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Neural Mechanisms underlying the planning of sequential saccades
No full textSaccades are rapid eye movements that we continually make (about 2-3 times per second) to look around and scan our visual environment. Though we effortlessly execute saccadic eye movements all the time, they are not just reflexive movements; saccades have been shown to involve multifaceted cognitive control mechanisms. This property of saccades, combined with the fact that saccadic parameters are easily measurable, and that the neural circuitry for saccade generation is fairly well established, has established saccadic eye movements to be an excellent tool to study motor planning and decision-making. However, much of the work done on saccade planning has been limited to understanding the production of single saccades. Natural behavior entails making multiple saccadic movements in a sequence to achieve day-to-day tasks such as reading a book. How are sequential saccades planned? This question forms the broad theme of this thesis.
The neural correlates of sequential saccade planning were scouted for in the macaque frontal eye field (FEF), a prefrontal area containing neuronal populations that undertake saccadic decision-making. Visual-salience neurons of the FEF have been shown to encode targets for upcoming saccades and movement-related neurons of the FEF have been shown to control the time of saccade initiation, providing a good link between neural activity and behaviorally measured reaction times. However, much of the neural underpinnings of saccade programming in the FEF have been uncovered using tasks involving single, isolated saccades. Motivated by this, I explored the mechanisms by which FEF neurons contributed to the programming of saccade sequences for this thesis, using single-unit electrophysiological recordings from the FEF of two macaques as they performed a sequential saccade task.
Sequential saccade programming can, in principle, operate through two major modes: serial or parallel. Behavioral measures, like short inter-saccadic intervals, strongly
indicate that multiple saccade plans can proceed in parallel. However, direct neural evidence of parallel programming in the FEF neuronal population that strongly link to behavior, i.e. movement neurons, is lacking. First, I show that FEF movement-related activity can start ramping-up for the second saccade before the first saccade execution is complete, and much before visual feedback from the first saccade can reach FEF, thereby providing neural correlates of parallel programming of sequential saccades. Perceptual processing in the FEF has been shown to precede motor processing in visual search tasks, and consistent with that notion, FEF neurons with visual activity were also able to augment activity related to the second target whilst the first saccade plan was still underway.
After finding neural evidence of parallel programming, I characterized the limits of parallel programming. Numerous studies have shown that when two motor plans overlap closely, processing bottlenecks arise to inhibit the programming of the second plan, and is behaviorally manifested by the progressive lengthening of the second task reaction time, as the temporal gap between the two tasks decreases. This feature of increase in the second task latency has been observed in sequential saccade tasks as well. Neural correlates of processing bottlenecks were found in the responses of FEF movement neurons, wherein for the second saccade plan, the rate of the growth of activity was perturbed and the threshold of saccade initiation was increased, in a degree proportional to the level of concurrence of the two saccade plans. The locus of processing bottlenecks was found to be at the level of FEF movement-related neurons, whereas the activity of the visual neurons indicated that visual processing for perceptually simple tasks might constitute a pre-bottleneck stage. Evidence of activity perturbations was also found for the first saccade plan, supporting capacity-sharing theories of processing bottlenecks, as opposed to single-channel bottleneck theories which postulate that only the second plan is gated by inhibitory control while the first can pass unabated. Together, the results suggest that processing bottlenecks in sequential saccades originate in the partitioning of the brain’s limited processing ‘capacity’ by simultaneously active motor plans, due to which, inhibitory control is applied on both the first and second saccade plans, to prevent straining of the aforesaid capacity.
Finally, I have examined peripheral signatures of sequential saccade planning. Recent studies using single saccade paradigms have shown that the function of FEF as a center of cognitive control is not limited to saccade eye movements, but can be generalized to the control of eye-head gaze shifts. Rapid presaccadic recruitment of dorsal neck muscle activity has been shown to occur after FEF both with single-unit microstimulation and trans-cranial magnetic stimulation, even under head-restrained conditions where no overt head movement is being brought about by the neck muscles. To investigate whether such
presaccadic recruitment occurs during sequential saccade planning or is gated out by inhibitory control, I recorded electromyographic (EMG) activity of motor units of the dorsal neck muscle as macaques performed the same sequential saccade task used for neural recordings. Neck muscle EMG showed leakage of FEF planning signals even for sequential saccades: peripheral correlates of parallel programming and processing bottlenecks were observed, with the activity mirroring that of FEF movement neurons. The correspondence of the results between the FEF and periphery suggest that a tight link exists between the eye and head systems, validating the hypothesis of a common gaze command originating in the FEF. The rapid recruitment of neck muscle activity observed for the second saccade before the completion of the first, also suggested that inhibitory control gates like basal ganglia do not preferentially intercept sequential saccade signals in the FEF-neck muscle circuit.
In summary, the results in this thesis provide direct neurophysiological evidence of behaviorally established features of sequential saccade planning such as parallel programming and processing bottlenecks. The fact that signatures of FEF movement responses can be captured at the level of the dorsal neck muscle suggests that the functional channel connecting FEF and the motor periphery is preserved even during sequential saccade planning and allows central responses to rapidly pass downstream by default, and perhaps prepare for an anticipated head movement in conjunction with the upcoming saccade. In cases where no head movement is elicited or where the head is restrained, inhibitory control mechanisms might come into play later and prevent supra-threshold rise of neck muscle activity
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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