings of words and objects, we need to have knowledge about these items themselves plus executive mechanisms that compute and manipulate semantic information in a task-appropriate way. The neural basis for semantic control remains controversial. Neuroimaging studies have focused on the role of the left inferior frontal gyrus (LIFG), whereas neuropsychological research suggests that damage to a widely distributed network elicits impairments of semantic control. There is also debate about the relationship between semantic and executive control more widely. We used TMS in healthy human volunteers to create "virtual lesions" in structures typically damaged in patients with semantic control deficits: LIFG, left posterior middle temporal gyrus (pMTG), and intraparietal sulcus (IPS). The influence of TMS on tasks varying in semantic and nonsemantic control demands was examined for each region within this hypothesized network to gain insights into (i) their functional specialization (i.e., involvement in semantic representation, controlled retrieval, or selection) and (ii) their domain dependence (i.e., semantic or cognitive control). The results revealed that LIFG and pMTG jointly support both the controlled retrieval and selection of semantic knowledge. IPS specifically participates in semantic selection and responds to manipulations of nonsemantic control demands. These observations are consistent with a large-scale semantic control network, as predicted by lesion data, that draws on semantic-specific (LIFG and pMTG) and domain-independent executive components (IPS).

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.

Lexicality Effects in Word and Nonword Recall of Semantic Dementia and Progressive Nonfluent Aphasia

BACKGROUND

Verbal working memory is an essential component of many language functions, including sentence comprehension and word learning. As such, working memory has emerged as a domain of intense research interest both in aphasiology and in the broader field of cognitive neuroscience. The integrity of verbal working memory encoding relies on a fluid interaction between semantic and phonological processes. That is, we encode verbal detail using many cues related to both the sound and meaning of words. Lesion models can provide an effective means of parsing the contributions of phonological or semantic impairment to recall performance.

METHODS AND PROCEDURES

We employed the lesion model approach here by contrasting the nature of lexicality errors incurred during recall of word and nonword sequences by 3individuals with progressive nonfluent aphasia (a phonological dominant impairment) compared to that of 2 individuals with semantic dementia (a semantic dominant impairment). We focused on psycholinguistic attributes of correctly recalled stimuli relative to those that elicited a lexicality error (i.e., nonword → word OR word → nonword).

OUTCOMES AND RESULTS

Patients with semantic dementia showed greater sensitivity to phonological attributes (e.g., phoneme length, wordlikeness) of the target items relative to semantic attributes (e.g., familiarity). Patients with PNFA showed the opposite pattern, marked by sensitivity to word frequency, age of acquisition, familiarity, and imageability.

CONCLUSIONS

We interpret these results in favor of a processing strategy such that in the context of a focal phonological impairment patients revert to an over-reliance on preserved semantic processing abilities. In contrast, a focal semantic impairment forces both reliance upon and hypersensitivity to phonological attributes of target words. We relate this interpretation to previous hypotheses about the nature of verbal short-term memory in progressive aphasia.

Lichtheim 2: synthesizing aphasia and the neural basis of language in a neurocomputational model of the dual dorsal-ventral language pathways

Traditional neurological models of language were based on a single neural pathway (the dorsal pathway underpinned by the arcuate fasciculus). Contemporary neuroscience indicates that anterior temporal regions and the "ventral" language pathway also make a significant contribution, yet there is no computationally-implemented model of the dual pathway, nor any synthesis of normal and aphasic behavior. The "Lichtheim 2" model was implemented by developing a new variety of computational model which reproduces and explains normal and patient data but also incorporates neuroanatomical information into its architecture. By bridging the "mind-brain" gap in this way, the resultant "neurocomputational" model provides a unique opportunity to explore the relationship between lesion location and behavioral deficits, and to provide a platform for simulating functional neuroimaging data.

Wernicke's aphasia reflects a combination of acoustic-phonological and semantic control deficits: a case-series comparison of Wernicke's aphasia, semantic dementia and semantic aphasia

Wernicke's aphasia (WA) is the classical neurological model of comprehension impairment and, as a result, the posterior temporal lobe is assumed to be critical to semantic cognition. This conclusion is potentially confused by (a) the existence of patient groups with semantic impairment following damage to other brain regions (semantic dementia and semantic aphasia) and (b) an ongoing debate about the underlying causes of comprehension impairment in WA. By directly comparing these three patient groups for the first time, we demonstrate that the comprehension impairment in Wernicke's aphasia is best accounted for by dual deficits in acoustic-phonological analysis (associated with pSTG) and semantic cognition (associated with pMTG and angular gyrus). The WA group were impaired on both nonverbal and verbal comprehension assessments consistent with a generalised semantic impairment. This semantic deficit was most similar in nature to that of the semantic aphasia group suggestive of a disruption to semantic control processes. In addition, only the WA group showed a strong effect of input modality on comprehension, with accuracy decreasing considerably as acoustic-phonological requirements increased. These results deviate from traditional accounts which emphasise a single impairment and, instead, implicate two deficits underlying the comprehension disorder in WA.

Density pervades: an analysis of phonological neighbourhood density effects in aphasic speakers with different types of naming impairment

We investigated the influence of phonological neighbourhood density (PND) on the performance of aphasic speakers whose naming impairments differentially implicate phonological or semantic stages of lexical access. A word comes from a dense phonological neighbourhood if many words sound like it. Limited evidence suggests that higher density facilitates naming in aphasic speakers, as it does in healthy speakers. Using well-controlled stimuli, Experiment 1 confirmed the influence of PND on accuracy and phonological error rates in two aphasic speakers with phonological processing deficits. In Experiments 2 and 3, we extended the investigation to an aphasic speaker who is prone to semantic errors, indicating a semantic deficit and/or a deficit in the mapping from semantics to words. This individual had higher accuracy, and fewer semantic errors, in naming targets from high- than from low-density neighbourhoods. It is argued that the Results provide strong support for interactive approaches to lexical access, where reverberatory feedback between word- and phoneme-level lexical representations not only facilitates phonological level processes but also privileges the selection of a target word over its semantic competitors.

The neurobiology of semantic memory

Semantic memory includes all acquired knowledge about the world and is the basis for nearly all human activity, yet its neurobiological foundation is only now becoming clear. Recent neuroimaging studies demonstrate two striking results: the participation of modality-specific sensory, motor, and emotion systems in language comprehension, and the existence of large brain regions that participate in comprehension tasks but are not modality-specific. These latter regions, which include the inferior parietal lobe and much of the temporal lobe, lie at convergences of multiple perceptual processing streams. These convergences enable increasingly abstract, supramodal representations of perceptual experience that support a variety of conceptual functions including object recognition, social cognition, language, and the remarkable human capacity to remember the past and imagine the future.

Multifractal dynamics in the emergence of cognitive structure

The complex-systems approach to cognitive science seeks to move beyond the formalism of information exchange and to situate cognition within the broader formalism of energy flow. Changes in cognitive performance exhibit a fractal (i.e., power-law) relationship between size and time scale. These fractal fluctuations reflect the flow of energy at all scales governing cognition. Information transfer, as traditionally understood in the cognitive sciences, may be a subset of this multiscale energy flow. The cognitive system exhibits not just a single power-law relationship between fluctuation size and time scale but actually exhibits many power-law relationships, whether over time or space. This change in fractal scaling, that is, multifractality, provides new insights into changes in energy flow through the cognitive system. We survey recent findings demonstrating the role of multifractality in (a) understanding atypical developmental outcomes, and (b) predicting cognitive change. We propose that multifractality provides insights into energy flows driving the emergence of cognitive structure.