1,872 research outputs found
LIDA: A Working Model of Cognition
In this paper we present the LIDA architecture as a working model of cognition. We argue that such working models are broad in scope and address real world problems in comparison to experimentally based models which focus on specific pieces of cognition. While experimentally based models are useful, we need a working model of cognition that integrates what we know from neuroscience, cognitive science and AI. The LIDA architecture provides such a working model. A LIDA based cognitive robot or software agent will be capable of multiple learning mechanisms. With artificial feelings and emotions as primary motivators and learning facilitators, such systems will ‘live’ through a developmental period during which they will learn in multiple ways to act in an effective, human-like manner in complex, dynamic, and unpredictable environments. We discuss the integration of the learning mechanisms into the existing IDA architecture as a working model of cognition
Dutch Dike Breach, Wilnis 2003
In August 2003, after a very dry and warm summer, one of the dikes along a canal in The Netherlands failed at night. Because of this dike breach, the water in the canal started to run into a housing quarter of the village Wilnis, which is about 30 km Southeast of Amsterdam. A local contractor immediately started to close off the canal. By the time this was finished, 600 houses were already half a meter under water. The 2000 residents were evacuated in the early morning. Almost all residents could return to their homes the same evening after the water was pumped out of this area. Like many other small dikes in The Netherlands, the complete dike consists of peat. Since peat has a relatively low specific weight, a peat dike has a higher risk of being pushed aside by water pressure than sand or clay dikes. This horizontal sliding is a rare type of failure mechanism. Though, when the stability of this dike is checked with a simple one page computation, it becomes clear that the failure of the dike after a dry period was a realistic threat. For many years it was known that this part of dike was at risk. This was reported to the minister of Public Works in 1993, but the two involved provincial authorities, the provincial government and the water board, did not take steps until after the dike failure
Radio Emission from the Intermediate-mass Black Hole in the Globular Cluster G1
We have used the Very Large Array (VLA) to search for radio emission from the
globular cluster G1 (Mayall-II) in M31. G1 has been reported by Gebhardt et al.
to contain an intermediate-mass black hole (IMBH) with a mass of ~2 x 10^4
solar masses. Radio emission was detected within an arcsecond of the cluster
center with an 8.4 GHz power of 2 x 10^{15} W/Hz. The radio/X-ray ratio of G1
is a few hundred times higher than that expected for a high-mass X-ray binary
in the cluster center, but is consistent with the expected value for accretion
onto an IMBH with the reported mass. A pulsar wind nebula is also a possible
candidate for the radio and X-ray emission from G1; future high-sensitivity
VLBI observations might distinguish between this possibility and an IMBH. If
the radio source is an IMBH, and similar accretion and outflow processes occur
for hypothesized ~ 1000-solar-mass black holes in Milky Way globular clusters,
they are within reach of the current VLA and should be detectable easily by the
Expanded VLA when it comes on line in 2010.Comment: ApJ Letters, accepted, 11 pages, 1 figur
Radio Emission Associated with Ultraluminous X-ray Sources in the Galaxy Merger NGC 3256
We present new 6, 3.6, and 2 cm VLA radio observations of the nearby merger
system NGC 3256, with resolutions of ~100 pc, which reveal compact radio
sources embedded in more diffuse emission at all three wavelengths. The two
radio nuclei are partially resolved, but the two dominant compact sources that
remain coincide with the two most powerful compact Ultraluminous X-ray sources
(ULXs) recently reported by Lira et al. The radio/X-ray ratios for these two
sources are too high by factors of >100-1000 to be normal X-ray binaries.
However, their radio and X-ray powers and ratios are consistent with
low-luminosity active galactic nuclei (LLAGNs), and optical emission lines
suggest the presence of a nuclear disk around the northern nucleus. If the two
nuclear ULXs are LLAGNs, their associated black holes are separated by only
\~1kpc, about 6 times closer to one another than those found recently in the
merger galaxy NGC 6240. A third ULX on the outskirts of the merger is also a
radio source, and probably is a collection of supernova remnants. The remaining
ULXs are not coincident with any source of compact radio emission, and are
consistent with expectations for beamed X-ray binaries or intermediate-mass
black holes.Comment: Accepted for ApJ (10 pages, 5 figures
L1CAM binds ErbB receptors through Ig-like domains coupling cell adhesion and neuregulin signalling.
During nervous system development different cell-to-cell communication mechanisms operate in parallel guiding migrating neurons and growing axons to generate complex arrays of neural circuits. How such a system works in coordination is not well understood. Cross-regulatory interactions between different signalling pathways and redundancy between them can increase precision and fidelity of guidance systems. Immunoglobulin superfamily proteins of the NCAM and L1 families couple specific substrate recognition and cell adhesion with the activation of receptor tyrosine kinases. Thus it has been shown that L1CAM-mediated cell adhesion promotes the activation of the EGFR (erbB1) from Drosophila to humans. Here we explore the specificity of the molecular interaction between L1CAM and the erbB receptor family. We show that L1CAM binds physically erbB receptors in both heterologous systems and the mammalian developing brain. Different Ig-like domains located in the extracellular part of L1CAM can support this interaction. Interestingly, binding of L1CAM to erbB enhances its response to neuregulins. During development this may synergize with the activation of erbB receptors through L1CAM homophilic interactions, conferring diffusible neuregulins specificity for cells or axons that interact with the substrate through L1CAM
Cicrumnuclear Supernova Remnants and HII Regions in NGC 253
Archival VLA data has been used to produce arcsecond-resolution 6- and 20-cm
images of the region surrounding the nuclear 200-pc (~15") starburst in NGC
253. Twenty-two discrete sources stronger than 0.4 mJy have been detected
within ~2 kpc (~3') of the galaxy nucleus; almost all these sources must be
associated with the galaxy. None of the radio sources coincides with a detected
X-ray binary, so they appear to be due to supernova remnants and H II regions.
The region outside the central starburst has a derived radio supernova rate of
<~0.1/yr, and may account for at least 20% of the recent star formation in NGC
253. Most of the newly identified sources have steep, nonthermal radio spectra,
but several relatively strong thermal sources also exist, containing the
equivalent of tens of O5 stars. These stars are spread over tens of parsecs,
and are embedded in regions having average ionized gas densities of
20-200/cm^3, much lower than in the most active nuclear star-forming regions in
NGC 253 or in the super star clusters seen in other galaxies. The strongest
region of thermal emission coincides with a highly reddened area seen at
near-infrared wavelengths, possibly containing optically obscured H II regions.Comment: 17 pages, 3 postscript figures, AASTeX format, in press for
Astronomical Journal, July 200
The late time radio emission from SN 1993J at meter wavelengths
We present the investigations of SN 1993J using low frequency observations
with the Giant Meterwave Radio Telescope. We analyze the light curves of SN
1993J at 1420, 610, 325 and 243 MHz during years since explosion.The
supernova has become optically thin early on in the 1420 MHz and 610 MHz bands
while it has only recently entered the optically thin phase in the 325 MHz
band. The radio light curve in the 235 MHz band is more or less flat. This
indicates that the supernova is undergoing a transition from an optically thick
to optically thin limit in this frequency band. In addition, we analyze the SN
radio spectra at five epochs on day 3000, 3200, 3266, 3460 and 3730 since
explosion. Day 3200 spectrum shows a synchrotron cooling break. SN 1993J is the
only young supernova for which the magnetic field and the size of the radio
emitting region are determined through unrelated methods. Thus the mechanism
that controls the evolution of the radio spectra can be identified. We suggest
that at all epochs, the synchrotron self absorption mechanism is primarily
responsible for the turn-over in the spectra. Light curve models based on free
free absorption in homogeneous or inhomogeneous media at high frequencies
overpredict the flux densities at low frequencies. The discrepancy is
increasingly larger at lower and lower frequencies. We suggest that an extra
opacity, sensitively dependent on frequency, is likely to account for the
difference at lower frequencies. The evolution of the magnetic field
(determined from synchrotron self absorption turn-over) is roughly consistent
with . Radio spectral index in the optically thin part
evolves from at few tens of days to in about
10 years.Comment: 37 pages, 9 figures in LaTex; scheduled for ApJ 10 September 2004,
v612 issue; send comments to: [email protected]
A Search for Active Galactic Nuclei in Sc Galaxies with H II Spectra
(Abridged) We have searched for nuclear radio emission from a statistically
complete sample of 40 Sc galaxies within 30 Mpc that are optically classified
as star-forming objects, in order to determine whether weak AGNs might be
present. Only three nuclear radio sources were detected, in NGC 864, NGC 4123,
and NGC 4535. These galaxies have peak 6-cm radio powers of 10^{20} W/Hz at
arcsecond resolution, while upper limits of the non-detected galaxies typically
range from 10^{18.4} to 10^{20} W/Hz. The three nuclear radio sources all are
resolved and appear to have diffuse morphologies, with linear sizes of ~300 pc.
This strongly indicates that circumnuclear star formation has been detected in
these three H II galaxies. Comparison with previous 20-cm VLA results for the
detected galaxies shows that the extended nuclear radio emission has a flat
spectrum in two objects, and almost certainly is generated by thermal emission
from gas ionized by young stars in the centers of those galaxies. The 6-cm
radio powers are comparable to predictions for thermal emission that are based
on the nuclear H-alpha luminosities, and imply nuclear star formation rates of
0.08-0.8 solar masses/yr, while the low-resolution NRAO VLA Sky Survey implies
galaxy-wide star formation rates of 0.3-1.0 solar masses/yr in stars above 5
solar masses. Although the presence of active nuclei powered by massive black
holes cannot be definitively ruled out, the present results suggest that they
are likely to be rare in these late-type galaxies with H II spectra.Comment: To appear in ApJ. 7 page
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