How to Achieve the Capacity of Asymmetric Channels

We survey coding techniques that enable reliable transmission at rates that approach the capacity of an arbitrary discrete memoryless channel. In particular, we take the point of view of modern coding theory and discuss how recent advances in coding for symmetric channels help provide more efficient solutions for the asymmetric case. We consider, in more detail, three basic coding paradigms. The first one is Gallager's scheme that consists of concatenating a linear code with a non-linear mapping so that the input distribution can be appropriately shaped. We explicitly show that both polar codes and spatially coupled codes can be employed in this scenario. Furthermore, we derive a scaling law between the gap to capacity, the cardinality of the input and output alphabets, and the required size of the mapper. The second one is an integrated scheme in which the code is used both for source coding, in order to create codewords distributed according to the capacity-achieving input distribution, and for channel coding, in order to provide error protection. Such a technique has been recently introduced by Honda and Yamamoto in the context of polar codes, and we show how to apply it also to the design of sparse graph codes. The third paradigm is based on an idea of B\"ocherer and Mathar, and separates the two tasks of source coding and channel coding by a chaining construction that binds together several codewords. We present conditions for the source code and the channel code, and we describe how to combine any source code with any channel code that fulfill those conditions, in order to provide capacity-achieving schemes for asymmetric channels. In particular, we show that polar codes, spatially coupled codes, and homophonic codes are suitable as basic building blocks of the proposed coding strategy.Comment: 32 pages, 4 figures, presented in part at Allerton'14 and published in IEEE Trans. Inform. Theor

Testing for a Cultural Influence on Reading for Meaning in the Developing Brain: The Neural Basis of Semantic Processing in Chinese Children

Functional magnetic resonance imaging was used to explore the neural correlates of semantic judgments in a group of 8- to 15-year-old Chinese children. Participants were asked to indicate if pairs of Chinese characters presented visually were related in meaning. The related pairs were arranged in a continuous variable according to association strength. Pairs of characters with weaker semantic association elicited greater activation in the mid ventral region (BA 45) of left inferior frontal gyrus, suggesting increased demands on the process of selecting appropriate semantic features. By contrast, characters with stronger semantic association elicited greater activation in left inferior parietal lobule (BA 39), suggesting stronger integration of highly related features. In addition, there was a developmental increase, similar to previously reported findings in English, in left posterior middle temporal gyrus (BA 21), suggesting that older children have more elaborated semantic representations. There were additional age-related increases in the posterior region of left inferior parietal lobule and in the ventral regions of left inferior frontal gyrus, suggesting that reading acquisition relies more on the mapping from orthography to semantics in Chinese children as compared to previously reported findings in English

Consequences of bi-literacy in bilingual individuals: in the healthy and neurologically impaired

Background. In the current global, cross-cultural scenario, being bilingual or multilingual is a norm rather than an exception. In such an environment an individual may be actively involved in reading and writing in all their languages in addition to speaking them. Regular use of two or more languages is termed as bilingualism and being able to read and write in both of them is referred to as bi-literacy. Research indicates that bilingualism has an impact on language production and cognition, specifically executive functions. Given the impact of literacy and bilingualism, the reasonable question that arises, is whether bi-literacy would offer an additional impact on language production and cognition. This becomes even more relevant in a multilingual, multi-cultural society such as India. We examined the impact of bi-literacy on oral language production (at word and connected speech level), comprehension and on non-verbal executive function measures in bi-literate bilingual healthy adults in an immigrant diaspora living in the UK. In addition to English, they were speakers of one of the South Indian languages (Kannada, Malayalam, Tamil and Telugu). The significance of bi-literacy among bilinguals assumes further importance in aphasia (language impairment due to brain damage). For those who have aphasia in one or more languages due to brain damage, the severity of impairment maybe different in both languages, also the modalities of language may be differentially affected. In particular, reading and writing maybe impaired differently in the languages used by a bi/multilingual. Manifestation of reading impairments are also dependent on the nature of the script of the language being read [e.g., Raman & Weekes (2005) report differential dyslexia in a Turkish-English speaker who exhibited surface dyslexia in English and deep dysgraphia in Turkish]. Our study contributes to the field of bilingual aphasia by focusing specifically on reading differing from the existing literature of aphasia in bilinguals, where the focus has predominantly been on language production and comprehension. Studying reading impairments provides a better understanding of how the reading impairments are manifested in the two languages, which will aid appropriate assessment and intervention. This research investigated the impact of bi-literacy in both populations (healthy adults and neurologically impaired) in two phases: Phase I (in UK) and Phase II (in India). Aim. Phase I investigated the impact of bi-literacy on oral language production (at word level and connected speech), comprehension and non-verbal executive function in bi-literate bilingual healthy adults. Phase II examined the reading impairments in two languages of bilingual persons with aphasia (BPWA). Methods. For Phase I, participants were thirty-four bi-literate bilingual healthy adults with English as their L2 and one of the Dravidian languages (Kannada, Malayalam, Tamil and Telugu) as their L1. We have used the term ‘print exposure’ as a proxy for literacy. They were divided into a high print exposure (HPE, n=22) and a low print exposure (LPE, n=12) group based on their performance on two tasks measuring L2 print exposure- grammaticality judgement task and sentence verification task. We also quantified their bilingual characteristics- proficiency, reading and writing characteristics and dominance. The groups were matched on years of education, age and gender. Participants completed a set of oral language production tasks in L2 (at word level) namely -verbal fluency, word and non-word repetition; comprehension tasks in L2 namely synonymy triplets task and sentence comprehension task (Chapter 2); oral narrative task in L2 (at connected speech level) (Chapter 3) followed by non-verbal executive function tasks tapping into inhibitory control (Spatial Stroop and Flanker tasks), working memory (visual n-back and auditory n-back) and task switching (colour-shape task) (Chapter 4). For Phase II, we characterized the reading abilities of four BPWA who spoke one of the Dravidian languages (Kannada, Tamil, Telugu) (alpha-syllabic) as their L1 and English (alphabetic) as their L2. We quantified their bilingual characteristics- proficiency, reading and writing characteristics and dominance. Subtests from the Psycholinguistic Assessment of Language Processing in Aphasia (PALPA; Kay, Lesser & Coltheart, 1992) were used to document the reading profile of BPWA in English and reading subtests from Reading Acquisition Profile (RAP-K; Rao, 1997) and words from Bilingual Aphasia test -Hindi (BAT; Paradis & Libben, 1987) were used to document the reading profile of BPWA in Kannada and Hindi respectively. Findings. Based on the findings of Phase I (i.e., results from Chapter 2-4), we found prominent differences between HPE and LPE on comprehension measures (synonymy triplets and sentence comprehension tasks). This is in contrast to the results observed in monolingual adults, were semantics is less impacted by print exposure. Moreover, our predictions that HPE will result in better oral language production skills were borne out in specific conditions-semantic fluency and non-word repetition task (at word level) and higher number of words in the narrative, higher verbs per utterance and fewer repetitions (at connected speech level). In addition, the non-verbal executive functions, we found no direct link between print exposure (in L2) and non-verbal executive functions in bi-literate bilinguals excepting working memory (auditory N-back task). Additionally, another consistency in our findings is that there seems to be a strong link between print exposure and semantic processing in our research. The findings on the semantic tasks have been consistent across comprehension (synonymy triplets task and sentence comprehension task) and production (semantic fluency) favouring HPE. The findings from Phase II (Chapter 5) reveal differences of reading characteristics in the two languages (with different scripts) of the four BPWA. This research provides preliminary evidence that a script related difference exists in the manifestation of dyslexia in bi-scriptal BPWA speaking a combination of alphabetic and alpha-syllabic languages. Conclusions. Our research contributes to the existing literature by highlighting the relationship between bi-literacy and language production, comprehension and non-verbal cognition where bi-literacy seems to have a higher impact on language than cognition. The contrary findings from the monolinguals and children literature, highlight the importance for considering nuances of bilingual research and specifically challenges the notion that semantic comprehension is not significantly affected by literacy. In the neurologically impaired population, our research provides a comprehensive profiling of reading abilities in BPWA in the Indian population with languages having different scripts. Using this profiling and classification, we are able to affirm the findings previously found in literature emphasizing the importance of script in the assessment of reading abilities in BPWA. Such profiling and classification assist in the development of bilingual models of reading aloud and classifying different types of reading impairments

An Investigation on Cognitive-Linguistic Skills of English-Chinese Bilingual Learners with and without Dyslexia in Singapore

This thesis investigates dyslexia and the cognitive-linguistics skills, namely phonological awareness, orthographic knowledge, morphological awareness and rapid naming, of bilingual learners in Singapore whose first language is English and second language is Chinese. The two main research aims are to investigate whether the English-Chinese bilingual learners with dyslexia diagnosed only in English are weaker than their typical counterparts in reading and all cognitive-linguistic skills in both languages or either language, and to investigate which cognitive-linguistic skills are strong predictors of reading in each language. Results show that the bilingual learners with dyslexia performed significantly poorer than their typical counterparts in reading and all cognitive-linguistic skills in both languages, although their dyslexia were diagnosed only in English. Results also found all English cognitive-linguistic skills predictive of English word reading, especially the unique predictive roles of morphological awareness and orthographic knowledge after rapid naming and phonological awareness were controlled. However, only rapid naming and morphological awareness were found to be predictive of Chinese word reading. The results suggest that dyslexia may manifest differently in reading and cognitive-linguistic skills of English and Chinese languages in the English-Chinese bilingual learners, based on the two different predictive models with different empirically and theoretically supported orders of cognitive-linguistic skills as predictors for reading development in the two languages. The difference in the unique contributions of the four cognitive-linguistic skills underlying the reading development of both languages may suggest the difference lies in language structure and instruction. Keywords: dyslexia, bilingualism, English reading, Chinese reading, cognitive-linguistic skill