Disorders of lexical access and production

Disorders of lexical access are characterized by inconsistent lexical access such that individuals successfully comprehend or produce a word in some contexts but fail on other occasions. Therefore, the lexical representations are thought to be intact, but their retrieval or activation is impaired and/or competing representations are not effectively managed. Lexical access deficits are most well-studied in individuals with aphasia, though some degree of lexical access difficulty can occur in a wide variety of neurogenic and developmental disorders, as well as in typical aging. This chapter focuses on the intersections of language, cognitive control, and memory: (1) how inhibition of lexical competitors and selection among competitors may explain some lexical access deficit phenomena, and (2) learning and retrieval processes in lexical access deficits from both basic research and translational application perspectives

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Exploring the use of multiplex network science to guide selection of treatment stimuli for word finding disorders in individuals with Primary Progressive Aphasia

Primary progressive aphasia (PPA) is a neurodegenerative language disorder gradually affecting individuals before the age of 65 years. The hallmark of PPA onset is the progressive and isolated impairment of word retrieval and/or comprehension. Other cognitive skills such as memory, visuospatial, reasoning and behaviour-related skills generally remain undisturbed during the initial stage of the disorder. There are 3 identified variants of PPA: semantic PPA (svPPA), nonfluent PPA (nfvPPA) and logopenic PPA (lvPPA). Each subtype of PPA is classified based on specific speech and language characteristics of the variant. Word finding difficulties are present in all three variants of PPA, but each variant shows different profiles of impairment in lexical processes. Due to lvPPA being the most recently discovered, the processes of lexical retrieval have been underexplored in this clinical population. The aim of this thesis is to apply the principles of network science to explore the influence of lexicon structure on lexical processing in individuals with lvPPA. The two network variables selected for investigation are degree (i.e., the number of words immediately linked to a given word, by phonological or semantic similarity) and closeness centrality (i.e., how closely connected a word is to all other words in the network and, therefore, how central its position is in the network). This thesis consists of three experimental studies (Chapters 2-4) that aim to address the identified gaps

Dynamic speech networks in the brain: Dual contribution of incrementality and constraints in access to semantics

This thesis explores the spatiotemporal network dynamics underlying natural speech comprehension, as measured by electro-magnetoencephalography (E/MEG). I focus on the transient effects of incrementality and constraints in speech on access to lexical semantics. Through three E/MEG experiments I address two core issues in systems neuroscience of language: 1) What are the network dynamics underpinning cognitive computations that take place when we map sounds to rich semantic representations? 2) How do the prior semantic and syntactic contextual constraints facilitate this mapping? Experiment 1 investigated the cognitive processes and relevant networks that come online prior to a word’s recognition point (e.g. “f” for butterfly) as we access meaning through speech in isolation. The results revealed that 300 ms before the word is recognised, the speech incrementally activated matching phonological and semantic representations resulting in transient competition. This competition recruited LIFG, and modality specific regions (LSMG, LSTG for the phonological; LAG and MTG for the semantic domain). Immediately after the word’s recognition point the semantic representation of the target concept was boosted, and rapidly accessed recruiting bilateral MTG and AG. Experiment 2 explored the cortical networks underpinning contextual semantic processing in speech. Participant listened to two-word spoken phrases where the semantic constraint provided by the modifier was manipulated. To separate out cognitive networks that are modulated by semantic constraint from task positive networks I performed a temporal independent component analysis. Among 14 networks extracted, only the activity of bilateral AG was modulated by semantic constraint between -400 to -300 ms before the noun’s recognition point. Experiment 3 addressed the influence of sentential syntactic constraint on anticipation and activation of upcoming syntactic frames in speech. Participants listened to sentences with local syntactic ambiguities. The analysis of the connectivity dynamics in the left frontotemporal syntax network showed that the processing of sentences that contained the less anticipated syntactic structure showed early increased feedforward information flow in 0-100 ms, followed by increased recurrent connectivity between LIFG and LpMTG from the 200-500 ms from the verb onset. Altogether the three experiments reveal novel insights into transient cognitive networks recruited incrementally over time both in the absence of and with context, as the speech unfolds, and how the activation of these networks are modulated by contextual syntactic and semantic constraints. Further I provide neural evidence that contextual constraints serve to facilitate speech comprehension, and how the speech networks recover from failed anticipations.Istanbul Chamber of Commerce Turkish Petrol Foundatio

The neural basis of semantic processing across comprehension contexts

Current neurobiological models of semantic cognition have been predominately derived from studies of single-words or sentences which may provide an impoverished estimate of how semantic processing occurs in real-world contexts. Studies that make use of more ecologically valid stimuli such as natural language or narratives suggest that, counter to the hub-and-spoke framework in which the anterior temporal lobe (ATL) serves as a graded hub integrating information from proximal sensorimotor spokes, the semantic system displays voxel-wise category specialization tiled across a large, distributed network. A complicating factor in reconciling these seemingly conflicting claims is the over-reliance on concrete conceptual knowledge in describing the organization of the semantic system. A recent theoretical account argues that social knowledge, like other types of semantic knowledge, is processed within the ventrolateral ATL, but this claim has not been tested using naturalistic stimuli, which better sample abstract social knowledge, including pragmatic inference. This thesis investigates the organization of the semantic system across multiple scales, from isolated words to multimodal narratives, and across multiple types of semantic conceptual knowledge, from concrete to abstract. Using comprehension of concrete words as a starting point, the first study describes a critical examination of specialization within the semantic system for taxonomic (dog – bear) and thematic (dog – leash) relations using intracranial EEG recordings from an array of depth electrodes within ATL, inferior parietal lobule (IPL), and two regions within the semantic control network, inferior frontal gyrus (IFG) and posterior middle temporal gyrus (pMTG). Moving across the context and conceptual scale to build upon this work, the second study investigated how the concrete and abstract lexical and semantic properties of single-words, akin to those that informed the hub-and-spoke model, are processed in a complex, complete narrative presented to participants during fMRI scanning. In doing so, this study enabled comparisons between prior studies of isolated words and naturalistic work, thus moving toward an integrated cross-scale account of semantic cognition. Using the same neuroimaging data, the third study extended this work to investigate how context contributes to the construction of meaning by studying how the semantic and social cognitive systems are engaged by social and pragmatic sentence-level content. This enabled a direct, naturalistic test of the claim that social knowledge is housed within the semantic system. The fourth study investigated shared processing between social and semantic systems using fMRI data collected during movie-viewing, which captures the multimodal environment in which social knowledge is exchanged. The results of these studies collectively demonstrate that the semantic and social systems are differentially engaged across the scales investigated here. Concrete conceptual relations engage one (or more) specialized hubs within the semantic system, whereas processing of naturalistic verbal and event content co-varies with activation in large brain networks. There is evidence of functional gradations within ATL that are differentially sensitive to the demands of narrative comprehension – the anterior superior temporal gyrus (i.e., dorsolateral subregion) and anterior fusiform (i.e., ventral subregion) appear to be particularly sensitive to the quantity and informativeness of external input whereas the anterior middle and inferior temporal gyri (i.e., ventrolateral subregion) appear to be engaged by internal, or endogenous, semantic processing during narrative comprehension. Engagement of this same ventrolateral subregion is observed in response to social word and sentence content, providing support for the claim that social processing is subsumed within the semantic system. Taken together, the results suggest an extension to the current neurobiological model of semantic cognition that accommodates comprehension contexts. The studies undertaken as part of this thesis build upon the existing concept-level frameworks towards a narrative-level framework of semantic cognition

Task load modulation of tDCS effects on behavioural and neural correlates of phonological processing: a brain-state-dependent stimulation approach

This thesis investigates the task load modulation of transcranial direct current stimulation (tDCS) effects on the behavioural and neural correlates of phonological processing in the healthy, dyslexic and aphasic brain. Tasks selected from a range of speech perception to speech production were assumed to engage the two main regions of the dorsal pathway of phonological processing, the targets left inferior frontal gyrus (LIFG) and left superior temporal gyrus (LSTG), differently: increasing engagement for the LIFG and decreasing engagement for the LSTG across the range. Anodal tDCS (facilitatory) and cathodal tDCS (inhibitory) were expected to have an effect as a direct function of target engagement with the task. Cathodal tDCS was also expected to induce indirect facilitation via network compensation negatively related to target engagement. These effects should be weaker in dyslexia and aphasia, and consistent with their altered pattern of brain activity. The findings suggest that task load modulation of tDCS effects occurred. Outcomes such as improved performance in a speech perception task caused by cathodal tDCS would suggest that cathodal tDCS induced network compensation. Results in dyslexia and aphasia were consistent with their altered pattern of brain activity, indicating that tDCS shows promise as both diagnosis and treatment tool