Cortical Dynamics of Language

The human capability for fluent speech profoundly directs inter-personal communication and, by extension, self-expression. Language is lost in millions of people each year due to trauma, stroke, neurodegeneration, and neoplasms with devastating impact to social interaction and quality of life. The following investigations were designed to elucidate the neurobiological foundation of speech production, building towards a universal cognitive model of language in the brain. Understanding the dynamical mechanisms supporting cortical network behavior will significantly advance the understanding of how both focal and disconnection injuries yield neurological deficits, informing the development of therapeutic approaches

Network Dynamics of Visual Object Recognition

Visual object recognition is the principal mechanism by which humans and many animals interpret their surroundings. Despite the complexity of neural computation required, object recognition is achieved with such rapidity and accuracy that it appears to us almost effortless. Extensive human and non-human primate research has identified putative category-selective regions within higher-level visual cortex, which are thought to mediate object recognition. Despite decades of study, however, the functional organization and network dynamics within these regions remain poorly understood, due to a lack of appropriate animal models as well as the spatiotemporal limitations of current non-invasive human neuroimaging techniques (e.g. fMRI, scalp EEG). To better understand these issues, we leveraged the high spatiotemporal resolution of intracranial EEG (icEEG) recordings to study rapid, transient interactions between the disseminated cortical substrates within category-specific networks. Employing novel techniques for the topologically accurate and statistically robust analysis of grouped icEEG, we found that category-selective regions were spatially arranged with respect to cortical folding patterns, and relative to each other, to generate a hierarchical information structuring of visual information within higher-level visual cortex. This may facilitate rapid visual categorization by enabling the extraction of different levels of object detail across multiple spatial scales. To characterize network interactions between distributed regions sharing the same category-selectivity, we evaluated feed-forward, hierarchal and parallel, distributed models of information flow during face perception via measurements of cortical activation, functional and structural connectivity, and transient disruption through electrical stimulation. We found that input from early visual cortex (EVC) to two face-selective regions – the occipital and fusiform face areas (OFA and FFA, respectively) – occurred in a parallelized, distributed fashion: Functional connectivity between EVC and FFA began prior to the onset of subsequent re-entrant connectivity between the OFA and FFA. Furthermore, electrophysiological measures of structural connectivity revealed independent cortico- cortical connections between the EVC and both the OFA and FFA. Finally, direct disruption of the FFA, but not OFA, impaired face-perception. Given that the FFA is downstream of the OFA, these findings are incompatible with the feed-forward, hierarchical models of visual processing, and argue instead for the existence of parallel, distributed network interactions

Visual speech differentially modulates beta, theta, and high gamma bands in auditory cortex

Speech perception is a central component of social communication. While principally an auditory process, accurate speech perception in everyday settings is supported by meaningful information extracted from visual cues (e.g., speech content, timing, and speaker identity). Previous research has shown that visual speech modulates activity in cortical areas subserving auditory speech perception, including the superior temporal gyrus (STG), potentially through feedback connections from the multisensory posterior superior temporal sulcus (pSTS). However, it is unknown whether visual modulation of auditory processing in the STG is a unitary phenomenon or, rather, consists of multiple temporally, spatially, or functionally distinct processes. To explore these questions, we examined neural responses to audiovisual speech measured from intracranially implanted electrodes within the temporal cortex of 21 patients undergoing clinical monitoring for epilepsy. We found that visual speech modulates auditory processes in the STG in multiple ways, eliciting temporally and spatially distinct patterns of activity that differ across theta, beta, and high-gamma frequency bands. Before speech onset, visual information increased high-gamma power in the posterior STG and suppressed beta power in mid-STG regions, suggesting crossmodal prediction of speech signals in these areas. After sound onset, visual speech decreased theta power in the middle and posterior STG, potentially reflecting a decrease in sustained feedforward auditory activity. These results are consistent with models that posit multiple distinct mechanisms supporting audiovisual speech perception and provide a crucial map for subsequent studies to identify the types of visual features that are encoded by these separate mechanisms.This study was supported by NIH Grant R00 DC013828 A. Beltz was supported by the Jacobs Foundation.http://deepblue.lib.umich.edu/bitstream/2027.42/167729/1/OriginalManuscript.pdfDescription of OriginalManuscript.pdf : Preprint of the article "Multiple auditory responses to visual speech"SEL

Decomposing Tool-Action Observation: A Stereo-EEG Study

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Speech-brain synchronization: a possible cause for developmental dyslexia

152 p.Dyslexia is a neurological learning disability characterized by the difficulty in an individual¿s ability to read despite adequate intelligence and normal opportunities. The majority of dyslexic readers present phonological difficulties. The phonological difficulty most often associated with dyslexia is a deficit in phonological awareness, that is, the ability to hear and manipulate the sound structure of language. Some appealing theories of dyslexia attribute a causal role to auditory atypical oscillatory neural activity, suggesting it generates some of the phonological problems in dyslexia. These theories propose that auditory cortical oscillations of dyslexic individuals entrain less accurately to the spectral properties of auditory stimuli at distinct frequency bands (delta, theta and gamma) that are important for speech processing. Nevertheless, there are diverging hypotheses concerning the specific bands that would be disrupted in dyslexia, and which are the consequences of such difficulties on speech processing. The goal of the present PhD thesis was to portray the neural oscillatory basis underlying phonological difficulties in developmental dyslexia. We evaluated whether phonological deficits in developmental dyslexia are associated with impaired auditory entrainment to a specific frequency band. In that aim, we measured auditory neural synchronization to linguistic and non-linguistic auditory signals at different frequencies corresponding to key phonological units of speech (prosodic, syllabic and phonemic information). We found that dyslexic readers presented atypical neural entrainment to delta, theta and gamma frequency bands. Importantly, we showed that atypical entrainment to theta and gamma modulations in dyslexia could compromise perceptual computations during speech processing, while reduced delta entrainment in dyslexia could affect perceptual and attentional operations during speech processing. In addition, we characterized the links between the anatomy of the auditory cortex and its oscillatory responses, taking into account previous studies which have observed structural alterations in dyslexia. We observed that the cortical pruning in auditory regions was linked to a stronger sensitivity to gamma oscillation in skilled readers, but to stronger theta band sensitivity in dyslexic readers. Thus, we concluded that the left auditory regions might be specialized for processing phonological information at different time scales (phoneme vs. syllable) in skilled and dyslexic readers. Lastly, by assessing both children and adults on similar tasks, we provided the first evaluation of developmental modulations of typical and atypical auditory sampling (and their structural underpinnings). We found that atypical neural entrainment to delta, theta and gamma are present in dyslexia throughout the lifespan and is not modulated by reading experience

Speech-brain synchronization: a possible cause for developmental dyslexia

152 p.Dyslexia is a neurological learning disability characterized by the difficulty in an individual¿s ability to read despite adequate intelligence and normal opportunities. The majority of dyslexic readers present phonological difficulties. The phonological difficulty most often associated with dyslexia is a deficit in phonological awareness, that is, the ability to hear and manipulate the sound structure of language. Some appealing theories of dyslexia attribute a causal role to auditory atypical oscillatory neural activity, suggesting it generates some of the phonological problems in dyslexia. These theories propose that auditory cortical oscillations of dyslexic individuals entrain less accurately to the spectral properties of auditory stimuli at distinct frequency bands (delta, theta and gamma) that are important for speech processing. Nevertheless, there are diverging hypotheses concerning the specific bands that would be disrupted in dyslexia, and which are the consequences of such difficulties on speech processing. The goal of the present PhD thesis was to portray the neural oscillatory basis underlying phonological difficulties in developmental dyslexia. We evaluated whether phonological deficits in developmental dyslexia are associated with impaired auditory entrainment to a specific frequency band. In that aim, we measured auditory neural synchronization to linguistic and non-linguistic auditory signals at different frequencies corresponding to key phonological units of speech (prosodic, syllabic and phonemic information). We found that dyslexic readers presented atypical neural entrainment to delta, theta and gamma frequency bands. Importantly, we showed that atypical entrainment to theta and gamma modulations in dyslexia could compromise perceptual computations during speech processing, while reduced delta entrainment in dyslexia could affect perceptual and attentional operations during speech processing. In addition, we characterized the links between the anatomy of the auditory cortex and its oscillatory responses, taking into account previous studies which have observed structural alterations in dyslexia. We observed that the cortical pruning in auditory regions was linked to a stronger sensitivity to gamma oscillation in skilled readers, but to stronger theta band sensitivity in dyslexic readers. Thus, we concluded that the left auditory regions might be specialized for processing phonological information at different time scales (phoneme vs. syllable) in skilled and dyslexic readers. Lastly, by assessing both children and adults on similar tasks, we provided the first evaluation of developmental modulations of typical and atypical auditory sampling (and their structural underpinnings). We found that atypical neural entrainment to delta, theta and gamma are present in dyslexia throughout the lifespan and is not modulated by reading experience

From sequences to cognitive structures : neurocomputational mechanisms

Ph. D. Thesis.Understanding how the brain forms representations of structured information distributed in time is a challenging neuroscientific endeavour, necessitating computationally and neurobiologically informed study. Human neuroimaging evidence demonstrates engagement of a fronto-temporal network, including ventrolateral prefrontal cortex (vlPFC), during language comprehension. Corresponding regions are engaged when processing dependencies between word-like items in Artificial Grammar (AG) paradigms. However, the neurocomputations supporting dependency processing and sequential structure-building are poorly understood. This work aimed to clarify these processes in humans, integrating behavioural, electrophysiological and computational evidence. I devised a novel auditory AG task to assess simultaneous learning of dependencies between adjacent and non-adjacent items, incorporating learning aids including prosody, feedback, delineated sequence boundaries, staged pre-exposure, and variable intervening items. Behavioural data obtained in 50 healthy adults revealed strongly bimodal performance despite these cues. Notably, however, reaction times revealed sensitivity to the grammar even in low performers. Behavioural and intracranial electrode data was subsequently obtained in 12 neurosurgical patients performing this task. Despite chance behavioural performance, time- and time-frequency domain electrophysiological analysis revealed selective responsiveness to sequence grammaticality in regions including vlPFC. I developed a novel neurocomputational model (VS-BIND: “Vector-symbolic Sequencing of Binding INstantiating Dependencies”), triangulating evidence to clarify putative mechanisms in the fronto-temporal language network. I then undertook multivariate analyses on the AG task neural data, revealing responses compatible with the presence of ordinal codes in vlPFC, consistent with VS-BIND. I also developed a novel method of causal analysis on multivariate patterns, representational Granger causality, capable of detecting flow of distinct representations within the brain. This alluded to top-down transmission of syntactic predictions during the AG task, from vlPFC to auditory cortex, largely in the opposite direction to stimulus encodings, consistent with predictive coding accounts. It finally suggested roles for the temporoparietal junction and frontal operculum during grammaticality processing, congruent with prior literature. This work provides novel insights into the neurocomputational basis of cognitive structure-building, generating hypotheses for future study, and potentially contributing to AI and translational efforts.Wellcome Trust, European Research Counci

Improving the P300-Based Brain-Computer Interface by Examining the Role of Psychological Factors on Performance

The effects of neurodegenerative diseases such as amyotrophic-lateral sclerosis (ALS) eventually render those suffering from the illness unable to communicate, leaving their cognitive function relatively unharmed and causing them to be “locked-in” to their own body. With this primary function compromised there has been an increased need for assistive communication methods such as brain-computer interfaces (BCIs). Unlike several augmentative or alternative communication methods (AACs), BCIs do not require any muscular control, which makes this method ideal for people with ALS. The wealth of BCI research focuses mainly on increasing BCI performance through improving stimulus processing and manipulating paradigms. Recent research has suggested a need for studies focused on harnessing psychological qualities of BCI users, such as motivation, mood, emotion, and depression, in order to increase BCI performance through working with the user. The present studies address important issues related to P300-BCI performance: 1) the impact of mood, emotion, motivation, and depression on BCI performance were examined independently; and 2) pleasant, unpleasant, and neutral emotions were induced in order to determine the influence of emotion on BCI performance. By exploring psychological mechanisms that influence BCI performance, further insight can be gained on the best methods for improving BCI performance and increasing the number of potential BCI users. The results from Study 1 did not reveal a significant relationship between any of the four psychological factors and BCI performance. Since previous research has found a significant impact of motivation and mood on BCI performance, it may be the case that these factors only impact performance for some individuals. As this is the first study to directly investigate the impact of emotion and depression on BCI performance, future research should continue to explore these relationships. The results from Study 2 were inconclusive for the pleasant condition, since it appears the pleasant emotion manipulation was unsuccessful. The findings indicate that unpleasant emotions do not have a significant impact on BCI performance. This result is promising since it indicates that individuals should still be able to use the BCI system to communicate, even when they are experiencing unpleasant emotions. Future research should further explore the impact of pleasant emotions on BCI performance

Multimodal MRI characterization of visual word recognition: an integrative view

228 p.The ventral occipito-temporal (vOT) association cortex contributes significantly to recognize different types of visual patterns. It is widely accepted that a subset of this circuitry, including the visual word form area (VWFA), becomes trained to perform the task of rapidly identifying word forms. An important open question is the computational role of this circuitry: To what extent is part of a bottom-up hierarchical processing of information on visual word recognition and/or is involved in processing top-down signals from higher-level language regions. This doctoral dissertation thesis proposal is aimed at characterizing the vOT reading circuitry using behavioral, functional, structural and quantitative MRI indexes, and linking its computations to the other two important regions within the language network: the posterior parietal cortex (pPC) and the inferior frontal gyrus (IFG). Results revealed that two distinct word-responsive areas can be segregated in the vOT: one responsible for visual feature extraction that is connected to the intraparietal sulcus via the vertical occipital fasciculus and a second one responsible for semantic processing that is connected to the angular gyrus via the posterior arcuate fasciculus and to the IFG via the anterior arcuate fasciculus. Importantly, reading behavior was predicted by functional activation in regions identified along the vOT, pPC and IFG, as well as by structural properties of the white matter fiber tracts linking them. The present work constitutes a critical step in the creation of a highly detailed characterization of the early stages of reading at the individual-subject level and to establish a baseline model and parameter range that might serve to clarify functional and structural differences between typical, poor and atypical readers.BCBL: basque center on cognition, brain and languag