3,908 research outputs found
The effect of neural adaptation of population coding accuracy
Most neurons in the primary visual cortex initially respond vigorously when a
preferred stimulus is presented, but adapt as stimulation continues. The
functional consequences of adaptation are unclear. Typically a reduction of
firing rate would reduce single neuron accuracy as less spikes are available
for decoding, but it has been suggested that on the population level,
adaptation increases coding accuracy. This question requires careful analysis
as adaptation not only changes the firing rates of neurons, but also the neural
variability and correlations between neurons, which affect coding accuracy as
well. We calculate the coding accuracy using a computational model that
implements two forms of adaptation: spike frequency adaptation and synaptic
adaptation in the form of short-term synaptic plasticity. We find that the net
effect of adaptation is subtle and heterogeneous. Depending on adaptation
mechanism and test stimulus, adaptation can either increase or decrease coding
accuracy. We discuss the neurophysiological and psychophysical implications of
the findings and relate it to published experimental data.Comment: 35 pages, 8 figure
Panoramic Localization in the 4-Legged League:Removing the Dependence on Artificial Landmarks
Event-driven simulations of a plastic, spiking neural network
We consider a fully-connected network of leaky integrate-and-fire neurons
with spike-timing-dependent plasticity. The plasticity is controlled by a
parameter representing the expected weight of a synapse between neurons that
are firing randomly with the same mean frequency. For low values of the
plasticity parameter, the activities of the system are dominated by noise,
while large values of the plasticity parameter lead to self-sustaining activity
in the network. We perform event-driven simulations on finite-size networks
with up to 128 neurons to find the stationary synaptic weight conformations for
different values of the plasticity parameter. In both the low and high activity
regimes, the synaptic weights are narrowly distributed around the plasticity
parameter value consistent with the predictions of mean-field theory. However,
the distribution broadens in the transition region between the two regimes,
representing emergent network structures. Using a pseudophysical approach for
visualization, we show that the emergent structures are of "path" or "hub"
type, observed at different values of the plasticity parameter in the
transition region.Comment: 9 pages, 6 figure
Collimation and asymmetry of the hot blast wave from the recurrent nova V745 Scorpii
The recurrent symbiotic nova V745 Sco exploded on 2014 February 6 and was
observed on February 22 and 23 by the Chandra X-ray Observatory Transmission
Grating Spectrometers. By that time the supersoft source phase had already
ended and Chandra spectra are consistent with emission from a hot, shock-heated
circumstellar medium with temperatures exceeding 10^7K. X-ray line profiles are
more sharply peaked than expected for a spherically-symmetric blast wave, with
a full width at zero intensity of approximately 2400 km/s, a full width at half
maximum of 1200 +/- 30 km/s and an average net blueshift of 165 +/- 10 km/s.
The red wings of lines are increasingly absorbed toward longer wavelengths by
material within the remnant. We conclude that the blast wave was sculpted by an
aspherical circumstellar medium in which an equatorial density enhancement
plays a role, as in earlier symbiotic nova explosions. Expansion of the
dominant X-ray emitting material is aligned close to the plane of the sky and
most consistent with an orbit seen close to face-on. Comparison of an
analytical blast wave model with the X-ray spectra, Swift observations and
near-infrared line widths indicates the explosion energy was approximately
10^43 erg, and confirms an ejected mass of approximately 10^-7 Msun. The total
mass lost is an order of magnitude lower than the accreted mass required to
have initiated the explosion, indicating the white dwarf is gaining mass and is
a supernova Type 1a progenitor candidate.Comment: To appear in the Astrophysical Journa
Pan-chromatic observations of the remarkable nova LMC 2012
We present the results of an intensive multiwavelength campaign on nova LMC
2012. This nova evolved very rapidly in all observed wavelengths. The time to
fall two magnitudes in the V band was only 2 days. In X-rays the super soft
phase began 135 days after discovery and ended around day 50 after
discovery. During the super soft phase, the \Swift/XRT and \Chandra\ spectra
were consistent with the underlying white dwarf being very hot, 1 MK,
and luminous, 10 erg s. The UV, optical, and near-IR
photometry showed a periodic variation after the initial and rapid fading had
ended. Timing analysis revealed a consistent 19.240.03 hr period in all
UV, optical, and near-IR bands with amplitudes of 0.3 magnitudes which
we associate with the orbital period of the central binary. No periods were
detected in the corresponding X-ray data sets. A moderately high inclination
system, = 6010^{\arcdeg}, was inferred from the early optical
emission lines. The {\it HST}/STIS UV spectra were highly unusual with only the
\ion{N}{5} (1240\AA) line present and superposed on a blue continuum. The lack
of emission lines and the observed UV and optical continua from four epochs can
be fit with a low mass ejection event, 10 M, from a hot
and massive white dwarf near the Chandrasekhar limit. The white dwarf, in turn,
significantly illuminated its subgiant companion which provided the bulk of the
observed UV/optical continuum emission at the later dates. The inferred extreme
white dwarf characteristics and low mass ejection event favor nova LMC 2012
being a recurrent nova of the U Sco subclass.Comment: 18 figures, 6 tables (one online only containing all the photometry
Slim cables, compact cross-bonding and corrected distance protection
In the Netherlands, high voltage connections will increasingly be realized using cables as it is very difficult to get permits for overhead lines. Pirelli and Nuon are continuously improving the design and production of the cables for increased reliability and capacity as well as lower costs. To this end, the present XLPE insulated cable has been optimized using a thinner primary insulation and lead sheath. The layout of production has been improved to lower manufacturing costs. Cable capacity has been improved by the choice for a thinner lead screen as well as increased use of cross-bonding. A cross-bonding box has been developed that allows capacity to be increased significantly at limited additional costs. The distance protection of cables has been revised to correct the calculation error caused by the return current in the screen. It is shown that adequate correction can be achieved using a real value which depends only on cable characteristics. This considerably eases implementation into new and in existing systems
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