1,952 research outputs found
Synchronization of electrically coupled resonate-and-fire neurons
Electrical coupling between neurons is broadly present across brain areas and
is typically assumed to synchronize network activity. However, intrinsic
properties of the coupled cells can complicate this simple picture. Many cell
types with strong electrical coupling have been shown to exhibit resonant
properties, and the subthreshold fluctuations arising from resonance are
transmitted through electrical synapses in addition to action potentials. Using
the theory of weakly coupled oscillators, we explore the effect of both
subthreshold and spike-mediated coupling on synchrony in small networks of
electrically coupled resonate-and-fire neurons, a hybrid neuron model with
linear subthreshold dynamics and discrete post-spike reset. We calculate the
phase response curve using an extension of the adjoint method that accounts for
the discontinuity in the dynamics. We find that both spikes and resonant
subthreshold fluctuations can jointly promote synchronization. The subthreshold
contribution is strongest when the voltage exhibits a significant post-spike
elevation in voltage, or plateau. Additionally, we show that the geometry of
trajectories approaching the spiking threshold causes a "reset-induced shear"
effect that can oppose synchrony in the presence of network asymmetry, despite
having no effect on the phase-locking of symmetrically coupled pairs
Reconciling memories: narrative as an approach to Aboriginal reconciliation in Australia
https://place.asburyseminary.edu/ecommonsatsdissertations/1709/thumbnail.jp
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Nitrosamine Formation in Carbon Capture
Carbon capture using amine scrubbing is an effective way to reduce CO2 emissions, but nitrosamines, a class of carcinogenic compounds, form from nitrogen oxides (NOx) in the process. Kinetic analysis of reactions involving nitrite and ethanol amine (MEA), piperazine (PZ), diethanol amine (DEA), methylethanol amine (MMEA), and methyldiethanol amine (MDEA) determined the reaction rate of each amine under various conditions. The reactions involving MEA, PZ, DEA, and MMEA were first order in nitrite, carbamate species, and hydronium ion. The tertiary amine, MDEA, did not fit the same rate law. A model accurately predicts reaction kinetics for unhindered primary and secondary amines. The rates of reaction revealed that primary amines react approximately 10 times slower than secondary amines under identical reaction conditions. Increased reactivity was noted in secondary amines which have more electron withdrawing groups attached to the amine. Two proposed mechanisms involve protonation of the carbamate species, nucleophilic attack of carbamic acid by nitrite, and formation of bicarbonate and a nitrosamine. The comparative kinetics can be applied to the analysis of the steady state concentration of nitrosamines in carbon capture, can help identify inhibitors for
this reaction, and can be applied to the use of blends to mitigate nitrosation.Chemical Engineerin
Modeling of the interaction of rigid wheels with dry granular media
We analyze the capabilities of various recently developed techniques, namely
Resistive Force Theory (RFT) and continuum plasticity implemented with the
Material Point Method (MPM), in capturing dynamics of wheel--dry granular media
interactions. We compare results to more conventionally accepted methods of
modeling wheel locomotion. While RFT is an empirical force model for
arbitrarily-shaped bodies moving through granular media, MPM-based continuum
modeling allows the simulation of full granular flow and stress fields. RFT
allows for rapid evaluation of interaction forces on arbitrary shaped intruders
based on a local surface stress formulation depending on depth, orientation,
and movement of surface elements. We perform forced-slip experiments for three
different wheel types and three different granular materials, and results are
compared with RFT, continuum modeling, and a traditional terramechanics
semi-empirical method. Results show that for the range of inputs considered,
RFT can be reliably used to predict rigid wheel granular media interactions
with accuracy exceeding that of traditional terramechanics methodology in
several circumstances. Results also indicate that plasticity-based continuum
modeling provides an accurate tool for wheel-soil interaction while providing
more information to study the physical processes giving rise to resistive
stresses in granular media
Population-scale organization of cerebellar granule neuron signaling during a visuomotor behavior.
Granule cells at the input layer of the cerebellum comprise over half the neurons in the human brain and are thought to be critical for learning. However, little is known about granule neuron signaling at the population scale during behavior. We used calcium imaging in awake zebrafish during optokinetic behavior to record transgenically identified granule neurons throughout a cerebellar population. A significant fraction of the population was responsive at any given time. In contrast to core precerebellar populations, granule neuron responses were relatively heterogeneous, with variation in the degree of rectification and the balance of positive versus negative changes in activity. Functional correlations were strongest for nearby cells, with weak spatial gradients in the degree of rectification and the average sign of response. These data open a new window upon cerebellar function and suggest granule layer signals represent elementary building blocks under-represented in core sensorimotor pathways, thereby enabling the construction of novel patterns of activity for learning
Empirical results on scheduling and dynamic backtracking
At the Honeywell Technology Center (HTC), we have been working on a scheduling problem related to commercial avionics. This application is large, complex, and hard to solve. To be a little more concrete: 'large' means almost 20,000 activities, 'complex' means several activity types, periodic behavior, and assorted types of temporal constraints, and 'hard to solve' means that we have been unable to eliminate backtracking through the use of search heuristics. At this point, we can generate solutions, where solutions exist, or report failure and sometimes why the system failed. To the best of our knowledge, this is among the largest and most complex scheduling problems to have been solved as a constraint satisfaction problem, at least that has appeared in the published literature. This abstract is a preliminary report on what we have done and how. In the next section, we present our approach to treating scheduling as a constraint satisfaction problem. The following sections present the application in more detail and describe how we solve scheduling problems in the application domain. The implemented system makes use of Ginsberg's Dynamic Backtracking algorithm, with some minor extensions to improve its utility for scheduling. We describe those extensions and the performance of the resulting system. The paper concludes with some general remarks, open questions and plans for future work
Tuning Curves, Neuronal Variability, and Sensory Coding
Tuning curves are widely used to characterize the responses of sensory neurons to external stimuli, but there is an ongoing debate as to their role in sensory processing. Commonly, it is assumed that a neuron's role is to encode the stimulus at the tuning curve peak, because high firing rates are the neuron's most distinct responses. In contrast, many theoretical and empirical studies have noted that nearby stimuli are most easily discriminated in high-slope regions of the tuning curve. Here, we demonstrate that both intuitions are correct, but that their relative importance depends on the experimental context and the level of variability in the neuronal response. Using three different information-based measures of encoding applied to experimentally measured sensory neurons, we show how the best-encoded stimulus can transition from high-slope to high-firing-rate regions of the tuning curve with increasing noise level. We further show that our results are consistent with recent experimental findings that correlate neuronal sensitivities with perception and behavior. This study illustrates the importance of the noise level in determining the encoding properties of sensory neurons and provides a unified framework for interpreting how the tuning curve and neuronal variability relate to the overall role of the neuron in sensory encoding
Lower Limits on Aperture Size for an ExoEarth-Detecting Coronagraphic Mission
The yield of Earth-like planets will likely be a primary science metric for
future space-based missions that will drive telescope aperture size. Maximizing
the exoEarth candidate yield is therefore critical to minimizing the required
aperture. Here we describe a method for exoEarth candidate yield maximization
that simultaneously optimizes, for the first time, the targets chosen for
observation, the number of visits to each target, the delay time between
visits, and the exposure time of every observation. This code calculates both
the detection time and multi-wavelength spectral characterization time required
for planets. We also refine the astrophysical assumptions used as inputs to
these calculations, relying on published estimates of planetary occurrence
rates as well as theoretical and observational constraints on terrestrial
planet sizes and classical habitable zones. Given these astrophysical
assumptions, optimistic telescope and instrument assumptions, and our new
completeness code that produces the highest yields to date, we suggest lower
limits on the aperture size required to detect and characterize a
statistically-motivated sample of exoEarths.Comment: Accepted for publication in ApJ; 38 pages, 16 Figures, 3 Table
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