872 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
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
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
Unsupervised discovery of temporal sequences in high-dimensional datasets, with applications to neuroscience.
Identifying low-dimensional features that describe large-scale neural recordings is a major challenge in neuroscience. Repeated temporal patterns (sequences) are thought to be a salient feature of neural dynamics, but are not succinctly captured by traditional dimensionality reduction techniques. Here, we describe a software toolbox-called seqNMF-with new methods for extracting informative, non-redundant, sequences from high-dimensional neural data, testing the significance of these extracted patterns, and assessing the prevalence of sequential structure in data. We test these methods on simulated data under multiple noise conditions, and on several real neural and behavioral datas. In hippocampal data, seqNMF identifies neural sequences that match those calculated manually by reference to behavioral events. In songbird data, seqNMF discovers neural sequences in untutored birds that lack stereotyped songs. Thus, by identifying temporal structure directly from neural data, seqNMF enables dissection of complex neural circuits without relying on temporal references from stimuli or behavioral outputs
Weather on Other Worlds. II. Survey Results: Spots Are Ubiquitous on L and T Dwarfs
We present results from the "Weather on Other Worlds" Spitzer Exploration
Science program to investigate photometric variability in L and T dwarfs,
usually attributed to patchy clouds. We surveyed 44 L3-T8 dwarfs, spanning a
range of colors and surface gravities. We find that 14/23 (61%; 95%
confidence interval: 41%-78%) of our single L3-L9.5 dwarfs are variable with
peak-to-peak amplitudes between 0.2% and 1.5%, and 5/16 (31%; 95% confidence
interval: 14%-56%) of our single T0-T8 dwarfs are variable with amplitudes
between 0.8% and 4.6%. After correcting for sensitivity, we find that 80% (95%
confidence interval: 53%-100%) of L dwarfs vary by >0.2%, and 36% (95%
confidence interval: 19%-52%) of T dwarfs vary by >0.4%. Given viewing geometry
considerations, we conclude that photospheric heterogeneities causing >0.2%
3-5-micron flux variations are present on virtually all L dwarfs, and probably
on most T dwarfs. A third of L dwarf variables show irregular light curves,
indicating that L dwarfs may have multiple spots that evolve over a single
rotation. Also, approximately a third of the periodicities are on time scales
>10 h, suggesting that slowly-rotating brown dwarfs may be common. We observe
an increase in the maximum amplitudes over the entire spectral type range,
revealing a potential for greater temperature contrasts in T dwarfs than in L
dwarfs. We find a tentative association (92% confidence) between low surface
gravity and high-amplitude variability among L3-L5.5 dwarfs. Although we can
not confirm whether lower gravity is also correlated with a higher incidence of
variables, the result is promising for the characterization of directly imaged
young extrasolar planets through variability.Comment: 42 pages, 11 figures, 2 tables, accepted by Ap
Stimulating a Response: Does Exposure to the Confederate Flag Impact People’s Attitudes Regarding Social Dominance Orientation, Ethnocultural Empathy, and their Political Beliefs?
Minimal psychological research has looked at whether priming participants with the Confederate flag impacts psychological functioning. The current study examined whether Confederate flag priming and people’s political orientation would account for various indicators of how people reconcile in-group/out-group divisions- social dominance orientation (SDO) and ethnocultural empathy (EE). Previous research noted that exposing people to the Confederate flag activates schemas resulting in biased judgments of out-group members (Becker, Enders-Comber, Wagner, Christ, & Butz, 2012; Callahan & Ledgewood, 2016; Kemmelmeier & Winter, 2008). Other studies noted that exposure to the Confederate flag changed voter’s preferences for political candidates, such as Barack Obama due to the activation of negative feelings towards Blacks (Ehrlinger et al., 2011). The current study builds from such work by examining the political attitudes of 194 participants enrolled in a midsized, primarily minority-serving university in the southern United States. Participants were exposed to 30 seconds of either the Confederate flag or a control group condition (an Olympic flag). After the exposure, participants’ attitudes towards SDO and EE were reported. Findings indicated that political ideology interacted with the flag condition and that the disparities were most pronounced when people of a particular political orientation were exposed to the Confederate rather than Olympic flag condition. Specifically, when exposed to the Confederate flag conservatives reported more SDO (i.e., less endorsement of group equality) and less EE (e.g., empathic feelings). By contrast, for liberals EE empathic awareness subscale scores were especially higher in the Confederate flag condition. Multiple main effects also emerged in which liberals generally reported lower SDO and higher EE than conservatives did (regardless of the flag priming). Implications concerning the current political climate in the U. S. and information shortcuts for potential voting behavior are also discussed
ATLAST detector needs for direct spectroscopic biosignature characterization in the visible and near-IR
Are we alone? Answering this ageless question will be a major focus for
astrophysics in coming decades. Our tools will include unprecedentedly large
UV-Optical-IR space telescopes working with advanced coronagraphs and
starshades. Yet, these facilities will not live up to their full potential
without better detectors than we have today. To inform detector development,
this paper provides an overview of visible and near-IR (VISIR;
) detector needs for the Advanced Technology
Large Aperture Space Telescope (ATLAST), specifically for spectroscopic
characterization of atmospheric biosignature gasses. We also provide a brief
status update on some promising detector technologies for meeting these needs
in the context of a passively cooled ATLAST.Comment: 8 pages, Presented 9 August 2015 at SPIE Optics + Photonics, San
Diego, C
Abiotic Ozone and Oxygen in Atmospheres Similar to Prebiotic Earth
The search for life on planets outside our solar system will use
spectroscopic identification of atmospheric biosignatures. The most robust
remotely-detectable potential biosignature is considered to be the detection of
oxygen (O_2) or ozone (O_3) simultaneous to methane (CH_4) at levels indicating
fluxes from the planetary surface in excess of those that could be produced
abiotically. Here, we use an altitude-dependent photochemical model with the
enhanced lower boundary conditions necessary to carefully explore abiotic O_2
and O_3 production on lifeless planets with a wide variety of volcanic gas
fluxes and stellar energy distributions. On some of these worlds, we predict
limited O_2 and O_3 build up, caused by fast chemical production of these
gases. This results in detectable abiotic O_3 and CH_4 features in the
UV-visible, but no detectable abiotic O_2 features. Thus, simultaneous
detection of O_3 and CH_4 by a UV-visible mission is not a strong biosignature
without proper contextual information. Discrimination between biological and
abiotic sources of O_2 and O_3 is possible through analysis of the stellar and
atmospheric context - particularly redox state and O atom inventory - of the
planet in question. Specifically, understanding the spectral characteristics of
the star and obtaining a broad wavelength range for planetary spectra should
allow more robust identification of false positives for life. This highlights
the importance of wide spectral coverage for future exoplanet characterization
missions. Specifically, discrimination between true- and false-positives may
require spectral observations that extend into infrared wavelengths, and
provide contextual information on the planet's atmospheric chemistry.Comment: Accepted for publication in The Astrophysical Journal. 43 pages, 6
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