The Role of Visual Evoked Potentials in the Diagnosis of Optic Nerve Injury as a Result of Mild Head Trauma

The curve of visual evoked potentials was observed and compared with changes of visual acuity and visual field during a 24-month period in a group of 39 patients with optic nerve injury as a result of mild cerebral trauma. Results of the study showed great improvement of visual acuity and visual field after treatment, and slower and continuous improvement of visual evoked potentials. The main abnormality of visual evoked potentials is the shortening of amplitude, which is recorded to gradually recede after treatment in half of the patients. Authors conclude that the shortening of amplitude can be partially explained by the edema and the compression of fibers in the optic canal. Also, they emphasize that in this type of optic nerve injury visual acuity testing is the best indicator of the promptness and scope of the injury, while visual field research presents the best method for following later delicate changes of visual function

Spin and orbital effects in a 2D electron gas in a random magnetic field

Using the method of superbosonization we consider a model of a random magnetic field (RMF) acting on both orbital motion and spin of electrons in two dimensions. The method is based on exact integration over one particle degrees of freedom and reduction of the problem to a functional integral over supermatrices $Q({\bf r},{\bf r^{\prime}})$. We consider a general case when both the direction of the RMF and the g-factor of the Zeeman splitting are arbitrary. Integrating out fast variations of $Q$ we come to a standard collisional unitary non-linear $\sigma$-model. The collision term consists of orbital, spin and effective spin-orbital parts. For a particular problem of a fixed direction of RMF, we show that additional soft excitations identified with spin modes should appear. Considering $\delta$% -correlated weak RMF and putting g=2 we find the transport time $\tau_{tr}$. This time is 2 times smaller than that for spinless particles.Comment: 9 pages, no figure

Localization of non-interacting electrons in thin layered disordered systems

Localization of electronic states in disordered thin layered systems with b layers is studied within the Anderson model of localization using the transfer-matrix method and finite-size scaling of the inverse of the smallest Lyapunov exponent. The results support the one-parameter scaling hypothesis for disorder strengths W studied and b=1,...,6. The obtained results for the localization length are in good agreement with both the analytical results of the self-consistent theory of localization and the numerical scaling studies of the two-dimensional Anderson model. The localization length near the band center grows exponentially with b for fixed W but no localization-delocalization transition takes place.Comment: 6 pages, 5 figure

Neuroophthalmologic Diagnosis of the Sella Turca Region

A chiasm n. optici is shaped by crossed optic nerves. Approximate vertical distance of 10 mm separates chiasm from dorsum sella turca and pituitary fosse. Twenty-five percent of all cerebral tumors appear in the chiasm area and almost half of them cause damages of visual function. Major and sometimes the only symptom is progressive visual loss. This damage appears because tumor masses are huge and are compressing optic nerves. Sellar region lesions must already be spreading suprasellary before patient can notice visual field defects. The sign of advanced process is symmetric or asymmetric loss of visual acuity or visual field defects. The aim of this study was to show the importance of visual field in the diagnosis of sellar disorders as well as in treatments. Eighteen patients with chiasm disorders were analyzed. Visual field was analyzed before and after treatment according to visual field classifications. The study has shown the importance of visual field testing in diagnosis of sellar processes, as well as in following therapy, surgical or conservative

Bond-disordered Anderson model on a two dimensional square lattice - chiral symmetry and restoration of one-parameter scaling

Bond-disordered Anderson model in two dimensions on a square lattice is studied numerically near the band center by calculating density of states (DoS), multifractal properties of eigenstates and the localization length. DoS divergence at the band center is studied and compared with Gade's result [Nucl. Phys. B 398, 499 (1993)] and the powerlaw. Although Gade's form describes accurately DoS of finite size systems near the band-center, it fails to describe the calculated part of DoS of the infinite system, and a new expression is proposed. Study of the level spacing distributions reveals that the state closest to the band center and the next one have different level spacing distribution than the pairs of states away from the band center. Multifractal properties of finite systems furthermore show that scaling of eigenstates changes discontinuously near the band center. This unusual behavior suggests the existence of a new divergent length scale, whose existence is explained as the finite size manifestation of the band center critical point of the infinite system, and the critical exponent of the correlation length is calculated by a finite size scaling. Furthermore, study of scaling of Lyapunov exponents of transfer matrices of long stripes indicates that for a long stripe of any width there is an energy region around band center within which the Lyapunov exponents cannot be described by one-parameter scaling. This region, however, vanishes in the limit of the infinite square lattice when one-parameter scaling is restored, and the scaling exponent calculated, in agreement with the result of the finite size scaling analysis.Comment: 23 pages, 11 figures. RevTe

Generalized Inverse Participation Numbers in Metallic-Mean Quasiperiodic Systems

From the quantum mechanical point of view, the electronic characteristics of quasicrystals are determined by the nature of their eigenstates. A practicable way to obtain information about the properties of these wave functions is studying the scaling behavior of the generalized inverse participation numbers $Z_q \sim N^{-D_q(q-1)}$ with the system size $N$. In particular, we investigate $d$-dimensional quasiperiodic models based on different metallic-mean quasiperiodic sequences. We obtain the eigenstates of the one-dimensional metallic-mean chains by numerical calculations for a tight-binding model. Higher dimensional solutions of the associated generalized labyrinth tiling are then constructed by a product approach from the one-dimensional solutions. Numerical results suggest that the relation $D_q^{d\mathrm{d}} = d D_q^\mathrm{1d}$ holds for these models. Using the product structure of the labyrinth tiling we prove that this relation is always satisfied for the silver-mean model and that the scaling exponents approach this relation for large system sizes also for the other metallic-mean systems.Comment: 7 pages, 3 figure

Spectral Statistics in Chiral-Orthogonal Disordered Systems

We describe the singularities in the averaged density of states and the corresponding statistics of the energy levels in two- (2D) and three-dimensional (3D) chiral symmetric and time-reversal invariant disordered systems, realized in bipartite lattices with real off-diagonal disorder. For off-diagonal disorder of zero mean we obtain a singular density of states in 2D which becomes much less pronounced in 3D, while the level-statistics can be described by semi-Poisson distribution with mostly critical fractal states in 2D and Wigner surmise with mostly delocalized states in 3D. For logarithmic off-diagonal disorder of large strength we find indistinguishable behavior from ordinary disorder with strong localization in any dimension but in addition one-dimensional $1/|E|$ Dyson-like asymptotic spectral singularities. The off-diagonal disorder is also shown to enhance the propagation of two interacting particles similarly to systems with diagonal disorder. Although disordered models with chiral symmetry differ from non-chiral ones due to the presence of spectral singularities, both share the same qualitative localization properties except at the chiral symmetry point E=0 which is critical.Comment: 13 pages, Revtex file, 8 postscript files. It will appear in the special edition of J. Phys. A for Random Matrix Theor

Anterior Ischaemic Optic Neuropathy in Patient with Rheumatoid Arthritis – Case Report

This case report presents a patient with long-lasting rheumatoid arthritis (RA) of fourth clinical grade, having ocular complications. RA was diagnosed according to current modified ARA criteria from 1987. Upon admission to the Department of Ophthalmology clinical examination revealed anterior ischaemic optic neuropathy (AION), which is not characteristic manifestation of RA in the eye. The occurrence of AION in patients with RA has been explained in literature as a secondary manifestation of hypertension in these patients or, by the presence of other connective tissue disease apart from RA (for example, MCTD – mixed connective tissue disease). Both mentioned causes were excluded in our case, as well as any other condition that could lead to AION. Therefore, we had concluded that AION presented a late complication of RA

The Role of Visual Evoked Potentials in the Diagnosis of Optic Nerve Injury as a Result of Mild Head Trauma

The curve of visual evoked potentials was observed and compared with changes of visual acuity and visual field during a 24-month period in a group of 39 patients with optic nerve injury as a result of mild cerebral trauma. Results of the study showed great improvement of visual acuity and visual field after treatment, and slower and continuous improvement of visual evoked potentials. The main abnormality of visual evoked potentials is the shortening of amplitude, which is recorded to gradually recede after treatment in half of the patients. Authors conclude that the shortening of amplitude can be partially explained by the edema and the compression of fibers in the optic canal. Also, they emphasize that in this type of optic nerve injury visual acuity testing is the best indicator of the promptness and scope of the injury, while visual field research presents the best method for following later delicate changes of visual function

Fast photoluminescence quenching in thin films of 4,4 '-bis(2,2-diphenylvinyl)-1,1 '-biphenyl exposed to air

The photoluminescence (PL) quenching mechanism of UV light and air-exposed amorphous thin films of 4,4'-bis(2,2-diphenylvinyl)-1,1'-biphenyl (DPVBi), a well-known hole-transport material used in organic light-emitting diodes, is studied. Thin films of DPVBi are stable when exposed to UV light in vacuum but tend to degrade if oxygen is present simultaneously. This is evident from the changes in UV-vis absorption spectra of the latter, showing that degradation rate of DPVBi films is linearly proportional to both oxygen concentration and UV light intensity. Mass spectrometry study of such films revealed a number of different oxygen-containing molecules and fragments of DPVBi thus confirming apparent photo-oxidation process. Also, DFT study of molecular DPVBi with and without oxygen was carried out, the IR spectra calculated for the lowest energy molecules found and the results are compared with the experiment. The most sensitive to photo-oxidation is DPVBi photoluminescence, which decays exponentially with respect to the concentration of photo-oxidized DPVBi molecules (impurities). The PL quantum yield of DPVBi thin film drops to a half of its original value for 0.2% of the impurities present, at which point an average distance between DPVBi molecules (the donors) and photo-oxidized DPVBi species (acceptors) is an order of magnitude larger than the separation between two adjacent molecules. This implies a need for a long-range Forster energy transfer, which we rule out based on the lack of a donor-acceptor spectral overlap. The apparent discrepancy can be removed by postulating exciton self-diffusion in DPVBi thin films, for which there is supporting evidence in existing literature