1,721,317 research outputs found
Transcranial magnetic stimulation in heterogeneous brain tissue: clinical impact on focality, reproducibility and true sham stimulation
No full textBACKGROUND: Transcranial magnetic stimulation (TMS) is an attractive research and possibly therapeutic tool for non-invasive central nervous system stimulation. However, relatively little is known about the direction, magnitude and distribution of induced electric field and current flows in tissue, and optimal setup characteristics as well as appropriate sham stimulation conditions remain largely undetermined, hampering reproducibility. METHODS: We reconstruct the conductive phenomena induced by TMS by implementing digitized coil geometry and realistic stimulator parameters and solving the electromagnetic problem over an MRI-based, realistic head model of 1mm resolution. Findings are validated by recording motor evoked potentials from the right abductor pollicis brevis muscle from healthy subjects stimulated in a stereotaxic framework. RESULTS: Several commonly used sham stimulation configurations elicit conductive patterns which achieve up to 40% of the strength of real stimulation. Also, variations in coil position of the order of a 7 degrees tilt, which are expected to occur in non-stereotaxic stimulation, can alter the stimulation intensity by up to 25%. CONCLUSIONS: In accordance with our findings, several clinical studies observe measurable effects during sham stimulation or no significant difference between sham and real stimulation, and the sensitivity of stimulation intensity to tiny coil rotations affords a partial explanation for the poor reproducibility and partial disagreements observed across clinical TMS studies. Knowledge of coil and stimulator specifications alone is hence not sufficient to control stimulation conditions, and a stereotaxic setup coupled with individually adjusted field solvers appear essential in performing reliable TMS studies
A reconstruction of the conductive phenomena elicited by transcranial magnetic stimulation in heterogeneous brain tissue
No full textTranscranial magnetic stimulation is an attractive research and possibly therapeutic tool for non-invasive stimulation of brain tissue. However, relatively little is known about the direction, magnitude and distribution of induced fields and current flow in tissue, and optimal setup characteristics remain largely undetermined. Further, the profound influence of brain size and shape as well as of brain tissue irregularity on actual stimulation patterns is unclear. We model the conductive phenomena induced in brain tissue by TMS by solving the quasistatic problem over a realistic head model of 1mm resolution derived from anatomical MRI scans using a finite difference successive overrelaxation procedure. The magnetic field is calculated from digitized coil geometry and realistic stimulator parameters. Stimulation with a symmetrical primary electric field results in electric field and current density distributions which are highly asymmetrical both in magnitude and in direction (i.e. distributed, non-contiguous stimulation peaks, deviation of stimulated area from coil "hot spot", sudden jumps in stimulation intensity and non-zero current flow across tissue interfaces). Knowledge of coil and stimulator specifications alone is hence not sufficient to control stimulation conditions, and a stereotaxic setup coupled with an individually adjusted field solver appears essential in performing reliable TMS studies. Our results bear direct relevance to any application of TMS, both investigative and therapeutic
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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