How visual attention span and phonological skills contribute to N170 print tuning: An EEG study in French dyslexic students

Developmental dyslexia is a disorder characterized by a sustainable learning deficit in reading. Based on ERP-driven approaches focusing on the visual word form area, electrophysiological studies have pointed a lack of visual expertise for written word recognition in dyslexic readers by contrasting the left-lateralized N170 amplitudes elicited by alphabetic versus non-alphabetic stimuli. Here, we investigated in 22 dyslexic participants and 22 age-matched control subjects how two behavioural abilities potentially affected in dyslexic readers (phonological and visual attention skills) contributed to the N170 expertise during a word detection task. Consistent with literature, dyslexic participants exhibited poorer performance in these both abilities as compared to healthy subjects. At the brain level, we observed (1) an unexpected preservation of the N170 expertise in the dyslexic group suggesting a possible compensatory mechanism and (2) a modulation of this expertise only by phonological skills, providing evidence for the phonological mapping deficit hypothesis

Ventral occipito-temporal cortex function and anatomical connectivity in reading

Previous functional neuroimaging studies of reading in skilled readers, acquired dyslexia and developmental dyslexia have all shown that the left ventral occipito-temporal cortex (vOT) is involved in visual word recognition. Specifically, a region in the left posterior occipito-temporal sulcus lateral to fusiform gyrus and medial to inferior temporal gyrus has been reported to play an important role. However, the precise functional contribution of this area in reading is yet to be fully explored. In this thesis, I empirically evaluated a claim that vOT responds not only to bottom-up processing demands of the visual stimuli but is also influenced by automatic, top-down non-visual processing demands, as proposed by the Interactive Account of vOT functioning. The first part of this thesis investigated the functional properties of vOT during reading, using functional magnetic resonance imaging. In the first project, the top-down influences on vOT were investigated, teasing apart visual and non-visual properties of written stimuli. In the second project, using the Japanese orthography I disentangled a word’s lexical frequency from the frequency of its visual form – an important distinction for understanding the neural information processing in regions engaged by reading and further explored the interactive nature of the vOT responses. The second part then investigated the anatomical basis of these functional interactions between vOT and other cortical regions. I used diffusion-weighted magnetic resonance imaging and tractography, the only method currently available to identify and measure white matter fibre pathways non-invasively and in vivo. My research has demonstrated that vOT integrates bottom-up visual information and top-down predictions from regions encoding non-visual attributes of the stimulus in an interactive fashion. It also illustrated the putative anatomical basis for functional connectivity during reading, which is consistent with the parallel cortical visual pathways seen in other primates. Altogether, the results provide strong support for the Interactive Account

Time-varying effective connectivity during visual object naming as a function of semantic demands

Accumulating evidence suggests that visual object understanding involves a rapid feedforward sweep, after which subsequent recurrent interactions are necessary. The extent to which recurrence plays a critical role in object processing remains to be determined. Recent studies have demonstrated that recurrent processing is modulated by increasing semantic demands. Differentially from previous studies, we used dynamic causal modeling to model neural activity recorded with magnetoencephalography while 14 healthy humans named two sets of visual objects that differed in the degree of semantic accessing demands, operationalized in terms of the values of basic psycholinguistic variables associated with the presented objects (age of acquisition, frequency, and familiarity). This approach allowed us to estimate the directionality of the causal interactions among brain regions and their associated connectivity strengths. Furthermore, to understand the dynamic nature of connectivity (i.e., the chronnectome; Calhoun et al., 2014) we explored the time-dependent changes of effective connectivity during a period (200–400 ms) where adding semantic-feature information improves modeling and classifying visual objects, at 50 ms increments. First, we observed a graded involvement of backward connections, that became active beyond 200 ms. Second, we found that semantic demands caused a suppressive effect in the backward connection from inferior frontal cortex (IFC) to occipitotemporal cortex over time. These results complement those from previous studies underscoring the role of IFC as a common source of top-down modulation, which drives recurrent interactions with more posterior regions during visual object recognition. Crucially, our study revealed the inhibitory modulation of this interaction in situations that place greater demands on the conceptual system

Abstractness of human speech sound representations

We argue, based on a study of brain responses to speech sound differences in Japanese, that memory encoding of functional speech sounds-phonemes-are highly abstract. As an example, we provide evidence for a theory where the consonants/p t k b d g/ are not only made up of symbolic features but are underspecified with respect to voicing or laryngeal features, and that languages differ with respect to which feature value is underspecified. In a previous study we showed that voiced stops are underspecified in English [Hestvik, A., & Durvasula, K. (2016). Neurobiological evidence for voicing underspecification in English. Brain and Language], as shown by asymmetries in Mismatch Negativity responses to /t/ and /d/. In the current study, we test the prediction that the opposite asymmetry should be observed in Japanese, if voiceless stops are underspecified in that language. Our results confirm this prediction. This matches a linguistic architecture where phonemes are highly abstract and do not encode actual physical characteristics of the corresponding speech sounds, but rather different subsets of abstract distinctive features

The Crisis of Language in Contemporary Japan: Reading, Writing, and New Technology

My dissertation is an ethnographically inspired theoretical exploration of the crises of reading and writing in contemporary Japan. Each of the five chapters examines concrete instances of reading and writing practices that have been problematized in recent decades. By calling attention to underlying moral assumptions, established sociocultural protocols, and socio-technological conditions of the everyday, I theorize the concept of embodied reading and writing thresholds. The scope of analysis is partly informed by popular discourse decrying a perceived decline in reading and writing proficiency among Japanese youth. This alleged failing literacy figures as a national crisis under the assumption that the futurity of children's national language proficiency metonymically correlates with the future well being of its national cultural body. In light of heightened interests in the past, present, and future of books, and a series of recent state interventions on the prospect of "national" text culture, it is my argument that ongoing tensions surrounding the changing media landscape and symbolic relations to the world do not merely reflect changes in styles of language, structures of spatiotemporal awareness, or forms of knowledge production. Rather, they indicate profound transformations and apprehensions among the lives mediated and embodied by the very system of signification that has come under scrutiny in the post-Lost Decade Japan (03/1991-01/2002). My dissertation offers an unique point of critical intervention into 1) various forms of tension arising from the overlapping media technologies and polarized population, 2) formations of reading and writing body (embodiment) at an intersection of heterogeneous elements and everyday disciplining, 3) culturally specific conditions and articulations of the effects of "universal" technologies, 4) prospects of "proper" national reading and writing culture, and 5) questions of cultural transformation and transmission. I hope that the diverse set of events explored in respective chapters provide, as a whole, a broader perspective of the institutional and technological background as well as an intimate understanding of culturally specific circumstances in Japan. Insofar as this is an attempt to conduct a nuanced inquiry into the culturally specific configurations and articulations of a global phenomenon, each ethnographic moment is carefully contextualized to reflect Japan specific conditions while avoiding the pitfall of culturalist assumptions. Understanding how an existing system of representation, technological imperatives and sociohistorical predicaments have coalesced to form a unique constellation is the first step in identifying how the practice of reading and writing becomes a site of heated national debate in Japan. Against theories that problematize the de-corporealizing effects of digital technology within reading and writing, I emphasize the material specificity of contemporary reading and writing practices

Characterization of high-gamma activity in electrocorticographic signals

INTRODUCTION: Electrocorticographic (ECoG) high-gamma activity (HGA) is a widely recognized and robust neural correlate of cognition and behavior. However, fundamental signal properties of HGA, such as the high-gamma frequency band or temporal dynamics of HGA, have never been systematically characterized. As a result, HGA estimators are often poorly adjusted, such that they miss valuable physiological information. METHODS: To address these issues, we conducted a thorough qualitative and quantitative characterization of HGA in ECoG signals. Our study is based on ECoG signals recorded from 18 epilepsy patients while performing motor control, listening, and visual perception tasks. In this study, we first categorize HGA into HGA types based on the cognitive/behavioral task. For each HGA type, we then systematically quantify three fundamental signal properties of HGA: the high-gamma frequency band, the HGA bandwidth, and the temporal dynamics of HGA. RESULTS: The high-gamma frequency band strongly varies across subjects and across cognitive/behavioral tasks. In addition, HGA time courses have lowpass character, with transients limited to 10 Hz. The task-related rise time and duration of these HGA time courses depend on the individual subject and cognitive/behavioral task. Task-related HGA amplitudes are comparable across the investigated tasks. DISCUSSION: This study is of high practical relevance because it provides a systematic basis for optimizing experiment design, ECoG acquisition and processing, and HGA estimation. Our results reveal previously unknown characteristics of HGA, the physiological principles of which need to be investigated in further studies

Applications of Sparse Representations

In this dissertation I explore the properties and uses of sparse representations. Sparse representations use high dimensional binary vectors for representing information. They have many properties which make this representation useful for applications involving pattern recognition in highly noisy and complex environments. Sparse representations have a very high capacity. A typical sparse representation vector has a capacity of 10^84 distinct vectors, which is more than the number of atoms in the universe. Sparse representations are highly noise robust. They can tolerate even up to 50% noise. A very powerful and useful property of sparse representations is that they allow us to easily measure similarity between two things by directly comparing their representations. These properties allow them to have applications in a variety of fields, like Artificial Intelligence and Molecular Biology, that need to encode information that is complex and noisy in nature. In this dissertation, I show how sparse representations can be used for representing complex environments for an agent based on Learning Intelligent Decision Agent (LIDA) model. Sparse representations allowed us to achieve a two-fold goal of producing information rich representations of things in the environment while proposing a method of generating grounded representations for the LIDA model. Sparse representations also allowed us to ground the representations used by LIDA in the sensory apparatus of the agent while still allowing a perfect fidelity communication between the sensory memory of LIDA and the rest of the model. I also show how sparse representations are useful in Molecular Biology for discovering data-driven patterns in heterogeneous and noisy gene expression data. We used a sparse auto-encoder to learn sparse representations of transcriptomics experiments taken from a huge publicly available dataset. These representations were then used to identify biological patterns in the form of gene sets. The representation provided a unique signature for a set of samples originating from the same experimental condition. Applications of our method include the identification of previously undiscovered gene sets as well as supervised classification of samples from different biological classes. Overall, our results show that sparse representations are useful in a variety of fields that involve finding patterns in a complex and noisy environment