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

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

A New Generating Function for Calculating the Igusa Local Zeta Function

A new method is devised for calculating the Igusa local zeta function $Z_f$ of a polynomial $f(x_1,\dots,x_n)$ over a $p$-adic field. This involves a new kind of generating function $G_f$ that is the projective limit of a family of generating functions, and contains more data than $Z_f$. This $G_f$ 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 $p$-adic fields with $p$ odd are presented, as well as for polynomials over unramified $2$-adic fields of the form $Q+L$ where $Q$ is a quadratic form and $L$ is a linear form where $Q$ and $L$ have disjoint variables. For a quadratic form over an arbitrary $p$-adic field with odd $p$, this new technique makes clear precisely which of the three candidate poles are actual poles.Comment: 54 page

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

The imposters (An historical novelette)

Thesis (M.S.)--Boston Universit

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

Measurement and Analysis of III-V & II-VI Infrared Detectors: Radiometric, Noise Spectrum, and Radiation Tolerance Performance

Infrared (IR) hybrid detector arrays and discrete detectors operated in the space environment may be subjected to a variety of sources of natural radiation while in orbit. This means IR detectors intended for applications such as space-based intelligence, surveillance, and reconnaissance (ISR) or space-situational awareness (SSA) must not only have high performance (high quantum efficiency, h and low dark-current density, JD, and preferably minimal 1/f noise content), but also their radiation tolerance or ability to withstand the effects of the radiation they would expect to encounter in space must be characterized and well understood. As the effects of proton interactions with hybrid detector arrays can dominate in space, a specific detector’s radiation tolerance is typically characterized by measuring its performance degradation as a function of proton fluence, FP, up to a total ionizing dose (TID) of typically 100 krad(Si), which is 3-5 times the maximum expected on-orbit TID value for typical space-based E/O applications. Now for other applications such as astronomy, planetary science, and imaging associated with nuclear medicine applications, the TID requirement can be much higher. When comparing the performance of novel IR detector technologies, it has also proven valuable to determine the rate of performance degradation induced by radiation, referred to as a damage factor. It has also proven valuable to perform temperature-dependent measurements of JD, which are used to determine the dark current limiting mechanism via an Arrhenius-analysis, and the degree to which any thermal annealing of the irradiation induced defects may occur have provided unique insights. Finally, given the potential sensor/system impact it is of the upmost importance to understand the frequency dependent contributions to the overall noise in IR detectors. This body of work contains in-depth measurements and analysis of these performance metrics for both III-V- and II-VI-based IR detectors of various detector architectures. In this dissertation, the results of IR III-V-based InAs/GaSb and InAs/InAsSb type-II strained layer superlattice (T2SLS) and bulk detectors that employ unipolar barriers in their detector architecture and II-VI-based HgCdTe IR detectors are characterized in both clear and radiation environments. III-V-based IR detectors that employ unipolar barriers are now being considered for space applications due to their relative advantage in manufacturability as compared with conventional HgCdTe IR detectors that dominant space-based IR E/O imaging. T2SLS detectors are theoretically predicted to have lower Auger-limited dark currents compared with HgCdTe. However, this advantage is yet to be realized due to the lack of reliable passivation schemesand higher bulk defect densities in these materials, which lead to surface- and Shockley-Read-Hall (SRH)-limited dark currents, respectively. Unipolar-barrier architecture detectors, including the nBn, pBp, pBiBn, etc. detectors reported on here, have been introduced in an effort to mitigate these dark current limiting mechanisms. By deliberate choices of the absorber materials and device structure, the potential barriers in these detectors appear only in either the conduction or valence band to block the majority-carrier bulk and surface currents (e.g. in a nBn detector the potential barrier appears only in the conduction band). This results in an elegant detector architecture in which the ideal barrier layer limits the depletion by an external bias to itself so that the absorbing layer remains in the flatband condition, which eliminates Generation Recombination currents due to SRH defects that may be present in the absorbing layer that ultimately limit the diffusion length. Subjecting IR detectors to proton irradiation may lead to both TID and displacement damage effects, both of which occur on orbit. TID effects occur as incoming protons lose their kinetic energy to ionization of the detector material’s constituent atoms and the additional charges become trapped in oxide layers or surface traps. This additional charging may result in flat-band voltage shifts and increased surface leakage currents. TID effects generally are more visible at lower device temperatures, where charges generated in oxide layers are less mobile, and tend to anneal out at higher temperatures. Displacement damage effects result from the occasional non-ionizing energy loss of an incoming proton due to elastic or inelastic scattering with an atomic nucleus that is sufficient to knock the atom from its lattice site and generate vacancy-interstitial pairs, anti-sites, and defect complexes. In this work these defects were shown to manifest in lower h, due to the consequent reduction in minority carrier lifetime t, and higher JD, due to the SRH mechanism. The proton fluence required to alter the background doping levels, such that the fundamental Auger mechanism is enhanced, when using protons with an energy of 63 MeV is expected to be order’s of magnitude higher than the fluence levels used in this work. Thus, a vital step to characterizing a detector’s radiation tolerance is measuring h and JD as a function of FP, with all irradiation and measurements conducted in-situ stepwise at the detector’s expected operating temperature and bias. In this research, it was found that rate of degradation in quantum efficiency when irradiated with 63 MeV protons for a family of Sb-based MWIR detectors that employed unipolar barrier architectures was greater than 3 times that of conventional p-on-n HgCdTe photodiodes with similar cut-off wavelengths. Likewise, it was found that the rate of degradation in the lateral optical collection length for these same devices was greater than 20 times that of the equivalent MWIR HgCdTe photodiodes. This has been attributed to a degradation in minority carrier lifetime leading to a reduction in the diffusion length. This body of research provides unique insights into the radiation susceptibility and fundamental mechanisms taking place that directly contribute to performance degradation of III-V- and II-V-based IR detectors of various detector architectures

The forests of Kalimpong : an ecological account

Although the vegetation of Sikkim is as well known as that of any part of India, comparatively little attention has been paid to the area now known as the Kaiimpong Sub -Division of the Darjeeling District.The territory, though originally part of the Sikkim state, was annexed by the Bhutanese in 1706 and until 1865 it remained in their hands. Under the Senchula Treaty of 1865 it became part of British India and was added to the Darjeeling District. Although both Sikkim and Bhutah were explored by Dr. Griffith and Sir Joseph Hooker early in the nineteenth century, little attention was paid to this area.The most complete accont of the vegetation of Kalimpong which has hitherto been written was published by the late Mr. J.S.Gamble in the Indian Forester in 1875, in an article entitled "Darjeeling Forests ", and yet in this article Mr. Gamble describes the area east of the Tista, which is the Kalimpong Sub -Division, as "practically unexplored ". Before leaving the District finally in 1882 Mr. Gamble had visited a considerable part of this area and noted the prevalence of certain plants in various - localities. These he recorded in his "List of the - Trees, Shrubs and Climbers of the Darjeeling District.Uf subsequent publications relating to Darjeeling or Sikkim the most important have dealt with the Alpine Flora of tue higher levels in - Sikkim proper or with plants of a particular family only. Nothing more than notes of cursory tours in this area have appeared. The District is included in the Flora of British India but not in Prain's Bengal Plants, and for many years Gamble's List has been the only convenient Forest Flora of the District. This List was revised by myself and my wife in 1925 and is now being published under the title "The 'frees of Northern Bengal:'Altogether Three Forest Working Plans for the Kalimpong Forest Division have been published. The Third Working Plan was published by myself in 1924 and is the only plan which contains a detailed description of the forests. The data given in the present work were collected mainly from 1922 -1924 during the preparation of the Third Forest Working - Plan and part of the material has already been published in a different form in the Plan itself. It has now been supplemented by further observations made in 1926 and it is believed that the account of the forest climax communities is fairly complete although much work still remains to be done before our knowledge of the herbaceous communities reaches the same standard. The study of the cryptogamic plants has only just been begun