Orientation Contrast Research Articles

Ca-carbonate in the Orgueil (CI) carbonaceous chondrite: Mineralogy, microstructure and implications for parent body history

The Orgueil CI carbonaceous chondrite is a breccia and some of its constituent clasts contain Ca-carbonate grains whose mineralogy is undetermined, but may yield new information on the history of its asteroidal or cometary parent body. Characterisation of one such clast shows that in addition to Ca-carbonate it contains magnetite, olivine and orthopyroxene grains that are supported within a finely crystalline saponite–serpentine–ferrihydrite groundmass. The Ca-carbonate grains are typically rounded, less than 10 μm in diameter, and backscatter Kikuchi patterns identify all those analysed as calcite. Electron backscatter diffraction orientation contrast maps show that most calcites have sub-μm sized e-twins, some of which are cut by corroded grain margins. These microstructures demonstrate that the clast experienced very mild shock during aqueous alteration and probably accompanying impact ‘gardening’ of the parent body regolith. The sample studied hosts several other millimeter sized clasts and a fine grained clastic matrix that are free of Ca-carbonate, olivine and orthopyroxene although can contain apatite, dolomite and pyrrhotite. Within one of these clasts is a network of phyllosilicate–magnetite veins that are inferred to have acted as fluid flow conduits during parent body aqueous alteration. Sulphur-rich reaction rims within the same clast are likely to have formed during post-fall remobilization and crystallization of pre-terrestrial sulphates. This study provides good evidence for alteration of the CI carbonaceous chondrites in a dynamic parent body regolith.

Context influences contour integration

Much research over the last decade has examined how the brain links local activity within primary visual cortex to signal the presence of extended global structure. Here we bring together two themes within this area by addressing how the immediate context that features arise in influences how they are integrated into contours. Specifically, observers were required to detect and discriminate the shape of contours that were surrounded by elements with a fixed orientation offset compared to contour elements. By comparing performance with contours made of elements oriented either near parallel ("snakes") or near perpendicular ("ladders") to the contour orientation, we were able to isolate the effect of orientation contrast on observers' ability to perform our task with near-collinear contour structure. We report both substantial facilitation of contour integration in the presence of near-perpendicular surrounds and inhibition in the presence of near-parallel surrounds. These results are consistent with known orientation dependence of suppressive surround interactions in the primary visual cortex and suggest that the "rules of association" for contour integration must incorporate the influence of local orientation context. Specifically we show that our results are consistent with contour integration relying on an opponent-orientation energy response from a bank of first-stage oriented filters.

Effects of GABAa somatic inhibition on orientation tuning and contrast sensitivity in visual cortex

Event Abstract Back to Event Effects of GABAa somatic inhibition on orientation tuning and contrast sensitivity in visual cortex The selectivity of neurons in sensory cortex is thought to be at least partially shaped by inhibitory connections. In primary visual cortex (V1) many studies have examined how orientation tuning is affected by application of the GABAa antagonist bicuculline. These studies, however, have reported inconsistent results: some have seen a loss of selectivity and others only minor effects. A possible reason lies in the broad spectrum of effects of bicuculline; much more selective blockage of GABAa receptors is achieved with the potent antagonist gabazine (SR95531). We examined the effect of local gabazine iontophoresis on orientation tuning in V1 of anesthetized cats. Using extracellular recordings, we measured orientation tuning with drifting gratings. Gabazine was iontophoresed immediately above the recording electrode. An array of 96 electrodes was placed ~0.5 mm away to examine the spatial spread of gabazine. If gabazine did not affect responses measured with the array, we assumed that its effects were mostly restricted to the soma and proximal dendrites of neurons at the site of iontophoresis. Blocking GABAa-receptor mediated, somatic inhibition increased responses to all orientations by 150%, broadened tuning width by 22%, and reduced direction selectivity by 32% (medians, n=23). The resulting tuning curves resembled responses typically measured in subthreshold membrane potential, suggesting that GABAa provides untuned inhibition, keeping responses to most orientations below threshold. We tested this hypothesis using a simple rectification model. The model relies on the finding that the shape of orientation tuning curves for firing rates can be predicted by applying an appropriate threshold and gain to noisy membrane potential responses. Given these parameters, we sought to determine two hypothetical membrane potential tuning curves, which were allowed to differ only in their mean responses across orientations, to predict the tuning curve for firing rates in both the control and gabazine condition. This rectification model captured the observed effects of blocking GABAa-receptor mediated, somatic inhibition, i.e. increase in responses, broadening of orientation tuning, and decrease of direction selectivity. The success of the model suggests that inhibition hyperpolarizes the membrane potential by a constant amount, resulting in fewer threshold crossings, and therefore decreased firing rates and increased selectivity. Next, we asked whether inhibition depended on overall contrast. We measured contrast response functions with drifting gratings of optimal orientation before, during, and after gabazine administration. Blocking inhibition led to increases in firing rates that were stronger for high-contrast than for low-contrast stimuli. These effects are consistent with the notion that inhibition grows with contrast. We conclude that GABAa-receptor mediated, somatic inhibition plays an important role in orientation and direction selectivity by keeping membrane potential responses to most stimulus orientations below threshold, but not necessarily by shaping the tuning curve itself. Our results are consistent with the view that somatic inhibition provides a constant hyperpolarization, whose magnitude depends on stimulus contrast, but not on orientation. Support: NIH EY017396. Conference: Computational and systems neuroscience 2009, Salt Lake City, UT, United States, 26 Feb - 3 Mar, 2009. Presentation Type: Poster Presentation Topic: Poster Presentations Citation: (2009). Effects of GABAa somatic inhibition on orientation tuning and contrast sensitivity in visual cortex. Front. Syst. Neurosci. Conference Abstract: Computational and systems neuroscience 2009. doi: 10.3389/conf.neuro.06.2009.03.203 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 03 Feb 2009; Published Online: 03 Feb 2009. 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Within-texture collinearity improves human texture segmentation

Spatial arrangement has been shown to facilitate both detection of a threshold target by collinear flankers and detection of smooth chains within random arrays of suprathreshold elements. Here, we investigate the effect of alignment between texture elements on orientation-based texture segmentation. Textures composed of Gabor elements were used in a figure-discrimination task. The degree of collinearity within the texture was manipulated, and threshold figure-ground orientation differences found. A facilitative effect of collinearity on segmentation was seen, which was insensitive to Gabor carrier phase at the texture-element co-axial spacing of 3λ used here. The pattern of results with respect to collinearity could not be attributed simply to improved linkage of local orientation contrast at figure borders in isolation, and instead suggests a role for the figure interior in texture segmentation.

Diffusion tensor quantification of the macrostructure and microstructure of human midsagittal corpus callosum across the lifespan

The midsagittal cross-sectional area of the human corpus callosum (CC) has been used by many researchers as a marker of development, natural aging, and neurodegenerative and acquired pathologies. The availability of non-invasive MRI methods for quantifying the macrostructural and microstructural organization of the CC would help to clarify the CC contribution to behavior and cognition in both health and disease. In this report, we extended and validated the ability of a recently described semi-automated diffusion tensor imaging tissue segmentation method to utilize the high orientation contrast of the CC on diffusion tensor imaging. Using a cohort of healthy right-handed children and adults aged 7-59 years, we show gender-independent non-linear (quadratic) and strongly correlated growth trends in the CC area and the corresponding diffusion tensor fractional anisotropy (r = 0.67; P < 1 x 10(-10)). Our results provide preliminary evidence that diffusion tensor anisotropy in the living CC may be related to the number of small myelinated fibers.

Diffusion tensor quantification of the human midsagittal corpus callosum subdivisions across the lifespan

The midsagittal corpus callosum (CC) cross-sectional area subdivisions have been used as early and sensitive markers of human brain white matter connectivity, development, natural aging and disease. Despite the simplicity and conspicuity of the appearance of the CC on anatomical magnetic resonance imaging (MRI), the published quantitative MRI literature on its regional sex and age trajectories are contradictory. The availability of noninvasive quantitative methods to assess the CC regions across the human lifespan would help clarify its contribution to behavior and cognition. In this report, we extended the utility of a recently described semi-automated diffusion tensor imaging (DTI) tissue segmentation method to utilize the high orientation contrast of the CC on DTI. Using optimized DTI methods on a cohort of 121 right-handed children and adults aged 6–68 years, we examined the CC areas and corresponding DTI metrics of the different functionally specialized sectors of the CC. Both the area and fractional anisotropy metrics followed inverted U-shaped curves, while the mean and radial diffusivities followed U-curves reflecting white matter progressive and regressive myelination dynamics that continue into young adulthood.

SU‐GG‐I‐128: Biophysical Studies of Tendon to Elucidate Magic Angle MRI

Purpose: The “Magic Angle Effect” observed in cartilage, tendon and other collagen rich tissues of the extracellular matrix have largely unused potential for evaluation of musculo‐skeletal disease processes. These studies seek to improve understanding of the molecular basis for magic angle phenomena based on collagen hydration. We hypothesize that improved understanding will provide a conceptual framework and extend the ability of musculo‐skeletal radiologists to design protocols using MRI orientational contrast for better accuracy and specificity. Method and Materials: These studies use Differential Scanning Calorimetry (DSC) to directly measure the enthalpy and entropy of water bound to rat tail collagen/tendon at different hydration levels. Water vapor sorption and recovery rehydration rates of rat tail collagen/tendon at 22 °C were also measured. Measured bound water fractions are compared to the theoretical values (h=0.263 g/g) predicted from the molecular structure of collagen. Bound water hydration measured with both methods agreed with the molecular prediction of water bound to the protein back bone. Statistical analyses performed using Graphpad Prism. Results: The water vapor sorption and DSC studies of collagen/tendon at 22 °C show that both equilibrium hydration and enthalpy are linear functions of relative humidity up to critical hydration of the protein backbone h = 0.26 g/g. The water bridge hydration hypothesis identifies three hydration water fractions in direct contact with the protein that differ in motional/orientational properties from bulk water. Comparison to T1 and T2 relaxation rate and orientational studies shows the relaxation rate of tendon is determined by fast exchange of water between these motional and orientational restricted water fractions. Conclusion: This work offers evidence for the important role of protein main chain hydration in determining MRI contrast due to dielectric binding of polar water molecules interacting with partial electric charges separated on the backbone by steric restrictions of the collagen molecule.

Attention capture by eye of origin singletons even without awareness—A hallmark of a bottom-up saliency map in the primary visual cortex

Human observers are typically unaware of the eye of origin of visual inputs. This study shows that an eye of origin or ocular singleton, e.g., an item in the left eye among background items in the right eye, can nevertheless attract attention automatically. Observers searched for a uniquely oriented bar, i.e., an orientation singleton, in a background of horizontal bars. Their reports of the tilt direction of the search target in a brief (200 ms) display were more accurate in a dichoptic congruent (DC) condition, when the target was also an ocular singleton, than in a monocular (M) condition, when all bars were presented to the same single eye, or a dichoptic incongruent (DI) condition, when an ocular singleton was a background bar. The better performance in DC did not depend on the ability of the observers to report the presence of an ocular singleton by making forced choices in the same stimuli (though without the orientation singleton). This suggests that the ocular singleton exogenously cued attention to its location, facilitating the identification of the tilt singleton in the DC condition. When the search display persisted without being masked, observers' reaction times (RTs) for reporting the location of the search target were shorter in the DC, and longer in the DI, than the M condition, regardless of whether the observers were aware that different conditions existed. In an analogous design, similar RT patterns were observed for the task of finding an orientation contrast texture border. These results suggest that in typical trials, attention was more quickly attracted to or initially distracted from the target in the DC or DI condition, respectively. Hence, an ocular singleton, though elusive to awareness, can effectively compete for attention with an orientation singleton (tilted 20 or 50 degrees from background bars in the current study). Similarly, it can also make a difficult visual search easier by diminishing the set size effect. Since monocular neurons with the eye of origin information are abundant in the primary visual cortex (V1) and scarce in other cortical areas, and since visual awareness is believed to be absent or weaker in V1 than in other cortical areas, our results provide a hallmark of the role of V1 in creating a bottom-up saliency map to guide attentional selection.

Surface Preparation of Uranium by Ion Milling

Abstract Metallographic examinations of uranium, as carried out with both optical and orientation contrast microscopies (electron backscatter diffraction or EBSD), typically require a final preparation step that involves acid-etching or electropolishing. Uranium oxidizes relatively rapidly after mechanical polishing, making EBSD impossible without additional preparation steps. Conventional specimen preparation generates a mixture of acids and radioactive waste, so-called “mixed waste” as defined by the Resource Conservation and Recovery Act (RCRA). Mixed waste is more costly to dispose of than either separate component and there is a desire to eliminate mixed waste streams. One simple method to avoid the use of acids in optical metallography is to wait a few minutes for the sample to differentially oxidize to obtain the desired contrast. However, this method is disadvantageous when more than a few micrographs are needed because the sample continues to change with each micrograph.

Laser powered heating stage in a scanning electron microscope for microstructural investigations at elevated temperatures

A laser powered heating stage designed for application in high vacuum environment of a scanning electron microscope (SEM) is presented. It was developed to observe and characterize microstructural changes in crystalline materials at elevated temperatures up to 1000 degrees C. The approach utilizes the power output of a commercial infrared diode laser in order to heat up specimens without interference with the electronic system of the SEM. The heating stage can be used in combination with any standard characterization technique applicable for SEMs--electron backscatter diffraction, orientation contrast imaging, x-ray energy dispersive spectrometry, etc. The results of test measurements are presented.

Leaning Tower Illusion: Orientation Contrast or Perspective Distortion? Reply to Maniatis

Leaning Tower Illusion: Orientation Contrast or Perspective Distortion? Reply to Maniatis

Change Detection is Easier at Texture Border Bars When They are Parallel to the Border: Evidence for V1 Mechanisms of Bottom-up Salience

A vertical bar is salient among horizontal ones by orientation contrast, and, traditionally, bottom up salience is viewed as caused only by feature contrast. Recently, it has been proposed that the primary visual cortex (V1) creates a bottom-up salience map in its outputs, which depends on direct inputs and on contextual inputs by intra-cortical interactions (Li Zhaoping, 2002 Trends in Cognitive Sciences 6 9-16). Since the interactions include iso-feature (eg iso-orientation) suppression, responsible for salience by feature contrast, and collinear facilitation, the V1 proposal predicts that collinear grouping contributes to salience additionally. Accordingly, in orientation textures, texture bars are more salient near texture borders owing to orientation contrast, and are even more salient when they are additionally parallel to the border by collinear grouping. We show that colour changes in texture bars are more detectable when the bars are parallel to the texture border. Since changes are more detectable at salient locations, our finding supports V1 mechanisms for bottom-up salience.

The Conjoint Effect of Divisive Normalization and Orientation Selectivity on Redundancy Reduction in Natural Images

Bandpass filtering, orientation selectivity, and contrast gain control are prominent features of sensory coding at the level of V1 simple cells. While the effect of bandpass filtering and orientation selectivity can be assessed within a linear model, contrast gain control is an inherently nonlinear computation. Here we employ the class of Lp elliptically contoured distributions to investigate the extent to which the two features—orientation selectivity and contrast gain control—are suited to model the statistics of natural images. Within this framework we find that contrast gain control can play a significant role for the removal of redundancies in natural images. Orientation selectivity, in contrast, has only a very limited potential for redundancy reduction.

Polarization microscopy probe for applications in a high magnetic field

A microscopy probe designed for use in high field magnets is presented. It was developed to observe and continuously measure the position and shape of a magnetically driven grain boundary in a magnetically anisotropic material. The approach utilizes the anisotropy of reflectivity of visible light for orientation contrast to determine the boundary location. The major components of the system are the polarizing microscope with video camera and the sample chamber with a resistive heater for annealing at elevated temperatures up to 500 degrees C in an inert gas atmosphere. The results of test measurements are presented.

Spatial Resolution in ACOM - What Will Come After EBSD

Abstract Automated Crystal Orientation Microscopy (ACOM) on a grain specific level has proved to be an invaluable new tool for characterizing polycrystalline materials. It is usually based on scanning facilities using electron diffraction , due to its high sensitivity and spatial resolution, but also attempts have been made which rely upon X-ray or hard synchrotron radiation diffraction. The grain orientations are commonly mapped in pseudo-colors on the scanning grid to construct Crystal Orientation Maps (COM), which represent “images” of the microstructure with the advantage of providing quantitative orientation contrast. In a similar way, misorientations across grain boundaries, Σ values of grain boundaries, or other microstructural characteristics are visualized by mapping the grains in the micrograph with specific colors. The principal objectives are the determination of quantitative, statistically meaningful data sets of crystal orientations, misorientations, the CSL character (Σ) of grain boundaries, local crystal texture (pole figures, ODF, MODF, OCF) and derived entities, phase discrimination and phase identification.

Texture segmentation in human perception: A combined modeling and fMRI study

The human visual system uses texture information to segment visual scenes into figure and ground. We developed a computational model of human texture processing [Thielscher A, Neumann H (2003) Neural mechanisms of cortico-cortical interaction in texture boundary detection: a modeling approach. Neuroscience 122:921–939] which allows us to examine the functional roles of early and intermediate stages of the ventral visual pathway in figure–ground segmentation. In particular, the model highlights the central role of cells in mid-level areas (such as V4) with larger receptive fields in the robust identification of texture boundaries and pop-out stimuli even under noisy conditions. A straightforward prediction of the model is that the activity of cells in mid-level, but not early visual areas directly co-varies with the saliency of the texture borders in the visual scene. Consequently, their activity should directly correlate with the saliency of pop-out texture regions as accessed in psychophysical studies [Nothdurft HC (1991) Texture segmentation and pop-out from orientation contrast. Vision Res 31:1073–1078]. This prediction explicitly derived from the model was tested using functional magnetic resonance imaging. The saliency of texture bars composed of oriented line items was varied by parametrically changing the defining orientation contrast between fore- and background lines. Consistent with the model, increasing contrast at texture boundaries resulted in a monotonic increase of blood oxygen level dependent responses in mid-level, but not early visual areas. Our modeling and imaging results indicate that mid-level visual areas form a key stage in figure–ground segregation by gradually signaling the salience of region boundaries defined by orientation contrast.

Focus on materials

Focus on materials

Epitaxial SiC Growth Morphology and Extended Defects Investigated by Electron Backscatter Diffraction and Electron Channeling Contrast Imaging

Electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) were employed to investigate epitaxial SiC growth on 4H-SiC mesa structures. SiC polytypes were identified by indexing Kikuchi maps recorded from various points on the mesa surfaces. Orientation contrast was observed between different polytype surfaces using ECCI by forescattered electron detection. Extended defects in 3C-SiC were imaged directly by ECCI. Additionally, the ECCI technique was utilized to correlate dislocations with atomic step morphologies for various mesa surfaces. Evidence of vertical growth enhancement in the form of additional faceting was attributed to the presence of threading screw dislocations at mesa surfaces. Atomic steps were observed very near the edges of some mesa surfaces free of dislocations.

Femtosecond versus nanosecond laser micro-machining of InP: a nondestructive three-dimensional analysis of strain

Ultra-fast femtosecond laser micro-machining can lead to improved surface morphology and a reduction in the heat-affected zone. In this paper, synchrotron x-ray topography (SXRT) and micro-Raman spectroscopy have been used as nondestructive tools to compare the residual strain in InP substrates after femtosecond (fs) and nanosecond (ns) laser processing. Two-dimensional stain distributions with varying probing depth and cross-section images across the four laser machined grooves were obtained. The recrystallized poly-InP layer on the laser machined groove surface has been found to be highly strained in tension, and the stress magnitude is much bigger than the shear stress introduced by crystal distortion underneath. After comparing the simulation results of SXRT orientation contrast with the topography images, the nature of the crystal plane distortion induced by both fs and ns laser machining methods was elucidated.

Structural Dependence of Grain Boundary Energy in Fe-Based Polycrystals Identified by OIM Measurements

The (relative) grain boundary energy of random high angle boundaries has been measured in several Fe-based polycrystals. Crystallographic data obtained by orientation contrast microscopy (OIM) are combined with the geometrical configuration of grain boundaries at triple junctions. A two-parameter representation of the relative grain boundary energy in terms of misorientation angle and misorientation axis is presented. In the applied procedure a variation of the energy values assigned to one boundary was observed depending on the triangulation path chosen by the operator to connect the arbitrary initial boundary with the boundary under consideration. Results show no evidence of correspondence between the observed energy cusps and the presence of CSL boundaries.