Shared processes resolve competition within and between episodic and semantic memory: Evidence from patients with LIFG lesions

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.

A reconceptualization of sentence production in post-stroke agrammatic aphasia: the synergistic processing bottleneck model

The language production deficit in post-stroke agrammatic aphasia (PSA-G) tends to result from lesions to the left inferior frontal gyrus (LIFG) and is characterized by a triad of symptoms: fragmented sentences, errors in functional morphology, and a dearth of verbs. Despite decades of research, the mechanisms underlying production patterns in PSA-G have been difficult to characterize. Two major impediments to progress may have been the view that it is a purely morphosyntactic disorder and the (sometimes overzealous) application of linguistic theory without interceding psycholinguistic evidence. In this paper, empirical evidence is examined to present an integrated portrait of language production in PSA-G and to evaluate the assumption of a syntax-specific syndrome. In light of extant evidence, it is proposed that agrammatic language production results from a combination of morphosyntactic, phonomotor, and processing capacity limitations that cause a cumulative processing bottleneck at the point of articulatory planning. This proposed Synergistic Processing Bottleneck model of PSA-G presents a testable framework for future research. The paper ends with recommendations for future research on PSA-G.

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.

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.

Creativity in verbal associations is linked to semantic control

Although memory is known to play a key role in creativity, previous studies have not isolated the critical component processes and networks. We asked participants to generate links between words that ranged from strongly related to completely unrelated in long-term memory, delineating the neurocognitive processes that underpin more unusual versus stereotypical patterns of retrieval. More creative responses to strongly associated word-pairs were associated with greater engagement of episodic memory: in highly familiar situations, semantic, and episodic stores converge on the same information enabling participants to form a personal link between items. This pattern of retrieval was associated with greater engagement of core default mode network (DMN). In contrast, more creative responses to weakly associated word-pairs were associated with the controlled retrieval of less dominant semantic information and greater recruitment of the semantic control network, which overlaps with the dorsomedial subsystem of DMN. Although both controlled semantic and episodic patterns of retrieval are associated with activation within DMN, these processes show little overlap in activation. These findings demonstrate that controlled aspects of semantic cognition play an important role in verbal creativity.

A middle ground where executive control meets semantics: the neural substrates of semantic control are topographically sandwiched between the multiple-demand and default-mode systems

Semantic control is the capability to operate on meaningful representations, selectively focusing on certain aspects of meaning while purposefully ignoring other aspects based on one's behavioral aim. This ability is especially vital for comprehending figurative/ambiguous language. It remains unclear why and how regions involved in semantic control seem reliably juxtaposed alongside other functionally specialized regions in the association cortex, prompting speculation about the relationship between topography and function. We investigated this issue by characterizing how semantic control regions topographically relate to the default-mode network (associated with memory and abstract cognition) and multiple-demand network (associated with executive control). Topographically, we established that semantic control areas were sandwiched by the default-mode and multi-demand networks, forming an orderly arrangement observed both at the individual and group level. Functionally, semantic control regions exhibited "hybrid" responses, fusing generic preferences for cognitively demanding operation (multiple-demand) and for meaningful representations (default-mode) into a domain-specific preference for difficult operation on meaningful representations. When projected onto the principal gradient of human connectome, the neural activity of semantic control showed a robustly dissociable trajectory from visuospatial control, implying different roles in the functional transition from sensation to cognition. We discuss why the hybrid functional profile of semantic control regions might result from their intermediate topographical positions on the cortex.

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.

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.

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.

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.

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.

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.

Mapping eloquent cortex: A voxel-based lesion-symptom mapping study of core speech production capacities in brain tumour patients

This study used voxel-based lesion-symptom mapping to examine the cortical and white matter regions associated with language production impairments in a sample of 63 preoperative tumour patients. We identified four cognitive functions considered crucial for spoken language production: semantic-to-lexical mapping (selecting the appropriate lexical label for the intended concept); phonological encoding (retrieving the word's phonological form); articulatory-motor planning (programming the articulatory motor movements); and goal-driven language selection (exerting top-down control over the words selected for production). Each participant received a score estimating their competence on each function. We then mapped the region(s) where pathology was significantly associated with low scores. For semantic-to-lexical mapping, the critical map encompassed portions of the left posterior middle and inferior temporal gyri, extending into posterior fusiform gyrus, overlapping substantially with the territory of the inferior longitudinal fasciculus. For phonological encoding, the map encompassed the left inferior parietal lobe and posterior middle temporal gyrus, overlapping with the territory of the inferior longitudinal and posterior arcuate fasciculi. For articulatory-motor planning, the map encompassed parts of the left frontal pole, frontal operculum, and inferior frontal gyrus, and overlapped with the territory of the frontal aslant tract. Finally, the map for goal-driven language selection encompassed the left frontal pole and the anterior cingulate cortex. We compare our findings with those from other neuropsychological samples, and conclude that the study of tumour patients offers evidence that complements that available from other populations.

Reduced competition between tool action neighbors in left hemisphere stroke

When pantomiming the use of tools, patients with limb apraxia after left hemisphere stroke (LCVA) produce more spatiotemporal hand action errors with tools associated with conflicting actions for use versus grasp-to-pick-up (e.g., corkscrew) than tools having a single action for both use and grasp (e.g., hammer). There are two possible accounts for this pattern of results. Reduced performance with 'conflict' tools may simply reflect weakened automaticity of use action activation, which is evident only when the use and grasp actions are not redundant. Alternatively, poor use performance may reflect a reduced ability of appropriate tool use actions to compete with task-inappropriate action representations. To address this issue, we developed a Stroop-like experiment in which 21 LCVA and 8 neurotypical participants performed pantomime actions in blocks containing two tools that were similar ("neighbors") in terms of hand action or function, or were unrelated on either dimension. In a congruent condition, they pantomimed the use action associated with the visually presented tool, whereas in an incongruent condition, they pantomimed the use action for the other tool in the block. Relative to controls and other task conditions, LCVA participants showed reductions in hand action errors in incongruent relative to congruent action trials; furthermore, the degree of reduction in this incongruence effect was related to the participants' susceptibility to grasp-on-use conflict in a separate test of pantomime to the sight of tools. Support vector regression lesion-symptom mapping analyses identified the left inferior frontal gyrus, supramarginal gyrus, and superior longitudinal fasciculus as core neuroanatomical sites associated with abnormal performance. Collectively, the results indicate that weakened activation of tool use actions in limb apraxia gives rise to reduced ability of these actions to compete for task-appropriate selection when competition arises within single tools (grasp-on-use conflict) as well as between two tools (reduced neighborhood effects).

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.

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.

Patterns of on-task thought in older age are associated with changes in functional connectivity between temporal and prefrontal regions

Humans spend a large proportion of their time engaged in thoughts unrelated to the task being performed, a tendency that declines with age. However, a clear neuro-cognitive account of what underlies this decrease is lacking. This study addresses the possibility that age-related changes in off-task thinking are correlated with changes in the intrinsic organisation of the brain. Laboratory measures of ongoing thought were recorded in young and older individuals, who also participated in a resting state fMRI experiment. Older individuals showed reduced connectivity between the left anterior temporal lobe with prefrontal aspects of the DMN. We found that off-task thinking did not increase when task demands were lower for older adults, which is a pattern repeatedly seen in younger individuals. Finally, we demonstrated that these neural and thought patterns were linked - for younger participants only, reductions in the strength of connectivity were related to a greater shift towards off-task thoughts when task demands decreased. Importantly, in the older individuals, lower connectivity between the same regions was linked to preserved performance on a creativity task. These data suggest that the age-related reduction of off-task thought may be related to reduced communication between temporal and prefrontal DMN regions in ageing.

Poor coherence in older people's speech is explained by impaired semantic and executive processes

The ability to speak coherently is essential for effective communication but declines with age: older people more frequently produce tangential, off-topic speech. The cognitive factors underpinning this decline are poorly understood. We predicted that maintaining coherence relies on effective regulation of activated semantic knowledge about the world, and particularly on the selection of currently relevant semantic representations to drive speech production. To test this, we collected 840 speech samples along with measures of executive and semantic ability from 60 young and older adults, using a novel computational method to quantify coherence. Semantic selection ability predicted coherence, as did level of semantic knowledge and a measure of domain-general executive ability. These factors fully accounted for the age-related coherence deficit. Our results indicate that maintaining coherence in speech becomes more challenging as people age because they accumulate more knowledge but are less able to effectively regulate how it is activated and used.