353 research outputs found
Learning flexible sensori-motor mappings in a complex network
Given the complex structure of the brain, how can synaptic plasticity explain the learning and forgetting of associations when these are continuously changing? We address this question by studying different reinforcement learning rules in a multilayer network in order to reproduce monkey behavior in a visuomotor association task. Our model can only reproduce the learning performance of the monkey if the synaptic modifications depend on the pre- and postsynaptic activity, and if the intrinsic level of stochasticity is low. This favored learning rule is based on reward modulated Hebbian synaptic plasticity and shows the interesting feature that the learning performance does not substantially degrade when adding layers to the network, even for a complex proble
Internal Representation of Task Rules by Recurrent Dynamics: The Importance of the Diversity of Neural Responses
Neural activity of behaving animals, especially in the prefrontal cortex, is highly heterogeneous, with selective responses to diverse aspects of the executed task. We propose a general model of recurrent neural networks that perform complex rule-based tasks, and we show that the diversity of neuronal responses plays a fundamental role when the behavioral responses are context-dependent. Specifically, we found that when the inner mental states encoding the task rules are represented by stable patterns of neural activity (attractors of the neural dynamics), the neurons must be selective for combinations of sensory stimuli and inner mental states. Such mixed selectivity is easily obtained by neurons that connect with random synaptic strengths both to the recurrent network and to neurons encoding sensory inputs. The number of randomly connected neurons needed to solve a task is on average only three times as large as the number of neurons needed in a network designed ad hoc. Moreover, the number of needed neurons grows only linearly with the number of task-relevant events and mental states, provided that each neuron responds to a large proportion of events (dense/distributed coding). A biologically realistic implementation of the model captures several aspects of the activity recorded from monkeys performing context-dependent tasks. Our findings explain the importance of the diversity of neural responses and provide us with simple and general principles for designing attractor neural networks that perform complex computation
The Dynamical Response Properties of Neocortical Neurons to Temporally Modulated Noisy Inputs In Vitro
Cortical neurons are often classified by current-frequency relationship. Such a static description is inadequate to interpret neuronal responses to time-varying stimuli. Theoretical studies suggested that single-cell dynamical response properties are necessary to interpret ensemble responses to fast input transients. Further, it was shown that input-noise linearizes and boosts the response bandwidth, and that the interplay between the barrage of noisy synaptic currents and the spike-initiation mechanisms determine the dynamical properties of the firing rate. To test these model predictions, we estimated the linear response properties of layer 5 pyramidal cells by injecting a superposition of a small-amplitude sinusoidal wave and a background noise. We characterized the evoked firing probability across many stimulation trials and a range of oscillation frequencies (1-1000 Hz), quantifying response amplitude and phase-shift while changing noise statistics. We found that neurons track unexpectedly fast transients, as their response amplitude has no attenuation up to 200 Hz. This cut-off frequency is higher than the limits set by passive membrane properties (∼50 Hz) and average firing rate (∼20 Hz) and is not affected by the rate of change of the input. Finally, above 200 Hz, the response amplitude decays as a power-law with an exponent that is independent of voltage fluctuations induced by the background nois
Age and structure parameters of a remote M31 globular cluster B514 based on HST, 2MASS, GALEX and BATC observations
B514 is a remote M31 globular cluster which locating at a projected distance
of R_p~55 kpc. Deep observations with the Advanced Camera for Surveys (ACS) on
the Hubble Space Telescope (HST) are used to provide the accurate integrated
light and star counts of B514. By coupling analysis of the distribution of the
integrated light with star counts, we are able to reliably follow the profile
of the cluster out to ~40". Based on the combined profile, we study in detail
its surface brightness distribution in F606W and F814W filters, and determine
its structural parameters by fitting a single-mass isotropic King model. The
results showed that, the surface brightness distribution departs from the
best-fit King model for r>10". B514 is quite flatted in the inner region, and
has a larger half-light radius than majority of normal globular clusters of the
same luminosity. It is interesting that, in the M_V versus log R_h plane, B514
lies nearly on the threshold for ordinary globular clusters as defined by
Mackey & van den Bergh. In addition, B514 was observed as part of the
Beijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey, using 13
intermediate-band filters covering a wavelength range of 3000--8500 \AA. Based
on aperture photometry, we obtain its SEDs as defined by the 13 BATC filters.
We determine the cluster's age and mass by comparing its SEDs (from 2267 to
20000{\AA}, comprising photometric data in the near-ultraviolet of GALEX, 5
SDSS bands, 13 BATC intermediate-band, and 2MASS near-infrared JHKs} filters)
with theoretical stellar population synthesis models, resulting in age of
Gyr. This age confirms the previous suggestion that B514 is an old
GC in M31. B514 has a mass of , and is a
medium-mass globular cluster in M31.Comment: Accepted for Publication in AJ, 18 pages, 6 figures and 9 table
Age and mass studies for young star clusters in M31 from SEDs-fit
In this paper, we present photometry for young star clusters in M31, which
are selected from Caldwell et al. These star clusters have been observed as
part of the Beijing--Arizona--Taiwan--Connecticut (BATC) Multicolor Sky Survey
from 1995 February to 2008 March. The BATC images including these star clusters
are taken with 15 intermediate-band filters covering 3000--10000 \AA. Combined
with photometry in the {\sl GALEX} far- and near-ultraviolet, broad-band
, SDSS , and infrared of Two Micron All Sky Survey,
we obtain their accurate spectral energy distributions (SEDs) from 1538-20000
\AA. We derive these star clusters' ages and masses by comparing their SEDs
with stellar population synthesis models. Our results are in good agreement
with previous determinations. The mean value of age and mass of young clusters
( Gyr) is about 385 Myr and , respectively. There
are two distinct peaks in the age distribution, a highest peak at age 60
Myr and a secondary peak around 250 Myr, while the mass distribution shows a
single peak around . A few young star clusters have two-body
relaxation times greater than their ages, indicating that those clusters have
not been well dynamically relaxed and therefore have not established the
thermal equilibrium. There are several regions showing aggregations of young
star clusters around the 10 kpc ring and the outer ring, indicating that the
distribution of the young star clusters is well correlated with M31's
star-forming regions. The young massive star clusters (age Myr and
mass ) show apparent concentration around the ring
splitting region, suggesting a recent passage of a satellite galaxy (M32)
through M31 disk.Comment: Accepted for Publication in AJ, 15 pages, 14 figures and 4 table
HST/ACS color-magnitude diagrams of candidate intermediate-age M 31 globular clusters. The role of blue horizontal branches
We present deep (V ~ 28.0) BV photometry obtained with the wide field channel
of the Advanced Camera for Surveys on board HST for four M31 globular clusters
that were identified as candidate intermediate-age (age ~ 1-9 Gyr) by various
authors, based on their integrated spectra and/or broad/intermediate-band
colors. Two of them (B292 and B350) display an obvious blue horizontal branch,
indicating that they are as old as the oldest Galactic globulars. On the other
hand, for the other two (B058 and B337), which display red horizontal branches,
it was not possible either to confirm or disconfirm the age estimate from
integrated spectra. The analysis of the distribution in the spectral indices
Mg2 and H_beta of the M31 and Milky Way clusters whose horizontal branch can be
classified as red or blue based on existing CMDs, strongly suggests that
classical age diagnostics from integrated spectra may be significantly
influenced by the HB morphology of the clusters and can lead to erroneous
age-classifications. We also provide the CMD for another two clusters that fall
into the field of the main targets, B336, an old and metal-poor globular with a
significant population of RR-Lyrae variables, and the newly discovered B531, a
cluster with a very red red giant branch.Comment: Accepted for publication in Astronomy and Astrophysics; 13 pages, 13
figures and 7 tables. Some figures have been decreased in quality, an higher
resolution version is available at http://www.bo.astro.it/M31/hstcatalog
Alpha-Tocopherol Counteracts the Cytotoxicity Induced by Ochratoxin A in Primary Porcine Fibroblasts
The aims of the current study were to determine the half-lethal concentration of ochratoxin A (OTA) as well as the levels of lactate dehydrogenase release and DNA fragmentation induced by OTA in primary porcine fibroblasts, and to examine the role of α-tocopherol in counteracting its toxicity. Cells showed a dose-, time- and origin-dependent (ear vs. embryo) sensitivity to ochratoxin A. Pre-incubation for 3 h with 1 nM α-tocopherol significantly (P < 0.01) reduced OTA cytotoxicity, lactate dehydrogenase release and DNA damage in both fibroblast cultures. These findings indicate that α-tocopherol supplementation may counteract short-term OTA toxicity, supporting its defensive role in the cell membrane
Learning flexible sensori-motor mappings in a complex network
Given the complex structure of the brain, how can synaptic plasticity explain the learning and forgetting of associations when these are continuously changing? We address this question by studying different reinforcement learning rules in a multilayer network in order to reproduce monkey behavior in a visuomotor association task. Our model can only reproduce the learning performance of the monkey if the synaptic modifications depend on the pre- and postsynaptic activity, and if the intrinsic level of stochasticity is low. This favored learning rule is based on reward modulated Hebbian synaptic plasticity and shows the interesting feature that the learning performance does not substantially degrade when adding layers to the network, even for a complex problem
2MASS photometry and age estimate of globular clusters in the outer halo of M31
We present the first photometric results in J, H, and K_s from 2MASS imaging
of 10 classical globular clusters in the far outer regions of M31. Combined
with the V and I photometric data from previous literature, we constructed the
color-color diagram between J-K_s and V-I. By comparing the integrated
photometric measurements with evolutionary models, we estimate the ages of
these clusters. The results showed that, all of these clusters are older than
yrs, of which 4 are older than 10 Gyrs and the other 6 are in
intermediate ages between 3-8 Gyrs. The masses for these outer halo GCs are
from to . We argued that, GC2 and
GC3, the ages, metallicities and the distance moduli of which are nearly the
same, were accreted from the same satellite galaxy, if they did not form {\it
in situ}. The statistical results show that, ages and metallicities for these
10 M31 outer halo GCs do not vary with projected radial position, and the
relationship between age and metallicity doest not exit.Comment: Accepted for Publication in RAA, 14 pages, 8 figures and 3 table
Oxygen-limited thermal tolerance is seen in a plastron-breathing insect and can be induced in a bimodal gas exchanger.
Thermal tolerance has been hypothesized to result from a mismatch between oxygen supply and demand. However, the generality of this hypothesis has been challenged by studies on various animal groups, including air-breathing adult insects. Recently, comparisons across taxa have suggested that differences in gas exchange mechanisms could reconcile the discrepancies found in previous studies. Here, we test this suggestion by comparing the behaviour of related insect taxa with different gas exchange mechanisms, with and without access to air. We demonstrate oxygen-limited thermal tolerance in air-breathing adults of the plastron-exchanging water bug Aphelocheirus aestivalis. Ilyocoris cimicoides, a related, bimodal gas exchanger, did not exhibit such oxygen-limited thermal tolerance and relied increasingly on aerial gas exchange with warming. Intriguingly, however, when denied access to air, oxygen-limited thermal tolerance could also be induced in this species. Patterns in oxygen-limited thermal tolerance were found to be consistent across life-history stages in these insects, with nymphs employing the same gas exchange mechanisms as adults. These results advance our understanding of oxygen limitation at high temperatures; differences in the degree of respiratory control appear to modulate the importance of oxygen in setting tolerance limits
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