bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural Requirements for Outside-In and Inside-Out Signaling by Drosophila Neuroglian, a Member of the L1 Family of Cell Adhesion Molecules
Authors: Hortsch Michael , Homer Diahann , Malhotra Jyoti Dhar , Chang Sherry , Frankel Jason , Jefford Gregory , Dubreuil Ronald R.
Published: July 2002

Expression of the Drosophila cell adhesion molecule neuroglian in S2 cells leads to cell aggregation and the intracellular recruitment of ankyrin to cell contact sites. We localized the region of neuroglian that interacts with ankyrin and investigated the mechanism that limits this interaction to cell contact sites. Yeast two-hybrid analysis and expression of neuroglian deletion constructs in S2 cells identified a conserved 36-amino acid sequence that is required for ankyrin binding. Mutation of a conserved tyrosine residue within this region reduced ankyrin binding and extracellular adhesion. However, residual recruitment of ankyrin by this mutant neuroglian molecule was still limited to cell contacts, indicating that the lack of ankyrin binding at noncontact sites is not caused by tyrosine phosphorylation. A chimeric molecule, in which the extracellular domain of neuroglian was replaced with the corresponding domain from the adhesion molecule fasciclin II, also selectively recruited ankyrin to cell contacts. Thus, outside-in signaling by neuroglian in S2 cells depends on extracellular adhesion, but does not depend on any unique property of its extracellular domain. We propose that the recruitment of ankyrin to cell contact sites depends on a physical rearrangement of neuroglian in response to cell adhesion, and that ankyrin binding plays a reciprocal role in stabilizing the adhesive interaction.
In Vivo Importance of Actin Nucleotide Exchange Catalyzed by Profilin
Authors: Wolven Amy K. , Belmont Lisa D. , Mahoney Nicole M. , Almo Steven C. , Drubin David G.
Published: July 2002

The actin monomer-binding protein, profilin, influences the dynamics of actin filaments in vitro by suppressing nucleation, enhancing nucleotide exchange on actin, and promoting barbed-end assembly. Profilin may also link signaling pathways to actin cytoskeleton organization by binding to the phosphoinositide PIP2 and to polyproline stretches on several proteins. Although activities of profilin have been studied extensively in vitro, the significance of each of these activities in vivo needs to be tested. To study profilin function, we extensively mutagenized the Saccharomyces cerevisiae profilin gene (PFY1) and examined the consequences of specific point mutations on growth and actin organization. The actin-binding region of profilin was shown to be critical in vivo. act1-157, an actin mutant with an increased intrinsic rate of nucleotide exchange, suppressed defects in actin organization, cell growth, and fluid-phase endocytosis of pfy1-4, a profilin mutant defective in actin binding. In reactions containing actin, profilin, and cofilin, profilin was required for fast rates of actin filament turnover. However, Act1-157p circumvented the requirement for profilin. Based on the results of these studies, we conclude that in living cells profilin promotes rapid actin dynamics by regenerating ATP actin from ADP actin–cofilin generated during filament disassembly.
Patterns for RANTES Secretion and Intercellular Adhesion Molecule 1 Expression Mediate Transepithelial T Cell Traffic Based on Analyses In Vitro and In Vivo
Authors: Taguchi Masahiko , Sampath Deepak , Koga Takeharu , Castro Mario , Look Dwight C. , Nakajima Shin , Holtzman Michael J.
Published: July 2002

Immune cell migration into and through mucosal barrier sites in general and airway sites in particular is a critical feature of immune and inflammatory responses, but the determinants of transepithelial (unlike transendothelial) immune cell traffic are poorly defined. Accordingly, we used primary culture airway epithelial cells and peripheral blood mononuclear cells to develop a cell monolayer system that allows for apical-to-basal and basal-to-apical T cell transmigration that can be monitored with quantitative immunofluorescence flow cytometry. In this system, T cell adhesion and subsequent transmigration were blocked in both directions by monoclonal antibodies (mAbs) against lymphocyte function-associated antigen 1 (LFA-1) or intercellular adhesion molecule 1 (ICAM-1) (induced by interferon γ [IFN-γ] treatment of epithelial cells). The total number of adherent plus transmigrated T cells was also similar in both directions, and this pattern fit with uniform presentation of ICAM-1 along the apical and basolateral cell surfaces. However, the relative number of transmigrated to adherent T cells (i.e., the efficiency of transmigration) was increased in the basal-to-apical relative to the apical-to-basal direction, so an additional mechanism was needed to mediate directional movement towards the apical surface. Screening for epithelial-derived β-chemokines indicated that IFN-γ treatment caused selective expression of RANTES (regulated upon activation, normal T cell expressed and secreted), and the functional significance of this finding was demonstrated by inhibition of epithelial–T cell adhesion and transepithelial migration by anti-RANTES mAbs. In addition, we found that epithelial (but not endothelial) cells preferentially secreted RANTES through the apical cell surface thereby establishing a chemical gradient for chemotaxis across the epithelium to a site where they may be retained by high levels of RANTES and apical ICAM-1. These patterns for epithelial presentation of ICAM-1 and secretion of RANTES appear preserved in airway epithelial tissue studied either ex vivo with expression induced by IFN-γ treatment or in vivo with endogenous expression induced by inflammatory disease (i.e., asthma). Taken together, the results define how the patterns for uniform presentation of ICAM-1 along the cell surface and specific apical sorting of RANTES may serve to mediate the level and directionality of T cell traffic through epithelium (distinct from endothelium) and provide a basis for how this process is precisely coordinated to route immune cells to the mucosal surface and maintain them there under normal and stimulated conditions.
Extended Duration of DH–JH Rearrangement in Immunoglobulin Heavy Chain Transgenic Mice: Implications for Regulation of Allelic Exclusion
Authors: Chang Yung , Bosma Melvin J. , Bosma Gayle C.
Published: July 2002

Here we show that suppression of VH–DJH rearrangement in mice bearing a μ heavy (H) chain transgene (μ-tg mice) is associated with an extended period of DH–JH rearrangement, the first step of Immunoglobulin H chain gene rearrangement. Whereas DH–JH rearrangement is normally initiated and completed at the pro-B cell stage, in μ-tg mice it continues beyond this stage and occurs most frequently at the small (late) pre-B stage. Despite ongoing DH–JH rearrangement in late pre-B cells of μ-tg mice, VH–DJH rearrangement is not detectable in these cells. We infer that the lack of VH–DJH rearrangement primarily reflects tg-induced acceleration of B cell differentiation past the stage at which rearrangement of VH elements is permissible. In support of this inference, we find that the normal representation of early B lineage subsets is markedly altered in μ-tg mice. We suggest that the effect of a productive VH–DJH rearrangement at an endogenous H chain allele may be similar to that of a μ-tg; i.e., cells that make a productive VH–DJH rearrangement on the first attempt rapidly progress to a developmental stage that precludes VH–DJH rearrangement at the other allele (allelic exclusion).
New estimates for Ritz vectors
Authors: Knyazev Andrew
Published: July 2002

The following estimate for the Rayleigh–Ritz method is proved:
\[
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="StartAbsoluteValue lamda overTilde minus lamda EndAbsoluteValue StartAbsoluteValue left-parenthesis u overTilde comma u right-parenthesis EndAbsoluteValue less-than-or-equal-to double-vertical-bar upper A u overTilde minus lamda overTilde u overTilde double-vertical-bar sine normal angle left-brace u semicolon upper U overTilde right-brace comma double-vertical-bar u double-vertical-bar equals 1 period">

|

λ
~

−
λ

|

|

(

u
~

,
u
)

|

≤

‖
A

u
~

−

λ
~

u
~

‖

sin
⁡
∠
{
u
;

U
~

}
,
‖
u
‖
=
1.

| \tilde \lambda - \lambda | |( \tilde u , u )| \le { \| A \tilde u - \tilde \lambda \tilde u \| } \sin \angle \{ u ; \tilde U \}, \| u \| =1.

\]
Here

A
A

is a bounded self-adjoint operator in a real Hilbert/euclidian space,

{
λ
,
u
}

\{ \lambda , u \}

one of its eigenpairs,

U
~

\tilde U

a trial subspace for the Rayleigh–Ritz method, and

{

λ
~

,

u
~

}

\{ \tilde \lambda , \tilde u \}

a Ritz pair. This inequality makes it possible to analyze the fine structure of the error of the Rayleigh–Ritz method, in particular, it shows that

|

(

u
~

,
u
)

|

≤
C

ϵ
2

,

|( \tilde u , u )| \le C \epsilon ^2,

if an eigenvector

u
u

is close to the trial subspace with accuracy

ϵ
\epsilon

and a Ritz vector

u
~

\tilde u

is an

ϵ
\epsilon

approximation to another eigenvector, with a different eigenvalue. Generalizations of the estimate to the cases of eigenspaces and invariant subspaces are suggested, and estimates of approximation of eigenspaces and invariant subspaces are proved.
Prime clusters and Cunningham chains
Authors: Forbes Tony
Published: July 2002

We discuss the methods and results of a search for certain types of prime clusters. In particular, we report specific examples of prime 16-tuplets and Cunningham chains of length 14.
Finite simple groups which projectively embed in an exceptional Lie group are classified!
Authors: Griess Robert , Ryba A.
Published: July 2002

Since finite simple groups are the building blocks of finite groups, it is natural to ask about their occurrence “in nature”. In this article, we consider their occurrence in algebraic groups and moreover discuss the general theory of finite subgroups of algebraic groups.
Orthogonal harmonic analysis of fractal measures
Authors: Jorgensen Palle , Pedersen Steen
Published: July 2002

We show that certain iteration systems lead to fractal measures admitting an exact orthogonal harmonic analysis.
The LabBase system for data management in large scale biology research laboratories.
Authors: Goodman N , Rozen S , Stein L D , Smith A G
Published: July 2002

MOTIVATION: The development of laboratory information management systems (LIMSs) for large scale biology research projects can be a challenging problem. Many such projects generate complex datasets via complex procedures that undergo continuous refinement. A key software challenge is to simplify the database-development task so that databases can be built and modified quickly enough to keep pace with changing project-requirements. Results: LabBase extends the facilities offered by relational database systems to simplify the task of creating databases for large scale biology research projects. LabBase provides a structural object data model, similar to ACEDB, and adds to this the concepts of Materials, Steps, and States: Materials are objects representing the identifiable things that participate in a laboratory protocol; Steps are objects reporting the results of a laboratory or analytical procedure; and States are objects denoting places in a laboratory protocol. The system provides a data definition language for succinctly defining laboratory databases, and operations for conveniently storing and retrieving data in such databases. The system also provides support for workflow management. LabBase is implemented in Perl5 and provides a natural interface for laboratory application programs written in Perl. AVAILABILITY: The software is freely available. Contact the authors. CONTACT: nat@jax.org
In-vitro activity of levofloxacin, a new fluoroquinolone: evaluation against Haemophilus influenzae and Moraxella catarrhalis
Authors: Kitzis M.-D. , Goldstein F. W. , Miegi M. , Acar J. F.
Published: July 2002

The in-vitro activity of levofloxacin was studied against 10 β-lactamase-negative and 93 β -lactamase-positive Moraxella catarrhalis isolates, and 65 β -lactamase-negative and 35 β-lactamase-positive Haemophilus influenzae isolates. The MICs of levofloxacin were determined by agar dilution on Mueller–Hinton agar (with the addition of 5% horse blood for M. catarrhalis ) or on Haemophilus Test Medium for H. influenzae, and were compared with those of ofloxacin, ciprofloxacin and sparfloxacin, as well as pefloxacin and d -ofloxacin for M. catarrhalis. The fluoroquinolones showed similar activity against isolates of H. influenzae and M. catarrhalis, irrespective of β-lactamase production. Levofloxacin (MIC 50/90 0.06 mg/L) was 64 times more active against M. catarrhalis than d -ofloxacin (MIC 50/90 4/8 mg/L) and twice as active as ofloxacin (MIC 50/90 0.125 mg/L). Ciprofloxacin had an MIC 50/90 of 0.03/0.06 mg/L and sparfloxacin showed an MIC 50/90 of 0.015 mg/L against M. catarrhalis irrespective of the resistance phenotype of the isolates. Against H. influenzae, levofloxacin was twice as active as ofloxacin (MIC 90 values 0.03 mg/L versus 0.06 mg/L), while the MIC 90 s of ciprofloxacin and sparfloxacin were both 0.015 mg/L. Our results therefore suggest that levofloxacin has potential for treating respiratory tract infections caused by H. influenzae and M. catarrhalis.
Temporal resolution in mesopelagic crustaceans
Authors: Frank Tamara M.
Published: July 2002

Mesopelagic crustaceans occupy a dim–light environment that is similar to that of nocturnal insects. In a light–limited environment, the requirement for greater sensitivity may result in slower photoreceptor transduction and increased summation time. This should be reflected by a lower temporal resolution, as indicated by a lower critical flicker fusion frequency (CFF). Therefore, one would predict that the CFFs of mesopelagic organisms would be relatively low compared with those of their shallow–water relatives, just as nocturnal insects tend to have lower CFFs than diurnal insects. Using an electrophysiological apparatus that was adapted for shipboard use, the dark–adapted CFFs of a variety of species of mesopelagic crustaceans were determined using the electroretinogram. The parameter examined was the maximum CFF—the point at which further increases in irradiance no longer result in a faster flicker fusion frequency. The results summarized here indicate that there is a trend towards lower CFFs with increasing habitat depth, with some interesting exceptions.
Thermodynamics of single peptide bond cleavage in bovine pancreatic trypsin inhibitor (BPTI)
Authors: Buczek Olga , Krowarsch Daniel , Otlewski Jacek
Published: July 2002

A major goal of this paper was to estimate a dynamic range of equilibrium constant for the opening of a single peptide bond in a model protein, bovine pancreatic trypsin inhibitor (BPTI). Ten mutants of BPTI containing a single Xaa→Met substitution introduced in different parts of the molecule were expressed in Escherichia coli. The mutants were folded, purified to homogeneity, and cleaved with cyanogen bromide to respective cleaved forms. Conformation of the intact mutants was similar to the wildtype, as judged from their circular dichroism spectra. Substantial conformational changes were observed on the chemical cleavage of three single peptide bonds—Met46‐Ser, Met49‐Cys, and Met53‐Thr—located within the C‐terminal helix. Cleavage of those peptide bonds caused a significant destabilization of the molecule, with a drop of the denaturation temperature by 56.4°C to 68°C at pH 4.3. Opening of the remaining seven peptide bonds was related to a 10.8°C to 39.4°C decrease in Tden. Free energies of the opening of 10 single peptide bonds in native mutants (ΔGop,N) were estimated from the thermodynamic cycle that links denaturation and cleavage free energies. To calculate those values, we assumed that the free energy of opening of a single peptide bond in the denatured state (ΔGop,D) was equal to −2.7 kcal/mole, as reported previously. Calculated ΔGop,N values in BPTI were in the range from 0.2 to 10 kcal/mole, which was equivalent to a >1 million–fold difference in equilibrium constants. The values of ΔGop,N were the largest for peptide bonds located in the C‐terminal helix and significantly lower for peptide bonds in the β‐structure or loop regions. It appears that opening constants for single peptide bonds in various proteins span across 33 orders of magnitude. Typical equilibrium values for a single peptide bond opening in a protein containing secondary structure elements fall into negligibly low values, from 10−3 to 10−8, and are efficient to ensure stability against proteolysis.
Neutron dosimetry for a general brachytherapy source
Authors: Rivard Mark J.
Published: July 2002

This paper extends previous work to characterize neutron dosimetry in the vicinity of brachytherapy sources. A general source is examined with an arbitrary length, diameter, and encapsulation using Monte Carlo methods. Fast neutron dosimetry and thermal neutron fluence rates were determined in a variety of clinically relevant media of varying dimensions. Applicator Tube, point source, high dose rate VariSource, and high dose rate μSelectron source geometries were analyzed. Fast neutron dosimetry was relatively independent of encapsulation thickness for an assortment of encapsulation materials less than 2 mm thick. Large variations in phantom size made minimal differences in the fast neutron dose close to the source. Specific source geometries were compared with dosimetry obtained from a simplified point model. The consequence of these results is a convenient means of accurately predicting clinical fast neutron dosimetry characteristics around a general brachytherapy source in a variety of media without requiring neutron transport. Thermal neutron fluence rates were determined for a variety of source encapsulation materials, encapsulation thicknesses, and phantom sizes. At a distance of 3 cm from the source center, the thermal neutron fluence rate for a 30 cm diameter phantom was a 2.65 times greater than for a 10 cm diameter water phantom. These results demonstrate thermal neutron fluence rate is relatively independent of encapsulation thickness and composition, yet highly dependent on hydrogen mass density and phantom size for phanta with diameters <30 cm.
Model‐based scatter correction for positron emission tomography (in German)
Authors: Werling Alexander
Published: July 2002

Positron emission tomography (PET) images include a significant scatter component. The presence of the scatter manifests itself as a loss of spatial resolution and as an apparent migration of activity from hot to cold regions. Monte Carlo (MC) based approaches have proved to be very useful in simulating the image‐forming process in the PET scanner. However, fully 3D MC simulations are computationally too intensive to be applied in clinical routine. Consequently, many efforts have been undertaken to develop approximate scatter correction techniques for PET. Scatter correction is generally performed prior to image reconstruction using an appropriate model of the scatter process. These models require estimates of the correct emission and attenuation distribution in the imaged object. The problem is that these estimates are computed from measured data and, therefore, already contain scattered events. The purpose of this work was to overcome this problem by incorporating scatter characteristics directly into the process of iterative image reconstruction. This was achieved by an optimized implementation of the Single Scatter Simulation (SSS) algorithm resulting in a significant speed‐up of the scatter estimation procedure (from about 10 min to 30 s). The computationally improved SSS algorithm was then included in the forward projection step of maximum likelihood image reconstruction. Results obtained from phantom measurements demonstrate that this approach leads to a better estimation of the scatter component than can be obtained by a simple sequential data processing strategy.
Improved contrast in multispectral phase images derived from magnetic resonance exams of multiple sclerosis patients
Authors: Mitchell J. R. , Rutt B. K.
Published: July 2002

We describe a method to extract data from multispectral MR exams of patients with Multiple Sclerosis (MS). The technique produces images of “spectral phase” (SP) relative to a reference tissue. SP images allow retrospective suppression of signal in the reference tissue, while maintaining high spatial resolution. Image quality in SP images was determined from MR exams of 5 MS patients selected at random from a clinical trial underway at our institute. Exams consisting of proton density weighted (PDw), T2 weighted (T2w), T1 weighted (T1w), and gadolinium‐DTPA enhanced T1w (GAD) images were acquired from each patient. The MR exams were corrected for intensity nonuniformity, then filtered with an algorithm based upon anisotropic diffusion, to reduce noise. Principal component (PC) images and SP images relative to cerebrospinal fluid normal appearing white matter gray matter and temporalis muscle were then calculated. Contrast between tissues and MS lesions in the MR and derived images was then determined by measuring the signal‐difference‐to‐noise ratio (dSNR) between tissues. Our new SP images provided better tissue contrast than the original MR, filtered MR, and PC images. Contrast improved between CSF and NAWM (from 19.5 to 56), CSF and GM (from 15 to 36), GM and NAWM (from 8 to 14), MS lesions and CSF (from 16 to 35), and between MS lesions and NAWM (from 24 to 47). (Maximum contrast in the original MR images compared to maximum contrast in the SP images.) The additional contrast in SP images may aid the quantification and analysis of lesion activity in MR exams of MS patients.
Evaluation of a diode detector array for use as a linear accelerator QC device
Authors: Watts Ronald J.
Published: July 2002

A series of tests were conducted to evaluate the accuracy of a diode detector array in measuring flatness and symmetry of x‐ray and electron energies, and the ability to determine change in beam energy. The results show the diode detector array accurately measures flatness and symmetry and can accurately measure or detect change in beam energy. The evaluation also demonstrates the utility of using this model detector system in a linear accelerator QC program.
Photon beam skin dose analyses for different clinical setups
Authors: Kim Siyong , Liu Chihray R. , Zhu Timothy C. , Palta Jatinder R.
Published: July 2002

A comprehensive set of data on skin dose for 8 MV and 18 MV photon beams from a medical linear accelerator was measured using a parallel‐plate chamber to document the effect of field size, source‐to‐surface distance (SSD), off‐axis distance, acrylic block tray, wedge (external standard wedge), Lipowitz's metal block, multileaf collimator (MLC), and dynamic wedge. The skin dose increased as field size increased from to (6% to 38% for 8 MV and 5% to 44% for 18 MV beam). With the use of an acrylic block tray, the skin dose increased for all field sizes (7% to 59% for 8 MV and 5% to 62% for 18 MV beam), but the increase was minimal for small fields. The skin dose with a wedge showed a much more complex trend. It was generally lower than the dose for an open field, but higher in the case of large fields and higher degree wedges. When both wedge and block tray were used, the tray was a major contributor to the skin dose because some of the contaminant electrons from the wedge assembly were absorbed by the block tray. Field‐shaping blocks increased the skin dose, but, interestingly, the block tray reduced the skin dose for small blocked fields treated with a high‐energy photon beam. The effect of an MLC on skin dose was very similar to that of a Lipowitz's metal block, but its magnitude was less. The skin dose was higher for dynamic wedge fields than it was for standard wedge fields. As SSD decreased, the skin dose increased, and this effect was dominant in larger field sizes. The SSD effect was enhanced in the presence of an acrylic block tray. The skin dose off‐axis was the same as at the central axis, or smaller. A similar pattern of behavior of the skin dose is expected for photon beams from other linear accelerators.
Linear accelerator output variations and their consequences for megavoltage imaging
Authors: Partridge M. , Evans P. M. , Mosleh‐Shirazi M. A.
Published: July 2002

An experimental study of radiation output intensity fluctuations of a Philips SL25 linear accelerator is presented. Measurements are obtained using an electronic portal imaging device, and the consequences of the measured fluctuations for various different applications of megavoltage imaging including portal imaging, transit dosimetry and megavoltage computed tomography (MVCT) are discussed with examples. Fluctuations in output of 0.7% (1 SD) are seen on every radiation pulse after photon noise and uncertainties caused by the detection system have been accounted for. Large fluctuations are also seen during the initial beam stabilization period (15%), during normal accelerator operation after the beam has been on for more than 1 min (4.5%) and during arc therapy as a repeatable function of gantry angle (9%). Such output intensity fluctuations are shown to produce image artifacts in portal imaging devices with scanned detector readout and can also produce systematic errors in detector calibration that would lead to uncertainty in transit dose calculations. The propagation of these intensity fluctuations through MVCT image reconstruction is shown to produce ring artifacts in the reconstructed image. Sample portal and MVCT images are presented. All observed fluctuations in accelerator output are well within the manufacturer's specifications and do not affect the total dose delivered during normal treatment. Finally, megavoltage imaging is shown to be a powerful tool for accelerator quality assurance and treatment verification.
Magnetic resonance imaging based digitally reconstructed radiographs, virtual simulation, and three‐dimensional treatment planning for brain neoplasms
Authors: Ramsey Chester R. , Oliver Adrian L.
Published: July 2002

Currently, patients with brain neoplasms must undergo both computed tomography (CT) and magnetic resonance (MR) imaging to take advantage of CT's density information and MR's soft tissue imaging capabilities. A method has been developed that allows virtual simulation, digitally reconstructed radiographs (DRRs), and 3‐D treatment planning of patients with brain neoplasms to be generated using only one T1‐weighted MR data set. DRRs of an anthropomorphic RANDO head phantom were generated using MR and CT imaging. The MR based DRRs provided structural information equivalent to CT based DRRs. The spatial linearity of CT and MR image sets was evaluated by measuring the percent distortion and spatial error. There was no statistical difference in spatial linearity or accuracy between the CT and MR image sets. MR and CT based treatment planning were compared using a variety of different treatment accessories, field sizes, photon energies, and gantry positions. Doses at various points throughout the head phantom were used as comparison points between CT based heterogeneous, CT based homogenous, and MR based homogenous treatment planning of the head phantom. Lithium fluoride thermoluminescent dosimeters were used to verify the dosimetric accuracy of MR based treatment planning by taking measurements at these points. For treatment plans with fields that pass through large air cavities, such as the maxillary sinus, homogenous treatment planning produces unacceptable dosimetric error (2%–4%). For treatment plans with fields that pass through the skull, MR homogenous treatment planning can be used with a dosimetric accuracy of ±2%.
An undergraduate laboratory experiment on quantized conductance in nanocontacts
Authors: Foley E. L. , Candela D. , Martini K. M. , Tuominen M. T.
Published: July 2002

We describe an undergraduate laboratory experiment on conductance steps observed to occur near integer multiples of 2e2/h as nanocontacts form and break between gold wires in loose contact. This experimental arrangement was first described by Costa-Krämer et al. [Surf. Sci. 342, L1144 (1995)]. A simple op-amp circuit in conjunction with an inexpensive storage oscilloscope suffice to observe the steps. The experiment may be extended by interfacing to a computer, which accumulates a histogram of conductance values as the wires are brought into and out of contact many times. The histogram shows peaks near integer multiples of 2e2/h. We emphasize the pedagogical issues involved in bringing an experiment from forefront condensed-matter physics research into the undergraduate laboratory.
On the frequency dependence of the otoacoustic emission latency in hypoacoustic and normal ears
Authors: Sisto R. , Moleti A.
Published: July 2002

Experimental measurements of the otoacoustic emission (OAE) latency of adult subjects have been obtained, as a function of frequency, by means of wavelet time-frequency analysis based on the iterative application of filter banks. The results are in agreement with previous OAE latency measurements by Tognola et al. [Hear. Res. 106, 112–122 (1997)], as regards both the latency values and the frequency dependence, and seem to be incompatible with the steep 1/f law that is predicted by scale-invariant full cochlear models. The latency-frequency relationship has been best fitted to a linear function of the cochlear physical distance, using the Greenwood map, and to an exponential function of the cochlear distance, for comparison with derived band ABR latency measurements. Two sets of ears [94 audiometrically normal and 42 impaired with high-frequency (f>3 kHz) hearing loss] have been separately analyzed. Significantly larger average latencies were found in the impaired ears in the mid-frequency range. Theoretical implications of these findings on the transmission of the traveling wave are discussed.
Detectability of simultaneously masked signals as a function of masker bandwidth and configuration for different signal delays
Authors: Wright Beverly A.
Published: July 2002

Signal detectability was measured in three temporal conditions as a function of the bandwidth and configuration of simultaneous maskers that either did or did not spectrally overlap the signal. The 20-ms signal was 250 Hz wide and was centered at 2500 Hz (fs). Although there were marked individual differences, performance was typically poorer when signal onset came 1 ms rather than 250 ms after the onset of a 420-ms masker, and poorest when signal onset came 1 ms after the onset of a 23-ms masker. The results support the idea that two separate across-channel processes contribute to temporal changes in signal detectability. One process contributes to the improvement observed as signal onset is delayed from masker onset, and its influence is reduced by the presence of masking components at fs only when the masker extends exclusively below fs. The other process is associated with the improvement observed as masker offset is delayed from signal offset, and its influence is reduced by the presence of masking components at fs when the masker extends exclusively above, or both below and above fs. Both of these processes are primarily activated by frequencies ranging from 0.6 to 0.8fs and 1.2 to 1.4fs. The data also demonstrate that the measured critical bandwidth narrows as signal onset is delayed from masker onset.
Exploration of the perceptual magnet effect using the mismatch negativity auditory evoked potential
Authors: Sharma Anu , Dorman Michael F.
Published: July 2002

The goals of this study were (i) to assess the replicability of the “perceptual magnet effect” [, J. Acoust. Soc. Am. 97(1), 553–561 (1995)] and (ii) to investigate neurophysiologic processes underlying the perceptual magnet effect by using the mismatch negativity (MMN) auditory evoked potential. A stimulus continuum from /i/ to /e/ was synthesized by varying F1 and F2 in equal mel steps. Ten adult subjects identified and rated the goodness of the stimuli. Results revealed that the prototype was the stimulus with the lowest F1 and highest F2 values and the nonprototype stimulus was close to the category boundary. Subjects discriminated stimulus pairs differing in equal mel steps. The results indicated that discrimination accuracy was not significantly different in the prototype and the nonprototype condition. That is, no perceptual magnet effect was observed. The MMN evoked potential (a preattentive, neurophysiologic index of auditory discrimination) revealed that despite equal mel differences between the stimulus pairs the MMN was largest for the prototype pair (i.e., the pair that had the lowest F1 and highest F2 values). Therefore the MMN appears to be sensitive to within category acoustic differences. Taken together, the behavioral and electrophysiologic results indicate that discrimination of stimulus pairs near a prototype is based on the auditory structure of the stimulus pairs.
Conservation of energy and absorption in acoustic fields for linear and nonlinear propagation
Authors: Wójcik Janusz
Published: July 2002

In the present paper, the energy effects accompanying a strong sound disturbance of a medium are analyzed. The waves may be, in time, periodic — continuous or pulsed — or have the form of single pulses. The description is based on equations which are commonly applied in nonlinear acoustics. The Fourier analysis, elements of the theory of linear operators, and analytical functions are applied. A general method is given for the construction of the absorption operator in the domain of space–time coordinates (x,t), to which the small-signal absorption coefficient corresponds. By analogy to linear equations and the corresponding dispersions equations, the quasi-dispersion equations in the case of nonlinear description are introduced. Simplification of the “classical” equation of nonlinear acoustics was performed. The relations between absorption operators in the space and time domains are shown. It is demonstrated that in nonlinear interactions, where terms of such type — nonlinear function of pressure — dominate, the power (energy) of the disturbance is conserved. Just as in the linear notation, the only reason why the total power (energy) changes is linear absorption, but that one which occurs under the conditions of nonlinear propagation. In consequence, the equations of power (and energy) balancing the disturbance have the same formal shape in nonlinear and linear descriptions. The equations provide a theoretical basis for different, easier, and more accurate methods than those used previously for determination (numerical and experimental) of, e.g., the power density of heat sources generated by sound. The function of the nonlinear gain of absorption and the function of effective absorption were also introduced. On the basis of quasi-dispersion equations the phenomenon of overtone generation (not harmonics) is shortly discussed.
Using tubephones for wideband auditory stimulation in brain research
Authors: Airas Matti , Jarvinen Antti , Piirila Esa
Published: July 2002

This study was motivated by the need for a high-quality auditory stimulation system for brain research purposes. Magnetic imaging techniques such as functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) prevent the use of typical headphones. In fMRI, a high magnetic field does not allow placing magnetic materials inside the imaging system. In MEG, the extremely weak magnetic fields present in the human brain are measured and any magnetic material in the magnetically shielded room would corrupt the results. In these cases, one possible solution is to place electromagnetic transducers outside of the measurement room. Sound has then to be transmitted to the patient’s ears by tubes which produce a highly distorted response. It is shown that by combining acoustical design with analog and digital equalization, wideband flat frequency response is achieved. The design and construction of the apparatus are described and problems like the effect of nonlinear distortion of the transducers are discussed. Practical applications of the system are shown and other possible constructions are compared with the implemented one.
Strategies used to detect auditory signals in small sets of random maskers
Authors: Wright Beverly A. , Saberi Kourosh
Published: July 2002

Detection performance for a masked auditory signal of fixed frequency can be substantially degraded if there is uncertainty about the frequency content of the masker. A quasimolecular psychophysical approach was used to examine response strategies in masker-uncertainty conditions, and to investigate the influence of uncertainty when the number of different masker samples was limited to ten or fewer. The task of the four listeners was to detect a 1000-Hz signal that was presented simultaneously with one of ten ten-tone masker samples. The masker sample was either fixed throughout a block of two-interval forced-choice trials or was randomized across or within trials. The primary results showed that: (1) When the signal level was low and the masker sample differed between the two intervals of a trial, most listeners based their responses more on the presence of specific masker samples than on the signal. (2) The detrimental effect of masker uncertainty was clearly evident when only four maskers were randomly presented, and grew as the size of the masker set was increased from two to ten. (3) The slopes of psychometric functions measured with the same masker samples differed among the fixed and two random-masker conditions. (4) There were large differences in the influence of masker uncertainty across masker samples and listeners. These data demonstrate the great susceptibility of human listeners to the influence of masker uncertainty and the ability of quasimolecular investigations to reveal important aspects of behavior in uncertainty condition.
Maturation of medial efferent system function in humans
Authors: Abdala Carolina , Ma Ellen , Sininger Yvonne S.
Published: July 2002

Otoacoustic emissions are typically reduced in amplitude when broadband noise is presented to the contralateral ear. This contralateral suppression is attributed to activation of the medial olivocochlear system, which has an inhibitory effect on outer hair-cell activity. By studying the effects of contralateral noise on cochlear output at different stages of auditory maturation in human neonates, it is possible to describe the timecourse for development of medial efferent system function in humans. The present study recorded 2 f1–f2 distortion product otoacoustic emissions (DPOAE) in human adults, term and premature neonates at three f2 frequencies: 1500, 3000, and 6000 Hz, using fixed primary tone frequency ratio (f2/f1=1.2) and level separation (10 dB, L1>L2). Average DPOAE growth functions were recorded with and without contralateral broadband noise. Results indicate that contralateral suppression of DPOAEs is absent at 6000 Hz, but present at 1500 and 3000 Hz for all ages. However, DPOAE amplitude from premature neonates was not altered by noise in an adult-like manner; in this age group, DPOAE amplitude was equally likely to be suppressed or enhanced by noise presented contralaterally. Contralateral enhancement may reflect a temporary stage of immaturity in outer hair cell-medial efferent fiber synapses just prior to term birth.
Auditory temporal integration in the rhesus macaque (Macaca mulatta)
Authors: O’Connor Kevin N. , Barruel Phillip , Hajalilou Reza , Sutter Mitchell L.
Published: July 2002

Temporal integration for pure tones was examined in two rhesus macaques. The subjects were required to respond to a brief sound (a tone burst) that deviated from a previous series of sounds (noise bursts) on a trial (a deviant-stimulus detection paradigm). Psychometric functions and thresholds were determined from correct detections (hit proportions) alone, and from d′ scores. Two models describing the decline in threshold as a function of stimulus duration, one a power function the other an exponential, were tested against the data. When the decline (slope) in threshold per log stimulus duration is used as a rate measure, our results yield a lower estimate of temporal integration rate in rhesus than did a previous study [Clack, J. Acoust. Soc. Am. 40, 1140–1146 (1966)]. Both studies, however, gave slope estimates of integration rate that were higher than in most other species. Comparison of the models using data from several species, revealed that the exponential, but not the power model, could account for two sources of variation in threshold measurement. One source is due to the range across threshold as a function of duration (the linear rate component), and is described by the constant of proportionality Ik in the model. The other source of variation arises from the rate of decline within this range (the nonlinear rate component), and is described by the time constant τ. In terms of this model, differences in rate estimates between Clack’s study and ours (and between rhesus and other species) are primarily due to the linear component. The nonlinear rate component was about equal for our study and Clack’s (τ=∼150 ms): a time constant that is just slightly larger (indicating a rate of temporal integration slightly slower) than for most other species examined.
Stress Dependence of Poisson's Ratio and of the Softening Temperature of Plastics
Authors: Nielsen Lawrence E.
Published: July 2002

Poisson's ratio was measured on a variety of plastics as a function of load up to 4000 psi by two methods. Poisson's ratio generally remained constant or decreased slightly with load. However, with crazed polyblends the ratio drastically decreased from 0.33 to 0.15 at high loads applied for long times. The low value can be explained by the growth of cracks perpendicular to the direction of the applied tensile stress. It has been shown that the lowering of the heat-distortion temperature with stress can not be explained in terms of the lowering of the glass temperature resulting from the volume increase on application of a stress. Instead, the lowering of the softening temperature can be explained in terms of the shape of the modulus vs. temperature curve. The new theory agrees quite well with the experimental values. Materials with a broad softening range have a heat-distortion temperature which is strongly dependent upon the load.
Flow Properties of a Partially Crystalline Polyethylene
Authors: Porter Roger S. , Johnson Julian F.
Published: July 2002

This study defines experimentally the steady flow of a partially crystalline polyethylene. The polymer has been exposed to high and variable shear rates at temperatures from about 40°C above to about 40°C below the polymer melting point. The polymer tested was a branched polyethylene, specific gravity 0.92 (20°C), melting point 103°C with a viscosity-average molecular weight of 2800. Viscosities measured above the melting point were independent of shear over the experimental shear rate range, 30–10,000 sec−1. Viscosities measured below the melting point became progressively more shear dependent with decreasing test temperature. A rotational concentric cylinder viscometer and a pressure capillary viscometer were used. The results have been interpreted in terms of temperature and shear dependence in order to evaluate possible mechanisms in flow such as orientation and destruction of crystalline regions. Estimates of the magnitude of molecular associations from several approaches have been made. Flow activation energies were calculated for the melt and as a function of shear rate for the partially crystalline state.
The Rheology of Phase Growth in Elastic Liquids
Authors: Street J. R.
Published: July 2002

A theoretical analysis of the spherically symmetric growth of vapor bubbles in viscoelastic liquids of the Oldroyd type is presented. The momentum transfer process is described by a nonlinear integro-differential equation for which analytical and numerical solutions have been obtained. The overall picture of bubble growth in such liquids compared to growth in a corresponding Newtonian liquid of the same shear viscosity is one of a high initial growth rate followed essentially by a cessation of growth when the bubbles interact to form thin liquid films. Furthermore, under certain conditions the analysis shows that because of the elastic nature of the liquid, tensile stresses can build up near the bubble surface in a rapid manner. This could lead to rupture of the liquid. Simplified methods are presented for the evaluation of elastic effects in expanding systems. This work sheds light on the general applicability of existing rheological models of viscoelastic liquids to engineering flow problems. Furthermore, it provides these existing models with critical tests which lie at the foundations of the conceptualization of viscoelastic liquids.
A Study of the Density of Polymer Melts During Extrusion
Authors: Rudin Alfred , Vlasschaert A. Gerry
Published: July 2002

Measurements have been made of the apparent density of polymer melts extruding from a reservoir into a capillary orifice. The technique involves simultaneous measurement of extrudate mass and the corresponding decrease in volume of polymer in the reservoir. Data at low shear rates agree closely with density values from other experimental methods. Sequential density readings fluctuate because variations in speed of the driving piston are damped by successive compressions and expansions of the polymer melt. A linear polyethylene was shown to suffer a significant density decrease as the shear rate was increased. This dilation is due to the triaxial action of normal stresses and can be used, with an appropriate model, to estimate the primary normal stress difference and recoverable shear in the flowing melt.
Wall slip of polyisobutylene: Interfacial and pressure effects
Authors: Koran François , Dealy John M.
Published: July 2002

A high-pressure sliding plate rheometer was used to investigate the flow behavior of polyisobutylene in simple shear. Experiments carried out using smooth steel surfaces revealed that wall slip is a dominant feature of the flow at the pressures, temperatures, and rates of deformation typically found in the processing of elastomers. A set of grooved plates made it possible to obtain viscosity data at stresses up to 20 kPa, but at higher stresses, the sample “slipped” even on the grooved plates. These data were fitted to a Cross viscosity model and extrapolated in order to estimate the viscosity at higher stresses. Flow curves (shear stress versus nominal shear rate) generated using fluoroelastomer–steel and steel–steel pairs of plates exhibited four distinct flow regimes: no-slip, adhesive slip, mixed adhesive, and cohesive slip, and primarily cohesive slip. Slip velocities were calculated by comparing the no-slip stresses estimated using the Cross model with measured values. The effect of pressure on cohesive wall slip was found to scale with pressure in the same way as viscosity, while the effect of pressure on adhesive wall slip did not.