1,720,968 research outputs found
Eye movement recording and nonlinear dynamics analysis - The case of saccades
Full text linkEvidence of a chaotic behavioral trend in eye movement dynamics was examined in the case of a saccadic temporal series collected from a healthy human subject. Saccades are highvelocity eye movements of very short duration, their recording being relatively accessible, so that the resulting data series could be studied computationally for understanding the neural processing in a motor system. The aim of this study was to assess the complexity degree in the eye movement dynamics. To do this we analyzed the saccadic temporal series recorded with an infrared camera eye tracker from a healthy human subject in a special experimental arrangement which provides continuous records of eye position, both saccades (eye shifting movements) and fixations (focusing over regions of interest, with rapid, small fluctuations). The semi-quantitative approach used in this paper in studying the eye functioning from the viewpoint of non-linear dynamics was accomplished by some computational tests (power spectrum, portrait in the state space and its fractal dimension, Hurst exponent and largest Lyapunov exponent) derived from chaos theory. A high complexity dynamical trend was found. Lyapunov largest exponent test suggested bi-stability of cellular membrane resting potential during saccadic experiment
What clinical disorders tell us about the neural control of saccadic eye movements
No full textSaccades are rapid eye movements that redirect the fovea from one object to another. A great deal has been learned about the anatomy and physiology of saccades, making them an ideal system for studying the neural control of movement. Basic research on normal eye movements has greatly increased our understanding of saccadic performance, anatomy and physiology, and led to a large number of control system models. These models simulate normal saccades well, but are challenged by clinical disorders because they often do not incorporate the specific anatomical and physiological substrates needed to model clinically important abnormalities. Historically, studies of saccadic abnormalities in patients have played a critical role in understanding the neural control of saccades because they provide information that complements basic research and thus restricts hypotheses to those that are biologically plausible. This review presents four examples of clinical disorders (slow saccades, interrupted saccades, high-frequency saccadic oscillations and macrosaccadic oscillations) that have provided insights into the neurobiology of saccades, have driven the development of new models, and have suggested an explanation or treatment for these disorders. We raise general questions for both scientists and clinicians that will assist in their efforts to understand the neural control of movement, improve diagnostic criteria and develop new treatments
Novel molecular mechanism of saccadic oscillations.
No full textProgram No. 819.15/Y21 2006 Abstract Viewer/Itinerary Planner, online
Ocular oscillations generated by coupling of brainstem excitatory and inhibitory saccadic burst neurons
No full textAbstract The human saccadic system is potentially
unstable and may oscillate if the burst neurons, which
generate saccades, are not inhibited by omnipause
neurons. A previous study showed that combined saccade
vergence movements can evoke oscillations in normal
subjects. We set out to determine: 1) whether similar
oscillations can be recorded during other paradigms
associated with inhibition of omnipause neurons; 2)
whether lesions of the fastigial nuclei disrupt such
oscillations; and 3) whether such oscillations can be
reproduced using a model based on the coupling of
excitatory and inhibitory burst neurons. We recorded
saccadic oscillations during vergence movements, combined
saccade-vergence movements, vertical saccades,
pure vergence and blinks in three normal subjects, and in a
patient with saccadic hypermetria due to a surgical lesion
affecting both fastigial nuclei. During combined saccadevergence,
normal subjects and the cerebellar patient
developed small-amplitude (0.1–0.5°), high-frequency
(27–35 Hz), conjugate horizontal saccadic oscillations.
Oscillations of a similar amplitude and frequency occurred
during blinks, pure vergence and vertical saccades. One
normal subject could generate saccadic oscillations
voluntarily (~0.7° amplitude, 25 Hz) during sustained
convergence. Previous models proposed that high-frequency
eye oscillations produced by the saccadic system
(saccadic oscillations), occur because of a delay in a
negative feedback loop around high-gain, excitatory burst
neurons in the brainstem. The feedback included the
cerebellar fastigial nuclei. We propose another model that
accounts for saccadic oscillations based on 1) coupling of
excitatory and inhibitory burst neurons in the brainstem
and 2) the hypothesis that burst neurons show postinhibitory
rebound discharge. When omnipause neurons
are inhibited (as during saccades, saccade-vergence movements
and blinks), this new model simulates oscillations
with amplitudes and frequencies comparable to those in
normal human subjects. The finding of saccadic oscillations
in the cerebellar patient is compatible with the new
model but not with the recent models including the
fastigial nuclei in the classic negative-feedback loop
model. Our model proposes a novel mechanism for
generating oscillations in the oculomotor system and
perhaps in other motor systems too
Dynamical complexity analysis of saccadic eye movements in two different psychological conditions
Full text linkSaccadic eye movements of a normal subject were assessed through semi-quantitative analysis algorithms based on linear and non-linear test application in order to highlight the dynamics type characterizing saccadic neural system behavior. These movements were recorded during a simple visually-guided saccade test and one with a cognitive load involving button pressing to show a decision. Following the application of specific computational tests, chaotic dynamical trend dominancy was mostly revealed with some differences between the two saccade recording conditions: auto-correlation time was increased from 170 to 240 by cognitive task superposition and the Hurst exponent was enhanced from 0.52 to 0.76, denoting more persistence in the dynamics of saccadic system during increased neural activity related to cognitive task
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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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