University of Regensburg Publication Server

    Homogeneous Clifford structures

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    We give an upper bound for the rank r of homogeneous (even) Clifford structures on compact manifolds of non-vanishing Euler characteristic. More precisely, we show that if r = 2a � b with b odd, then r � 9 for a = 0, r � 10 for a = 1, r � 12 for a = 2 and r � 16 for a � 3. Moreover, we describe the four limiting cases and show that there is exactly one solution in each case

    Motor activity following the administration of selective D-1 and D-2 dopaminergic drugs to normal common marmosets

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    In normal common marmosets administration of the D-1/D-2 agonist apomorphine or the selective D-2 agonist quinpirole caused a dose-dependent increase in motor activity and induced stereotyped behaviour. Both the selective D-2 antagonist raclopride and the selective D-1 antagonist SCH 23390 inhibited normal locomotor activity and induced catalepsy. Quinpirole- and apomorphine-induced motor activity were potently inhibited by pretreatment with raclopride. The effects of quinpirole, but not apomorphine, were weakly inhibited by SCH 23390. The selective D-1 partial agonist SKF 38393 decreased motor activity and did not induce grooming, oral movements or other behaviours. SKF 38393 inhibited motor activity induced by the administration of quinpirole but did not alter apomorphine-induced motor behaviour. Locomotor activity in normal common marmosets appears to be mediated mainly via D-2 systems. In contrast to rodents, administration of SKF 38393 does not induce behavioural activation and there does not appear to be a facilitating effect of D-1 systems on D-2 function in the normal common marmoset. However, the ability of both SKF 38393 and SCH 23390 to inhibit quinpirole locomotor activity suggests some interaction between D-1 and D-2 systems to occur in this species

    Electrosensory basis for individual recognition in a weakly electric, mormyrid fish, Pollimyrus adspersus (Günther, 1866)

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    Pollimyrus adspersus discriminates the individually variable waveforms of Electric Organ Discharges (EODs) of conspecifics of only 150–250 ms duration. We examined: (1) the discrimination threshold for artificially generated EODs of similar waveform, (2) the mechanism of signal analysis (spectral vs temporal) present, by determining the discrimination between different waveforms of identical amplitude spectra, and (3) the threshold field intensity and reach of discrimination. The triphasic P. adspersus EOD waveform was artificially generated by superimposing two Gaussians, one wide, the second narrow, inverted, and of threefold amplitude. The natural variability among individual EOD waveforms was simulated by phase-shifting one Gaussian relative to the other. The symmetrical waveform where the peaks of the two Gaussians coincided was used as a reference (phase shift=0, rewarded stimulus S+). Results were: (1) in foodrewarded conditioning experiments, trained fish (N=7) detected a phase-shift in artificial EOD stimuli as low as 2 ms (N=2 fish), 6 ms (N=1) and 10 ms (N=1). (2) All fish tested (N=3) discriminated between artificial EODs of identical amplitude spectra but different waveforms (hence, different phase spectra), demonstrating a temporal mechanism of signal analysis. (3) The maximum reach of waveform discrimination was 130 cm at 4.9 mVp-p/cm and 100 mS/cm water conductivity (test signal generated at natural amplitude), that is, similar to the reach of EOD detection. Therefore, among the three kinds of electroreceptor organ present in mormyrids, we consider Knollenorgane the relevant sensory organs for EOD waveform discrimination

    Modification of HLA expression as a possible factor in the pathogenesis of Burkitt's lymphoma

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    Our data show that: Most BL cells express significantly less HLA class I and II than the corresponding LCLs. Lymphoblastoid cell lines have a much higher HLA class I and II density than normal peripheral blood B cells. There are qualitative differences in class I and II expression in most BL/LCL pairs. These qualitative differences are not caused by EBV infection. They might be due to a selective downregulation of HLA specifities by c-myc activation. We conclude that these modifications of HLA expression may well play a role in the reduced specific immune recognition of Burkitt's lymphoma cells

    Contractile cells in the human scleral spur

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    The scleral spur in 37 human (age 17-87 years) and six cynomolgus monkey eyes (2-4 years) was investigated. Serial meridional and tangential sections were studied with ultrastructural and immunocytochemical methods. The bundles of the ciliary muscle do not enter the scleral spur, but their tendons, which consist of elastic fibres join the elastic fibres in the scleral spur. Within the scleral spur a population of circularly oriented and spindle-shaped cells is found. In contrast to the ciliary muscle cells, the scleral spur cells form no bundles, but are loosely aggregated. They have long cytoplasmic processes and are connected to each other by adherens-type and gap junctions. They stain intensely for alpha-smooth muscle actin, myosin and vimentin. In contrast to the ciliary muscle cells, they do not stain for desmin. Ultrastructurally, the scleral spur cells contain abundant thin (actin) filaments, but do not otherwise show the typical ultrastructural features of ciliary muscle cells. The scleral spur cells do not express a complete basal lamina. They form individual tendinous connections with the elastic fibres in the scleral spur, which are continuous with the elastic fibres of the trabecular meshwork. The scleral spur cells are in close contact with nerve terminals containing small agranular (30-60 nm) and large granular (65-110 nm) vesicles but also with terminals containing small granular (30-60 nm) vesicles which are regarded as typical for adrenergic terminals. We conclude that the scleral spur cells are contractile myofibroblasts. Their contraction might influence the rate of the aqueous outflow

    Induction and the empiricist model of knowledge

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    Numerical computations of facetted pattern formation in snow crystal growth

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    Facetted growth of snow crystals leads to a rich diversity of forms, and exhibits a remarkable sixfold symmetry. Snow crystal structures result from diffusion limited crystal growth in the presence of anisotropic surface energy and anisotropic attachment kinetics. It is by now well understood that the morphological stability of ice crystals strongly depends on supersaturation, crystal size and temperature. Until very recently it was very difficult to perform numerical simulations of this highly anisotropic crystal growth. In particular, obtaining facet growth in combination with dendritic branching is a challenging task. We present numerical simulations of snow crystal growth in two and three space dimensions using a new computational method recently introduced by the authors. We present both qualitative and quantitative computations. In particular, a linear relationship between tip velocity and supersaturation is observed. The computations also suggest that surface energy effects, although small, have a larger effect on crystal growth than previously expected. We compute solid plates, solid prisms, hollow columns, needles, dendrites, capped columns and scrolls on plates. Although all these forms appear in nature, most of these forms are computed here for the first time in numerical simulations for a continuum model

    Coherent Backscattering in Fock Space: a Signature of Quantum Many-Body Interference in Interacting Bosonic Systems

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    We predict a generic signature of quantum interference in many-body bosonic systems resulting in a coherent enhancement of the average return probability in Fock space. This enhancement is robust with respect to variations of external parameters even though it represents a dynamical manifestation of the delicate superposition principle in Fock space. It is a genuine quantum many-body effect that lies beyond the reach of any mean-field approach. Using a semiclassical approach based on interfering paths in Fock space, we calculate the magnitude of the backscattering peak and its dependence on gauge fields that break time-reversal invariance. We confirm our predictions by comparing them to exact quantum evolution probabilities in Bose-Hubbard models, and discuss their relevance in the context of many-body thermalization

    Far-infrared absorption due to impurities in CsBr crystals

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    The far-infrared absorption in the phonon band of CsBr due to Na+, K+, Rb+, In+, and Tl+ impurities has been measured and compared with the calculated density of phonon states of the host lattice
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