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Sequential presentation protects working memory from catastrophic interference
Neural network models of memory are notorious for catastrophic interference: old items are forgotten as new items are memorized (e.g., French, 1999; McCloskey & Cohen, 1989). While Working Memory (WM) in human adults shows severe capacity limitations, these capacity limitations do not reflect neural-network style catastrophic interference. However, our ability to quickly apprehend the numerosity of small sets of objects (i.e., subitizing) does show catastrophic capacity limitations, and this subitizing capacity and WM might reflect a common capacity. Accordingly, computational investigations (Knops, Piazza, Sengupta, Eger, & Melcher, 2014; Sengupta, Surampudi, & Melcher, 2014) suggest that mutual inhibition among neurons can explain both kinds of capacity limitations as well as why our ability to estimate the numerosity of larger sets is limited according to a Weber ratio signature. Based on simulations with a saliency map-like network and mathematical proofs, we provide three results. First, mutual inhibition among neurons leads to catastrophic interference when items are presented simultaneously. The network can remember a limited number of items, but when more items are presented, the network forgets all of them. Second, if memory items are presented sequentially rather than simultaneously, the network remembers the most recent items rather than forgetting all of them. Hence, the tendency in WM tasks to sequentially attend even to simultaneously presented items might not only reflect attentional limitations, but an adaptive strategy to avoid catastrophic interference. Third, the mean activation level in the network can be used to estimate the number of items in small sets, but does not accurately reflect the number of items in larger sets. Rather, we suggest that the Weber ratio signature of large number discrimination emerges naturally from the interaction between the limited precision of a numeric estimation system and a multiplicative gain control mechanism
Intrusive origin of the Sudbury Igneous Complex: Structural and sedimentological evidence
In recent years, many geoscientists have come to believe that the Sudbury event was exogenic rather than endogenic. Critical to a recent exogenic hypothesis is the impact melt origin of the Sudbury Igneous Complex (SIC). Such origin implies that the SIC was emplaced before deposition of the Whitewater Group, in contrast to origins in which the SIC postdates the lithification of the Onaping Formation. Structural and sedimentological evidence is summarized herein that supports an intrusion of the SIC after lithification of all Whitewater Group strata, and conflicts with the hypothesis advanced by other researchers
A New Generating Function for Calculating the Igusa Local Zeta Function
A new method is devised for calculating the Igusa local zeta function
of a polynomial over a -adic field. This involves a new
kind of generating function that is the projective limit of a family of
generating functions, and contains more data than . This resides in
an algebra whose structure is naturally compatible with operations on the
underlying polynomials, facilitating calculation of local zeta functions. This
new technique is used to expand significantly the set of quadratic polynomials
whose local zeta functions have been calculated explicitly. Local zeta
functions for arbitrary quadratic polynomials over -adic fields with odd
are presented, as well as for polynomials over unramified -adic fields of
the form where is a quadratic form and is a linear form where
and have disjoint variables. For a quadratic form over an arbitrary
-adic field with odd , this new technique makes clear precisely which of
the three candidate poles are actual poles.Comment: 54 page
Caldolysin, a highly active protease from an extremely Thermophilic Bacterium
Proteases comprise a significant proportion of those proteins which have been subject to detailed characterisation (amino acid sequence and high resolution crystallographic analysis). The extent of research interest in proteolytic enzymes reflects both their historical status, and the practical advantages of proteases as research subjects (available in quantity, extracellular etc.) widely occurring
The industrial potential of enzymes from extremely thermophilic bacteria
The thermal regions of the central North Island of New Zealand are some of the most extensive in the world. In addition, they are readily accessible and contain a diversity of ecological habitats, including a large number at 100°C. These areas are regarded as an important tourist attraction, and as a source of geothermal power, It is now clear that they also contain an important and unique genetic resource
Theoretical study of resonant x-ray emission spectroscopy of Mn films on Ag
We report a theoretical study on resonant x-ray emission spectra (RXES) in
the whole energy region of the Mn white lines for three prototypical
Mn/Ag(001) systems: (i) a Mn impurity in Ag, (ii) an adsorbed Mn monolayer on
Ag, and (iii) a thick Mn film. The calculated RXES spectra depend strongly on
the excitation energy. At excitation, the spectra of all three systems
are dominated by the elastic peak. For excitation energies around , and
between and , however, most of the spectral weight comes from
inelastic x-ray scattering. The line shape of these inelastic ``satellite''
structures changes considerably between the three considered Mn/Ag systems, a
fact that may be attributed to changes in the bonding nature of the Mn-
orbitals. The system-dependence of the RXES spectrum is thus found to be much
stronger than that of the corresponding absorption spectrum. Our results
suggest that RXES in the Mn region may be used as a sensitive probe
of the local environment of Mn atoms.Comment: 9 pages, 11 figure
The imposters (An historical novelette)
Thesis (M.S.)--Boston Universit
Deviation from one-dimensionality in stationary properties and collisional dynamics of matter-wave solitons
By means of analytical and numerical methods, we study how the residual
three-dimensionality affects dynamics of solitons in an attractive
Bose-Einstein condensate loaded into a cigar-shaped trap. Based on an effective
1D Gross-Pitaevskii equation that includes an additional quintic self-focusing
term, generated by the tight transverse confinement, we find a family of exact
one-soliton solutions and demonstrate stability of the entire family, despite
the possibility of collapse in the 1D equation with the quintic self-focusing
nonlinearity. Simulating collisions between two solitons in the same setting,
we find a critical velocity, , below which merger of identical in-phase
solitons is observed. Dependence of on the strength of the transverse
confinement and number of atoms in the solitons is predicted by means of the
perturbation theory and investigated in direct simulations. Symmetry breaking
in collisions of identical solitons with a nonzero phase difference is also
shown in simulations and qualitatively explained by means of an analytical
approximation.Comment: 10 pages, 7 figure
Photonic crystal laser-driven accelerator structures
Laser-driven acceleration holds great promise for significantly improving
accelerating gradient. However, scaling the conventional process of
structure-based acceleration in vacuum down to optical wavelengths requires a
substantially different kind of structure. We require an optical waveguide that
(1) is constructed out of dielectric materials, (2) has transverse size on the
order of a wavelength, and (3) supports a mode with speed-of-light phase
velocity in vacuum. Photonic crystals--structures whose electromagnetic
properties are spatially periodic--can meet these requirements.
We discuss simulated photonic crystal accelerator structures and describe
their properties. We begin with a class of two-dimensional structures which
serves to illustrate the design considerations and trade-offs involved. We then
present a three-dimensional structure, and describe its performance in terms of
accelerating gradient and efficiency. We discuss particle beam dynamics in this
structure, demonstrating a method for keeping a beam confined to the waveguide.
We also discuss material and fabrication considerations. Since accelerating
gradient is limited by optical damage to the structure, the damage threshold of
the dielectric is a critical parameter. We experimentally measure the damage
threshold of silicon for picosecond pulses in the infrared, and determine that
our structure is capable of sustaining an accelerating gradient of 300 MV/m at
1550 nm. Finally, we discuss possibilities for manufacturing these structures
using common microfabrication techniques.Comment: Ph.D. Thesis, Stanford Universit
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