Anisotropy in the chromatic channel: a horizontal-vertical effect

We compared the chromatic contrast thresholds of drifting (2Hz) red-green sine-wave gratings of horizontal, vertical, and two oblique orientations at three spatial frequencies (2, 4, 8 cpd). Luminance contrast thresholds for yellow-black gratings were also obtained. The classic oblique effect was found for high spatial frequency luminance and chromatic stimuli. For chromatic thresholds, a significant difference was found between the horizontal and vertical thresholds of all observers. One observer was retested with her head tilted 45 deg and demonstrated that the anisotropy was specific to retinal coordinates. These results give evidence for orientation selectivity in the chromatic channel which is at least partially independent of that in the luminance channel. We estimated the degree of lateral chromatic aberration in our observers' eyes and discuss the possible contribution of this aberration to the horizontal-vertical difference in the chromatic channel.     

Changes in perceived contrast of suprathreshold gratings as a function of orientation and spatial frequency

Orientation anisotropy for suprathreshold gratings of different spatial frequencies was measured using a contrast matching procedure. Observers matched the contrast of sine-wave gratings of various orientations to a vertical reference grating set at different reference contrasts. At threshold, the size of the anisotropy increased with spatial frequency, confirming previous results. When the reference grating contrast was set above threshold, the anisotropy declined, and eventually disappeared for gratings of medium spatial frequencies. At higher spatial frequencies, although the relative anisotropy became smaller, it did not disappear within the range of contrasts used in this study. For medium, but not for high spatial frequencies, the data are consistent with Kulikowski's (1976) model of effective contrast constancy.     

Rotational diffusion of the α(2a) adrenergic receptor revealed by FlAsH labeling in living cells

The fluorescein arsenical hairpin binder (FlAsH) shows much promise to determine the relative orientations of protein regions and structures even in living cells and in the plasma membrane. In this study, we characterized FlAsH's photophysical properties by steady-state anisotropy and time-resolved single photon counting for further applications with G-protein coupled receptors. We find that FlAsH has a relatively high initial anisotropy of 0.31 ± 0.01 and a three-component fluorescence lifetime with an average of 4.1 ± 0.1 ns. We characterized the FlAsH fluorophore orientation in the α(2A) adrenergic receptor revealing rigid orientations of FlAsH in the membrane plane for rotational correlation times of ～50 ns in living cells. To elucidate the fluorophore-membrane orientation and rotational correlation time, an anisotropy treatment similar to that of another researcher (Axelrod, D. 1979. Biophys. J. 26:557-573) was developed. The rotational correlation times were observed to increase by up to 16 ns after agonist addition. The rotational correlation time also allowed for a comparison to the theoretical relationship between translational and rotational diffusion (originally proposed by Saffman, P. G., and M. Delbrück. 1975. Proc. Natl. Acad. Sci. USA. 72:3111-3113) and revealed a discrepancy of a factor between 10 and 100.     

Testing Neuronal Accounts of Anisotropic Motion Perception with Computational Modelling

There is an over-representation of neurons in early visual cortical areas that respond most strongly to cardinal (horizontal and vertical) orientations and directions of visual stimuli, and cardinal- and oblique-preferring neurons are reported to have different tuning curves. Collectively, these neuronal anisotropies can explain two commonly-reported phenomena of motion perception – the oblique effect and reference repulsion – but it remains unclear whether neuronal anisotropies can simultaneously account for both perceptual effects. We show in psychophysical experiments that reference repulsion and the oblique effect do not depend on the duration of a moving stimulus, and that brief adaptation to a single direction simultaneously causes a reference repulsion in the orientation domain, and the inverse of the oblique effect in the direction domain. We attempted to link these results to underlying neuronal anisotropies by implementing a large family of neuronal decoding models with parametrically varied levels of anisotropy in neuronal direction-tuning preferences, tuning bandwidths and spiking rates. Surprisingly, no model instantiation was able to satisfactorily explain our perceptual data. We argue that the oblique effect arises from the anisotropic distribution of preferred directions evident in V1 and MT, but that reference repulsion occurs separately, perhaps reflecting a process of categorisation occurring in higher-order cortical areas.     

Probing the Orientational Distribution of Dyes in Membranes through Multiphoton Microscopy

Numerous dyes are available or under development for probing the structural and functional properties of biological membranes. Exogenous chromophores adopt a range of orientations when bound to membranes, which have a drastic effect on their biophysical behavior. Here, we present a method that employs optical anisotropy data from three polarization-imaging techniques to establish the distribution of orientations adopted by molecules in monolayers and bilayers. The resulting probability density functions, which contain the preferred molecular tilt μ and distribution breadth γ, are more informative than an average tilt angle 〈φ〉. We describe a methodology for the extraction of anisotropy data through an image-processing technology that decreases the error in polarization measurements by about a factor of four. We use this technique to compare di-4-ANEPPS and di-8-ANEPPS, both dipolar dyes, using data from polarized 1-photon, 2-photon fluorescence and second-harmonic generation imaging. We find that di-8-ANEPPS has a lower tilt but the same distributional width. We find the distribution of tilts taken by di-4-ANEPPS in two phospholipid membrane models: giant unilamellar vesicles and water-in-oil droplet monolayers. Both models result in similar distribution functions with average tilts of 52° and 47°, respectively.     

Stability of cortical responses and the statistics of natural scenes.

The primary visual cortex (V1) of higher mammals contains maps of stimulus features; how these maps influence vision remains unknown. We have examined the functional significance of an asymmetry in the orientation map in cat V1, i.e., the fact that a larger area of V1 is preferentially activated by vertical and horizontal contours than by contours at oblique orientations. Despite the fact that neurons tuned to cardinal and oblique orientations have indistinguishable tuning characteristics, cardinal neurons remain more stable in their response properties after selective perturbation induced by adaptation. Similarly, human observers report different adaptation-induced changes in orientation tuning between cardinal and oblique axes. We suggest that the larger cortical area devoted to cardinal orientations imposes stability on the processing of cardinal contours during visual perception, by retaining invariant cortical responses along cardinal axes.     

Modelling of the UV Index on vertical and 40° tilted planes for different orientations

In this study, estimated data of the UV Index on vertical planes are presented for the latitude of Valencia, Spain. For that purpose, the UVER values have been generated on vertical planes by means of four different geometrical models a) isotropic, b) Perez, c) Gueymard, d) Muneer, based on values of the global horizontal UVER and the diffuse horizontal UVER, measured experimentally. The UVER values, obtained by any model, overestimate the experimental values for all orientations, with the exception of the Perez model for the East plane. The results show statistical values of the MAD parameter (Mean Absolute Deviation) between 10% and 25%, the Perez model being the one that obtained a lower MAD for all levels. As for the statistic RMSD parameter (Root Mean Square Deviation), the results show values between 17% and 32%, and again the Perez model provides the best results in all vertical planes. The difference between the estimated UV Index and the experimental UV Index, for vertical and 40° tilted planes, was also calculated. 40° is an angle close to the latitude of Burjassot, Valencia, (39.5°), which, according to various studies, is the optimum angle to capture maximum radiation on tilted planes. We conclude that the models provide a good estimate of the UV Index, as they coincide or differ in one unit compared to the experimental values in 99% of cases, and this is valid for all orientations. Finally, we examined the relation between the UV Index on vertical and 40° tilted planes, both the experimental and estimated by the Perez model, and the experimental UV Index on a horizontal plane at 12 GMT. Based on the results, we can conclude that it is possible to estimate with a good approximation the UV Index on vertical and 40° tilted planes in different directions on the basis of the experimental horizontal UVI value, thus justifying the interest of this study.     

The Spherical Expansion of H2 Dimer Interaction Energy by Double Symmetry-Adapted Perturbation Theory

Abstract

The weak interaction energy of H₂ dimer is studied by double symmetry-adapted perturbation theory (SAPT) within second-order of intermolecular and intramonomer perturbation for molecular simulations. The assumed orientations of H₂ dimer are linear, parallel, T type and X type. Among four orientations T orientation is the most stable, while linear orientation is the most repulsive. The second-order dispersion energy E _disp ⁽²⁾ is the most attractive contribution in all orientations. The interaction energy has the anisotropy, so we expressed our total interaction energy by the spherical expansion to compare with the experimental value. The isotropic interaction energy is about 85% of the experimental value.     

Inference Based on Kernel Estimates of the Relative Risk Function in Geographical Epidemiology

Kernel smoothing is a popular approach to estimating relative risk surfaces from data on the locations of cases and controls in geographical epidemiology. The interpretation of such surfaces is facilitated by plotting of tolerance contours which highlight areas where the risk is sufficiently high to reject the null hypothesis of unit relative risk. Previously it has been recommended that these tolerance intervals be calculated using Monte Carlo randomization tests. We examine a computationally cheap alternative whereby the tolerance intervals are derived from asymptotic theory. We also examine the performance of global tests of hetereogeneous risk employing statistics based on kernel risk surfaces, paying particular attention to the choice of smoothing parameters on test power (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Directional anisotropies reveal a functional segregation of visual motion processing for perception and action

Human exhibits an anisotropy in direction perception: discrimination is superior when motion is around horizontal or vertical rather than diagonal axes. In contrast to the consistent directional anisotropy in perception, we found only small idiosyncratic anisotropies in smooth pursuit eye movements, a motor action requiring accurate discrimination of visual motion direction. Both pursuit and perceptual direction discrimination rely on signals from the middle temporal visual area (MT), yet analysis of multiple measures of MT neuronal responses in the macaque failed to provide evidence of a directional anisotropy. We conclude that MT represents different motion directions uniformly, and subsequent processing creates a directional anisotropy in pathways unique to perception. Our data support the hypothesis that, at least for visual motion, perception and action are guided by inputs from separate sensory streams. The directional anisotropy of perception appears to originate after the two streams have segregated and downstream from area MT.     

Effect of orientational order on the decay of the fluorescence anisotropy in membrane suspensions. A new approximate solution of the rotational diffusion equation

We discussed the time-dependence of fluorescent emission anisotropy of a cylindrical probe in membrane vesicles. We showed that, if the motion of the probe were described as diffusion in an anisotropic environment, it would be possible to determine not only the second-rank but also the fourth-rank orientational order parameter from the decay of the fluorescence anisotropy. The approximations involved were based on an interpolation of short-time and long-time behavior of the relevant correlation functions. A general expression was derived for the time dependence of the fluorescence anisotropy in closed form, which applies to any particular distribution model. It was shown to be in good agreement with previously reported results for the cone model and the Gaussian model. Finally, the applicability of the theory to time-resolved and differential phase fluorescence depolarization experiments was discussed.     

Collagen fiber alignment does not explain mechanical anisotropy in fibroblast populated collagen gels

Many load-bearing soft tissues exhibit mechanical anisotropy. In order to understand the behavior of natural tissues and to create tissue engineered replacements, quantitative relationships must be developed between the tissue structures and their mechanical behavior. We used a novel collagen gel system to test the hypothesis that collagen fiber alignment is the primary mechanism for the mechanical anisotropy we have reported in structurally anisotropic gels. Loading constraints applied during culture were used to control the structural organization of the collagen fibers of fibroblast populated collagen gels. Gels constrained uniaxially during culture developed fiber alignment and a high degree of mechanical anisotropy, while gels constrained biaxially remained isotropic with randomly distributed collagen fibers. We hypothesized that the mechanical anisotropy that developed in these gels was due primarily to collagen fiber orientation. We tested this hypothesis using two mathematical models that incorporated measured collagen fiber orientations: a structural continuum model that assumes affine fiber kinematics and a network model that allows for nonaffine fiber kinematics. Collagen fiber mechanical properties were determined by fitting biaxial mechanical test data from isotropic collagen gels. The fiber properties of each isotropic gel were then used to predict the biaxial mechanical behavior of paired anisotropic gels. Both models accurately described the isotropic collagen gel behavior. However, the structural continuum model dramatically underestimated the level of mechanical anisotropy in aligned collagen gels despite incorporation of measured fiber orientations; when estimated remodeling-induced changes in collagen fiber length were included, the continuum model slightly overestimated mechanical anisotropy. The network model provided the closest match to experimental data from aligned collagen gels, but still did not fully explain the observed mechanics. Two different modeling approaches showed that the level of collagen fiber alignment in our uniaxially constrained gels cannot explain the high degree of mechanical anisotropy observed in these gels. Our modeling results suggest that remodeling-induced redistribution of collagen fiber lengths, nonaffine fiber kinematics, or some combination of these effects must also be considered in order to explain the dramatic mechanical anisotropy observed in this collagen gel model system.     

The influence of anisotropy on brain injury prediction

Traumatic Brain Injury (TBI) occurs when a mechanical insult produces damage to the brain and disrupts its normal function. Numerical head models are often used as tools to analyze TBIs and to measure injury based on mechanical parameters. However, the reliability of such models depends on the incorporation of an appropriate level of structural detail and accurate representation of the material behavior. Since recent studies have shown that several brain regions are characterized by a marked anisotropy, constitutive equations should account for the orientation-dependence within the brain. Nevertheless, in most of the current models brain tissue is considered as completely isotropic. To study the influence of the anisotropy on the mechanical response of the brain, a head model that incorporates the orientation of neural fibers is used and compared with a fully isotropic model. A simulation of a concussive impact based on a sport accident illustrates that significantly lowered strains in the axonal direction as well as increased maximum principal strains are detected for anisotropic regions of the brain. Thus, the orientation-dependence strongly affects the response of the brain tissue. When anisotropy of the whole brain is taken into account, deformation spreads out and white matter is particularly affected. The introduction of local axonal orientations and fiber distribution into the material model is crucial to reliably address the strains occurring during an impact and should be considered in numerical head models for potentially more accurate predictions of brain injury.     

Effects of non-local diffusion on structural MRI preprocessing and default network mapping: statistical comparisons with isotropic/anisotropic diffusion

Neuroimaging community usually employs spatial smoothing to denoise magnetic resonance imaging (MRI) data, e.g., Gaussian smoothing kernels. Such an isotropic diffusion (ISD) based smoothing is widely adopted for denoising purpose due to its easy implementation and efficient computation. Beyond these advantages, Gaussian smoothing kernels tend to blur the edges, curvature and texture of images. Researchers have proposed anisotropic diffusion (ASD) and non-local diffusion (NLD) kernels. We recently demonstrated the effect of these new filtering paradigms on preprocessing real degraded MRI images from three individual subjects. Here, to further systematically investigate the effects at a group level, we collected both structural and functional MRI data from 23 participants. We first evaluated the three smoothing strategies' impact on brain extraction, segmentation and registration. Finally, we investigated how they affect subsequent mapping of default network based on resting-state functional MRI (R-fMRI) data. Our findings suggest that NLD-based spatial smoothing maybe more effective and reliable at improving the quality of both MRI data preprocessing and default network mapping. We thus recommend NLD may become a promising method of smoothing structural MRI images of R-fMRI pipeline.     

AN OBJECTIVE NUMERICAL METHOD OF ASSESSING THE RELIABILITY OF TIME-INTENSITY PANELISTS

A measure of the reliability (T-IR) of time-intensity measurements was developed based on the concept of standard deviation as a measure of panelist variability. The T-IR measure was applied to time-intensity data collected from 10 panelists evaluating the sweetness of 4 model sweetener solutions on horizontal and vertical time-intensity line orientations. T-IR scores showed that the panelists were similarly reliable across the sweeteners and orientations. As well, independent of scale orientation, responses to sweeteners were similarly reliable. The T-IR measure can be used to maintain a high level of performance by monitoring time-intensity panelists. T-IR also provides an objective method of selecting panelists for time-intensity panels.     

“Taller and Shorter”: Human 3-D Spatial Memory Distorts Familiar Multilevel Buildings

Animal experiments report contradictory findings on the presence of a behavioural and neuronal anisotropy exhibited in vertical and horizontal capabilities of spatial orientation and navigation. We performed a pointing experiment in humans on the imagined 3-D direction of the location of various invisible goals that were distributed horizontally and vertically in a familiar multilevel hospital building. The 21 participants were employees who had worked for years in this building. The hypothesis was that comparison of the experimentally determined directions and the true directions would reveal systematic inaccuracy or dimensional anisotropy of the localizations. The study provides first evidence that the internal representation of a familiar multilevel building was distorted compared to the dimensions of the true building: vertically 215% taller and horizontally 51% shorter. This was not only demonstrated in the mathematical reconstruction of the mental model based on the analysis of the pointing experiments but also by the participants’ drawings of the front view and the ground plan of the building. Thus, in the mental model both planes were altered in different directions: compressed for the horizontal floor plane and stretched for the vertical column plane. This could be related to human anisotropic behavioural performance of horizontal and vertical navigation in such buildings.     

The stereoscopic anisotropy affects manual pointing

Although binocular disparity can in principle provide absolute depth information, perceived stereoscopic depth depends on the relative disparities between points and their spatial arrangement. An example of this is the stereoscopic anisotropy--observers typically perceive less depth for stereoscopic surfaces when depth varies in the horizontal direction than in the vertical direction. We investigated whether this anisotropy also affects manual pointing. Participants were presented with stereograms depicting surfaces that were slanted in depth about either a horizontal axis (inclination) or a vertical axis (slant), and were asked either to point to the edge of a surface, or to estimate its inclination or slant. For both tasks, a clear anisotropy was observed, with participants perceiving greater depth, and also pointing out steeper surfaces, for inclined surfaces than for slanted surfaces. We conclude that both perception and the control of action are subject to a similar stereoscopic anisotropy, and that performance on the two tasks relies on similar depth processing mechanisms.     

Orientation of lutein in a lipid bilayer - revisited

Lutein is present in the human retina and lens, where it plays a protective role. As lutein is associated with the lipid matrix of biomembranes, the role depends on its membrane location. Experimental studies predicted two orientations of lutein in a phosphatidylcholine (PC) bilayer: vertical and horizontal. Using a molecular dynamics simulation, we observed, in two different PC bilayers, both orientations of lutein, and in each bilayer, a single change from vertical to horizontal orientation or vice versa. Both orientations were stabilized by hydrogen bonding of lutein OH groups with mainly carbonyl but also phosphate oxygen atoms of PC.     

Kalman smoothing improves the estimation of joint kinematics and kinetics in marker-based human gait analysis

We developed a Kalman smoothing algorithm to improve estimates of joint kinematics from measured marker trajectories during motion analysis. Kalman smoothing estimates are based on complete marker trajectories. This is an improvement over other techniques, such as the global optimisation method (GOM), Kalman filtering, and local marker estimation (LME), where the estimate at each time instant is only based on part of the marker trajectories. We applied GOM, Kalman filtering, LME, and Kalman smoothing to marker trajectories from both simulated and experimental gait motion, to estimate the joint kinematics of a ten segment biomechanical model, with 21 degrees of freedom. Three simulated marker trajectories were studied: without errors, with instrumental errors, and with soft tissue artefacts (STA). Two modelling errors were studied: increased thigh length and hip centre dislocation. We calculated estimation errors from the known joint kinematics in the simulation study. Compared with other techniques, Kalman smoothing reduced the estimation errors for the joint positions, by more than 50% for the simulated marker trajectories without errors and with instrumental errors. Compared with GOM, Kalman smoothing reduced the estimation errors for the joint moments by more than 35%. Compared with Kalman filtering and LME, Kalman smoothing reduced the estimation errors for the joint accelerations by at least 50%. Our simulation results show that the use of Kalman smoothing substantially improves the estimates of joint kinematics and kinetics compared with previously proposed techniques (GOM, Kalman filtering, and LME) for both simulated, with and without modelling errors, and experimentally measured gait motion.     

Rotational Diffusion of the α2a Adrenergic Receptor Revealed by FlAsH Labeling in Living Cells

The fluorescein arsenical hairpin binder (FlAsH) shows much promise to determine the relative orientations of protein regions and structures even in living cells and in the plasma membrane. In this study, we characterized FlAsH's photophysical properties by steady-state anisotropy and time-resolved single photon counting for further applications with G-protein coupled receptors. We find that FlAsH has a relatively high initial anisotropy of 0.31 ± 0.01 and a three-component fluorescence lifetime with an average of 4.1 ± 0.1 ns. We characterized the FlAsH fluorophore orientation in the α_2A adrenergic receptor revealing rigid orientations of FlAsH in the membrane plane for rotational correlation times of ∼50 ns in living cells. To elucidate the fluorophore-membrane orientation and rotational correlation time, an anisotropy treatment similar to that of another researcher (Axelrod, D. 1979. Biophys. J. 26:557–573) was developed. The rotational correlation times were observed to increase by up to 16 ns after agonist addition. The rotational correlation time also allowed for a comparison to the theoretical relationship between translational and rotational diffusion (originally proposed by Saffman, P. G., and M. Delbrück. 1975. Proc. Natl. Acad. Sci. USA. 72:3111–3113) and revealed a discrepancy of a factor between 10 and 100.