bluuebunny/crossref_metadata_2025 huggingface dataset:
Induction of long‐term depression is associated with decreased dendritic length and spine density in layers III and V of sensorimotor neocortex
Authors: Monfils Marie‐H. , Teskey G. Campbell
Published: May 2004

Long‐term potentiation (LTP) and long‐term depression (LTD) are currently the most widely investigated models of the synaptic mechanisms underlying learning and memory. Previous research has shown that induction of LTP increases measures of pyramidal cell dendritic morphology in the hippocampus and layers III and V of the neocortex. However, to date there are no reports on the direct effects of LTD induction on dendritic morphology. Here, we investigated the effects of LTD induction on sensorimotor pyramidal cell dendritic morphology. Rats carried a stimulating electrode in the corpus callosum (midline) and a recording electrode in the right sensorimotor cortex. Each rat received low‐frequency stimulation composed of 900 pulses at 1 Hz or handling daily for a total 15 days. Evoked potentials (EPs) of the transcallosal pathway were recorded in the right hemisphere before and after the 15 days of stimulation or handling. The rats were then perfused with saline and the brains were immediately processed for Golgi‐Cox staining. Our results show that LTD induction is related to decreases in dendritic length and spine density both in layers III and V as well as a decrease in dendritic branch complexity in layer V of the sensorimotor cortex. Thus, neuronal alterations following modifications in neocortical synaptic efficacy may provide a general mechanism for the physical instantiation of learning and memory. Synapse 53:114–121, 2004. © 2004 Wiley‐Liss, Inc.
Gravity waves and mesospheric clouds in the summer middle atmosphere: A comparison of lidar measurements and ray modeling of gravity waves over Sondrestrom, Greenland
Authors: Gerrard Andrew J. , Kane Timothy J. , Eckermann Stephen D. , Thayer Jeffrey P.
Published: May 2004

We conducted gravity wave ray‐tracing experiments within an atmospheric region centered near the ARCLITE lidar system at Sondrestrom, Greenland (67°N, 310°E), in efforts to understand lidar observations of both upper stratospheric gravity wave activity and mesospheric clouds during August 1996 and the summer of 2001. The ray model was used to trace gravity waves through realistic three‐dimensional daily‐varying background atmospheres in the region, based on forecasts and analyses in the troposphere and stratosphere and climatologies higher up. Reverse ray tracing based on upper stratospheric lidar observations at Sondrestrom was also used to try to objectively identify wave source regions in the troposphere. A source spectrum specified by reverse ray tracing experiments in early August 1996 (when atmospheric flow patterns produced enhanced transmission of waves into the upper stratosphere) yielded model results throughout the remainder of August 1996 that agreed best with the lidar observations. The model also simulated increased vertical group propagation of waves between 40 km and 80 km due to intensifying mean easterlies, which allowed many of the gravity waves observed at 40 km over Sondrestrom to propagate quasi‐vertically from 40–80 km and then interact with any mesospheric clouds at 80 km near Sondrestrom, supporting earlier experimentally‐inferred correlations between upper stratospheric gravity wave activity and mesospheric cloud backscatter from Sondrestrom lidar observations. A pilot experiment of real‐time runs with the model in 2001 using weather forecast data as a low‐level background produced less agreement with lidar observations. We believe this is due to limitations in our specified tropospheric source spectrum, the use of climatological winds and temperatures in the upper stratosphere and mesosphere, and missing lidar data from important time periods.
Enhanced strain relaxation in a two-step process of GexSi1−x/Si(001) heterostructures grown by low-temperature molecular-beam epitaxy
Authors: Bolkhovityanov Yu. B. , Deryabin A. S. , Gutakovskii A. K. , Revenko M. A. , Sokolov L. V.
Published: May 2004

Two-layer GexSi1−x heterostructures, with a finite fraction of germanium up to x=0.48 and a thickness of at most 0.65 μm, were grown by molecular-beam epitaxy. It is shown that plastic relaxation of the second step is significantly enhanced. It is assumed that threading dislocations with a density of 105–106 cm−2, which appear in the first step in the process of its plastic relaxation, are sources of misfit dislocations positioned between the first and second steps. Cross-sectional transmission electron microscopy showed the superior quality of the dislocation network in the stepped regions. Threading dislocation densities in the second step were determined with the help of etching pits and were found to be close to 105 cm−2.
Identification of pre-sliding friction dynamics
Authors: Parlitz U. , Hornstein A. , Engster D. , Al-Bender F. , Lampaert V. , Tjahjowidodo T. , Fassois S. D. , Rizos D. , Wong C. X. , Worden K. , Manson G.
Published: May 2004

The hysteretic nonlinear dependence of pre-sliding friction force on displacement is modeled using different physics-based and black-box approaches including various Maxwell-slip models, NARX models, neural networks, nonparametric (local) models and dynamical networks. The efficiency and accuracy of these identification methods is compared for an experimental time series where the observed friction force is predicted from the measured displacement. All models, although varying in their degree of accuracy, show good prediction capability of pre-sliding friction. Finally, we show that even better results can be achieved by using an ensemble of the best models for prediction.
Photon-stimulated desorption of F− ions from CF3Cl adsorbed on Si(111)-7×7
Authors: Wen C.-R. , Chou L.-C.
Published: May 2004

We report the photon-stimulated desorption of negative ions induced by direct dipolar dissociation and dissociative electron attachment. The photon-stimulated desorption of F− ions from CF3Cl physisorbed on a Si(111)-7×7 surface at 30 K in the photon energy range 12–35 eV was studied. The F− ion yield exhibits four resonances, at 12.8, 16.2, 19.5, and 22.3 eV, quite unlike the gas phase photodissociation cross section. The intensities of these resonances depend strongly on the CF3Cl coverage in a manner which varies from peak to peak. The resonances at 19.5 and 22.3 eV, which have a significant enhancement in the monolayer regime, are due to electron mediated dipolar dissociation of adsorbed CF3Cl molecules. The enhancement is attributed to surface electron attachment following molecular excitation. A significant enhancement in the monolayer regime has also been observed for the resonances at 12.8 and 16.2 eV. These two resonances are ascribable to a combination of electron mediated dipolar dissociation and dissociative electron attachment driven by photoelectrons generated in the neighboring molecules.
On the valence shell electronic spectroscopy of 2-vinyl furan
Authors: Giuliani A. , Walker I. C. , Delwiche J. , Hoffmann S. V. , Kech C. , Limão-Vieira P. , Mason N. J. , Hubin-Franskin M.-J.
Published: May 2004

The vacuum ultraviolet (VUV) absorption spectrum (3.50–10.33 eV, 350–120 nm) of gaseous 2-vinyl furan has been measured for the first time using both synchrotron radiation source and electron energy loss spectroscopies with absolute cross section determinations. The He I photoelectron spectrum obtained at higher resolution than previously has been interpreted with the aid of semiempirical molecular orbital calculations. Three excited states of type ππ*1 are found responsible for an intense and structured first band observed between 4.2 and 5.8 eV (295–214 nm). Three triplet states were detected for the first time at about 2.46, 3.35, and 3.8 eV (477, 370, and 328 nm) which are, from the calculations, assigned as ππ*3. Some partial Rydberg series, linked to IE1 and IE2 are identified. The VUV absorption spectrum bears little resemblance to that of the parent compound, furan. The electronically excited molecule is found akin to a linear polyene.
Interfacial properties of the nanostructured dye-sensitized solid heterojunction TiO2/RuL2(NCS)2/CuI
Authors: Karlsson P. G. , Bolik S. , Richter J. H. , Mahrov B. , Johansson E. M. J. , Blomquist J. , Uvdal P. , Rensmo H. , Siegbahn H. , Sandell A.
Published: May 2004

The interfaces of the nanostructured dye-sensitized solid heterojunction TiO2/Ru–dye/CuI have been studied using photoelectron spectroscopy of core and valence levels, x-ray absorption spectroscopy and atomic force microscopy. A nanostructured anatase TiO2 film sensitized with RuL2(NCS)2 [cis-bis(4,4′-dicarboxy-2,2′-bipyridine)-bis(isothio-cyanato)-ruthenium(II)] was prepared in a controlled way using a novel combined in-situ and ex-situ (Ar atmosphere) method. Onto this film CuI was deposited in-situ. The formation of the dye–CuI interface and the changes brought upon the dye–TiO2 interface could be monitored in a stepwise fashion. A direct interaction between the dye NCS groups and the CuI is evident in the core level photoelectron spectra. Concerning the energy matching of the valence electronic levels, the photoelectron spectra indicate that the dye HOMO overlaps in energy with the Cu 3d–I 5p hydrid states. The CuI grow in the form of particles, which at the initial stages displace the dye molecules causing dye–TiO2 bond breaking. Consequently, the very efficient charge injection channel provided by the dye–TiO2 carboxylic bonding is directly affected for a substantial part of the dye molecules. This may be of importance for the functional properties of such a heterojunction.
Excitation migration in trimeric cyanobacterial photosystem I
Authors: Şener Melih K. , Park Sanghyun , Lu Deyu , Damjanović Ana , Ritz Thorsten , Fromme Petra , Schulten Klaus
Published: May 2004

A structure-based description of excitation migration in multireaction center light harvesting systems is introduced. The description is an extension of the sojourn expansion, which decomposes excitation migration in terms of repeated detrapping and recapture events. The approach is applied to light harvesting in the trimeric form of cyanobacterial photosystem I (PSI). Excitation is found to be shared between PSI monomers and the chlorophylls providing the strongest respective links are identified. Excitation sharing is investigated by computing cross-monomer excitation trapping probabilities. It is seen that on the average there is a nearly 40% chance of excitation cross transfer and trapping, indicating efficient coupling between monomers. The robustness and optimality of the chlorophyll network of trimeric PSI is examined.
Photoinduced dynamics of ethene in the N, V, and Z valence states: A six-dimensional nonadiabatic quantum dynamics investigation
Authors: Viel Alexandra , Krawczyk Robert P. , Manthe Uwe , Domcke Wolfgang
Published: May 2004

The photoinduced dynamics of ethene following π→π* excitation is investigated by quantum wave-packet dynamics on three coupled six-dimensional diabatic potential-energy surfaces representing the N, V, and Z valence states, which have been developed previously [J. Chem. Phys. 119, 1397 (2003)]. The C–C stretching and torsion, as well as the pyramidalization and scissoring of both CH2 groups are included in this description. The wave-packet calculations have been performed using the multiconfigurational time-dependent Hartree method for a time period up to 100 fs. While a small amount of population transfer to the electronic ground state is found within this period, the overall population decay time of the V state is found to exceed the 100 fs range significantly. The autocorrelation function of the wave packet and the stationary absorption spectrum of the V state also have been calculated. It is found that both the torsional mode as well as the C–C stretching mode contribute to the very extended vibrational structure of the absorption spectrum, and that both modes are strongly coupled. At least on the present ab initio surface of limited dimensionality, the speed of pyramidalization of 90° twisted ethene appears as the bottleneck for the ultrafast radiationless decay of the V state.
Collision-energy-resolved Penning ionization electron spectroscopy of p-benzoquinone: Study of electronic structure and anisotropic interaction with He*(2 3S) metastable atoms
Authors: Kishimoto Naoki , Okamura Kohji , Ohno Koichi
Published: May 2004

Collision energy dependence of partial ionization cross sections (CEDPICS) of p-benzoquinone with He*(2 3S) metastable atoms indicates that interaction potentials between p-benzoquinone and He*(2 3S) are highly anisotropic in the studied collision energy range (100–250 meV). Attractive interactions were found around the C=O groups for in-plane and out-of-plane directions, while repulsive interactions were found around CH bonds and the benzenoid ring. Assignment of the first four ionic states of p-benzoquinone and an analogous methyl-substituted compound was examined with CEDPICS and anisotropic distributions of the corresponding two nonbonding oxygen orbitals (nO+,nO−) and two πCC orbitals (πCC+,πCC−). An extra band that shows negative CEDPICS was observed at ca. 7.2 eV in Penning ionization electron spectrum.
Freezing and folding behavior in simple off-lattice heteropolymers
Authors: Magee J. E. , Warwicker J. , Lue L.
Published: May 2004

We have performed parallel tempering Monte Carlo simulations using a simple continuum heteropolymer model for proteins. All 10 heteropolymer sequences which we have studied have shown first-order transitions at low temperature to ordered states dominated by single chain conformations. These results are in contrast with the theoretical predictions of the random energy model for heteropolymers, from which we would expect continuous transitions to glassy behavior at low temperatures.
Excess entropy scaling for the diffusion coefficient in expanded liquid metals
Authors: Bretonnet J. L.
Published: May 2004

Molecular-dynamics simulation is used to compute the pair correlation function and the velocity autocorrelation function of Cs and Rb along the liquid–vapor coexistence curve, from which the excess entropy Sex and the diffusion coefficient D are deduced. The numerical results of both physical properties are correlated and a scaling law between the excess entropy and the reduced diffusion coefficient D*(=D/D0) is investigated for different expressions of the reduction parameter D0. The choice of thermodynamic states along the liquid–vapor coexistence curve gives us the possibility to extend the investigation of the relation between the reduced diffusion coefficient and the excess entropy over a wide area and to test the adequacy of the scaling law confidently.
A novel fiber optic sensor for multiple and simultaneous measurement of vibration velocity
Authors: Di Sante Raffaella , Scalise Lorenzo
Published: May 2004

Simultaneous and multipoint assessment of vibration velocities is an important issue for the development of advanced noncontact vibrometers. In this article a novel fiber optic vibrometer is presented. The architecture of the sensor is based on a simple optical layout and it is characterized by multiple fiber optic interferometric sensors which are operated in the homodyne mode. Optical configuration and operation of the single-point version of the sensor, as well as the two-points measurement version, are described and typical measured signals with the operating range are shown. The sensor can easily be configured in order to perform a higher number of point measurements. Some details regarding signal acquisition and processing are also given and the ways in which Doppler demodulation is performed are discussed. Finally tests with sinusoidal target excitation in the range 0–1.8 kHz have been conducted.
Correlation method in period measurement of a torsion pendulum
Authors: Tian Y. L. , Tu Y. , Shao C. G.
Published: May 2004

The correlation method is used to determine the period of a torsion pendulum. Calculation shows that this method cannot only suppress the disturbance of white noise, but it is also insensitive to drift and damping of the torsion pendulum. An estimate of the frequency based on this method is an approximative minimum variance unbiased estimator.
Alpha: A large-area plasma helicon array
Authors: Watts Christopher
Published: May 2004

Helicon sources are extremely efficient in creating high density, current free plasma discharges. However, these sources typically have a small cross-sectional area and are unsuitable for studies that require a large diameter plasma. We report the construction of a plasma facility that overcomes this limitation by using multiple helicon sources in a close-packed array. A large-area plasma helicon array (Alpha) is a 4 m long, 50 cm diam device that uses seven helicon sources to generate a large diameter rf plasma. The facility is designed to study basic plasma wave phenomena in a high-density, current-free plasma. The helicon array can operate in one of two modes, creating either seven distinct plasma columns or an integrated uniform large diameter plasma column. Details of the facility specifications, construction, and initial plasma parameters of the multihelicon source are reported.
Conical crystal spectrograph for high brightness x-ray Kα spectroscopy in subpicosecond laser–solid interaction
Authors: Martinolli E. , Koenig M. , Boudenne J. M. , Perelli E. , Batani D. , Hall T. A.
Published: May 2004

A high brightness crystal spectrograph was designed and successfully used to study the x-ray Kα spectrum of aluminum as a diagnostic for target heating due to suprathermal electrons in subpicosecond laser–solid interaction experiments. Conical geometry was chosen in order to enhance spatial focusing, since an extremely low signal-to-noise ratio was expected for the photon flux, and to have a reasonable spectral range while occupying only a small solid angle within the target chamber. Very high image brightness is obtained through strong spatial focusing, as well as good spectral resolution. A simple analytical model and three-dimensional numerical simulation are presented to describe the crystal characteristics. The performance of the spectrograph was tested both on an optical bench and with a ray-tracing code. The experimental spectra allowed us to estimate the target temperature and characterize the fast electron transport. The spectrograph is considered to be particularly useful, in the configuration described here, for high power laser experiments where the solid angle accessible to the spectrograph is small and blast and debris damage from the laser produced plasma is significant.
Radio frequency magneto-impedance effect in spin tunneling junctions
Authors: Hirabayashi Naoaki , Kaiju Hideo , Shiiki Kazuo
Published: May 2004

The rf magneto-impedance effect of spin tunneling junctions Co/Al-oxide/Co fabricated on glass substrate by ion-beam sputtering is investigated. To measure junction impedance, a measurement method is established. Junction impedance can be determined from the voltages of two cobalt electrodes and a standard resistor at room temperature over a frequency range of 0.1–100 MHz using a lock-in amplifier by taking input impedance into account. The real part of magneto-impedance ratio exhibits a positive value at frequencies less than the roll-off frequency and a negative value at frequencies higher than the roll-off frequency. Furthermore, the imaginary part of the magneto-impedance ratio is found to be positive across the entire frequency region measured and to approach zero at high frequencies.
Optically injected laser system: Characterization of chaos, bifurcation, and control
Authors: Banerjee Santo , Saha Papri , Chowdhury A. Roy
Published: May 2004

A single mode semiconductor laser subjected to optical injection, described by a set of three coupled nonlinear ordinary differential equations, exhibiting chaos is considered. By means of a recurrence analysis, quantification of the strange attractor is made. Analytical studies of the system using asymptotic averaging technique, derive certain conditions describing the prediction of 1→2 bifurcation, which have subsequently been verified on numerical simulation. Furthermore, the locus of points on the parameter phase space representing Hopf bifurcation has been derived. The problem of control of chaos by a new procedure based on adaptive stabilization is also addressed. The results of such control are shown explicitly. Though this analysis deals with a very specific set of equations, the overall features that come out of the study remains valid for almost all laser systems.
Chaotic digital communication by encoding initial conditions
Authors: Xiaofeng Gong , Xingang Wang , Meng Zhan , Lai C. H.
Published: May 2004

We investigate the possibility to improve the noise performance of a chaotic digital communication scheme by utilizing further dynamical information. We show that by encoding the initial information of the chaotic carrier according to the transmitting bits, extra redundance can be introduced into the segments of chaotic signals corresponding to the consecutive bits. Such redundant information can be exploited effectively at the receiver end to improve the noise performance of the system. Compared to other methods (e.g., differential chaos shift keying), straightforward application of the proposed modulation/demodulation scheme already provides significant performance gain in the low signal-to-noise ratio (SNR) region. Furthermore, maximum likelihood precleaning procedure based on the Viterbi algorithm can be applied before the demodulation step to overcome the performance degradation in the high SNR region. The study indicates that it is possible to improve the noise performance of the chaotic digital communication scheme if further dynamics information is added to the system.
End-bonding multiwalled carbon nanotubes in alumina templates: Superconducting proximity effect
Authors: Haruyama J. , Tokita A. , Kobayashi N. , Nomura M. , Miyadai S. , Takazawa K. , Takeda A. , Kanda Y.
Published: May 2004

We cut off the top parts of multiwalled carbon nanotubes (MWNTs), which are synthesized in the nanopores of an alumina template and grown over the template surface from the pores, by utilizing ultrasonic cleaning. This leads to highly regularly aligned MWNTs with open top ends and tubes of the same length in the template. These open top ends can be end-bonded by evaporating gold/niobium electrodes and performing optimal annealing. We find that these niobium/MWNTs junctions result in highly transparent interfaces. As evidence, proximity-induced superconductivity and supercurrents are shown in the MWNTs.
Capillary detector with deuterated scintillator for inertial confinement fusion neutron images
Authors: Disdier L. , Lerche R. A. , Bourgade J. L. , Glebov V. Yu.
Published: May 2004

We demonstrate 325 μm spatial resolution in a 14 MeV neutron detector made with a capillary array filled with a deuterated liquid scintillator. A 10 μm source resolution in an image formed by 14 MeV neutrons is now achievable on a 25 m line-of-sight path for implosions at the Laser Mega Joule and the National Ignition Facilities. Analysis of penumbral and annular imaging techniques using this capillary array technology predicts good signal-to-noise ratios for images formed by targets yielding 1015 neutrons or more. Experimental images of individual scattered recoil ions and measurement of their resulting light distributions are presented and are well understood. Neutron imaging of deuterium-filled capsules is also found to be feasible, based on the light-yield calibration of the neutron detector.
Integrator-based circuit-independent chaotic oscillator structure
Authors: Elwakil A. S.
Published: May 2004

An integrator-based chaotic oscillator structure composed of three cascaded inverting, noninverting, and differential integrators is presented. The nonlinearity responsible for folding the trajectories is introduced by a single switching diode which is controlled by the output of the first integrator in the cascade. Chaotic behavior is verified on the functional level of the structure rendering it circuit-independent. A possible circuit realization is given and a canonical singleparameter-controlled ordinary differential equation capturing the qualitative dynamics of similar fourth-order integrator-based chaotic oscillators is proposed.
Planar Hall effect sensor for magnetic micro- and nanobead detection
Authors: Ejsing L. , Hansen M. F. , Menon A. K. , Ferreira H. A. , Graham D. L. , Freitas P. P.
Published: May 2004

Magnetic bead sensors based on the planar Hall effect in thin films of exchange-biased permalloy have been fabricated and characterized. Typical sensitivities are 3 μV/Oe mA. The sensor response to an applied magnetic field has been measured without and with coatings of commercially available 2 μm and 250 nm magnetic beads used for bioapplications (Micromer-M and Nanomag-D, Micromod, Germany). Detection of both types of beads and single bead detection of 2 μm beads is demonstrated, i.e., the technique is feasible for magnetic biosensors. Single 2 μm beads yield 300 nV signals at 10 mA and 15 Oe applied field.
Ultrafast valence intersubband hole relaxation in InGaN multiple-quantum-well laser diodes
Authors: Gan Kian-Giap , Sun Chi-Kuang , DenBaars Steven P. , Bowers John E.
Published: May 2004

The ultrafast carrier dynamics in InGaN multiple-quantum-well (MQW) laser diodes were investigated using a time-resolved bias-lead monitoring technique. From the optical selection rules of TE and TM polarized light, one can selectively excite and probe different valence-subband-to-conduction-subband transitions in the MQW structure with different polarized pump and probe light. The subband structure of the MQW structure of the laser diode was calculated and is verified by electroluminescence measurement. Using this technique, ultrafast valence intersubband hole relaxation processes (τ<0.35 ps) were found to dominate the observed carrier dynamics.
Femtosecond optical pulse propagation in subwavelength metallic slits
Authors: Dechant A. , Elezzabi A. Y.
Published: May 2004

Subwavelength slit arrays embedded in thin silver films demonstrate a marked ability to modify an incident optical pulse’s temporal profile. Ultrashort pulse propagation through these arrays is accompanied by both an increased initial transmission, as well as a subsequent pulse train re-radiation. The role of impulsively excited surface plasmons is investigated as a mechanism for this enhanced transmission and re-radiation. By simply changing the geometrical parameters of the array, pulse reshaping is easily achieved, and in some cases superluminal light flow is observed.
Type-II GaAsSb/InP heterojunction bipolar light-emitting transistor
Authors: Feng M. , Holonyak N. , Chu-Kung B. , Walter G. , Chan R.
Published: May 2004

We report radiative recombination in the base layer of Type-II InP/GaAsSb/InP double heterojunction bipolar light-emitting transistors (HBLET) operating in the common-emitter configuration. The typical current gain, β, for a 120×120 μm2 emitter area of the HBLET is 38. The optical emission wavelength from a 30 nm GaAs0.51Sb0.49 base is centered at λpeak=1600 nm. Three-port operation of the Type-II HBLET with simultaneously an amplified electrical output and an optical output with signal modulation is demonstrated at 10 kHz.
The influence of the annealing sequence on p+/n junctions observed by scanning capacitance microscopy
Authors: Chang M. N. , Chen C. Y. , Wan W. W. , Liang J. H.
Published: May 2004

This letter reports on the investigation of p+–n junction variation produced by various annealing sequences. With well-controlled photoperturbation, we have employed scanning capacitance microscopy to directly observe the junction narrowing induced by post-spike furnace annealing. For p+–n junctions, it is revealed that post-spike furnace annealing may degrade the electrical activation of boron atoms, leading to junction narrowing without significant boron diffusion. The mechanism and the stability of electrical junctions formed by spike annealing are also discussed. The experimental results also clearly show that furnace annealing followed by spike annealing can result in junction broadening with a more concentrated boron profile.
Microscale polymeric helical structures produced by electrospinning
Authors: Kessick Royal , Tepper Gary
Published: May 2004

Microscale helical coils consisting of a composite of one conducting and one nonconducting polymer were produced using electrospinning. The nonconducting polymer was poly(ethylene oxide) and the conducting polymer was poly(aniline sulfonic acid). The coil structures were studied over a range of processing conditions and fiber composition. The data suggest that the helical structures are formed due to viscoelastic contraction upon partial neutralization of the charged fibers. Polymeric microcoils may find applications in microelectromechanical systems, advanced optical components, and drug delivery systems.
Solid-Phase Synthesis of some Linear and Cyclic Peptidomimetics Based on 2-Aminoalkyloxazole-4- and 5-carboxylates
Authors: Cox Matthew O. , Prager Rolf H.
Published: May 2004

The synthesis of several small linear and cyclic peptidomimetics based on the coupling of 2-aminoalkyloxazole-4- and -5-carboxylic acids has been investigated using Wang and Rink-linked resins.
Utilization of 1-Aryl-2,2-dibromocyclopropanes in Synthetic Approaches to Phenanthroquinolizidine and Phenanthroindolizidine Alkaloids
Authors: Banwell Martin G. , Sydnes Magne O.
Published: May 2004

The 1-aryl-2,2-dibromocyclopropane 8 was treated with silver acetate in acetic acid to give the allylic acetate 19. Exposure of the derived chloride 26 to racemic 2-piperidinemethanol (7) then afforded the allylic amine 6, which engaged in a Pd(0)-catalyzed cross-coupling reaction with arylboronate 29 to provide the cis-stilbene 25. The chloride 4 (X = Cl) derived from compound 25 failed to cyclize in the presence of base to give the desired phenanthroquinolizidine alkaloid julandine 1. This outcome resulted in a re-examination of the closing stages of a previously reported synthesis of the phenanthroindolizidine alkaloid ent-septicine (ent-3). Towards this end, chloride 26 was reacted with (S)-2-pyrrolidinemethanol (32) to give the allylic amine 33, which was cross-coupled with boronate ester 34 to afford cis-stilbene 35. Contrary to literature reports, compound 35 was converted into the corresponding chloride 37, rather than the expected mesylate, upon reaction with methanesulfonyl chloride in the presence of pyridine. Moreover, chloride 37 was not converted into ent-septicine (ent-3) on exposure to sodium hydride, thus calling into question a previously reported synthesis of this natural product.
Magnetic tests for magnetosome chains in Martian meteorite ALH84001
Authors: Weiss Benjamin P. , Kim Soon Sam , Kirschvink Joseph L. , Kopp Robert E. , Sankaran Mohan , Kobayashi Atsuko , Komeili Arash
Published: May 2004

Transmission electron microscopy studies have been used to argue that magnetite crystals in carbonate from Martian meteorite ALH84001 have a composition and morphology indistinguishable from that of magnetotactic bacteria. It has even been claimed from scanning electron microscopy imaging that some ALH84001 magnetite crystals are aligned in chains. Alignment of magnetosomes in chains is perhaps the most distinctive of the six crystallographic properties thought to be collectively unique to magnetofossils. Here we use three rock magnetic techniques, low-temperature cycling, the Moskowitz test, and ferromagnetic resonance, to sense the bulk composition and crystallography of millions of ALH84001 magnetite crystals. The magnetic data demonstrate that although the magnetite is unusually pure and fine-grained in a manner similar to terrestrial magnetofossils, most or all of the crystals are not arranged in chains.
Negative Functional Response to Sensory Stimulation and its Origins
Authors: Kannurpatti Sridhar S. , Biswal Bharat B.
Published: May 2004

Functional imaging studies typically give prominence to positive responses. However, negative changes that accompany activation-induced positive responses are not yet clearly understood. The authors investigated the origin of sustained negative CBF responses that accompanied positive CBF changes. Measurements were made in the rat somatosensory cortex in response to whisker stimulation using laser-Doppler imaging. Flux images indicative of CBF were obtained at rest and during whisker stimulation with a spatial resolution of 200 μm. Large and intermediate blood vessels in the cortical surface exhibiting high flux values were clearly resolved. This greatly reduced the contamination of the tissue pixel volume with macroscopic blood vessels. Regions that responded positively to whisker stimulation were from areas with intermediate to low baseline flux and distinctly away from high flux areas. Stimulation-induced change in signal intensity was the largest in pixels with low baseline flux, presumably from tissue and microvessels. Simultaneously, a sustained decrease in signal intensity was observed in regions with high baseline flux values. The temporal coherence, macrovascular origin, lesser trial-to trial variability, and complete absence of the negative CBF response in the microvascular regions suggest that it may be purely hemodynamic in nature.
Origin of PACAP-immunoreactive Nerve Fibers Innervating the Subarachnoidal Blood Vessels of the Rat Brain
Authors: Martin Florian , Baeres Moesgaard , Møller Morten
Published: May 2004

The subarachnoidal cerebral blood vessels of the rat are innervated by nerve fibers containing different neuropeptides, e.g. pituitary adenylatecyclase activating polypeptide (PACAP). PACAP dilates brain arterioles and immunohistochemical studies of the rat have indicated that PACAP binds to a VPAC1-receptor in the cerebral vasculature of this species. We have investigated the perikaryal origin of the nerve fibers innervating the subarachnoidal blood vessels of the rat by combined retrograde tracing with Fluorogold and immunohistochemistry. The in vivo neuronal retrograde tracings were done by injection of 2% Fluorogold in water into the subarachnoidal space in the area of the middle cerebral artery. The retrograde transported tracer was detected by use of an antibody against Fluorogold. One week after the injections, the animals were vascularly perfused with Stephanini's fixative and labeled perikarya were found bilaterally in the trigeminal, sphenopalatine, and otic ganglia. The retrograde Fluorogold tracings were combined with immunohistochemistry for PACAP using a mouse monoclonal antibody and the biotinylated tyramide amplification system. Double labeled perikarya containing both Fluoro-gold and PACAP were found predominantly in the trigeminal ganglion, and only rarely in the otic and sphenopalatine ganglion. Summarizing, our retrograde tracings combined with immunohistochemistry indicate that the perikarya in the trigeminal ganglion are the main origin of PACAPergic nerve fibers projecting to the cerebral vasculature of the rat.
Unconditional Exact Tests for Equivalence or Noninferiority for Paired Binary Endpoints
Authors: Hsueh Huey‐Miin , Liu Jen‐Pei , Chen James J.
Published: May 2004

Summary. Problems of establishing equivalence or noninferiority between two medical diagnostic procedures involve comparisons of the response rates between correlated proportions. When the sample size is small, the asymptotic tests may not be reliable. This article proposes an unconditional exact test procedure to assess equivalence or noninferiority. Two statistics, a sample‐based test statistic and a restricted maximum likelihood estimation (RMLE)‐based test statistic, to define the rejection region of the exact test are considered. We show the p‐value of the proposed unconditional exact tests can be attained at the boundary point of the null hypothesis. Assessment of equivalence is often based on a comparison of the confidence limits with the equivalence limits. We also derive the unconditional exact confidence intervals on the difference of the two proportion means for the two test statistics. A typical data set of comparing two diagnostic procedures is analyzed using the proposed unconditional exact and asymptotic methods. The p‐value from the unconditional exact tests is generally larger than the p‐value from the asymptotic tests. In other words, an exact confidence interval is generally wider than the confidence interval obtained from an asymptotic test.
Drawbacks to Integer Scoring for Ordered Categorical Data
Authors: Ivanova Anastasia , Berger Vance W.
Published: May 2004

Summary. Linear rank tests are widely used when testing for independence against stochastic order in a 2 ×J contingency table with two treatments and J ordered outcome levels. For this purpose, numerical scores are assigned, possibly by default, to the J outcome levels. When the choice of scores is not apparent, integer (equally spaced) scores are often assigned. We show that this practice generally leads to unnecessarily conservative tests. The use of slightly perturbed scores will result in a less conservative and uniformly more powerful test.
Latent Class Modeling Approaches for Assessing Diagnostic Error without a Gold Standard: With Applications to p53 Immunohistochemical Assays in Bladder Tumors
Authors: Albert Paul S. , McShane Lisa M. , Shih Joanna H. ,   The U.S. National Cancer Institute Bladder Tumor Marker Network
Published: May 2004

Summary. Improved characterization of tumors for purposes of guiding treatment decisions for cancer patients will require that accurate and reproducible assays be developed for a variety of tumor markers. No gold standards exist for most tumor marker assays. Therefore, estimates of assay sensitivity and specificity cannot be obtained unless a latent class model‐based approach is used. Our goal in this article is to estimate sensitivity and specificity for p53 immunohistochemical assays of bladder tumors using data from a reproducibility study conducted by the National Cancer Institute Bladder Tumor Marker Network. We review latent class modeling approaches proposed by previous authors, and we find that many of these approaches impose assumptions about specimen heterogeneity that are not consistent with the biology of bladder tumors. We present flexible mixture model alternatives that are biologically plausible for our example, and we use them to estimate sensitivity and specificity for our p53 assay example. These mixture models are shown to offer an improvement over other methods in a variety of settings, but we caution that, in general, care must be taken in applying latent class models.
Relating Amino Acid Sequence to Phenotype: Analysis of Peptide‐Binding Data
Authors: Segal Mark R. , Cummings Michael P. , Hubbard Alan E.
Published: May 2004

Summary. We illustrate data analytic concerns that arise in the context of relating genotype, as represented by amino acid sequence, to phenotypes (outcomes). The present application examines whether peptides that bind to a particular major histocompatibility complex (MHC) class I molecule have characteristic amino acid sequences. However, the concerns identified and addressed are considerably more general. It is recognized that simple rules for predicting binding based solely on preferences for specific amino acids in certain (anchor) positions of the peptide's amino acid sequence are generally inadequate and that binding is potentially influenced by all sequence positions as well as between‐position interactions. The desire to elucidate these more complex prediction rules has spawned various modeling attempts, the shortcomings of which provide motivation for the methods adopted here. Because of (i) this need to model between‐position interactions, (ii) amino acids constituting a highly (20) multilevel unordered categorical covariate, and (iii) there frequently being numerous such covariates (i.e., positions) comprising the sequence, standard regression/classification techniques are problematic due to the proliferation of indicator variables required for encoding the sequence position covariates and attendant interactions. These difficulties have led to analyses based on (continuous) properties (e.g., molecular weights) of the amino acids. However, there is potential information loss in such an approach if the properties used are incomplete and/or do not capture the mechanism underlying association with the phenotype. Here we demonstrate that handling unordered categorical covariates with numerous levels and accompanying interactions can be done effectively using classification trees and recently devised bump‐hunting methods. We further tackle the question of whether observed associations are attributable to amino acid properties as well as addressing the assessment and implications of between‐position covariation.
Survival Analysis Using Auxiliary Variables Via Multiple Imputation, with Application to AIDS Clinical Trial Data
Authors: Faucett Cheryl L. , Schenker Nathaniel , Taylor Jeremy M. G.
Published: May 2004

Summary. We develop an approach, based on multiple imputation, to using auxiliary variables to recover information from censored observations in survival analysis. We apply the approach to data from an AIDS clinical trial comparing ZDV and placebo, in which CD4 count is the time‐dependent auxiliary variable. To facilitate imputation, a joint model is developed for the data, which includes a hierarchical change‐point model for CD4 counts and a time‐dependent proportional hazards model for the time to AIDS. Markov chain Monte Carlo methods are used to multiply impute event times for censored cases. The augmented data are then analyzed and the results combined using standard multiple‐imputation techniques. A comparison of our multiple‐imputation approach to simply analyzing the observed data indicates that multiple imputation leads to a small change in the estimated effect of ZDV and smaller estimated standard errors. A sensitivity analysis suggests that the qualitative findings are reproducible under a variety of imputation models. A simulation study indicates that improved efficiency over standard analyses and partial corrections for dependent censoring can result. An issue that arises with our approach, however, is whether the analysis of primary interest and the imputation model are compatible.
Estimation of Survival Distributions of Treatment Policies in Two‐Stage Randomization Designs in Clinical Trials
Authors: Lunceford Jared K. , Davidian Marie , Tsiatis Anastasios A.
Published: May 2004

Summary. Some clinical trials follow a design where patients are randomized to a primary therapy at entry followed by another randomization to maintenance therapy contingent upon disease remission. Ideally, analysis would allow different treatment policies, i.e., combinations of primary and maintenance therapy if specified up‐front, to be compared. Standard practice is to conduct separate analyses for the primary and follow‐up treatments, which does not address this issue directly. We propose consistent estimators for the survival distribution and mean restricted survival time for each treatment policy in such two‐stage studies and derive large‐sample properties. The methods are demonstrated on a leukemia clinical trial data set and through simulation.
Bayesian Models for Multivariate Current Status Data with Informative Censoring
Authors: Dunson David B. , Dinse Gregg E.
Published: May 2004

Summary. Multivariate current status data consist of indicators of whether each of several events occur by the time of a single examination. Our interest focuses on inferences about the joint distribution of the event times. Conventional methods for analysis of multiple event‐time data cannot be used because all of the event times are censored and censoring may be informative. Within a given subject, we account for correlated event times through a subject‐specific latent variable, conditional upon which the various events are assumed to occur independently. We also assume that each event contributes independently to the hazard of censoring. Nonparametric step functions are used to characterize the baseline distributions of the different event times and of the examination times. Covariate and subject‐specific effects are incorporated through generalized linear models. A Markov chain Monte Carlo algorithm is described for estimation of the posterior distributions of the unknowns. The methods are illustrated through application to multiple tumor site data from an animal carcinogenicity study.
Effect of List Errors on the Estimation of Population Size
Authors: Lee Alan
Published: May 2004

Summary. In many situations, it is possible to estimate the size of a closed population if some members of the population are recorded on one or more administrative lists. An important example is estimating the prevalence of a disease, where some members of the disease population may be recorded on lists such as disease registries, hospital admission records, and general practitioner records. An incomplete contingency table is formed by matching the lists and the missing cell count estimated by prediction based on a fitted model, which assumes that the matching is done without error. In practice, matching errors do occur, and in this article, we examine the effect of these errors on the estimation process and show how the standard models may be modified to allow for matching errors.
A Unified Parametric Regression Model for Recapture Studies with Random Removals in Continuous Time
Authors: Yip Paul S. F. , Wang Yan
Published: May 2004

Summary. Conditional likelihood based on counting processes are combined with a Horvitz‐Thompson estimator to yield a population size estimator that is more efficient than the existing ones. Random removals are allowed in the recapturing process. Simulation studies are shown to assess the performance of the proposed estimators. Examples on a bird banding and a small mammal recapturing study are given.
Levene Tests of Homogeneity of Variance for General Block and Treatment Designs
Authors: O'Neill Michael E. , Mathews Ky L.
Published: May 2004

Summary. This article develops a weighted least squares version of Levene's test of homogeneity of variance for a general design, available both for univariate and multivariate situations. When the design is balanced, the univariate and two common multivariate test statistics turn out to be proportional to the corresponding ordinary least squares test statistics obtained from an analysis of variance of the absolute values of the standardized mean‐based residuals from the original analysis of the data. The constant of proportionality is simply a design‐dependent multiplier (which does not necessarily tend to unity). Explicit results are presented for randomized block and Latin square designs and are illustrated for factorial treatment designs and split‐plot experiments. The distribution of the univariate test statistic is close to a standard F‐distribution, although it can be slightly underdispersed. For a complex design, the test assesses homogeneity of variance across blocks, treatments, or treatment factors and offers an objective interpretation of residual plots.