1,720,966 research outputs found
Flexibility in neuronal computations and reward learning
No full textIn ever-changing environments, where animals encounter diverse stimuli, flexibility is crucial for survival. This flexibility encompasses the ability to adapt by manipulating inputs and modifying behavior. In this thesis, we demonstrate that 'flexibility' is a pervasive property found at multiple levels of brain organization. It is inherent to individual neurons and also manifests in neural networks, contributing to adaptive behavior.
We begin by investigating the substrate of computations, by exploring how neurons represent their inputs. Our findings reveal that auditory neurons possess composite receptive fields, a capability that enables them to process complex stimuli effectively. We then examine how neurons combine their inputs, revealing their capacity for flexible computations. In these two studies, we probe the neuron with natural stimuli, which are ethologically relevant to the animal in its environment.
The ability to process and operate over information flexibly should confer a survival advantage to the animal. Expanding our scope from single neurons to behaviour, and from instantaneous computations to learning from experiences over longer timescales, we delve into the neural bases of flexible behaviour within a reward-learning framework. We show that the control of neuromodulators over synaptic plasticity enhances reversal learning for agents navigating reward changes.
These investigations, which span single-neuron computations to complex behaviours, offer insight into the neural mechanisms for adaptive responses and representations for navigating a dynamic environment.Open Acces
Development of methods to mechanically stimulate hair cells
No full textThe cochlea is one of the most sensitive organs in the human body. It can detect
atomic-scale vibrations, amplify sound energies just above thermal noise, and
discriminate frequencies with a resolution of 0.2% of its frequency spectrum, up to
twenty thousand cycles per second. These astonishing capabilities of the cochlea
are the work of the hair cells, the sensory receptors of sound pressure waves.
Hair cells convert mechanical energy from sound to electrical energy, a process
known as mechanoelectrical transduction, or mechanotransduction. This process
causes the release of neurotransmitters that trigger afferent neurons to the brain.
Since the discovery of the mechanotransduction process in the 1970s, methods
for direct stimulation of hair cells to study mechanotransduction mechanisms have
been limited to about 10% of the human frequency spectrum. Thus, the lack of a
direct stimulation method at high frequencies has limited our understanding of key
processes about human hearing. First, how does the ear adapt and reset itself, a
process known as adaptation, in a matter of microseconds at high frequencies?
And how do hair cells in the high-frequency regime amplify the stimulus?
In this thesis, I describe two methods that were developed in parallel to mechanically
stimulate hair cells, and that have the potential to stimulate at frequencies up to
hundreds of kilohertz.
In the first method, we use photonic pressure from a laser pulse to deliver piconewton-
level force onto the hair bundle. The hair bundle is a collection of stereocilia
atop the hair cell. First, we developed the theoretical formalism using Maxwell’s
electromagnetic radiation concepts and analyzed the range of force needed to
deflect the hair bundles by 140 nm, the full physiological range of the hair bundle
movement in normal hearing. The idea is to place an optical fiber with a microlens
at the tip a few microns away from the hair bundle on its symmetric axis
and pulse the laser to apply photonic pressure. Because the hair cell is about 15
times smaller than the smallest commercially-available optical fiber (125 μm), we
chemically etched the optical fiber to match the required spot size and melted a
4 μm diameter ball lens at the tip of the optical fiber to focus the laser beam onto
the hair bundle.
In addition to developing the tapered fiber-optic (TFO) probe, we developed a
high-resolution photometric system to measure the nanometer scale deflections.
We illuminate the hair-cell preparation with a bright LED from the basal side of the
hair cell and image the bright spot of stereocilia on the apical side of the cell with a
microscope. This bright image is further magnified, externally to the microscope,
and projected on to a dual-photodiode. The deflection of the hair bundle changes
the light levels between the two sides of the dual-photodiode. This differential
output gives rise to the signal that is calibrated in volts per nanometer.
In order to understand the quality of the laser beam spot and the divergence
characteristics of the laser beam, we developed an optical setup. We characterize
the light output from the TFO by measuring the numerical aperture of the focusing
optic. This allows us to place the TFO at a known distance from the hair bundle
during experimentation.
Using the TFO we localize the photonic pressure to the apex of the hair bundle.
We stimulate both frog and rat hair cells and measured the mechanical response of
the hair bundle on the order of a few hundred microseconds. We found that, on
average, the beam diameter of the TFO is approximately 10 μm when the fiber-tip
is 10 μm away from the target. We show that the photonic force produced with 30
mW of laser power directed onto a hair bundle is capable of deflecting a hair bundle
of a frog approximately 50nm, and a hair bundle from the second apical turn of the
rat cochlea 40nm. This method opens up the exploration of fast-mechanotransduction
kinetics of hair cells that have been elusive for over forty years.
In the second method, I developed a micro-spring that has the stiffness to match
the mechanical impedance (500 μN/m - 2 mN/m) of the hair bundle and use the
spring to apply force (push) on the hair bundle using a piezo-electric actuator. For
this, I made a PDMS polymer spring that is 5 μm in length, end area of 1 μm-square, and
stiffness of 500 μN/m using lithography techniques. I used soft-material synthesis
techniques to derive a 1 μm thickness polymer layer, a novel thickness for PDMS
layers. I used electron-beam lithography and dry reactive ion etching (DRIE) to
pattern an accordion-type spring on the polymer layer. However, this project was
set aside before it was tested because we realized the photonic-force method is more
advantageous.Open Acces
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
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
- …
