1,721,038 research outputs found
Transcranial magnetic stimulation patterns in heterogeneous brain tissue: focality, reproducibility and true sham stimulation
No full textTranscranial magnetic stimulation (TMS) has been gaining status as an innovative research and possibly therapeutic tool for non-invasive stimulation of brain tissue. However, optimal values of key stimulation parameters such as coil size, shape and orientation and current pulse characteristics remain largely undetermined. Further, the influence of brain size and shape as well as of the strong heterogeneity of brain tissue on stimulation efficacy is unclear. In order to calculate the current density distribution induced in brain tissue by TMS, a complete set of MRI images of the human brain is discretised over a cubic mesh, constructing a spatial map for tissue electrical conductivity. The magnetic induction field is calculated as a function of coil geometry and position as well as incoming current pulse characteristics, after which Maxwell’s equations in integral form are discretised and solved over the mesh by a successive overrelaxation procedure. Findings are validated by recording motor evoked potentials from the right abductor pollicis brevis muscle from a healthy subject in a stereotaxic framework. The inhomogeneous electrical characteristics of brain tissue are seen to significantly influence stimulation patterns, as a symmetrical primary electric field results in a highly asymmetrical current density distribution. Also, several commonly used sham stimulation configurations elicit conductive patterns which achieve up to 40% of the strength of real stimulation. Further, variations in coil position of the order of a 7 degree tilt, which are expected to occur in non-stereotaxic stimulation, can alter the stimulation intensity by up to 38%. Knowledge of coil specifications alone is clearly not sufficient to control stimulation conditions. In accordance with our findings, several clinical studies observe measurable effects during sham stimulation, and the sensitivity of stimulation intensity to tiny coil rotations afford a partial explanation for the poor reproducibility and partial disagreements observed across TMS studies. Our findings bear direct relevance to any application of TMS, both investigative and therapeutic
A high resolution simulation study of the time-dependent effects of tissue anisotropy in Transcranial Magnetic Stimulation
No full textA 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
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
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
Quantifying uncertainty in transcranial magnetic stimulation: a high resolution simulation study in ICBM space
No full textTranscranial Magnetic Stimulation offers enormous potential for noninvasive brain stimulation. While it is known that brain tissue significantly "reshapes" induced field and charge distributions, most modeling investigations to-date have focused on single-subject data with limited generality. Further, the effects of the significant uncertainties which exist in the simulation (i.e. brain conductivity distributions) and stimulation (e.g. coil positioning and orientations) setup have not been quantified. In this study, we construct a high-resolution anisotropic head model in standard ICBM space, which can be used as a population-representative standard for bioelectromagnetic simulations. Further, we employ Monte-Carlo simulations in order to quantify how uncertainties in conductivity values propagate all the way to induced field and currents, demonstrating significant, regionally dependent dispersions in values which are commonly assumed "ground truth". This framework can be leveraged in order to quantify the effect of any type of uncertainty in noninvasive brain stimulation and bears relevance in all applications of TMS, both investigative and therapeutic
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
Haase (Wolfgang) éd Aufstieg und Niedergang der römischen Welt. T. II: Principat
No full textSchmidt Francis. Haase (Wolfgang) éd Aufstieg und Niedergang der römischen Welt. T. II: Principat. In: Archives de sciences sociales des religions, n°55/2, 1983. pp. 236-237
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