Oblique Effect Research Articles

Evidence for tilt normalization can be explained by anisotropic orientation sensitivity.

Some data have been taken as evidence that after prolonged viewing, near-vertical orientations "normalize" to appear more vertical than they did previously. After almost a century of research, the existence of tilt normalization remains controversial. The most recent evidence for tilt normalization comes from data suggesting a measurable "perceptual drift" of near-vertical adaptors toward vertical, which can be nulled by a slight physical rotation away from vertical (Müller, Schillinger, Do, & Leopold, 2009). We argue that biases in estimates of perceptual stasis could, however, result from the anisotropic organization of orientation-selective neurons in V1, with vertically-selective cells being more narrowly tuned than obliquely-selective cells. We describe a neurophysiologically plausible model that predicts greater sensitivity to orientation displacements toward than away from vertical. We demonstrate the predicted asymmetric pattern of sensitivity in human observers by determining threshold speeds for detecting rotation direction (Experiment 1), and by determining orientation discrimination thresholds for brief static stimuli (Experiment 2). Results imply that data suggesting a perceptual drift toward vertical instead result from greater discrimination sensitivity around cardinal than oblique orientations (the oblique effect), and thus do not constitute evidence for tilt normalization.

Visual mismatch negativity (vMMN): a prediction error signal in the visual modality.

Our visual field contains much more information at everymoment than we can attend and consciously process. How isthe multitude of unattended events processed in the brain andselected for the further attentive evaluation? Current theories ofvisual change detection emphasize the importance of consciousattention to detect changes in the visual environment. However,an increasing body of studies shows that the human brain iscapable of detecting even small visual changes if such changesviolate non-conscious probabilistic expectations based on priorexperiences. In other words, our brain automatically representsenvironmental statistical regularities.Since the discovery of the auditory mismatch negativity(MMN) event-related potential (ERP) component, the majorityof research in the field has focused on auditory deviance detec-tion.Suchautomaticchangedetectionmechanismsoperateinthevisual modality too, as indicated by the visual mismatch negativ-ity (vMMN) brain potential to rare changes. vMMN is typicallyelicited by stimuli with infrequent (deviant) features embeddedin a stream of frequent (standard) stimuli, outside the focus ofattention. Information about both simple and more complexcharacteristics of stimuli is rapidly processed and stored by thebrain in the absence of conscious attention.InthisresearchtopicweaimtopresentvMMNasapredictionerror signal and put it in context of the hierarchical predictivecoding framework. Predictive coding theories account for phe-nomena such as MMN and repetition suppression, and placetheminabroadercontextofageneraltheoryofcorticalresponses(Friston,2005,2010).EachpaperinthisResearchTopicisavalu-able contribution to the field of automatic visual change detec-tion and deepens our understanding of the short term plasticityunderlying predictive processes of visual perceptual learning.A wide range of vMMN studies has been presented in sev-enteen articles in this Research Topic. Twelve articles addressroughly four general sub-themes including attention, language,faceprocessing,andpsychiatricdisorders.Additionally,fourarti-cles focused on particular subjects such as the oblique effect,object formation, and development and time-frequency analysisof vMMN. Furthermore, a review paper presented vMMN in ahierarchical predictive coding framework.Four articles investigated the relationship between attentionand vMMN. Kremlaˇcek et al. (2013) presented subjects withradial motion stimuli in the periphery of the visual field usingan oddball paradigm and manipulated the attentional load byvarying the difficulty of a central distractor tasks. They aimedto manipulate the amount of available attentional resources thatmight have been involuntarily captured by the vMMN-evokingstimulipresentedintheperipheryoutsideoftheattentionalfocus.The distractor task had three difficulty levels: (1) a central fix-ation (easy), and a target number detection task with (2) onetarget number (moderate), and (3) three target numbers (diffi-cult). Analysis of deviant minus standard differential waveformsrevealed a significant posterior negativity in the ∼140–200msinterval,whichwasunaffectedbythedifficultyofthecentraltask,indicatingthattheautomaticprocessesunderlyingregistrationofchanges in motion are independent of attentional resources usedto detect target numbers.Kimura and Takeda (2013) investigated whether characteris-tics of vMMN depended on the difficulty of an attended primarytask, i.e., they tested the level of automaticity of the vMMN. Taskdifficulty was manipulated as the magnitude of change of a cir-cle at fixation, and vMMN was elicited by deviant orientationof bar patterns. An equal probability control condition was alsoused. The difference potential between the deviant-related ERPand the ERP elicited by identical orientation pattern in the con-trol condition appeared to be influenced by the difficulty of theattentive task. As a function of task difficulty, the latency of thedifference potential (i.e., the vMMN) increased, indicating thatprocesses underlying vMMN to orientation changes are not fullyindependent of the attention demands of the ongoing tasks.Kuldkepp et al. (2013) used rare changes in direction ofperipheral motion to evoke vMMN applying a novel continuouswhole-display stimulus configuration. The demanding distractortask involved motion onset detection and was presented in thecenter of the visual field. The level of attention to the vMMN-evoking stimuli was varied by manipulating their task-relevanceusing “Ignore” and “Attend” conditions. Deviant minus standardwaveforms in the “Ignore” condition showed significant vMMNin the 100–200, 250–300, and 235–375ms intervals, whereas in

Optical properties of thin graphitic nanopetal arrays

Thermal radiative properties of thin graphitic petal arrays are theoretically and experimentally investigated. Finite-difference time-domain (FDTD) simulations are first performed to calculate optical properties of vertical graphitic arrays of different structures, namely, graphitic gratings, periodic graphitic cavities, and random graphitic cavities. For graphitic gratings, the absorptance and reflectance are relatively larger when the incident electric field is parallel to the graphitic plane, while the absorptance and reflectance are both significantly lower when the electric field is polarized perpendicular to the graphitic plane. Ordered graphitic petal cavity arrays show optical properties falling between the above two cases of different polarizations. Random vertical cavity arrays with various angles of orientation show similar properties to ordered petal cavities. For oblique gratings, the reflectance will increase with oblique angle for both polarizations, while the absorptance decreases with oblique angle for the in-plane polarization and increases with oblique angle for the out-of-plane polarization. The oblique effects are explained by the strong anisotropic nature of graphitic petals. The FDTD results are compared to effective medium theory to find that the latter describes the optical properties of the graphitic grating and cavity well, and we propose an approach based on effective medium theory to approximate the dielectric function of graphitic petals with random orientation. The predicted hemispherical total reflectance based on this model gives about 2% reflectance in the visible spectrum and agrees well with experimental data from a fabricated graphitic petals sample.

Nonlinear slow magnetoacoustic waves in coronal plasma structures

Context. There is abundant observational evidence of longitudinal waves in the plasma structures of the solar corona. These essentially compressive waves are confidently interpreted as slow magnetoacoustic waves. The use of the slow waves in plasma diagnostics and estimating their possible contribution to plasma heating and acceleration require detailed theoretical modelling. Aims. We investigate the role of obliqueness and magnetic effects in the evolution of slow magnetoacoustic waves, also called tube waves, in field-aligned plasma structures. Special attention is paid to the wave damping caused by nonlinear steepening. Methods. We considered an untwisted straight axisymmetric field-aligned plasma cylinder and analysed the behaviour of the slow magnetoacoustic waves that are guided by this plasma structure. We adopted a thin flux tube approximation. We took into account dissipation caused by viscosity, resistivity and thermal conduction, and nonlinearity. Effects of stratification and dispersion caused by the finite radius of the flux tube were neglected. Results. We derive the Burgers-type evolutionary equation for tube waves in a uniform plasma cylinder. Compared with a plane acoustic wave, the formation of shock fronts in tube waves is found to occur at a larger distance from the source. In addition, tube waves experience stronger damping. These effects are most pronounced in plasmas with the parameter β at about or greater than unity. In a low-β plasma, the evolution of tube waves can satisfactorily be described with the Burgers equation for plane acoustic waves.

The effect of impact obliquity on shock heating in planetesimal collisions

AbstractCollisions between planetesimals in the early solar system were a common and fundamental process. Most collisions occurred at an oblique incidence angle, yet the influence of impact angle on heating in collisions is not fully understood. We have conducted a series of shock physics simulations to quantify oblique heating processes, and find that both impact angle and target curvature are important in quantifying the amount of heating in a collision. We find an expression to estimate the heating in an oblique collision compared to that in a vertical incidence collision. We have used this expression to quantify heating in the Rhealsilvia‐forming impact on Vesta, and find that there is slightly more heating in a 45° impact than in a vertical impact. Finally, we apply these results to Monte Carlo simulations of collisional processes in the early solar system, and determine the overall effect of impact obliquity from the range of impacts that occurred on a meteorite parent body. For those bodies that survived 100 Myr without disruption, it is not necessary to account for the natural variation in impact angle, as the amount of heating was well approximated by a fixed impact angle of 45°. However, for disruptive impacts, this natural variation in impact angle should be accounted for, as around a quarter of bodies were globally heated by at least 100 K in a variable‐angle model, an order of magnitude higher than under an assumption of a fixed angle of 45°.

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.

Stabilizing action of pressure in homogeneous compressible shear flows: effect of Mach number and perturbation obliqueness

AbstractCompressibility exerts a stabilizing influence on a variety of high-speed shear flows such as turbulent mixing layers, transitioning boundary layers and homogeneously sheared turbulence. An important stabilizing feature that is common amongst all shear flows is the velocity–pressure interaction dynamics. In this study, velocity–pressure interactions of individual perturbation or fluctuation modes are investigated using direct numerical simulations and linear analysis in high-Mach-number homogeneous shear flow. For a given perturbation wave mode, the action of pressure is shown to depend on two important factors: the orientation of the perturbation wavevector with respect to the shear plane and the Mach number. It is shown that the streamwise perturbation wave mode rapidly develops a high level of kinetic energy but is self-limiting owing to the action of pressure. On the other hand, the energy of spanwise perturbation wave modes grows unaffected by pressure or Mach number. Oblique modes combine spanwise and streamwise characteristics and are shown to be chiefly responsible for stabilizing effects seen in shear flows. Three regimes of obliqueness of different linear stability characteristics are identified. The critical role of perturbation obliqueness on stabilization is established.

MEASUREMENT OF WAVE PRESSURE ON PERFORATED CAISSON WALL UNDER OBLIQUE WAVE INCIDENCE

Three dimensional experiments were carried out in a wave basin in order to examine the effects of wave obliquity on the wave pressure acting on perforated caisson wall. The magnitudes of pressures measured under oblique waves did not show decaying trend predicted by Goda’s formula, for both the front and rear walls of the caisson. On the contrary, the wave force on a single caisson, estimated by integration of the vertical pressure profiles on the caisson front, showed slightly increasing trend on average with increase of wave incidence angle. Meanwhile, the simultaneously measured wave force for the whole caissons is found to be apparently reduced with wave obliquity as a result of peak-delay force reduction in lateral direction along the caisson walls.

Dissecting the oblique effect: replicating the expansion of motion direction space around the cardinal axes with a computer model built from V1 and MT neuron inputs

Dissecting the oblique effect: replicating the expansion of motion direction space around the cardinal axes with a computer model built from V1 and MT neuron inputs

Dissecting the oblique effect: Replicating the expansion of motion direction space around the cardinal axes with a computer model built from V1 and MT neuron inputs

Dissecting the oblique effect: Replicating the expansion of motion direction space around the cardinal axes with a computer model built from V1 and MT neuron inputs

Two mechanisms subserve the oblique effect

Two mechanisms subserve the oblique effect

Spin evolution of Earth-sized exoplanets, including atmospheric tides and core–mantle friction

AbstractPlanets with masses between 0.1 and 10 M⊕are believed to host dense atmospheres. These atmospheres can play an important role on the planet's spin evolution, since thermal atmospheric tides, driven by the host star, may counterbalance gravitational tides. In this work, we study the long-term spin evolution of Earth-sized exoplanets. We generalize previous works by including the effect of eccentric orbits and obliquity. We show that under the effect of tides and core–mantle friction, the obliquity of the planets evolves either to 0° or 180°. The rotation of these planets is also expected to evolve into a very restricted number of equilibrium configurations. In general, none of these equilibria is synchronous with the orbital mean motion. The role of thermal atmospheric tides becomes more important for Earth-sized planets in the habitable zones of their systems; so they cannot be neglected when we search for their potential habitability.

Oblique propagation of dust ion-acoustic solitary waves in a magnetized dusty pair-ion plasma

We investigate the propagation characteristics of electrostatic waves in a magnetized pair-ion plasma with immobile charged dusts. It is shown that obliquely propagating (OP) low-frequency (in comparison with the negative-ion cyclotron frequency) long-wavelength “slow” and “fast” modes can propagate, respectively, as dust ion-acoustic (DIA) and dust ion-cyclotron (DIC)-like waves. The properties of these modes are studied with the effects of obliqueness of propagation (θ), the static magnetic field, the ratios of the negative to positive ion masses (m), and temperatures (T) as well as the dust to negative-ion number density ratio (δ). Using the standard reductive perturbation technique, we derive a Korteweg-de Vries (KdV) equation which governs the evolution of small-amplitude OP DIA waves. It is found that the KdV equation admits only rarefactive solitons in plasmas with m well below its critical value mc (≫ 1) which typically depends on T and δ. It is shown that the nonlinear coefficient of the KdV equation vanishes at m = mc, i.e., for plasmas with much heavier negative ions, and the evolution of the DIA waves is then described by a modified KdV (mKdV) equation. The latter is shown to have only compressive soliton solution. The properties of both the KdV and mKdV solitons are studied with the system parameters as above, and possible applications of our results to laboratory and space plasmas are briefly discussed.

TU-F-18C-02: Increasing Amorphous Selenium Thickness in Direct Conversion Flat-Panel Imagers for Contrast-Enhanced Dual-Energy Breast Imaging

Purpose: Contrast-enhanced (CE) breast imaging using iodinated contrast agents requires imaging with x-ray spectra at energies greater than those used in mammography. Optimizing amorphous selenium (a-Se) flat panel imagers (FPI) for this higher energy range may increase lesion conspicuity. Methods: We compare imaging performance of a conventional FPI with 200 μm a-Se conversion layer to a prototype FPI with 300 μm a-Se layer. Both detectors are evaluated in a Siemens MAMMOMAT Inspiration prototype digital breast tomosynthesis (DBT) system using low-energy (W/Rh 28 kVp) and high-energy (W/Cu 49 kVp) x-ray spectra. Detectability of iodinated lesions in dual-energy images is evaluated using an iodine contrast phantom. Effects of beam obliquity are investigated in projection and reconstructed images using different reconstruction methods. The ideal observer signal-to-noise ratio is used as a figure-of-merit to predict the optimal a-Se thickness for CE lesion detectability without compromising conventional full-field digital mammography (FFDM) and DBT performance. Results: Increasing a-Se thickness from 200 μm to 300 μm preserves imaging performance at typical mammographic energies (e.g. W/Rh 28 kVp), and improves the detective quantum efficiency (DQE) for high energy (W/Cu 49 kVp) by 30%. While the more penetrating high-energy x-ray photons increase geometric blur due to beam obliquity in the FPI with thicker a-Se layer, the effect on lesion detectability in FBP reconstructions is negligible due to the reconstruction filters employed. Ideal observer SNR for CE objects shows improvements in in-plane detectability with increasing a-Se thicknesses, though small lesion detectability begins to degrade in oblique projections for a-Se thickness above 500 μm. Conclusion: Increasing a-Se thickness in direct conversion FPI from 200 μm to 300 μm improves lesion detectability in CE breast imaging with virtually no cost to conventional FFDM and DBT. This work was partially supported by a research grant from Siemens Healthcare.

Dynamics of dust-ion acoustic shock waves in a magnetized charge variable superthermal complex plasma

A theoretical investigation is carried out to study the characteristic properties of dust-ion acoustic (DIA) shock waves in a magnetized, charge varying, complex (dusty) plasma which consists of immobile dust grains, fluid ions and superthermal electrons. The effects of collisions are not included here, but the dissipation leading to the formation of stable shock structures is provided by the dust charge fluctuations. The Korteweg–de Vries–Burgers (KdV–Burgers) equation is derived using the reductive perturbation technique. The combined effects of obliqueness, magnitude of the magnetic field, and electron superthermality on the DIA shock waves are then investigated. It is shown that the effects of obliqueness, electron non-thermality and dust charge fluctuation significantly modify the basic properties of DIA shock waves.

Effects of slab geometry and obliquity on the interplate thermal regime associated with the subduction of three-dimensionally curved oceanic plates

We investigated the relationships among slab geometry, obliquity, and the thermal regime associated with the subduction of oceanic plates using a three-dimensional (3D) parallelepiped thermal convection model. Various models with convex and concave slab shapes were constructed in the numerical simulation, and the temperature and mantle flow distributions were calculated. The results revealed that when the slab dip angle increases, or the obliquity of subduction becomes steeper, the interplate temperature decreases remarkably. Cooler (warmer) zones on the plate interface were identified from the modeling where there was a larger (smaller) subduction angle. Consequently, the interplate temperature distribution is partly controlled by the true subduction angle (TSA), which is a function of the slab dip angle and the obliquity of subduction. The rate of change of the interface temperature for the TSA was 10–50 °C (10°< TSA < 20°) at depths ranging from (TSA – 10) × 5 km to 60 + (TSA – 10) × 5 km for a flat slab after a subduction history of 7 Myrs. The along-arc slab curvature affects the variation in TSA. The slab radius also appeared to influence the radius of induced mantle flow.

Damage in low alloy steel produced by sweeping, interacting detonation waves

Detonation waves that sweep along the surface of a metal plate induce reduced pressure and enhanced shear, relative to the same detonation at normal incidence. Detonation waves at intermediate obliquity impress intermediate combined stress states. Release waves from the free surfaces may enter into play and contribute to the damage. Initiation of explosive at discrete points produces strong pressure, density, and velocity gradients in the gaseous explosive products in areas where the waves collide, are impressed in an adjacent metal, causing similar stress gradients within the metal that often leading to intense damage. In this work, we investigate damage generated in AISI 4130 steel by the combined effects of oblique drive and interacting detonation waves. The experimental data consist of multipoint velocimetry points probing the free surface in regions loaded by interacting detonation waves and regions between the interactions. Metallography on recovered plate records the plastic flow and damage correlated with the velocimetry data. Spall is indicated in most regions, but not some, and the alpha-epsilon stress-induced phase transformation appears in most regions, but not all.

Ballistic performance of titanium alloy encapsulated ceramic composite plates

Titanium alloy (Ti–6Al–4V) encapsulated ceramic (SiC) composite plates were fabricated using graphite mould casting. The ballistic penetration resistance of the plate was evaluated experimentally using a ballistic rifle. Numerical simulations using the finite element method were also conducted to predict the ballistic resistance. The underlying mechanisms were explored in terms of local stress distribution, energy absorption, ballistic limit velocity and obliquity effect. The influence of structural confinement and metal–ceramic interfaces on the overall ballistic performance of the plate was discussed.

Localized slip and distributed deformation in oblique settings: the example of the Denali fault system, Alaska

In most fault systems the direction of the relative plate motion is oblique to the azimuth of the existing faults. Hence, during earthquakes the displacement may be partitioned between several faults that accommodate different components of the total motion. Here, we quantify the effect of the obliquity of the fault system relatively to the plate-motion direction on the distribution of the deformation in the fault system, during distinct periods of the seismic cycle. The 2002 November, M w 7.9, Denali strike-slip earthquake ruptured 341 km of the Denali fault. The azimuth of the fault varies by more than 50 • over the total rupture length, making the Denali fault an ideal system to test the effect of obliquity. From west to east, thrust dominates the first part of the rupture while strike-slip dominates the central and eastern sections. Using a kinematic model that considers the obliquity of the plate-motion direction relative to the local fault azimuth, we explored how much of the far-field tectonic loading is accommodated on the main strike-slip fault during the earthquake, and how much is accommodated by distributed deformation off the main fault, on secondary structures. Using a dataset of 735 focal mechanisms, we represent the deformation using strain rosettes and we compare seismological data with model results using the areal strain. Then we developed the parameter Ca, the coefficient of accommodation, which allows a direct quantification of the efficiency of a fault to accommodate oblique motion. Using these indicators, we show that in oblique setting, such as in the Denali case, the aftershocks and the background seismicity are organized to accommodate a significant part of the deformation that is not taken on the Denali strike-slip fault during the main earthquakes. The westward increase of the obliquity actually increases the amount of such deformation accommodated through distributed thrust faults, leading to the westward widening of the Alaska Range, located north of the Denali fault. Therefore, the strain partitioning between localized slip on pre-established major faults and distributed deformation accommodated through aftershocks and background seismicity on smaller faults (highlighted here by the longer-term topography) seems to be needed during the seismic cycle to accommodate the boundary conditions in such oblique settings.

Two independent sources of anisotropy in the visual representation of direction in 2-D space

It is known that visual direction representation is more accurate for cardinal directions compared to oblique, a phenomenon named the "oblique effect". It has been hypothesized that there are two sources of oblique effect, a low level one confined to vision and a high level one extending to different modalities and corresponding to higher cognitive processes. In this study directional error (DE) was measured when normal individuals tried to align the direction of an arrow presented in the center of a computer monitor to the direction of a peripheral target located in one of 32 directions equally spaced on an imaginary circle of 60 mm radius. Task difficulty was manipulated by varying arrow length (15, 30, 45 and 60 mm). By measuring mean DE and its variance we identified two independent sources of the oblique effect. A low level oblique effect was manifested in higher accuracy or equivalently lower variance of DE in the alignment for cardinal orientations compared to oblique. A second oblique effect was manifested measuring mean DE resulting in space expansion in the vicinity of cardinal directions and space contraction in the vicinity of oblique directions. Only this latter source of oblique effect was modulated by arrow length as predicted from a theoretical model postulating that this oblique effect is produced by a cognitive process of 2-D space categorization.