A bibliometric analysis of global publication trends on rTMS and aphasia

Abstract

Background: 

Aphasia is one of the most devastating cognitive disorders caused by brain injury and seriously hinders patients’ rehabilitation and quality of life. Repetitive transcranial magnetic stimulation involves the repeated application of extracranial pulsed magnetic fields to the local central nervous system to alter the membrane potential of cortical nerve cells, generating induced currents that affect brain metabolism and electrical activity. As one of the most popular noninvasive brain stimulation techniques, it has been used to treat aphasia. However, only a few bibliometric studies have examined the research direction and main findings in the field.

Methods: 

To obtain an in-depth understanding of the research status and trend in this area, a bibliometric analysis based on the Web of Science database was conducted. VOSviewer (Leiden University, Leiden, Netherlands) and Microsoft Excel (Microsoft, Redmond, USA) were used to extract bibliometric information. Analysis of global distribution was conducted using the webpage mapping implement GunnMap2 (http://lert.co.nz/map/).

Results: 

Publications in this field were retrieved from the Web of Science Core Collection database, and 189 articles met the final inclusion criteria. The most influential authors, institutions, journals, and countries were Ralph MA from the University of Manchester, Harvard University, Neuropsychologia, and the USA, respectively.

Conclusion: 

This study revealed publication patterns and emerging trends in the literature, providing a detailed and objective overview of the current state of research on repetitive transcranial magnetic stimulation for the treatment of aphasia. This information will be of great benefit to anyone seeking information about this field and can serve as a reference guide for researchers aiming to conduct further research.

1. Introduction

Aphasia refers to a language disorder caused by damage to specific language centers and related areas in the brain,^[1] namely, impairment in 6 basic aspects of spontaneous communication: listening, understanding, retelling, naming, reading, and writing.^[2,3] Since aphasia has a negative impact^[4] on patients’ daily activities and quality of life, impaired understanding of speech is to aggravate depression or mood disorders,^[5,6] leading to social isolation and unemployment, which further reduce mental health^[7,8] and impose a heavy burden^[6,9,10] on caregivers and the health care system.

Repetitive transcranial magnetic stimulation (rTMS) is a noninvasive brain stimulation technique that can regulate the cortical excitability of patients with aphasia; rTMS combined with speech-language training can facilitate language recovery.^[11] Low- or high-frequency stimulation of targeted brain areas can be effective.^[12,13] rTMS differs from single-pulse TMS in that cortical activity remains altered after stimulation, making rTMS a promising approach for the treatment of neurological and psychiatric disorders.^[14] The advantage of rTMS is that its therapeutic effects can last for months after treatment. rTMS is mainly used to treat stroke, depression, and Parkinson disease.^[15–17] Bai et al^[18] reported that after treatment with low-frequency rTMS (twice a day) combined with speech-language training, patients with aphasia after stroke exhibited improvements in not only language function but also the expression of brain-derived neurotrophic factors, indicating that rTMS could promote nerve repair and protect brain tissue.

Bibliometrics, a quantitative method used to analyze the literature and evaluate trends in research activities,^[19,20] has become one of the best assessment tools.^[21] Through qualitative and quantitative analysis, research contributions are presented objectively according to country, institution, journal, and author of origin in a given research field, and predictions are made regarding research trends or hot spots.^[22] While it is difficult to assess the true importance of an article, citation frequency is used as a simple quantitative metric. In the context of limited healthcare resources, bibliometric research is helpful to guide researchers and institutions in expanding or narrowing the scope of their research.^[23] In addition, bibliometric analysis plays an important role in the formulation of policies and clinical guidelines for various diseases.

Although publications present a general overview of rTMS and aphasia,^[24–26] no bibliometric analysis of rTMS treatment for aphasia has been conducted, and research hotspots have yet to be predicted. Based on the literature from 1900 to 2021, a bibliometric analysis was conducted on rTMS and aphasia research in this study to determine the publishing patterns and new trends and to gain insights to guide future research and applications.

2. Research methods

2.1. Design

Articles that had been published and presented sufficient data for statistical and visualization bibliometric analysis were included in this study. Descriptive analysis and bibliometric maps were used to describe the bibliometric information (country, institute, author, publication year, journal, title, abstract, keywords, and article type).

2.2. Search strategy and data collection

A comprehensive literature search was conducted in the Web of Science (WoS) Core Collection database, which contains data from SCIE and SSCI, on August 10, 2021. To ensure the accuracy of the data, we included synonyms and related terms in the keyword search. The following search string was used: (TS = (repetitive transcranial magnetic stimulation or rTMS) and TS = (Aphasia* or language disorder* or muteness or speechlessness*). We also limited the retrieved articles to those published before August 1, 2021.

The inclusion criteria of this study were as follows: retrieved from the Core Collection database of the WoS, published before August 1, 2021, and was an article. Retrieved studies were excluded based on the following criteria: different article types (i.e., conference abstracts, action research, book reviews, news, or articles unpublished or subject to correction), or duplicate publications; only newly published studies were included.

2.3. Ethical considerations

Since there were no human participants or animal subjects in this study, ethical approval was not needed.

2.4. Literature data analysis

VOSviewer (Leiden University, Leiden, Netherlands) was developed based on the Java programming language and has a strong visualization ability. In this study, VOSviewer (version 1.6.16) and Microsoft Excel (Microsoft, Redmond, WA) were used to extract bibliometric information, such as country, institute, author, publication year, journal, title, abstract, keywords, and article type. This study relied on VOSviewer to extract and generate bibliometric maps to visualize, calculate and analyze the co-occurrence network of terms, intercountry collaboration axes, and co-citation and bibliographic coupling of inter-author network relationships extracted from publication titles and abstracts. In addition, a world map depicting the distribution of publications was produced using GunnMap 2 (http://lert.co.nz/map/).

2.5. Validity and reliability

All citation information was exported from the ISI WoS database in.txt format and imported into VOSviewer and Microsoft Excel. The title and abstract of each article were rechecked to exclude irrelevant articles by 2 reviewers in an independent manner. If the 2 researchers disagreed, a third researcher settled the debate.

3. Result

3.1. Characterization analysis

In summary, 189 articles published in the WoS Core Collection database before August 10, 2021, met the inclusion criteria, the first of which was published in 1999 (Fig. 1). Reports not published in English or as an article were excluded in our search strategy, as English publications and an article document type are required by most top global journals.

Figure 1.: Flowchart of the literature search.

3.1.1. Trajectories of the numbers of publications and citations.

The numbers of publications and citations were extracted from the WoS Core Collection database by year, and their evolution is shown in Figure 2. The number of publications per year increased steadily until 2012 and then dropped to approximately 10 publications per year by the end of 2020. Almost half of the 189 articles were published between 2009 and 2014. In contrast, the number of citations increased from 2011 to 2020. Based on this trend, research in this field has been a concern for researchers.

Figure 2.: Trend of the numbers of publications and citations from 1999 to 2021.

3.1.2. Publication distribution across countries.

As shown in Figure 3A, 189 articles were published in 32 countries. The top 10 most productive countries, based on the number of publications, are shown in Figure 3B; the top 5 are the USA (n = 58, 30.69%), Germany (n = 29, 15.34%), England (n = 24, 12.70%), Canada (n = 23, 12.17%), and Italy (n = 23, 12.17%), followed by South Korea (n = 14, 7.41%), Spain (n = 13, 6.88%), Australia (n = 12, 6.35%), China (n = 12, 6.35%), and Japan (n = 8, 4.23%).

Figure 3.: Global research on the use of rTMS for aphasia. (A) Distribution of the total number of publications by country, and (B) the top 10 most productive countries according to the number of publications. (C) Visualization of collaborative relationships. rTMS = repetitive transcranial magnetic stimulation.

Visualization of the collaboration network (Fig. 3C) was generated by VOSviewer. The number of links in each country reflects international collaborations. The higher the number of lines the greater the activity. Additionally, the thickness of the line between the 2 countries indicates their collaborative strength. The thicker the line is, the stronger the cooperative relationship.

Unsurprisingly, the USA, Italy, England, Canada, and Germany had the most links, similar to the ranking of country contributions. The USA had the highest collaborative strength, which was far greater than the others.

3.1.3. Publication distribution across journals.

A total of 96 journals published articles in the field of rTMS and aphasia. The top 5 journals (ranked by the number of publications) are shown in Table 1, together with journal information such as journal impact factor (JIF), JIF Quartile, WoS category, total articles, total citations, and citations per article (CA). The most popular journal was Neuropsychologia (n = 10, 5.3%, Q2), followed by Brain and Language (n = 9, 4.8%, Q1). Although only 21.3% of articles were published in the top 5 journals, the remaining journals published 1 or 2 articles each. CA indicates the contribution impact in this field; the higher the CA is, the greater the contribution. Although some journals did not publish similar articles, they played a significant role in this field, such as Cerebral Cortex (CA = 122.8). Among the WoS categories according to journal citation reports, neuroscience was the most popular.

Table 1 - Quantitative measurement of journal publishing research on rTMS for aphasis.

Rank	Journal title	JIF	JIF quartile	Categories	Article (%)	Total citations	Citation per article
1	NEUROPSYCHOLOGIA	3.130	Q2	Behavioural Science, Neuroscience	10 (5.3%)	356	35.6
2	BRAIN AND LANGUAGE	2.384	Q1	Linguistics	9 (4.8%)	1020	113.3
3	RESTORATIVE NEUROLOGY AND NEUROSCIENCE	2.404	Q4	Neuroscience	8 (4.3%)	219	27.4
4	STROKE	7.941	Q1	Clinical Neurology, Peripheral Vascular Disease	7 (3.7)	578	82.6
5	CEREBRAL CORTEX	5.357	Q1	Neuroscience	6 (3.2%)	737	122.8

JIF = journal impact factor, rTMS = repetitive transcranial magnetic stimulation.

3.1.4. Publication distribution across institutions.

The top 10 most prolific institutions are listed in Table 2; the USA was the most prolific country and had 4 of the most prolific institutions. Although Harvard University published the most articles (n = 13, 6.9%), the University of Manchester had the highest CA score (24.4). The institutions in Table 2 are in 6 countries, similar to the county contribution ranking. Notably, the University of Queensland (n = 9, 4.8%) in Australia and Jikei University (n = 7, 3.7%) in Japan ranked 6 and 9 out of 10, respectively, although Australia and Japan did not rank in the top 5 most productive countries.

Table 2 - Quantitative measurement of top 10 institutions on rTMS for aphasis.

Rank	Institution	Country	Article (%)	Citations	Citations per article
1	Harvard University	USA	13 (6.9%)	1096	13.4
1	University of Manchester	England	13 (6.9%)	1470	24.4
3	McGill University	Canada	12 (6.4%)	394	9.7
4	Boston University	USA	11 (5.9%)	804	10.1
5	University of Pennsylvania	USA	10 (5.3%)	306	12.4
6	Max Planck Institution	German	9 (4.8%)	644	7.9
6	University of Queensland	Australia	9 (4.8%)	272	5.2
8	Beth Israel Deaconess Medical Canter	USA	8 (4.3%)	870	8.5
9	JiKei University	Japan	7 (3.7%)	157	4.0
9	University of Cologne	German	7 (3.7%)	723	5.0
9	University of York	UK	7 (3.7%)	839	13.4

rTMS = repetitive transcranial magnetic stimulation.

3.1.5. Publication distribution across WoS categories.

The top 10 article categories are listed in Table 3. Nearly half of the 189 publications were classified as “Neuroscience” (n = 92, 48.4%), followed by “Clinical Neurology” (n = 68, 35.8%), “Psychology Experimental” (n = 31, 16.3%), “Rehabilitation” (n = 27, 14.2%), and “Linguistics” (n = 20, 10.5%).

Table 3 - Quantitative measurement of top 10 categories on rTMS for aphasis.

Rank	Web of Science category	Record	Rank	Web of Science category	Record
1	NEUROSCIENCES	92 (48.4%)	6	AUDIOLOGY SPEECH LANGUAGE PATHOLOGY	17 (8.9%)
2	CLINICAL NEUROLOGY	68 (35.8%)	6	BEHAVIORAL SCIENCES	17 (8.9%)
3	PSYCHOLOGY EXPERIMENTAL	31 (16.3%)	8	PSYCHIATRY	15 (7.9%)
4	REHABILITATION	27 (14.2%)	9	PSYCHOLOGY	13 (6.8%)
5	LINGUISTICS	20 (10.5%)	10	PERIPHERAL VASCULAR DISEASE	11 (5.8%)

rTMS = repetitive transcranial magnetic stimulation.

3.2. Analysis of keyword co-occurrence

Keywords were extracted from the titles and abstracts of the 189 publications and analyzed using VOSviewer. Keywords can provide information on how authors conceptualize their studies and have been proven to be vital in scientific monitoring. The more important the terms are, the larger the circles in Figure 4. Lines between terms indicate associations and thicker lines represent a stronger link between the 2 terms. A total of 67 terms were selected using the least coincidence set 5 times. The top 5 keywords other than rTMS and aphasia were “stroke” (63 times), “rehabilitation” (62 times) “language” (60 times), “fMRI” (33 times), and “brain” (31 times). According to the co-occurrence visualization, the terms were classified into 5 clusters of different colors. The research topics mainly focused on the mechanism by which rTMS improves aphasia after stroke.

Figure 4.: Visualization of keyword co-occurrence.

3.3. Top 10 most cited articles

The 10 most cited papers in the field of aphasia and rTMS are shown in Table 4. The 10 articles were published in 7 journals, spanning from 2001 to 2011. The first of these papers was written by Langhorne et al (2011); it was entitled “Stroke Care to Stroke Rehabilitation” and published in the Lancet and has 990 citations with an average of 99 citations per year. The Journal of Cognitive Neuroscience, Cerebral Cortex, and Brain and Language each published 2 of the top 10 articles. Nine of the 10 most cited articles received >10 citations per year, and 5 of them received >20 citations per year.

Table 4 - Top 10 most cited publications.

Rank	Article title	Author (year)	Source title	Citations	Citation/Year
1	Stroke care 2 stroke rehabilitation	Langhorne et al (2011)	LANCET	990	99.0
2	Semantic processing in the left inferior prefrontal cortex: a combined functional magnetic resonance imaging and transcranial magnetic stimulation study	Devlin et al (2003)	JOURNAL OF COGNITIVE NEUROSCIENCE	386	21.4
3	Improved picture naming in chronic aphasia after TMS to part of right Broca area: an open-protocol study	Naeser et al (2005)	BRAIN AND LANGUAGE	373	23.3
4	Transcranial magnetic stimulation of left temporoparietal cortex and medication-resistant auditory hallucinations	Hoffman et al (2003)	ARCHIVES OF GENERAL PSYCHIATRY	337	18.7
5	The Neural Organization of Semantic Control: TMS evidence for a distributed network in left inferior frontal and posterior middle temporal gyrus	Whitney et al (2011)	CEREBRAL CORTEX	264	26.4
6	The ventral and inferolateral aspects of the anterior temporal lobe are crucial in semantic memory: evidence from a novel direct comparison of distortion-corrected fMRI, rTMS, and semantic dementia	Binney et al (2010)	CEREBRAL CORTEX	255	23.2
7	Anterior temporal lobes mediate semantic representation: mimicking semantic dementia by using rTMS in normal participants	Pobric et al (2007)	PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA	238	17.0
8	Role of the contralateral inferior frontal gyrus in recovery of language function in poststroke aphasia – a combined repetitive transcranial magnetic stimulation and positron emission tomography study	Winhuisen et al (2006)	STROKE	214	14.3
9	A proposed regional hierarchy in recovery of poststroke aphasia	Heiss, Hiel (2005)	BRAIN AND LANGUAGE	214	13.4
10	Grammatical distinctions in the left frontal cortex	Shapiro et al (2001)	JOURNAL OF COGNITIVE NEUROSCIENCE	141	7.1

rTMS = repetitive transcranial magnetic stimulation.

3.4. Top 10 contributing authors

An author’s contribution is evaluated by the volume of publications. In Table 5, the top 10 contributing authors are ranked by their research outputs; ties were resolved by citation scores. Matthew AL (n = 11) from the University of Manchester and Alvaro PL (n = 11) from Harvard University were the 2 top contributing authors, followed by Alexander Thiel (n = 8) and Margaret AN (n = 8). Notably, Elizabeth Jefferies (n = 7) from the University of York ranked only 5 out of 10 but had the highest citation frequency (n = 137.4).

Table 5 - Top 10 authors with research outputs.

Rank	Author	Institution (country)	Article	Citations score	Citations per article
1	Matthew A Lambon Ralph	University of Manchester (UK)	11	1348	122.5
1	Alvaro Pascual-Leone	Harvard University (USA)	11	447	40.6
3	Alexander Thiel	McGill University (Canada)	8	371	46.4
3	Margaret A Naeser	Boston University (USA)	8	367	45.9
5	Elizabeth Jefferies	University of York (UK)	7	962	137.4
5	Roy H Hamilton	University of Pennsylvania (USA)	7	172	24.6
5	Wolf-Dieter Heiss	Max Planck Institution (German)	7	165	23.6
5	Masahiro Abo	Jikei University (Japan)	7	157	22.4
9	Paula I Martin	Boston University (USA)	6	361	60.2
9	Alexander K Hartmann	University Oldenburg (German)	6	253	42.2
9	Caroline H S Barwood	University of Queensland (Australia)	6	155	25.8
9	Bruce E Murdoch	University of Queensland (Australia)	6	155	25.8
9	Wataru Kakuda	Stanford University (USA)	6	149	24.8

4. Discussion

4.1. Overview

Aphasia is a medical challenge in the 21st century, and many therapies have been developed for patients; among these, rTMS has been extensively studied. This study analyzed the research trends of rTMS and aphasia. Although this study included only 189 publications, the trajectories of the numbers of publications and citations show that this field is increasingly recognized and has a promising future.

Related articles were published in 32 countries; the USA ranked first among contributing countries with 58 articles, accounting for almost one-third of all included publications. In addition, among the top 10 institutions, 4 were in the USA. Moreover, half of the top 10 contributing authors work in the USA, and Harvard University is one of the greatest contributing institutions worldwide in this field. Therefore, the USA is the leading country in terms of impact on this field. This may be closely related to the fact that the USA was the first to publish research in this field. Most countries that have published research in this field are developed countries, with some developing countries with better economic foundations. These countries may be more concerned about the quality of life of patients and have a close alignment of national health policies and strategies.

The included publications in the field of rTMS and aphasia were published in 96 journals, but only 5 journals published >5 articles. The IF scores of the top 5 journals with the most articles were between 2.404 and 7.941, with Stroke (IF = 7.941) having the highest IF score. Although Neuropsychologia had the largest number of published articles, its JIF (3.130) and the total number of citations (356) were not the highest. Among the top 5 journals in terms of publication volume, 3 had a JIF value in Q1. Additionally, as shown in Table 1, the 2 indices of total citations and CA were positively correlated with the JIF quartile. In summary, the number of rTMS- and aphasia-related articles published in journals with high IF scores are low, and there is great potential for exploration in this area. We look forward to more high-quality research published in the future to provide a reference for the majority of scientific researchers.

The fields of rTMS and aphasia have been studied for several decades. Epstein CM et al^[27] published the first study on rTMS and aphasia in Clinical Neurophysiology in 1999. This study determined the anatomical and physiological locations of rTMS-induced speech pauses and examined the relationship between speech pauses and language function. A meta-analysis demonstrated that rTMS has significant positive effects on aphasia,^[28] but the underlying brain mechanism is still unclear. It has been reported that rTMS facilitated the recovery of the affected Broca area in patients with chronic poststroke aphasia.^[29] Baltazar et al^[30] discovered that rTMS decreased neuronal loss and neurodegeneration caused by ischemia and increased the number and strength of synapses by modulating astrocytes. In 2001, Caramazza et al^[31] used rTMS to suppress the activity of the left prefrontal cortex and evaluate its role in the production of nouns and verbs. Over the past 2 decades, many highly cited articles have been published.^[32,33] Martin PI et al^[32] reported that patients exhibited improvements in aphasia after rTMS treatment. Although clinical trials have demonstrated its effectiveness, its specific mechanism of action remains unclear.

The most cited articles illustrate the typical findings and directions in this field. The most cited publication was Langhorne et al^[34] in the Lancet in 2011. This study has been cited 990 times; thus, it can be deduced that this study has important implications for research in this field. This study indicated that rTMS is an effective strategy for the treatment of aphasia after stroke and has broad application prospects. Devlin et al^[35] used transcranial magnetic stimulation to temporarily interfere with neural information processing in the anterior part of the left inferior prefrontal cortex, reporting that both repetitive and single-pulse TMS significantly slowed the participants’ responses in the semantic task. In a clinical trial conducted by Naeser et al,^[36] 1-Hz rTMS was applied to 4 aphasia patients at 5 to 11 years poststroke, and a significant improvement was observed in picture naming 2 months after rTMS treatment. In Whitney’s research,^[37] semantic control was found to be supported by an extended network involving the prefrontal cortex and posterior temporal regions. Binney et al^[38] indicated that rTMS significantly improved the functioning of key regions for semantic representation in the anterior temporal lobe, including the anterior inferior temporal gyrus, the anterior fusiform gyrus, and the anterior superior temporal sulcus. Highly cited articles are an important benchmark for research in this field, and their research quality is high, which provides important insight for the majority of researchers.

4.2. Limitations and suggestions for future research

Our study still has some limitations. First, the literature on rTMS for aphasia was retrieved only from the WoS database. Although the quality of the articles was effectively guaranteed, this selection still limits the comprehensiveness of the research. Second, this study was limited to articles published in English, which is also an important limitation on the comprehensiveness of the included articles. Third, due to the time limitation of the initial inclusion of articles in this study, articles published after August 2021 were not included in the study results. In addition, the retrieval choice led to inevitable tradeoffs. Regarding research in this field, more in-depth investigation of indicators is still needed.

5. Conclusion

In conclusion, this study reports publication patterns and emerging trajectories of research on rTMS and aphasia. The most influential authors, institutions, journals, and countries were Ralph MA from the University of Manchester, Harvard University, Neuropsychologia, and the USA, respectively. This study revealed emerging trends of rTMS and aphasia research according to related publications.

Author contributions

Conceptualization: Xuemiao Huang, Yuan Wang, Shuang Zhang, Tingyu Zhang.

Data curation: Taiwei Wang, Xuemiao Huang, Tingyu Zhang.

Formal analysis: Taiwei Wang, Xuemiao Huang, Junjie Jiang.

Funding acquisition: Taiwei Wang.

Investigation: Taiwei Wang, Xuemiao Huang, Junjie Jiang.

Methodology: Taiwei Wang, Xuemiao Huang, Shuang Zhang, Junjie Jiang.

Project administration: Junjie Jiang, Lijing Zhao.

Resources: Xuemiao Huang, Xiaochen Fu, Lijing Zhao.

Software: Xuemiao Huang, Yuan Wang, Tingyu Zhang.

Supervision: Xuemiao Huang, Yuan Wang, Xiaochen Fu.

Validation: Taiwei Wang, Xiaochen Fu.

Visualization: Lijing Zhao.Writing – original draft: Taiwei Wang, Xuemiao Huang.

Writing – review & editing: Taiwei Wang, Yuan Wang, Shuang Zhang, Tingyu Zhang, Junjie Jiang, Lijing Zhao.