Elucidating the Nature of Deregulated Semantic Cognition in Semantic Aphasia: Evidence for the Roles of Prefrontal and Temporo-parietal Cortices

The Impact of Lexical-semantic Impairment on Spoken Verb Production in Individuals With Mild Cognitive Impairment

BACKGROUND

Although episodic memory is the primary concern in individuals with mild cognitive impairment (MCI), other cognitive functions may also be affected, including language. Language impairment in individuals with MCI has been attributed primarily to the breakdown of semantic representations, difficulties in accessing semantic information, and the weakening of executive functions. However, in most prior studies of word processing in individuals with MCI, researchers have used measures focused on noun production.

OBJECTIVE

To investigate how verb production tasks might aid in detecting cognitive impairment in individuals with MCI.

METHODS

We compared the performance of 45 individuals with MCI and 45 healthy controls on action naming and action fluency tasks.

RESULTS

In the action naming task, the performance of participants with MCI was significantly impaired compared to healthy controls in terms of total score, the number of semantic errors produced, and the use of generic terms. In the action fluency task, participants with MCI produced significantly fewer verbs, fewer clusters, and fewer switches than healthy controls.

CONCLUSION

The results of our study emphasize the utility of verb production tasks in the identification of cognitive impairment in individuals with MCI and provide evidence of the importance of including action naming and action fluency tasks in the assessment of individuals with MCI.

Transcranial direct current stimulation over the left posterior temporal lobe modulates semantic control: Evidence from episodic memory distortions

Evidence accumulates to show that semantic cognition requires, in addition to semantic representations, control processes that regulate the accessibility and use of semantic knowledge in a task- and time-appropriate fashion. Semantic control has been recently proposed to rely on a distributed network that includes the posterior temporal cortex. Along these lines, recent meta-analyses of neuroimaging data and studies with patients suffering from semantic aphasia have suggested that the left posterior middle temporal gyrus (pMTG) is critically involved whenever situational context must constrain semantic retrieval. In the present experiment, we used transcranial direct current stimulation over the left posterior temporal lobe in an attempt to interfere with semantic control while participants performed a DRM task, a procedure for inducing conceptually-based false recognition that is contingent on both activation and control processes. Paralleling findings with patients suffering from brain damage restricted to the temporoparietal cortex, anodal stimulation (relative to sham stimulation) resulted in increased false recognition but intact true recognition. These findings fit well with the idea that the left pMTG is a key component of a semantic control network, the alteration of which results in memory performance that is affected by the intrusion of contextually-inappropriate semantic information.

White matter network underlying semantic processing: evidence from stroke patients

The hub-and-spoke theory of semantic representation fractionates the neural underpinning of semantic knowledge into two essential components: the sensorimotor modality-specific regions and a crucially important semantic hub region. Our previous study in patients with semantic dementia has found that the hub region is located in the left fusiform gyrus. However, because this region is located within the brain damage in patients with semantic dementia, it is not clear whether the semantic deficit is caused by structural damage to the hub region itself or by its disconnection from other brain regions. Stroke patients do not have any damage to the left fusiform gyrus, but exhibit amodal and modality-specific deficits in semantic processing. Therefore, in this study, we validated the semantic hub region from a brain network perspective in 79 stroke patients and explored the white matter connections associated with it. First, we collected data of diffusion-weighted imaging and behavioural performance on general semantic tasks and modality-specific semantic tasks (assessing object knowledge on form, colour, motion, sound, manipulation and function). We then used correlation and regression analyses to examine the association between the nodal degree values of brain regions in the whole-brain structural network and general semantic performance in the stroke patients. The results revealed that the connectivity of the left fusiform gyrus significantly predicted general semantic performance, indicating that this region is the semantic hub. To identify the semantic-relevant connections of the semantic hub, we then correlated the white matter integrity values of each tract connected to the left fusiform gyrus separately with performance on general and modality-specific semantic processing. We found that the hub region accomplished general semantic processing through white matter connections with the left superior temporal pole, middle temporal gyrus, inferior temporal gyrus and hippocampus. The connectivity between the hub region and the left hippocampus, superior temporal pole, middle temporal gyrus, inferior temporal gyrus and parahippocampal gyrus was differentially involved in object form, colour, motion, sound, manipulation and function processing. After statistically removing the effects of potential confounding variables (i.e. whole-brain lesion volume, lesion volume of regions of interest and performance on non-semantic control tasks), the observed effects remained significant. Together, our findings support the role of the left fusiform gyrus as a semantic hub region in stroke patients and reveal its crucial connectivity in the network. This study provides new insights and evidence for the neuroanatomical organization of semantic memory in the human brain.

Semantic impairment in aphasia: A problem of control?

PURPOSE

The relationship between cognitive processes and language ability in aphasia has recently gained increasing attention, with some authors suggesting that control impairments may underlie difficulties with semantic tasks in aphasia. This paper aims to present an overview of the current research on the involvement of cognitive processes in semantic processing tasks and discuss the proposed relationship between cognitive control and semantic processing in aphasia.

METHOD

The role of cognitive processes in semantic processing tasks commonly used in the aphasiology literature is discussed and two theoretical approaches to semantic processing that contribute to the understanding of the nature of semantic breakdown in aphasia are outlined. Finally, we examine the evidence put forward in the Controlled Semantic Cognition framework with regard to the interpretation of impaired performance on semantic processing tasks in people with aphasia.

RESULT

Non-linguistic cognitive abilities such as working memory, inhibition and control are required by semantic processing tasks, in addition to access to conceptual information, making it difficult to dissociate these abilities. Several issues exist regarding the evidence put forward for a control deficit as the underlying cause of poor performance on semantic processing tasks in aphasia.

CONCLUSION

It remains unclear whether impairment on semantic tasks in people with aphasia is related to problems with the representation and/or processing (activation/retrieval) of meaning or problems with cognitive control (or both). Further research is still needed to fully understand how non-linguistic cognitive processes interact with semantic processing, as well as clarify and consistency the definition of control.

Using lexical semantic cues to mitigate interference effects during real-time sentence processing in aphasia

We examined the auditory sentence processing of neurologically unimpaired listeners and individuals with aphasia on canonical sentence structures in real-time using a visual-world eye-tracking paradigm. The canonical sentence constructions contained multiple noun phrases and an unaccusative verb, the latter of which formed a long-distance dependency link between the unaccusative verb and its single argument (which was base generated in the object position and then displaced to the subject position). To explore the likelihood of similarity-based interference during the real time linking of the verb and the sentence's subject noun, we manipulated the animacy feature of the noun phrases (matched or mismatched). The study's objectives were to examine whether (a) reducing the similarity-based interference by mismatching animacy features would modulate the encoding and retrieval dynamics of noun phrases in real-time; and (b) whether individuals with aphasia would demonstrate on time sensitivity to this lexical-semantic cue. Results revealed a significant effect of this manipulation in individuals both with and without aphasia. In other words, the mismatch in the representational features of the noun phrases increased the distinctiveness of the unaccusative verb's subject target at the time of syntactic retrieval (verb offset) for individuals in both groups. Moreover, individuals with aphasia were shown to be sensitive to the lexical-semantic cue, even though they appeared to process it slower than unimpaired listeners. This study extends to the cue-based retrieval model by providing new insight on the real-time mechanisms underpinning sentence comprehension.

How do valence and meaning interact? The contribution of semantic control

The hub-and-spoke model of semantic cognition proposes that conceptual representations in a heteromodal 'hub' interact with and emerge from modality-specific features or 'spokes', including valence (whether a concept is positive or negative), along with visual and auditory features. As a result, valence congruency might facilitate our ability to link words conceptually. Semantic relatedness may similarly affect explicit judgements about valence. Moreover, conflict between meaning and valence may recruit semantic control processes. Here we tested these predictions using two-alternative forced-choice tasks, in which participants matched a probe word to one of two possible target words, based on either global meaning or valence. Experiment 1 examined timed responses in healthy young adults, while Experiment 2 examined decision accuracy in semantic aphasia patients with impaired controlled semantic retrieval following left hemisphere stroke. Across both experiments, semantically related targets facilitated valence matching, while related distractors impaired performance. Valence congruency was also found to facilitate semantic decision-making. People with semantic aphasia showed impaired valence matching and had particular difficulty when semantically related distractors were presented, suggesting that the selective retrieval of valence information relies on semantic control processes. Taken together, the results are consistent with the hypothesis that automatic access to the global meaning of written words affects the processing of valence, and that the valence of words is also retrieved even when this feature is task-irrelevant, affecting the efficiency of global semantic judgements.

Taxonomic and thematic relations rely on different types of semantic features: Evidence from an fMRI meta-analysis and a semantic priming study

Taxonomic and thematic relations are major components of semantic representation but their neurocognitive underpinnings are still debated. We hypothesised that taxonomic relations preferentially activate parts of anterior temporal lobe (ATL) because they rely more on colour and shape features, while thematic relations preferentially activate temporoparietal cortex (TPC) because they rely more on action and location knowledge. We first conducted activation likelihood estimation (ALE) meta-analysis to assess evidence for neural specialisation in the existing fMRI literature (Study 1), then used a primed semantic judgement task to examine if the two relations are primed by different feature types (Study 2). We find that taxonomic relations show minimal feature-based specialisation but preferentially activate the lingual gyrus. Thematic relations are more dependent on action and location features and preferentially engage TPC. The meta-analysis also showed that lateral ATL is preferentially engaged by Thematic relations, which may reflect their greater reliance on verbal associations.

The influences of working memory updating on word association effects and thematic role assignment during sentence processing

The current study investigated how individual variability in working memory (WM) updating affects real-time processing of thematic role assignment and word association during sentence reading comprehension when ERPs were recorded. By adopting a factorial design, four types of sentences were formed by crossing word association and role assignment as independent variables. The results indicated that associated words evoked a smaller N400 effect but a larger P600 effect than unassociated words in the high WM group, whereas no word association effect was found in the low WM group. In contrast, role reversal elicited larger N400 effects for both groups. These results suggest that individual differences in WM updating influenced whether and how readers retrieved and integrated the associated word in whole sentences but did not influence the online assignment of thematic roles during sentence reading. Individuals with high WM updating, in contrast to those with low WM updating, were good at making use of word-associated information provided by the preceding context in current processing.

Mapping lesion, structural disconnection, and functional disconnection to symptoms in semantic aphasia

Patients with semantic aphasia have impaired control of semantic retrieval, often accompanied by executive dysfunction following left hemisphere stroke. Many but not all of these patients have damage to the left inferior frontal gyrus, important for semantic and cognitive control. Yet semantic and cognitive control networks are highly distributed, including posterior as well as anterior components. Accordingly, semantic aphasia might not only reflect local damage but also white matter structural and functional disconnection. Here, we characterise the lesions and predicted patterns of structural and functional disconnection in individuals with semantic aphasia and relate these effects to semantic and executive impairment. Impaired semantic cognition was associated with infarction in distributed left-hemisphere regions, including in the left anterior inferior frontal and posterior temporal cortex. Lesions were associated with executive dysfunction within a set of adjacent but distinct left frontoparietal clusters. Performance on executive tasks was also associated with interhemispheric structural disconnection across the corpus callosum. In contrast, poor semantic cognition was associated with small left-lateralized structurally disconnected clusters, including in the left posterior temporal cortex. Little insight was gained from functional disconnection symptom mapping. These results demonstrate that while left-lateralized semantic and executive control regions are often damaged together in stroke aphasia, these deficits are associated with distinct patterns of structural disconnection, consistent with the bilateral nature of executive control and the left-lateralized yet distributed semantic control network.

Damage to temporoparietal cortex is sufficient for impaired semantic control

Semantic control allows us to focus semantic activation on currently relevant aspects of knowledge, even in the face of competition or when the required information is weakly encoded. Diverse cortical regions, including left prefrontal and posterior temporal cortex, are implicated in semantic control, however; the relative contribution of these regions is unclear. For the first time, we compared semantic aphasia (SA) patients with damage restricted to temporoparietal cortex (TPC; N = 8) to patients with infarcts encompassing prefrontal cortex (PF+; N = 22), to determine if prefrontal lesions are necessary for semantic control deficits. These SA groups were also compared with semantic dementia (SD; N = 10), characterised by degraded semantic representations. We asked whether TPC cases with semantic impairment show controlled retrieval deficits equivalent to PF+ cases or conceptual degradation similar to patients with SD. Independent of lesion location, the SA subgroups showed similarities, whereas SD patients showed a qualitatively distinct semantic impairment. Relative to SD, both TPC and PF+ SA subgroups: (1) showed few correlations in performance across tasks with differing control demands, but a strong relationship between tasks of similar difficulty; (2) exhibited attenuated effects of lexical frequency and concept familiarity, (3) showed evidence of poor semantic regulation in their verbal output - performance on picture naming was substantially improved when provided with a phonological cue, and (4) showed effects of control demands, such as retrieval difficulty, which were equivalent in severity across TPC and PF+ groups. These findings show that semantic impairment in SA is underpinned by damage to a distributed semantic control network, instantiated across anterior and posterior cortical areas.

The cross-domain functional organization of posterior lateral temporal cortex: insights from ALE meta-analyses of 7 cognitive domains spanning 12,000 participants

The posterior lateral temporal cortex is implicated in many verbal, nonverbal, and social cognitive domains and processes. Yet without directly comparing these disparate domains, the region's organization remains unclear; do distinct processes engage discrete subregions, or could different domains engage shared neural correlates and processes? Here, using activation likelihood estimation meta-analyses, the bilateral posterior lateral temporal cortex subregions engaged in 7 domains were directly compared. These domains comprised semantics, semantic control, phonology, biological motion, face processing, theory of mind, and representation of tools. Although phonology and biological motion were predominantly associated with distinct regions, other domains implicated overlapping areas, perhaps due to shared underlying processes. Theory of mind recruited regions implicated in semantic representation, tools engaged semantic control areas, and faces engaged subregions for biological motion and theory of mind. This cross-domain approach provides insight into how posterior lateral temporal cortex is organized and why.

Neural evidence for a separation of semantic and phonological control processes

There remain major doubts about the nature and domain specificity of inhibitory control processes, both within and between cognitive domains. This study examined inhibitory processes within the language domain, by contrasting semantic versus phonological inhibitory control. In an fMRI experiment, elderly participants performed phonological and semantic inhibitory control tasks involving resistance to highly or weakly interfering stimuli. In the semantic domain, inhibitory control effects, contrasting high vs. low interference control levels, were observed at univariate and multivariate levels in all fronto-parieto-temporal region-of-interests. In the phonological domain, inhibitory control effects were observed only at multivariate levels, and were restricted to the pars triangularis of the bilateral inferior frontal gyrus and to the left middle temporal gyrus. Critically, no reliable multivariate cross-domain prediction of neural patterns associated with inhibitory control was observed. This study supports a functional dissociation of the neural substrates associated with inhibitory control for phonological vs. semantic domains.

Training flexible conceptual retrieval in post-stroke aphasia

Semantic therapy in post-stroke aphasia typically focusses on strengthening links between conceptual representations and their lexical-articulatory forms to aid word retrieval. However, research has shown that semantic deficits in this group can affect both verbal and non-verbal tasks, particularly in patients with deregulated retrieval as opposed to degraded knowledge. This study, therefore, aimed to facilitate semantic cognition in a sample of such patients with post-stroke semantic aphasia (SA) by training the identification of both strong and weak semantic associations and providing explicit pictorial feedback that demonstrated both common and more unusual ways of linking concepts together. We assessed the effects of this training on (i) trained and untrained items; and (ii) trained and untrained tasks in eleven individuals with SA. In the training task, the SA group showed improvement with practice, particularly for trained items. A similar untrained task using pictorial stimuli (Camel and Cactus Test) also improved. Together, these results suggest that semantic training can be beneficial in patients with SA and may show some degree of generalization to untrained situations. Future research should seek to understand which patients are most likely to benefit from this type of training.

Distinguishing semantic control and phonological control and their role in aphasic deficits: A task switching investigation

People use cognitive control across many contexts in daily life, yet it remains unclear how cognitive control is used in contexts involving language. Distinguishing language-specific cognitive control components may be critical to understanding aphasia, which can co-occur with cognitive control deficits. For example, deficits in control of semantic representations (i.e., semantic control), are thought to contribute to semantic deficits in aphasia. Conversely, little is known about control of phonological representations (i.e., phonological control) in aphasia. We developed a switching task to investigate semantic and phonological control in 32 left hemisphere stroke survivors with aphasia and 37 matched controls. We found that phonological and semantic control were related, but dissociate in the presence of switching demands. People with aphasia exhibited group-wise impairment at phonological control, although individual impairments were subtle except in one case. Several individuals with aphasia exhibited frank semantic control impairments, and these individuals had relative deficits on other semantic tasks. The present findings distinguish semantic control from phonological control, and confirm that semantic control impairments contribute to semantic deficits in aphasia.

A generalised semantic cognition account of aesthetic experience

Given that aesthetic experiences typically involve extracting meaning from environment, we believe that semantic cognition research has much to offer the field of neuroaesthetics. In the current paper, we propose a generalised framework that is inspired by the semantic cognition literature and that treats aesthetic experience as just one example of how meaning accumulates. According to our framework, aesthetic experiences are underpinned by the same cognitive and brain systems that are involved in deriving meaning from the environment in general, such as modality-specific conceptual representations and controlled processes for retrieving the appropriate type of information. Our generalised semantic cognition view of aesthetic experience has substantial implications for theory development: it leads to novel, falsifiable predictions and it reconfigures foundational assumptions regarding the structure of the cognitive and brain systems that may be involved in aesthetic experiences.

Motivated semantic control: Exploring the effects of extrinsic reward and self-reference on semantic retrieval in semantic aphasia

Recent insights show that increased motivation can benefit executive control, but this effect has not been explored in relation to semantic cognition. Patients with deficits of controlled semantic retrieval in the context of semantic aphasia (SA) after stroke may benefit from this approach since 'semantic control' is considered an executive process. Deficits in this domain are partially distinct from the domain-general deficits of cognitive control. We assessed the effect of both extrinsic and intrinsic motivation in healthy controls and SA patients. Experiment 1 manipulated extrinsic reward using high or low levels of points for correct responses during a semantic association task. Experiment 2 manipulated the intrinsic value of items using self-reference, allocating pictures of items to the participant ('self') or researcher ('other') in a shopping game before participants retrieved their semantic associations. These experiments revealed that patients, but not controls, showed better performance when given an extrinsic reward, consistent with the view that increased external motivation may help ameliorate patients' semantic control deficits. However, while self-reference was associated with better episodic memory, there was no effect on semantic retrieval. We conclude that semantic control deficits can be reduced when extrinsic rewards are anticipated; this enhanced motivational state is expected to support proactive control, for example, through the maintenance of task representations. It may be possible to harness this modulatory impact of reward to combat the control demands of semantic tasks in SA patients.

Alpha oscillations in left perisylvian cortices support semantic processing and predict performance

Semantic processing is the ability to discern and maintain conceptual relationships among words and objects. While the neural circuits serving semantic representation and controlled retrieval are well established, the neuronal dynamics underlying these processes are poorly understood. Herein, we examined 25 healthy young adults who completed a semantic relation word-matching task during magnetoencephalography (MEG). MEG data were examined in the time-frequency domain and significant oscillatory responses were imaged using a beamformer. Whole-brain statistical analyses were conducted to compare semantic-related to length-related neural oscillatory responses. Time series were extracted to visualize the dynamics and were linked to task performance using structural equation modeling. The results indicated that participants had significantly longer reaction times in semantic compared to length trials. Robust MEG responses in the theta (3-6 Hz), alpha (10-16 Hz), and gamma (64-76 Hz and 64-94 Hz) bands were observed in parieto-occipital and frontal cortices. Whole-brain analyses revealed stronger alpha oscillations in a left-lateralized network during semantically related relative to length trials. Importantly, stronger alpha oscillations in the left superior temporal gyrus during semantic trials predicted faster responses. These data reinforce existing literature and add novel temporal evidence supporting the executive role of the semantic control network in behavior.

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

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

Intrinsic connectivity of left ventrolateral prefrontal cortex predicts individual differences in controlled semantic retrieval

Control processes allow us to constrain the retrieval of semantic information from long-term memory so that it is appropriate for the task or context. Control demands are influenced by the strength of the target information itself and by the circumstances in which it is retrieved, with more control needed when relatively weak aspects of knowledge are required and after the sustained retrieval of related concepts. To investigate the neurocognitive basis of individual differences in these aspects of semantic control, we used resting-state fMRI to characterise the intrinsic connectivity of left ventrolateral prefrontal cortex (VLPFC), implicated in controlled retrieval, and examined associations on a paced serial semantic task, in which participants were asked to detect category members amongst distractors. This task manipulated both the strength of target associations and the requirement to sustain retrieval within a narrow semantic category over time. We found that individuals with stronger connectivity between VLPFC and medial prefrontal cortex within the default mode network (DMN) showed better retrieval of strong associations (which are thought to be recalled more automatically). Stronger connectivity between the same VLPFC seed and another DMN region in medial parietal cortex was associated with larger declines in retrieval over the course of the category. In contrast, participants with stronger connectivity between VLPFC and cognitive control regions within the ventral attention network (VAN) had better controlled retrieval of weak associations and were better able to sustain their comprehension throughout the category. These effects overlapped in left insular cortex within the VAN, indicating that a common pattern of connectivity is associated with different aspects of controlled semantic retrieval induced by both the structure of long-term knowledge and the sustained retrieval of related information.

Impaired emotion perception and categorization in semantic aphasia

According to a constructionist model of emotion, conceptual knowledge plays a foundational role in emotion perception; reduced availability of relevant conceptual knowledge should therefore impair emotion perception. Conceptual deficits can follow both degradation of semantic knowledge (e.g., semantic 'storage' deficits in semantic dementia) and deregulation of retrieval (e.g., semantic 'access' deficits in semantic aphasia). While emotion recognition deficits are known to accompany degraded conceptual knowledge, less is known about the impact of semantic access deficits. Here, we examined emotion perception and categorization tasks in patients with semantic aphasia, who have difficulty accessing semantic information in a flexible and controlled fashion following left hemisphere stroke. In Study 1, participants were asked to sort faces according to the emotion they portrayed - with numbers, written labels and picture examples each provided as category anchors across tasks. Semantic aphasia patients made more errors and showed a larger benefit from word anchors that reduced the need to internally constrain categorization than comparison participants. They successfully sorted portrayals that differed in valence (positive vs. negative) but had difficulty categorizing different negative emotions. They were unimpaired on a control task that involved sorting faces by identity. In Study 2, participants matched facial emotion portrayals to written labels following vocal emotion prosody cues, miscues, or no cues. Patients presented with overall poorer performance and benefited from cue trials relative to within-valence miscue trials. This same effect was seen in comparison participants, who also showed deleterious effects of within-valence miscue relative to no cue trials. Overall, we found that patients with deregulated semantic retrieval have deficits in emotional perception but that word anchors and cue conditions can facilitate emotion perception by increasing access to relevant emotion concepts and reducing reliance on semantic control. Semantic control may be of particular importance in emotion perception when it is necessary to interpret ambiguous inputs, or when there is interference between conceptually similar emotional states. These findings extend constructionist accounts of emotion to encompass difficulties in controlled semantic retrieval.

Establishing a role of the semantic control network in social cognitive processing: A meta-analysis of functional neuroimaging studies

The contribution and neural basis of cognitive control is under-specified in many prominent models of socio-cognitive processing. Important outstanding questions include whether there are multiple, distinguishable systems underpinning control and whether control is ubiquitously or selectively engaged across different social behaviours and task demands. Recently, it has been proposed that the regulation of social behaviours could rely on brain regions specialised in the controlled retrieval of semantic information, namely the anterior inferior frontal gyrus (IFG) and posterior middle temporal gyrus. Accordingly, we investigated for the first time whether the neural activation commonly found in social functional neuroimaging studies extends to these 'semantic control' regions. We conducted five coordinate-based meta-analyses to combine results of 499 fMRI/PET experiments and identified the brain regions consistently involved in semantic control, as well as four social abilities: theory of mind, trait inference, empathy and moral reasoning. This allowed an unprecedented parallel review of the neural networks associated with each of these cognitive domains. The results confirmed that the anterior left IFG region involved in semantic control is reliably engaged in all four social domains. This supports the hypothesis that social cognition is partly regulated by the neurocognitive system underpinning semantic control.

Multiple dimensions underlying the functional organization of the language network

Understanding the different neural networks that support human language is an ongoing challenge for cognitive neuroscience. Which divisions are capable of distinguishing the functional significance of regions across the language network? A key separation between semantic cognition and phonological processing was highlighted in early meta-analyses, yet these seminal works did not formally test this proposition. Moreover, organization by domain is not the only possibility. Regions may be organized by the type of process performed, as in the separation between representation and control processes proposed within the Controlled Semantic Cognition framework. The importance of these factors was assessed in a series of activation likelihood estimation meta-analyses that investigated which regions of the language network are consistently recruited for semantic and phonological domains, and for representation and control processes. Whilst semantic and phonological processing consistently recruit many overlapping regions, they can be dissociated (by differential involvement of bilateral anterior temporal lobes, precentral gyrus and superior temporal gyri) only when using both formal analysis methods and sufficient data. Both semantic and phonological regions are further dissociable into control and representation regions, highlighting this as an additional, distinct dimension on which the language network is functionally organized. Furthermore, some of these control regions overlap with multiple-demand network regions critical for control beyond the language domain, suggesting the relative level of domain-specificity is also informative. Multiple, distinct dimensions are critical to understand the role of language regions. Here we present a proposal as to the core principles underpinning the functional organization of the language network.

Influence of homophone processing during auditory language comprehension on executive control processes: A dual-task paradigm

In the present preregistered study, we evaluated the possibility of a shared cognitive mechanism during verbal and non-verbal tasks and therefore the implication of domain-general cognitive control during language comprehension. We hypothesized that a behavioral cost will be observed during a dual-task including both verbal and non-verbal difficult processing. Specifically, to test this claim, we designed a dual-task paradigm involving: an auditory language comprehension task (sentence comprehension) and a non-verbal Flanker task (including congruent and incongruent trials). We manipulated sentence ambiguity and evaluated if the ambiguity effect modified behavioral performances in the non-verbal Flanker task. Under the assumption that ambiguous sentences induce a more difficult process than unambiguous sentences, we expected non-verbal flanker task performances to be impaired only when a simultaneous difficult language processing is performed. This would be specifically reflected by a performance cost during incongruent Flanker items only during ambiguous sentence presentation. Conversely, we observed a facilitatory effect for the incongruent Flanker items during ambiguous sentence suggesting better non-verbal inhibitory performances when an ambiguous sentence was simultaneously processed. Exploratory data analysis suggests that this effect is not only related to a more difficult language processing but also to the previous (n-1) Flanker item. Indeed, results showed that incongruent n-1 Flanker items led to a facilitation of the incongruent synchronized Flanker items only when ambiguous sentences were conjointly presented. This result, even if it needs to be corroborated in future studies, suggests that the recruitment of executive control mechanisms facilitates subsequent executive control implication during difficult language processing. The present study suggests a common executive control mechanism during difficult verbal and non-verbal tasks.

Incidental Verbal Semantic Processing Recruits the Fronto-temporal Semantic Control Network

The frontoparietal semantic network, encompassing the inferior frontal gyrus and the posterior middle temporal cortex, is considered to be involved in semantic control processes. The explicit versus implicit nature of these control processes remains however poorly understood. The present study examined this question by assessing regional brain responses to the semantic attributes of an unattended stream of auditory words while participants' top-down attentional control processes were absorbed by a demanding visual search task. Response selectivity to semantic aspects of verbal stimuli was assessed via a functional magnetic resonance imaging response adaptation paradigm. We observed that implicit semantic processing of an unattended verbal stream recruited not only unimodal and amodal cortices in posterior supporting semantic knowledge areas, but also inferior frontal and posterior middle temporal areas considered to be part of the semantic control network. These results indicate that frontotemporal semantic networks support incidental semantic (control) processes.

A case-study of language-specific executive disorder

Executive control is recruited for language processing, particularly in complex linguistic tasks. Although the issue of the existence of an executive control specific to language is still an open issue, there is much evidence that executively-demanding language tasks rely on domain-general rather than language-specific executive resources. Here, we addressed this issue by assessing verbal and non-verbal executive capacities in LG, an aphasic patient after a stroke. First, we showed that LG's performance was spared in all non-verbal tasks regardless of the executive demands. Second, by contrasting conditions of high and low executive demand in verbal tasks, we showed that LG was only impaired in verbal task with high executive demand. The performance dissociation between low and high executive demand conditions in the verbal domain, not observed in the non-verbal domain, shows that verbal executive control partly dissociates from non-verbal executive control. This language-specific executive disorder suggests that some executive processes might be language-specific.

Different types of abstract concepts: evidence from two neurodegenerative patients

The observation of neurological patients showing selective impairments for specific conceptual categories contributed in the development of semantic memory theories. Here, we studied two patients (P01, P02), affected, respectively, by the semantic variant of Primary Progressive Aphasia (sv-PPA) and Cortico-Basal Syndrome (CBS). An implicit lexical decision task, including concrete (animals, tools) and abstract (emotions, social, quantity) concepts, was administered to patients and healthy controls.P01 and P02 showed an abolished priming effect for social and quantity-related concepts, respectively. This double dissociation suggests a role of different brain areas in representing specific abstract categories, giving insights for current semantic memory theories.

Morpho-semantic analysis of ambiguous morphemes in Chinese compound word recognition: An fMRI study

The present fMRI study examined the neural basis of processing context-supported or -unsupported interpretations of ambiguous morphemes during Chinese compound word reading in a masked priming lexical decision task. Targets were Chinese bimorphemic words that contained ambiguous morphemes. Prime words contained the same ambiguous morphemes with either the same meanings (context-supported interpretation) or different ones (context-unsupported interpretation). Lexical-level semantic sharing and unrelated control conditions were also included. Compared to the unrelated control condition, the context-supported morphemic meaning was associated with increased activity in the left SFG and bilateral MTG, and this priming effect could be dissociated from that of the lexical-level semantic-related condition. In broader brain regions, including the left SFG, bilateral MTG, left STG, right IOG, and left precuneus, the context-unsupported meaning condition showed decreased activity compared with the unrelated control condition. These findings indicate that both the context-supported and -unsupported meanings evoke significant priming effects, however, they differ from each other with different brain basis, providing new insight into the neural substrates of ambiguous morpheme processing.

Evaluating the distinction between semantic knowledge and semantic access: Evidence from semantic dementia and comprehension-impaired stroke aphasia

Theories of semantic memory based on neuropsychological findings have posited a distinction between stored semantic representations and the mechanisms used to access and manipulate them (e.g., Lambon Ralph, Jefferies, Patterson, & Rogers, 2017; Warrington & Cipolotti, 1996). The most recent instantiation of this view, the controlled semantic cognition theory (Lambon Ralph et al., 2017), is supported by findings suggesting that multimodal (i.e., both verbal and nonverbal) semantic deficits may result from qualitatively different impairments: on the one hand, damage to a semantic access mechanism related to executive control, which is observed in semantic aphasia (SA), and on the other, damage to semantic representations, which is observed in semantic dementia (SD) (Jefferies & Lambon Ralph, 2006). In this study we compared SA and SD patients on several phenomena previously used to support these distinctions. Contrary to the prior results, we found that (1) overall, cross-task consistency was equivalent for the two groups; (2) neither patient group showed consistency driven by item identity across different semantic tasks; (3) correlations among task performance were not obviously driven by the semantic control demands of different tasks; (4) both groups showed executive function deficits; and (5) both groups showed strong effects of distractor interference in a synonym judgment task. Furthermore, we investigated the components of executive ability that could underlie semantic control deficits by correlating performance on updating, shifting, and inhibition tasks with performance on tasks testing semantic abilities. We found that updating was related to semantic processing generally, whereas shifting and inhibition were not. These results also suggest that complex executive function tasks relate to semantic tasks through their shared relationship with language abilities. Overall, evidence from SA and SD patients does not differentiate representations and access mechanisms in the semantic system, as has previously been suggested. Implications for the storage-access distinction are discussed.

Knowing what you need to know in advance: The neural processes underpinning flexible semantic retrieval of thematic and taxonomic relations

Semantic retrieval is flexible, allowing us to focus on subsets of features and associations that are relevant to the current task or context: for example, we use taxonomic relations to locate items in the supermarket (carrots are a vegetable), but thematic associations to decide which tools we need when cooking (carrot goes with peeler). We used fMRI to investigate the neural basis of this form of semantic flexibility; in particular, we asked how retrieval unfolds differently when participants have advanced knowledge of the type of link to retrieve between concepts (taxonomic or thematic). Participants performed a semantic relatedness judgement task: on half the trials, they were cued to search for a taxonomic or thematic link, while on the remaining trials, they judged relatedness without knowing which type of semantic relationship would be relevant. Left inferior frontal gyrus showed greater activation when participants knew the trial type in advance. An overlapping region showed a stronger response when the semantic relationship between the items was weaker, suggesting this structure supports both top-down and bottom-up forms of semantic control. Multivariate pattern analysis further revealed that the neural response in left inferior frontal gyrus reflects goal information related to different conceptual relationships. Top-down control specifically modulated the response in visual cortex: when the goal was unknown, there was greater deactivation to the first word, and greater activation to the second word. We conclude that top-down control of semantic retrieval is primarily achieved through the gating of task-relevant 'spoke' regions.

Neurocognitive mechanisms supporting the generalization of concepts across languages

The neurocognitive mechanisms that support the generalization of semantic representations across different languages remain to be determined. Current psycholinguistic models propose that semantic representations are likely to overlap across languages, although there is evidence also to the contrary. Neuroimaging studies observed that brain activity patterns associated with the meaning of words may be similar across languages. However, the factors that mediate cross-language generalization of semantic representations are not known. We here identify a key factor: the depth of processing. Human participants were asked to process visual words as they underwent functional MRI. We found that, during shallow processing, multivariate pattern classifiers could decode the word semantic category within each language in putative substrates of the semantic network, but there was no evidence of cross-language generalization in the shallow processing context. By contrast, when the depth of processing was higher, significant cross-language generalization was observed in several regions, including inferior parietal, ventromedial, lateral temporal, and inferior frontal cortex. These results are in keeping with distributed-only views of semantic processing and favour models based on multiple semantic hubs. The results also have ramifications for existing psycholinguistic models of word processing such as the BIA+, which by default assumes non-selective access to both native and second languages.

A meta-analysis of functional magnetic resonance imaging studies of divergent thinking using activation likelihood estimation

There are conflicting findings regarding brain regions and networks underpinning creativity, with divergent thinking tasks commonly used to study this. A handful of meta-analyses have attempted to synthesise findings on neural mechanisms of divergent thinking. With the rapid proliferation of research and recent developments in fMRI meta-analysis approaches, it is timely to reassess the regions activated during divergent thinking creativity tasks. Of particular interest is examining the evidence regarding large-scale brain networks proposed to be key in divergent thinking and extending this work to consider the role of the semantic control network. Studies utilising fMRI with healthy participants completing divergent thinking tasks were systematically identified, with 20 studies meeting the criteria. Activation Likelihood Estimation was then used to integrate the neuroimaging results across studies. This revealed four clusters: the left inferior parietal lobe; the left inferior frontal and precentral gyrus; the superior and medial frontal gyrus and the right cerebellum. These regions are key in the semantic network, important for flexible retrieval of stored knowledge, highlighting the role of this network in divergent thinking.

Multimodal comprehension in left hemisphere stroke patients

Hand gestures, imagistically related to the content of speech, are ubiquitous in face-to-face communication. Here we investigated people with aphasia's (PWA) processing of speech accompanied by gestures using lesion-symptom mapping. Twenty-nine PWA and 15 matched controls were shown a picture of an object/action and then a video-clip of a speaker producing speech and/or gestures in one of the following combinations: speech-only, gesture-only, congruent speech-gesture, and incongruent speech-gesture. Participants' task was to indicate, in different blocks, whether the picture and the word matched (speech task), or whether the picture and the gesture matched (gesture task). Multivariate lesion analysis with Support Vector Regression Lesion-Symptom Mapping (SVR-LSM) showed that benefit for congruent speech-gesture was associated with 1) lesioned voxels in anterior fronto-temporal regions including inferior frontal gyrus (IFG), and sparing of posterior temporal cortex and lateral temporal-occipital regions (pTC/LTO) for the speech task, and 2) conversely, lesions to pTC/LTO and sparing of anterior regions for the gesture task. The two tasks did not share overlapping voxels. Costs from incongruent speech-gesture pairings were associated with lesioned voxels in these same anterior (for the speech task) and posterior (for the gesture task) regions, but crucially, also shared voxels in superior temporal gyrus (STG) and middle temporal gyrus (MTG), including the anterior temporal lobe. These results suggest that IFG and pTC/LTO contribute to extracting semantic information from speech and gesture, respectively; however, they are not causally involved in integrating information from the two modalities. In contrast, regions in anterior STG/MTG are associated with performance in both tasks and may thus be critical to speech-gesture integration. These conclusions are further supported by associations between performance in the experimental tasks and performance in tests assessing lexical-semantic processing and gesture recognition.

Barking up the right tree: Univariate and multivariate fMRI analyses of homonym comprehension

Homonyms are a critical test case for investigating how the brain resolves ambiguity in language and, more generally, how context influences semantic processing. Previous neuroimaging studies have associated processing of homonyms with greater engagement of regions involved in executive control of semantic processing. However, the precise role of these areas and the involvement of semantic representational regions in homonym comprehension remain elusive. We addressed this by combining univariate and multivariate fMRI analyses of homonym processing. We tested whether multi-voxel activation patterns could discriminate between presentations of the same homonym in different contexts (e.g., bark following tree vs. bark following dog). The ventral anterior temporal lobe, implicated in semantic representation but not previously in homonym comprehension, showed this meaning-specific coding, despite not showing increased mean activation for homonyms. Within inferior frontal gyrus (IFG), a key site for semantic control, there was a dissociation between pars orbitalis, which also showed meaning-specific coding, and pars triangularis, which discriminated more generally between semantically related and unrelated word pairs. IFG effects were goal-dependent, only occurring when the task required semantic decisions, in line with a top-down control function. Finally, posterior middle temporal cortex showed a hybrid pattern of responses, supporting the idea that it acts as an interface between semantic representations and the control system. The study provides new evidence for context-dependent coding in the semantic system and clarifies the role of control regions in processing ambiguity. It also highlights the importance of combining univariate and multivariate neuroimaging data to fully elucidate the role of a brain region in semantic cognition.

Contrasting Semantic versus Inhibitory Processing in the Angular Gyrus: An fMRI Study

Recent studies of semantic memory have focused on dissociating the neural bases of two foundational components of human thought: taxonomic categories, which group similar objects like dogs and seals based on features, and thematic categories, which group dissimilar objects like dogs and leashes based on events. While there is emerging consensus that taxonomic concepts are represented in the anterior temporal lobe, there is disagreement over whether thematic concepts are represented in the angular gyrus (AG). We previously found AG sensitivity to both kinds of concepts; however, some accounts suggest that such activity reflects inhibition of irrelevant information rather than thematic activation. To test these possibilities, an fMRI experiment investigated both types of conceptual relations in the AG during two semantic judgment tasks. Each task trained participants to give negative responses (inhibition) or positive responses (activation) to word pairs based on taxonomic and thematic criteria of relatedness. Results showed AG engagement during both negative judgments and thematic judgments, but not during positive judgments about taxonomic pairs. Together, the results suggest that activity in the AG reflects functions that include both thematic (but not taxonomic) processing and inhibiting irrelevant semantic information.

Going off the rails: Impaired coherence in the speech of patients with semantic control deficits

The ability to speak coherently, maintaining focus on the topic at hand, is critical for effective communication and is commonly impaired following brain damage. Recent data suggests that executive processes that regulate access to semantic knowledge (i.e., semantic control) are critical for maintaining coherence during speech. To test this hypothesis, we assessed speech coherence in a case-series of stroke patients who exhibited deficits in semantic control. Patients were asked to speak about a series of topics and their responses were analysed using computational linguistic methods to derive measures of their global coherence (the degree to which they spoke about the topic given) and local coherence (the degree to which they maintained a topic from one utterance to the next). Compared with age-matched controls, patients showed severe impairments to global coherence but not to local coherence. Global coherence was strongly correlated with the patients' performance on tests of semantic control, with greater semantic control deficits associated with poorer ability to maintain global coherence. Other aspects of speech production were also impaired but were not significantly correlated with semantic control deficits. These results suggest that semantic control deficits give rise to speech that is poorly regulated at the macrolinguistic "message" level. The preservation of local coherence in the patients suggests that automatic activation of semantic associations is relatively intact, such that each utterance they produce is connected meaningfully to the next. However, in the absence of control processes to constrain semantic activation, the content of their speech becomes increasingly distant from the original topic of discourse. This study is the first to investigate the impact of semantic control impairments on speech production at the discourse level and suggests that patients with these impairments are likely to have difficulties maintaining coherence in conversation.

The role of default mode network in semantic cue integration

Recent accounts of large-scale cortical organisation suggest that the default mode network (DMN) is positioned at the top of a principal gradient, reflecting the separation between heteromodal and unimodal sensory-motor regions in patterns of connectivity and in geodesic distance along the cortical surface (Margulies et al., 2016). This isolation of DMN from external inputs might allow the integration of disparate sources of information that can constrain subsequent cognition. We tested this hypothesis by manipulating the degree to which semantic decisions for ambiguous words (e.g. jam) were constrained by preceding visual cues depicting relevant spatial contexts (e.g. supermarket or road) and/or facial emotions (e.g. happy vs. frustrated). We contrasted (i) the effects of a single preceding cue with a no-cue condition employing scrambled images, and (ii) convergent spatial and emotion cues with single cues. Single cues elicited stronger activation in the multiple demand network relative to no cues, consistent with the requirement to maintain information in working memory. The availability of two convergent cues elicited stronger activation within DMN regions (bilateral angular gyrus, middle temporal gyrus, medial prefrontal cortex, and posterior cingulate), even though behavioural performance was unchanged by cueing – consequently task difficulty is unlikely to account for the observed differences in brain activation. A regions-of-interest analysis along the unimodal-to-heteromodal principal gradient revealed maximal activation for the convergent cue condition at the heteromodal end, corresponding to the DMN. Our findings are consistent with the view that regions of DMN support states of information integration that constrain ongoing cognition and provide a framework for understanding the location of these effects at the heteromodal end of the principal gradient.

Consistently inconsistent: Multimodal episodic deficits in semantic aphasia

Semantic Aphasia (SA) patients have difficulty accessing semantic knowledge in both verbal and non-verbal tasks appropriately for the current context. Automatically activated semantic knowledge overwhelms the system, because it is no longer able to inhibit interference from dominant meanings in order to select weaker alternatives. Episodic memory, like semantic memory, requires control to select relevant memories amongst competing episodes. For example, our memory for what we ate for breakfast last Saturday is affected by competition from numerous other breakfast meals eaten on other days. Where one is unable to guide retrieval, we may rely on automatically activated knowledge about "breakfast foods", and therefore experience false memories. Brain systems that support semantic control are also implicated in episodic control, and therefore deficits in semantic control are likely to cause more widespread problems. Despite this, nearly all research to date focuses on semantic performance alone. This study explored the impact of this semantic impairment on episodic recall. We used a verbal and non-verbal episodic memory task: participants remembered nursery rhymes in the verbal condition and logos and their associated products in the visual condition (e.g. bowl of cereal and coco-pops). For both tasks, we manipulated a) congruency with pre-existing knowledge (e.g. expectancy of trials: baa baa blackbuild - instead of sheep) and b) whether these trial types were blocked by congruency or mixed, as well as (c) distractor strength. If SA patients experience overwhelming automatic activation, they should find incongruent items more difficult to suppress, particularly when presented in an unpredictable task format. A total of 13 SA patients were compared to 33 controls across three experiments. In line with our predictions, SA patients found it more difficult to retrieve episodic memories which were in conflict with pre-existing semantic knowledge. This was true across modalities. Moreover, these deficits were accentuated when the congruency was presented in a mixed fashion, and so unpredictable across trials. Evidence of these episodic control impairments in SA cases supports the idea of a shared mechanism for semantic and episodic memory control.

The neurocognitive basis of knowledge about object identity and events: dissociations reflect opposing effects of semantic coherence and control

Semantic memory encompasses knowledge of specific objects and their diverse associations, but the mechanisms that allow us to retrieve aspects of knowledge required for a given task are poorly understood. The dual hub theory suggests that separate semantic stores represent knowledge of (i) taxonomic categories (in the anterior temporal lobes, ATL) and (ii) thematic associations (in angular gyrus, AG or posterior middle temporal gyrus, pMTG). Alternatively, the controlled semantic cognition (CSC) framework suggests that semantic processing emerges from the flexible interaction of heteromodal semantic representations in ATL with a semantic control network, which includes pMTG as well as prefrontal regions. According to this view, ATL supports patterns of coherent auto-associative retrieval, while semantic control sites respond when ongoing conceptual activation needs to be altered to suit the task or context. These theories make different predictions about the nature of functional dissociations within the semantic network. We review evidence for these claims across multiple methods. First, we show ATL is sensitive to the strength of thematic associations as well as taxonomic relations. Next, we document functional dissociations between AG and pMTG: rather than these regions acting as comparable thematic hubs, AG is allied to the default mode network and supports more 'automatic' retrieval, while pMTG responds when control demands are high. However, the semantic control network, including pMTG, also shows a greater response to events/actions and verbs, supporting the claims of both theories. We propose that tasks tapping event semantics often require greater shaping of conceptual retrieval than comparison tasks, because these elements of our knowledge are inherently flexible, with relevant features depending on the context. In this way, the CSC account might be able to account for findings that suggest both a process and a content distinction within the semantic network. This article is part of the theme issue 'Towards mechanistic models of meaning composition'.

Inferior frontal gyrus involvement during search and solution in verbal creative problem solving: A parametric fMRI study

In verbal creative problems like compound remote associates (CRAs), the solution is semantically distant and there is no predefined path to the solution. Therefore, people first search through the space of possible solutions before retrieving the correct semantic content by extending their search space. We assume that search and solution are both part of a semantic control process which involves the inferior frontal gyrus (IFG). Furthermore, we expect the degree of relevant semantic control areas like the IFG to depend on how much the search space needs to be extended, i.e. how semantically distant the solution is.

To demonstrate this, we created a modified CRA paradigm which systematically modulates the semantic distance from the first target word to the solution via priming. We show that brain areas (left IFG and middle temporal gyrus) associated with semantic control are already recruited during search. In addition, BOLD response in the left angular gyrus linearly correlates with search space extension. Hence, there is evidence that this process already takes place during search. Furthermore, bilateral IFG (pars orbitalis and triangularis) also correlates with search space extension but during solution. We discuss the role of the IFG in accessing semantically distant information during verbal creative problem solving.

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We present a novel way how to quantify verbal restructuring in a parametric fMRI paradigm.

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Evidence is provided that search for a solution and verbal restructuring are both part of a cognitive control process.

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Inferior frontal gyrus (IFG) and the angular gyrus are identified as part of this cognitive control process.

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BOLD activation of bilateral IFG parametrically correlates with verbal restructuring during and before solution.

Control the source: Source memory for semantic, spatial and self-related items in patients with LIFG lesions

Patients with multimodal semantic deficits following stroke ('semantic aphasia') have largely intact knowledge, yet difficulty controlling conceptual retrieval to suit the circumstances. Although conceptual representations are thought to be largely distinct from episodic representations of recent events, controlled retrieval processes may overlap across semantic and episodic memory domains. We investigated this possibility by examining item familiarity and source memory for recent events in semantic aphasia following infarcts affecting left inferior frontal gyrus. We tested the hypothesis that the nature of impairment in episodic judgements reflects the need for control over retrieval: item familiarity might be relatively intact, given it is driven by strong cues (re-presentation of the item), while source recollection might be more impaired since this task involves resolving competition between several potential sources. This pattern was observed most strongly when the degree of competition between sources was higher, i.e., when non-meaningful sources had similar perceptual features, and existing knowledge was incongruent with the source. In contrast, when (i) spatial location acted as a strong cue for retrieval; (ii) existing knowledge was congruent with episodic memory and (iii) distinctiveness of sources was increased by means of self-referential processing, source memory reached normal levels. These findings confirm the association between deregulated control of semantic and episodic memory in patients with semantic aphasia and delineate circumstances that ameliorate or aggravate these deficits.

The white matter architecture underlying semantic processing: A systematic review

From a holistic point of view, semantic processes are subserved by large-scale subcortico-cortical networks. The dynamic routing of information between grey matter structures depends on the integrity of subcortical white matter pathways. Nonetheless, controversy remains on which of these pathways support semantic processing. Therefore, a systematic review of the literature was performed with a focus on anatomo-functional correlations obtained from direct electrostimulation during awake tumor surgery, and conducted between diffusion tensor imaging metrics and behavioral semantic performance in healthy and aphasic individuals. The 43 included studies suggest that the left inferior fronto-occipital fasciculus contributes to the essential connectivity that allows semantic processing. However, it remains uncertain whether its contributive role is limited to the organization of semantic knowledge or extends to the level of semantic control. Moreover, the functionality of the left uncinate fasciculus, inferior longitudinal fasciculus and the posterior segment of the indirect arcuate fasciculus in semantic processing has to be confirmed by future research.

The neural basis of semantic cognition in Mandarin Chinese: A combined fMRI and TMS study

While converging sources of evidence point to the possibility of a large‐scale distributed network for semantic cognition, a consensus regarding the underlying subregions and their specific function in this network has not been reached. In the current study, we combined functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) methodology to investigate the neural basis of semantic cognition in Mandarin Chinese. In the fMRI experiment, strong activations were observed in left inferior frontal gyrus (IFG) and left middle temporal gyrus (MTG) for semantic judgment task. Moreover, functional connectivity was found from seed region left IFG to left MTG. Meanwhile, negative correlation between performance and extracted parameter estimates from left IFG to left MTG was detected in semantic task. Subsequent TMS stimulation over left IFG resulted in performance deficits in semantic judgment task, in contrast to other three sites: left MTG, right intraparietal sulcus (IPS) and a control site. We concluded that the neural basis of semantic processing for Mandarin Chinese closely resembled that for alphabetic languages such as English, supporting a language‐universal view on semantic cognition.

The interplay between control processes and feature relevance: Evidence from dual-task methodology

Neuropsychological studies suggest a distinction between (a) semantic knowledge and (b) control processes that shape the retrieval of conceptual information to suit the task or context. These aspects of semantic cognition are specifically impaired in patients with semantic dementia and semantic aphasia, respectively. However, interactions between the structure of knowledge and control processes that are expected during semantic retrieval have not been fully characterised. In particular, domain-general executive resources may not have equal relevance for the capacity to promote weak yet task relevant features (i.e., "controlled retrieval) and to ignore or suppress distracting information (i.e., "selection"). Here, using a feature selection task, we tested the contribution of featural relevance to semantic performance in healthy participants under conditions of divided attention. Healthy participants showed greater dual-task disruption as the relevance value of the distractor feature linearly increased, supporting the emerging view that semantic relevance is one of the organising principles of the structure of semantic representation. Moreover, word frequency, and inter-correlational strength affected overall performance, but they did not show an interaction with dual-task conditions. These results suggest that domain-general control processes, disrupted by divided attention, are more important to the capacity to efficiently avoid distracting information during semantic decision-making than to the promotion of weak target features. The present study therefore provides novel information about the nature of the interaction between structured conceptual knowledge and control processes that support the retrieval of appropriate information and relates these results to a new theoretical framework, termed controlled semantic cognition.

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

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

Emotion and location cues bias conceptual retrieval in people with deficient semantic control

Visuo-spatial context and emotional valence are powerful cues to episodic retrieval, but the contribution of these inputs to semantic cognition has not been widely investigated. We examined the impact of visuo-spatial, facial emotion and prosody cues and miscues on the retrieval of dominant and subordinate meanings of ambiguous words. Cue photographs provided relevant visuo-spatial or emotional information, consistent with the interpretation of the ambiguous word being probed, while miscues were consistent with an alternative interpretation. We compared the impact of these cues in healthy controls and semantic aphasia patients with deficient control over semantic retrieval following left-hemisphere stroke. Patients showed greater deficits in retrieving the subordinate meanings of ambiguous words, and stronger effects of cueing and miscuing relative to healthy controls. These findings suggest that contextual cues that guide retrieval to the appropriate semantic information reduce the need to constrain semantic retrieval internally, while miscues that are not aligned with the task increase the need for semantic control. Moreover, both valence and visuo-spatial context can prime particular semantic interpretations, in line with theoretical frameworks that argue meaning is computed through the integration of these features. In semantic aphasia, residual comprehension relies heavily on facial expressions and visuospatial cues. This has important implications for patients, their families and clinicians when developing new or more effective modes of communication.

Semantic Processing in Bilingual Aphasia: Evidence of Language Dependency

Individuals with aphasia frequently show lexical retrieval deficits due to increased interference of semantically related competitors, a phenomenon that can be observed in tasks such as naming pictures grouped by semantic category. These deficits are explained in terms of impaired semantic control, a set of abilities that are to some extent dependent upon executive control (EC). However, the extent to which semantic control abilities can be affected in a second and non-dominant language has not been extensively explored. Additionally, findings in healthy individuals are inconclusive regarding the degree to which semantic processing is shared between languages. In this study, we explored the effect of brain damage on semantic processing by comparing the performance of bilingual individuals with aphasia on tasks involving semantic control during word production and comprehension. Furthermore, we explored whether semantic deficits are related to domain-general EC deficits. First, we investigated the naming performance of Catalan-Spanish bilinguals with fluent aphasia and age-matched healthy controls on a semantically blocked cyclic naming task in each of their two languages (Catalan and Spanish). This task measured semantic interference in terms of the difference in naming latencies between pictures grouped by the same semantic category or different categories. Second, we explored whether lexical deficits extend to comprehension by testing participants in a word-picture matching task during a mixed language condition. Third, we used a conflict monitoring task to explore the presence of EC deficits in patients with aphasia. We found two main results. First, in both language tasks, bilingual patients' performances were more affected than those of healthy controls when they performed the task in their non-dominant language. Second, there was a significant correlation between the speed of processing on the EC task and the magnitude of the semantic interference effect exclusively in the non-dominant language. Taken together, these results suggest that lexical retrieval may be selectively impaired in bilinguals within those conditions where semantic competition is higher, i.e.,- in their non-dominant language; this could possibly be explained by an excessive amount of inhibition placed upon this language. Moreover, lexico-semantic impairments seem to be at least somewhat related to conflict monitoring deficits, suggesting a certain degree of overlap between EC and semantic control.