The Block Spin Renormalization Group Approach and Two-Dimensional Quantum Gravity

A block spin renormalization group approach is proposed for the dynamical triangulation formulation of two-dimensional quantum gravity. The idea is to update link flips on the block lattice in response to link flips on the original lattice. Just as the connectivity of the original lattice is meant to be a lattice representation of the metric, the block links are determined in such a way that the connectivity of the block lattice represents a block metric. As an illustration, this approach is applied to the Ising model coupled to two-dimensional quantum gravity. The correct critical coupling is reproduced, but the critical exponent is obscured by unusually large finite size effects.Comment: 10 page

Linking cortical visual processing to viewing behavior using fMRI

One characteristic of natural visual behavior in humans is the frequent shifting of eye position. It has been argued that the characteristics of these eye movements can be used to distinguish between distinct modes of visual processing (Unema et al., 2005). These viewing modes would be distinguishable on the basis of the eye-movement parameters fixation duration and saccade amplitude and have been hypothesized to reflect the differential involvement of dorsal and ventral systems in saccade planning and information processing. According to this hypothesis, on the one hand, while in a “pre-attentive” or ambient mode, primarily scanning eye movements are made; in this mode fixation are relatively brief and saccades tends to be relatively large. On the other hand, in “attentive” focal mode, fixations last longer and saccades are relatively small, and result in viewing behavior which could be described as detailed inspection. Thus far, no neuroscientific basis exists to support the idea that such distinct viewing modes are indeed linked to processing in distinct cortical regions. Here, we used fixation-based event-related (FIBER) fMRI in combination with independent component analysis (ICA) to investigate the neural correlates of these viewing modes. While we find robust eye-movement-related activations, our results do not support the theory that the above mentioned viewing modes modulate dorsal and ventral processing. Instead, further analyses revealed that eye-movement characteristics such as saccade amplitude and fixation duration did differentially modulate activity in three clusters in early, ventromedial and ventrolateral visual cortex. In summary, we conclude that evaluating viewing behavior is crucial for unraveling cortical processing in natural vision

Assessing Uncertainty and Reliability of Connective Field Estimations From Resting State fMRI Activity at 3T

Connective Field (CF) modeling estimates the local spatial integration between signals in distinct cortical visual field areas. As we have shown previously using 7T data, CF can reveal the visuotopic organization of visual cortical areas even when applied to BOLD activity recorded in the absence of external stimulation. This indicates that CF modeling can be used to evaluate cortical processing in participants in which the visual input may be compromised. Furthermore, by using Bayesian CF modeling it is possible to estimate the co-variability of the parameter estimates and therefore, apply CF modeling to single cases. However, no previous studies evaluated the (Bayesian) CF model using 3T resting-state fMRI data. This is important since 3T scanners are much more abundant and more often used in clinical research compared to 7T scanners. Therefore in this study, we investigate whether it is possible to obtain meaningful CF estimates from 3T resting state (RS) fMRI data. To do so, we applied the standard and Bayesian CF modeling approaches on two RS scans, which were separated by the acquisition of visual field mapping data in 12 healthy participants. Our results show good agreement between RS- and visual field (VF)- based maps using either the standard or Bayesian CF approach. In addition to quantify the uncertainty associated with each estimate in both RS and VF data, we applied our Bayesian CF framework to provide the underlying marginal distribution of the CF parameters. Finally, we show how an additional CF parameter, beta, can be used as a data-driven threshold on the RS data to further improve CF estimates. We conclude that Bayesian CF modeling can characterize local functional connectivity between visual cortical areas from RS data at 3T. Moreover, observations obtained using 3T scanners were qualitatively similar to those reported for 7T. In particular, we expect the ability to assess parameter uncertainty in individual participants will be important for future clinical studies

Supported glass fibers catalysts for novel multi-phase reactor design

The catalytic properties of Pd and Pt supported on woven glass fabrics were explored for liq.-phase hydrogenation of benzaldehyde as a model for three-phase catalytic reaction. The catalysts demonstrated high activity and are suitable materials to be used in reactors with structured catalytic bed. The main characteristics of catalytic bed are discussed. [on SciFinder (R)

Structured multiphase reactors based on fibrous catalysts: nitrite hydrogenation as a case study

A reactor concept designed as a bubble column staged with fibrous catalytic layers is tested on pilot-scale for the redn. of nitrite to N by H. CO2 is used to neutralize the hydroxide ions produced during hydrogenation of nitrite. The effective reaction rate was found to depend on the superficial gas velocity ug0. The fibrous catalysts demonstrated a high selectivity towards nitrogen. The influence of the mass transport on the effective kinetics is discussed and the volumetric liq.-solid mass transfer coeff. kSaS is estd. as a function of the superficial gas velocity ug0. The selectivity was found to be independent of the hydrodynamics

Bubble columns staged with structured fibrous catalytic layers: Residence time distribution and mass transfer

A new reactor concept for catalyzed three-phase reactions based on the use of woven fibrous materials for bubble columns is suggested. In comparison to conventional multistage bubble columns, the trays are made from woven fibrous catalytic layers. The hydrodynamic parameters such as the residence time distribution (RTD) and the volumetric gas-liquid mass transfer coefficient k(L)a are investigated in an air/water system for different layer structures and different superficial gas (u(g0) < 60 cm/s) and liquid (u(10) < 6 cm/s) velocities. The major reactor design parameters, such as the thread diameter D, the distance between the woven threads w, and the distance between the fibrous layers b, are discussed

Reduction of nitrite-ions in water over Pd-supported on structured fibrous materials

Catalytic hydrogenation of NO2- in water to N2 over Pd-supported on different woven fibrous materials was studied. Low porosity C fiber, low porosity D-type glass fiber, and composite (glass covered by g-Al2O3) fiber were used. Pd dispersion was compared for different supports at various Pd loadings and correlated with catalytic activity. NH3 as an undesired byproduct was also obsd. and main parameters to suppress its formation are discussed. A reactor concept based on woven fibrous catalysts is suggested and main design parameters for tech. application were estd. [on SciFinder (R)

Pt and Pd supported on glass fibers as effective combustion catalysts

Pd and Pt supported on glass fiber materials with developed porosity and high sp. surface areas were studied in total propane oxidn. The reaction was carried out in recycling reactor and the kinetic parameters were detd. under different reaction conditions in the temp. range 200-500 DegC. Pt catalysts were seen to be more active than Pd for the same metal loading on identical support. Catalytic activity was seen to depend on support compn. The highest activity was obsd. on Pt supported on glass fiber modified by titania, demonstrating the ignition temps. around 200 DegC. The catalyst surface morphol. and surface dispersion of active metal were characterized by high-resoln. electron microscopy. [on SciFinder (R)