27,971 research outputs found
Monosynaptic connections between pairs of spiny stellate cells in layer 4 and pyramidal cells in layer 5A indicate that lemniscal and paralemniscal afferent pathways converge in the infragranular somatosensory cortex.
Monosynaptic interlaminar connections between spiny stellate cells in layer 4 (L4), the main cortical recipient layer for thalamic projections, and pyramidal cells in layer 5A (L5A), one of the main cortical output layers, were examined anatomically and functionally by paired recordings in acute brain slices. The somata of pairs forming interlaminar L4-to-L5A connections were located predominantly close to or directly under the barrel-septum wall in layer 4. Superposition of spiny stellate axon arbors and L5A pyramidal cell dendritic arbors suggested an innervation domain underneath an L4 barrel wall. Functionally, the L4-to-L5A connections were of high reliability and relatively low efficacy, with a unitary EPSP amplitude of 0.6 mV, and the connectivity was moderately high (one in seven pairs tested was connected). The EPSP amplitude was weakly depressing (paired-pulse ratio of approximately 0.8) during repetitive presynaptic action potentials at 10 Hz. The existence of Monosynaptic L4-to-L5A connections indicates that the specific 'lemniscal' thalamic input from the ventro-basal nucleus of the thalamus to the cortex and the more unspecific 'paralemniscal' afferent thalamic projections from the posterior medial nucleus of the thalamus merge already at an initial stage of cortical signal processing. These Monosynaptic connections establish a Monosynaptic coupling of the input to the cortex and its output, thereby effectively bypassing the supragranular layers
Efficiency versus Convergence of Boolean Kernels for On-Line Learning Algorithms
The paper studies machine learning problems where each example is described
using a set of Boolean features and where hypotheses are represented by linear
threshold elements. One method of increasing the expressiveness of learned
hypotheses in this context is to expand the feature set to include conjunctions
of basic features. This can be done explicitly or where possible by using a
kernel function. Focusing on the well known Perceptron and Winnow algorithms,
the paper demonstrates a tradeoff between the computational efficiency with
which the algorithm can be run over the expanded feature space and the
generalization ability of the corresponding learning algorithm. We first
describe several kernel functions which capture either limited forms of
conjunctions or all conjunctions. We show that these kernels can be used to
efficiently run the Perceptron algorithm over a feature space of exponentially
many conjunctions; however we also show that using such kernels, the Perceptron
algorithm can provably make an exponential number of mistakes even when
learning simple functions. We then consider the question of whether kernel
functions can analogously be used to run the multiplicative-update Winnow
algorithm over an expanded feature space of exponentially many conjunctions.
Known upper bounds imply that the Winnow algorithm can learn Disjunctive Normal
Form (DNF) formulae with a polynomial mistake bound in this setting. However,
we prove that it is computationally hard to simulate Winnows behavior for
learning DNF over such a feature set. This implies that the kernel functions
which correspond to running Winnow for this problem are not efficiently
computable, and that there is no general construction that can run Winnow with
kernels
Electroweak Precision Physics at e+ e- Colliders with RacoonWW
We present precise predictions for the processes e+ e- -> WW -> 4f(gamma) at
LEP2 and future Linear-Collider (LC) energies obtained with the Monte Carlo
generator RacoonWW. The program RacoonWW includes the complete O(alpha)
electroweak radiative corrections to e+ e- -> WW -> 4f in the double-pole
approximation (DPA). While the virtual corrections are treated in DPA, the
calculation of the bremsstrahlung corrections is based on the full lowest-order
matrix elements to the processes e+ e- -> 4f+gamma. This asymmetric treatment
of virtual and real photons requires a careful matching of the arising infrared
and collinear singularities. We also take into account higher-order
initial-state photon radiation via the structure-function method. Here, we
briefly describe the RacoonWW approach, give numerical results for the total
W-pair production cross sections, confront them with LEP2 data, and study the
impact of the radiative corrections on angular and W-invariant-mass
distributions at LEP2 and LC energies.Comment: 12 pages, 13 postscript figures, to appear in the Proceedings of the
22nd annual MRST Conference on Theoretical High Energy Physics (MRST 2000),
Rochester, New York, May 8-9, 200
W-pair production at future e+e- colliders: precise predictions from RACOONWW
We present numerical results for total cross sections and various
distributions for e+e- --> WW --> 4f(+gamma) at a future 500GeV linear
collider, obtained from the Monte Carlo generator RACOONWW. This generator is
the first one that includes O(alpha) electroweak radiative corrections in the
double-pole approximation completely. Owing to their large size the corrections
are of great phenomenological importance.Comment: 11 pages, latex, 10 postscript file
Electroweak Radiative Corrections to Off-Shell W-Pair Production
We briefly describe the RacoonWW approach to calculate radiative corrections
to e+ e- -> W W -> 4 fermions and present numerical results for the total
W-pair production cross section at LEP2.Comment: 3 pages, 2 figures, talk given at the DPF2000 meeting, Columbus, OH,
August 9-12, 200
Probing anomalous quartic gauge-boson couplings via e+e- --> 4fermions+gamma
All lowest-order amplitudes for e+e- --> 4f+gamma are calculated including
five anomalous quartic gauge-boson couplings that are allowed by
electromagnetic gauge invariance and the custodial SU(2)_c symmetry. Three of
these anomalous couplings correspond to the operators L_0, L_c, and L_n that
have been constrained by the LEP collaborations in WWgamma production. The
anomalous couplings are incorporated in the Monte Carlo generator RACOONWW.
Moreover, for the processes e+e- --> 4f+gamma RACOONWW is improved upon
including leading universal electroweak corrections such as initial-state
radiation. The discussion of numerical results illustrates the size of the
leading corrections as well as the impact of the anomalous quartic couplings
for LEP2 energies and at 500GeV.Comment: 27 pages, latex, 42 postscript files, some misprints correcte
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