Contributions of suppression and excitation to simultaneous masking: effects of signal frequency and masker-signal frequency relation

This study investigated the contributions of suppression and excitation to simultaneous masking for a range of masker frequencies both below and above three different signal frequencies (750, 2000, and 4850 Hz). A two-stage experiment was employed. In stage I, the level of each off-frequency simultaneous masker necessary to mask a signal at 10 or 30 dB sensation level was determined. In stage II, three different forward-masking conditions were tested: (1) an on-frequency condition, in which the signals in stage I were used to mask probes of the same frequency; (2) an off-frequency condition, in which the off-frequency maskers (at the levels determined in stage I) were used to mask the probes; and (3) a combined condition, in which the on- and off-frequency maskers were combined to mask the probes. If the off-frequency maskers simultaneously masked the signal via spread of excitation in stage I, then the off-frequency and combined maskers should produce considerable forward masking in stage II. If, on the other hand, they masked via suppression, they should produce little or no forward masking. The contribution of suppression was found to increase with increasing signal frequency; it was absent at 750 Hz, but dominant at 4850 Hz. These results have implications for excitation pattern analyses and are consistent with stronger nonlinear processing at high rather than at low frequencies.     

On the accurate measurement of amide one-bond 15N-1H couplings in proteins: effects of cross-correlated relaxation, selective pulses and dynamic frequency shifts

Amide one-bond 15N-1H scalar couplings of 15N- and [15N,2H]-isotopically enriched ubiquitin have been measured with the Quantitative J approach by monitoring NMR signal intensity modulation. Scalar couplings of the non-deuterated protein are in average approximately 0.6 Hz larger than values of deuterated ubiquitin. This deviation is 30 times the error derived from experiment reproducibility. Refocusing dipole/dipole cross-correlated relaxation decreases the discrepancy to approximately 0.1 Hz, suggesting that it likely originates from relaxation interference. Alternatively, the subtraction of J values obtained at different magnetic fields largely reduces the relaxation effects. In contrast, the dynamic frequency shift whose main contribution to 1J(15N-1H) arises from 15N chemical shielding anisotropy/NH dipole cross-correlation, is not eliminated by refocusing spin evolution under this interaction. Furthermore, the average difference of 1J(15N-1H) values at two magnetic fields closely agrees with the theoretical expected difference in the dynamic frequency shift.     

Curvature effects on magnetic susceptibility of 1D attractive two component fermions

We develop a bosonization approach for finding magnetic susceptibility of 1D attractive two component Fermi gas at the onset of magnetization taking into account the curvature effects. It is shown that the curvature of free dispersion at the Fermi points couples the spin and charge modes and leads to a linear critical behavior and finite susceptibility for a wide range of models. Possible manifestations of spin-charge coupling in cold atomic gases are also briefly discussed.     

Analysis of the high-temperature spin-glass susceptibility: Determination of the local magnetic exchange

New measurements are reported of the magnetic susceptibility above the freezing temperature T_f for noble-metal spin-glasses with $\text{1}\text{2}$ to 6 at.% Fe or Mn. The susceptibility for T_f<T?5 T_f is not Curie-Weiss, but local magnetic correlations manifest themselves and provide a key for the determination of the exchange interactions in spin-glasses. The exchange parameters J_n are resolved up to 5 neighbors for $\text{Au}$Fe, $\text{Cu}$Mn, $\text{Au}$Mn and $\text{Pt}$Mn through a configuration ensemble calculation which includes atomic short range order.     

Knight shifts of27Al and magnetic susceptibility of intermetallic compound NiAl with Fe additions

Atomic and electronic structures, magnetic properties of intermetallic compound NiAl with Fe additions (up to IO at .% for different types of alloying) were studied using NMR and magnetic susceptibility measurements. The effect of these parameters on the probability of occurence of phase transformation B2-LI_o was explored. Experimental results were compared with those obtained by computer calculations of the electronic structure for atomic arrangement of Fe impurity in both sublattices by means of the cluster variant of TBA-LCAO method. The responsibility of the density of 3d-states at the Fermi level increasing effects (Fe−Ni substitution) and corresponding contributions, associated with orbital interactions (Fe−Al substitution), for observed behaviour of measured parameters were determined. Appearance of B2-LI_o phase transition with increasing of antistructural Fe atoms (ASA) concentration was observed experimentally by means of transmission electron microscopy and X-ray diffraction analysis.     

The magnetic susceptibility of spin-one Ising and Heisenberg ferromagnets with biquadratic exchange and uniaxial anisotropy

For spin-one Ising and Heisenberg ferromagnets with biquadratic exchange interaction J' and uniaxial anisotropy D, the magnetic susceptibility has been calculated by the use of the pair model approximation and its dependences on J' and D are discussed in comparative way.     

Pion exchange and the H1 forward spectrometer data

We point out that the component of the nucleon wave function is vital to the interpretation of the recent H1 data for leading baryon production. While the n/p ratio is equal to two with the component alone, the inclusion of the component brings this ratio very near to unity, as observed in the experiment. This result demonstrates that pion exchange can not only account for leading neutron but also for a large fraction of the leading proton production. Received: 4 December 1998 / Revised version: 1 February 1999 / Published online: 27 April 1999     

Quantification of entanglement from magnetic susceptibility for a Heisenberg spin 1/2 system

We report temperature and magnetic field dependent magnetization and quantification of entanglement from the experimental data for dichloro (thiazole) copper (II), a Heisenberg spin chain system. The plot of magnetic susceptibility vs. temperature indicates an infinite spin chain. Isothermal magnetization measurements (as functions of magnetic field) were performed at various temperatures below the antiferromagnetic (AFM) ordering, where the AFM correlations persist significantly. These magnetization curves are fitted to the Bonner–Fisher model. Magnetic susceptibility is used as an entanglement witness to quantify the amount of entanglement in the system.     

Quark exchange effects on 3He and 3H charge densities

The exchange of quarks between nucleons bound in a nucleus leads to a definite and evaluable contribution to the nuclear charge density distribution. Although this exchange contribution is a consequence of finite nucleon size, it cannot be included by the conventional procedure of folding nucleon charge distributions with the density of point nucleons. For the A = 3 nuclei investigated here, the effects of quark exchange were found to be of significance for small distances away from the nuclear centre-of-mass.     

Accurate 1H tumor spectra quantification from acquisitions without water suppression

Monovoxel magnetic resonance spectroscopy (MRS) is a technique extensively used for the study of brain tumors in many imaging centers. However, given the fact that monovoxel spectrum quality depends upon voxel size, region of acquisition and the presence of metal and/or blood residue after surgery can make the comparison of MRS brain tumor spectra more difficult than that of other pathologies. This study was conducted in order to evaluate whether it is possible to predict in which cases a tumor spectrum will be quantifiable from acquisitions obtained without water suppression, allowing comparison to other spectra. Three different methods were employed: a qualitative, clinical method and two quantitative ones (Amares and Quest). It was found that by using Quest, it is possible to estimate the number of acquisitions needed to obtain a quantifiable spectrum before its acquisition, something which was not feasible with Amares (given the base used). On examining the spectra as physicians would, it was found that after a certain number of acquisitions, they did not change. The study shows that it is possible to optimize MRS acquisition time in brain tumors and guarantee spectrum quantification for comparison of different MRS studies, obtained both from a single patient or different patients.     

MR imaging of oscillatory magnetic field changes: Progressing from phantom to human

Detection of ultra-weak oscillatory magnetic field changes using MRI is of great research interest not only for neuronal current MRI of endogenous neuronal oscillations but also for direct visualization of exogenous transcranial currents or iron oxide contrast agent distribution. In this work, we present a novel oscillatory-selective detection (OSD) method that is magnitude-sensitive to the oscillatory magnetic field changes and immune to the main field inhomogeneity. In OSD, a train of 180° pulses with alternating polarity and mirror symmetry are used to refocus and accumulate magnetization changes induced by external oscillatory fields. After taking both the signal change and image signal-to-noise ratio (SNR) into account, a final 90° pulse with a phase offset of 45° is applied to store a combination of the current-induced signal change and background magnetization for the subsequent EPI acquisition. Its performance was demonstrated in phantom and human studies, both of which showed much better detection in the comparison with the recently proposed spin-lock oscillatory excitation (SLOE) method. OSD was further successfully applied in imaging transcranial alternating current stimulation (tACS) induced field changes in the human brain. These promising results suggest that OSD can overcome the limitation of field inhomogeneity impeding previous oscillatory current MRI sensitivity and be a viable tool in future tACS study.