Dyslexia: A Bibliometric and Visualization Analysis

Dyslexia is a disorder characterized by an impaired ability to understand written and printed words or phrases. Epidemiological longitudinal data show that dyslexia is highly prevalent, affecting 10–20% of the population regardless of gender. This study aims to provide a detailed overview of research status and development characteristics of dyslexia from types of articles, years, countries, institutions, journals, authors, author keywords, and highly cited papers. A total of 9,166 publications have been retrieved from the Social Sciences Citation Index (SSCI) and Science Citation Index Expanded (SCI-E) from 2000 to 2021. The United States of America, United Kingdom, and Germany were the top three most productive countries in terms of the number of publications. China, Israel, and Japan led the Asia research on dyslexia. University of Oxford had the most publications and won first place in terms of h-index. Dyslexia was the most productive journal in this field and Psychology was the most used subject category. Keywords analysis indicated that “developmental dyslexia,” “phonological awareness,” children and fMRI were still the main research topics. “Literacy,” “rapid automatized naming (RAN),” “assessment,” “intervention,” “meta-analysis,” “Chinese,” “executive function,” “morphological awareness,” “decoding,” “dyscalculia,” “EEG,” “Eye tracking,” “rhythm,” “bilingualism,” and “functional connectivity” might become the new research hotspots.


INTRODUCTION
The term dyslexia is derived from the Greek script and was first proposed in 1887 by Dr. Rudolf Berlin in his work "Eine besondere Art der Wortblindheit (Dyslexie)". In 1994, Lyon proposed a working definition of dyslexia, and later in 2002, a revised version of dyslexia was approved as "dyslexia is a specific learning that is neurobiological in origin that is characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities" (1,2). In 1896, Hinshelwood J. published a case of dyslexia (3). There were almost no publications on dyslexia from 1900 to 1945 and the possible reason might be the turmoil of society and the world wars. Since 1946, more scientific research gradually uncovers the reasons behind dyslexia including the causes, symptoms, clinical diagnosis, and improvement measures (4)(5)(6)(7)(8)(9)(10)(11)(12)(13). Although the root cause of dyslexia is still unclear, researchers do have some explanations that give us a better understanding of dyslexia and people with dyslexia (14)(15)(16)(17)(18)(19)(20)(21). According to the European dyslexia association (EDA), the incidence of dyslexia worldwide is about 9-12%. At present, some countries have passed a series of legislation to promote better identification of people with dyslexia, and to protect the rights in education, employment, and access to public services of individuals with dyslexia (22)(23)(24)(25).
Bibliometrics was proposed by Alan Pritchard in 1969, defined as "the application of mathematics and statistical methods to books and other media of communication" (26). Bibliometrics is an important branch of information science and philology. At the same time, it also shows important methodological value and becomes a special research method of information science. The number of bibliometrics academic papers published each year around the world is continually increasing, with about 3,000 in 2021. Bibliometric analyses are useful tools to quantitatively analyze academic literature to get a good understanding of the research trends in specific areas of science and technology, such as public health care (27)(28)(29)(30)(31)(32), drug discovery (33)(34)(35), nursing (36,37), biomass (38)(39)(40)(41)(42), and COVID-19 (43)(44)(45)(46)(47)(48)(49). Bibliometrics has become an academic link closely related to science communication and basic theories. To our knowledge, few comprehensive bibliometric studies have been performed on the dyslexia research literature. Ram (50) conducted an analysis of dyslexia literature  from Scopus, which mainly studied the document types, trends of the number of publications, most productive countries, journals, authors, and keywords. Recently, Zhang et al. (51) published a paper on the top 100 most-cited studies of dyslexia research. Due to the language and the stages of cognition of dyslexia, there is still a need to carry out a comprehensive analysis on the differences of bibliometric characters and research priorities and hotspots of dyslexia research from a country perspective.
To fill this research gap, this study (1) uses the bibliometric method to indicate the status and development trends using major research areas, productive institutes, and journals from a country perspective, (2) analyzes the collaboration patterns between countries and organizations, (3) explores the priorities and hotspots by analyzing the author keywords from temporal evolution and a country perspective. This study demonstrates the status of studies of dyslexia from a country perspective, which offers readers a fresh perspective and suggestions to dyslexia students and families, researchers, and policymakers for future challenges and policy formulation.

METHODS
The analysis was based on the publications related to "dyslexia" which were retrieved through the Social science citation index (SSCI) and science citation index expand (SCI-E) during the period 2000 to 2021. The data were obtained from the Web of Science (WoS) Core Collection by searching the title, abstract, author keywords, and KeyWords plus with search formula of "dyslexia" on January 14th, 2022. The graphical analysis of cooperation uses bibliographic coupling, co-citation, citation, co-authorship, and co-occurrence metrics. We used the Derwent Data Analyzer (DDA) software to present the outcomes of bibliometric analyses. Articles originating from England, Scotland, Wales, and Northern Ireland were grouped under the United Kingdom (UK) heading. The impact factor (IF) for each journal was determined according to the report from the 2020 Journal Citation Reports (JCR). Note that some related publications that did not use "dyslexia" in their topic parts may not be included in this analysis. This issue might produce some deviations.
Total 99 countries have published articles on the topic of dyslexia from 2000 to 2021.

International Cooperation Analysis
The academic collaboration networks of countries were extracted using Derwent Data Analyzer (DDA) software based on the cooccurrence matrix of author's country and country cooperation. The result of the top 20 most productive countries' cooperation (with a minimum of 5 shared publications) is shown in Figure 2. The size of nodes represents the number of publications. The lines between the nodes represent the cooperative frequency. The United States of America is the country with the highest number of papers in the dyslexia research field, followed by the United Kingdom, Germany, Italy, Canada, and China. As can be seen in Figure 2, the United States of America cooperated most frequently with the United Kingdom, Canada, and China. Furthermore, the United States of America and United Kingdom had the biggest collaboration network among the top 20 most productive countries. Researchers from Japan, Brazil, and Greece need to strengthen their international cooperation. China, Israel, and Japan led the research in Asian countries.  Table 1. The University of Oxford ranked first in terms of total publications and obtained the highest h-index (75), followed by UCL and the University of Jyvaskyla. Yale University has the highest number of ACCP. Figure 3 shows the cooperation between organizations with a minimum of 8 papers among the top 50 productive organizations. As shown in Figure 3, institutions from the same country were more closely connected. This was confirmed by the analysis of the top 3 most collaborative organizations for each institution (see Table 1). The University of Oxford has the largest collaborative network.

Organization Co-occurrence Analysis
In addition, we analyzed the share of cooperative publications between institutes (see Table 1). It can be seen that all the 15 most productive institutions except University Haifa have very high collaboration rates, especially the UCL, Harvard University, Yale University, and the University of Connecticut. Interestingly, all of the top 15 prolific organizations are universities. It suggests that dyslexia research is mostly held by universities. her research on dyslexia focuses on language impairment (67-69), comorbidity (70,71), and intervention (72)(73)(74)(75)(76). Lyytinen H is from the University of Jyvaskyla and his research on dyslexia focuses on the longitudinal study (21,(77)(78)(79), speech perception (80)(81)(82), auditory processing (83)(84)(85), and intervention (86,87).

Research Area and Journal Analysis From a Country Perspective
Research works on dyslexia have been carried out in about 101 research areas in SCI and SSCI databases. Figure 4 shows the number of papers published by the top 20 most productive countries in the top 20 most productive research areas. "Psychology" ranked first in terms of the total publications of all countries. "Neurosciences Neurology" and "Education Educational Research" are listed in the second or third position in all countries. Sweden, Spin, Norway, the Netherlands, China, and Greece had published more papers on "Education Educational Research" than "Neurosciences Neurology".
The 9,110 papers related to dyslexia research during 2000-2021 were published in 1,156 journals. Table 3    Journal of Learning Disabilities and Annals of Dyslexia than other countries. These suggested that the United Kingdom and United States of America researchers prefer to publish in journals hosted by their countries. dyslexia research, 9,562 author keywords which appeared 32,757 times from 9,110 papers were analyzed. Keywords with the same meanings were grouped and represented by one unified word or phrase, and the publications that lack author keywords may not be included in this analysis. Among the author keywords, 6,705 (70%) were used only once. The high percentage of once-only author keywords may indicate a lack of continuity in research and a wide range of interests in dyslexia research. Figure 5 shows a network map of author keywords cooccurrence analysis (frequency not <50 times) related to dyslexia. As seen in the analysis result in Figure 5, the keywords "dyslexia" and "reading" occupied the core positions. The top high-frequency nodes linked with "dyslexia" are "reading, " "children, " "attention, " "dyscalculia, " "magnocellular, " "adults, " "magnetoencephalography, " and "MRI". Keywords "fMRI, " "eye movements, " "spelling, " "intervention, " "phonology, " and "writing" were the top high-frequency nodes connected to "reading".
To better understand the development trend of research, we compared the top 50 high-frequency author keywords in the past 5 years and the first 16 years (see Table 4); "dyslexia" and "developmental dyslexia" were exceptions because these keywords were among the search terms of the data that were used in this study. "Phonological awareness, " "reading, " and "spelling" are the main research aspects; "children" are the main group studied. "fMRI" was still a strong and useful technique to measure the brain activity of dyslexia and remained among the top eight most frequently used keywords (94,95). "Literacy" refers to the quality or state of being literate, especially the ability to read and write. The rank of "literacy" increased from 27th in 2000-2016 to 15th in 2017-2021, suggesting that the research of literacy remained hot research during the past 20 years.
With the in-depth research and experience accumulation of dyslexia, early intervention and prevention of dyslexia have important social significance (96). "Rapid automatized naming (RAN)" as one of the effective cognitive measures drew the attention of researchers that involved in creating optimal assessments and interventions (97). In fact, the rank of "Rapid automatized naming" had an apparent upward movement from 37th in 2000-2016 to 27th in 2017-2021. Both "Assessment" and "intervention" also had a big upward movement from 40th in 2000-2016 to 29th in 2017-2021 and 36th in 2000-2016 to 21st in 2017-2021, respectively. In 1976, Gene Glass first used the name "Meta-analysis" to represent the process and method of integrating and analyzing many empirical studies on the same subject through statistical analysis to obtain the most representative conclusions. This method had become an important tool for analyzing various research results of dyslexia (98-101) and "meta-analysis" reached the 30th in 2017-2021 from 95th in 2000-2016.
In the 1980s and 1990s, Chinese scholars began to study dyslexia in reading Chinese, but most of the results were published in their Chinese journals. In recent years, with the enhancement of scientific research capabilities and international cooperation, increased research results have been published in international journals (102)(103)(104)(105)(106). The rank of "Chinese" had a dramatic increase from 49th in 2000-2016 to 32nd in 2017-2021. It is also worth mentioning that "executive function" (107,108), "morphological awareness" (109,110), "Meta-analysis, " "decoding" (16), "dyscalculia, " "EEG" (111, 112), "Eye tracking" (113, 114), "rhythm" (115,116), "bilingualism" (117,118), and "functional connectivity" (119, 120) entered the top 50 highfrequency keywords in 2017-2021, suggesting that these topics may become the new research hotspots. Table 5 shows the 20 countries with the highest scientific production in dyslexia research as well as the keywords most used by these countries. Not surprisingly, "dyslexia, " "reading, " and "developmental dyslexia" were the keywords most used by most of these countries, ranking first to third in 15 of the 20 countries. "fMRI" was one of the research hotspots in the United States of America, Norway, Switzerland, and Austria. The language of early research on dyslexia was mainly English. In the 1970s, some researchers believed that the writing system of Asian countries would not cause dyslexia. However, with the development of early reading education activities by educators in some Asian countries, the problem of children's dyslexia had gradually attracted the attention of researchers. Therefore, it was not surprising that Asian countries (China, Japan, and Israel) had their language as one of their research focuses (104,(121)(122)(123)(124)(125)(126)(127)(128)).

An Analysis of Highly Cited Papers Based on WoS
The citation account is an important indicator of academic influence and was widely used in research evaluation. According to the Essential Science Indicators (ESI) database, highly cited papers (HCPs) refers to papers with citations in the top 1% of all papers based on a cited threshold for an academic field and publication year during the past 10 years. To some extent, HCPs from the ESI database might reflect research directions and hotspots in an academic field (129).   (97,130) and two about meta-analysis (100,101), indicating that RAN and metaanalysis became the hotspots in dyslexia research. "Predictors of developmental dyslexia" (131) and "Early detection of dyslexia risk" (96) might be one of the new dyslexia research directions.

DISCUSSION
There is no doubt that more countries have taken dyslexia seriously over the past few decades. The United States of America, United Kingdom, and Germany had done well in publishing research papers in this field. Some Asian countries like China and Israel have started to play a role in dyslexia research. It is worth noting that in 2020-2021, the research results from China increased significantly, and the ranking jumped to third place based on the number of published papers in the past 2 years. North America, Western and Northern Europe, Asia, and Australia were the most active regions in the research of dyslexia. This was further confirmed by most active institutions and authors. There were no organizations from Africa in the top 15 most productive institutions that indicated that the issues relating to dyslexia in low-income regions lag far behind in developed countries and regions. The possible reason might be poor awareness of dyslexia among educators, the public, funding input, economic level, etc. As dyslexia is a world health issue, we expect more Asian and African nations join this research area. Although, most of the dyslexia research is held by universities, it will benefit sharing its knowledge and experiences between organizations such as hospitals, schools, and research centers.
According to the keywords analysis, 65% of publications were about children, suggesting that the most of research was about children with dyslexia. At present, MRI technology is mostly used to explore the brain function and mechanism of dyslexia, among which fMRI research is at the forefront. As can be seen from Figure 5 and Tables 4, 6, "developmental dyslexia, " "phonological awareness, " children, and fMRI are still the hotspots in dyslexia research. By comparing the keywords in papers published before and after 2017, we found that the keywords "literacy, " "rapid automatized naming (RAN), " "assessment, " "intervention, " "meta-analysis, " "Chinese, " "executive function, " "morphological awareness, " "decoding, " "dyscalculia, " "EEG, " "Eye tracking, " "rhythm, " "bilingualism, " and "functional connectivity" were increasingly attracting the attention of researchers and had become some new research hotspots in dyslexia research. With the rapid development of the Internet, more knowledge is mainly obtained through network resources, and the effect of dyslexia on "information seeking" behavior has gradually attracted the attention of dyslexia researchers (132,133). In addition, the emergence of a new keyword COVID-19 in the past 2 years also showed that during the COVID-19 epidemic, researchers began to study the impact of the epidemic on dyslexia research (115,(134)(135)(136)(137). As the international exchange of dyslexia research continues to grow, scientists are aware that differences in education-related legislation in different countries may lead to persistent differences  countries and had the biggest collaboration network in the dyslexia research. Currently, international cooperation is still insufficient in Asian and African countries. The advanced expertise and experience of developed countries can be shared with developing countries through international cooperation. To our knowledge, there is no cure for dyslexia, but early assessment and intervention will give the best outcome. And also, people with dyslexia can learn to read with structured literacy which helps to rewire their brains. This was confirmed by the topmost used author keywords "intervention, " "assessment, " and "literacy". This study provided an insight into the status of current dyslexia research. It can also provide useful information for relevant researchers to find potential collaborators. In addition, this study may help to increase public awareness and acceptance of dyslexia, disseminate knowledge of dyslexia to educators, policymakers, and especially parents of children with dyslexia.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

AUTHOR CONTRIBUTIONS
YWan and YC designed the study. WY is responsible for data collection. YC and XY analyzed the data. YWu analyzed, interpreted the data, and wrote the manuscript. All authors have read and agreed to the published version of the manuscript.