Orientation Contrast Research Articles

Modulation of V1 Activity by Shape: Image-Statistics or Shape-Based Perception?

It is current dogma that neurons in primary visual cortex extract local edges from the scene from which later visual areas reconstruct more meaningful shapes. Recent neuroimaging studies, however, have shown V1 modulations by the degree of structure in the image (shape). These V1 modulations due to the level of shape coherence have been explained in one of two possible ways: due to changes in image statistics or shape-based perceptual influences from higher visual areas. Here we compare both hypotheses using stimuli composed of Gabor arrays constructed to form circular shapes that can be successively degraded by manipulating the orientations of individual Gabors while maintaining local and global statistics. In a first experiment, we confirm that V1 responses are inversely correlated with the degree of structure in the image. In a second experiment, stimulus predictions are compared based on the degree of circular shape or change in the image statistic varied (orientation variance) in the image. We find that these V1 modulations to shape change are correlated with low-level changes in orientation contrast rather than shape perception per se.

Czochralski growth and X-ray topographic characterization of decagonal AlCoNi quasicrystals

Large single crystals of the decagonal AlCoNi phase were reproducibly grown by the Czochralski method from Al-rich melts. By using native seeds, it was possible to grow crystals with well-defined orientations, e.g. along the 10-fold periodic axis or parallel to one of the 2-fold axes. Quasicrystals, that had been pulled in periodic or quasiperiodic directions, were characterized by X-ray transmission topography at the synchrotron source (ESRF) and at laboratory sources using the Lang technique. Because of the metallurgical homogeneity of the grown quasicrystals, lattice parameter differences did not contribute to the topographic contrast, instead, orientation contrast dominated. A column-like substructure along the periodic direction could be revealed. From this it is concluded that crystal growth along a two-fold direction might be favourable with respect to minimizing defect spreading.

Tilt aftereffect for texture edges is larger than in matched subjective edges, but both are strong adaptors of luminance edges

The tilt aftereffect (TAE) has been used previously to probe whether contours defined by different attributes are subserved by the same or by different underlying mechanisms. Here, we compare two types of contours between texture surfaces, one with texture orientation contrast across the edge (orientation contrast contour; OC) and one without, commonly referred to as a subjective contour (SC). Both contour types produced curves of TAE versus adapting angle displaying typical positive and negative peaks at approximately 15 and 70 deg, respectively. The curves are well fit by difference of Gaussian (DoG) functions, with one Gaussian accounting for the contour adaptation effect and the other accounting for the texture orientation adaptation effect. Adaptation to OC elicited larger TAEs than did adaptation to SC, suggesting that they more effectively activate orientation-selective neurons in V1/V2 during prolonged viewing. Surprisingly, both contour types adapted a luminance contour (LC) as strongly as did an LC itself, suggesting that the second-order orientation cue contained in the texture edge activates the same set of orientation-selective neurons as does an LC. These findings have implications for the mechanisms by which the orientations of texture edges and SCs are encoded.

Crystal-plastic deformation of zircon: A defect in the assumption of chemical robustness

Research Article| April 01, 2006 Crystal-plastic deformation of zircon: A defect in the assumption of chemical robustness Steven M. Reddy; Steven M. Reddy 1Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia Search for other works by this author on: GSW Google Scholar Nicholas E. Timms; Nicholas E. Timms 1Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia Search for other works by this author on: GSW Google Scholar Patrick Trimby; Patrick Trimby 2HKL Technology A/S, Majsmarken 1, 9500 Hobro, Denmark Search for other works by this author on: GSW Google Scholar Peter D. Kinny; Peter D. Kinny 3Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia Search for other works by this author on: GSW Google Scholar Craig Buchan; Craig Buchan 3Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia Search for other works by this author on: GSW Google Scholar Kevin Blake Kevin Blake 4Advanced Analytical Centre, James Cook University, Townsville, Queensland 4811, Australia Search for other works by this author on: GSW Google Scholar Geology (2006) 34 (4): 257–260. https://doi.org/10.1130/G22110.1 Article history received: 29 Jul 2005 rev-recd: 31 Oct 2005 accepted: 14 Dec 2005 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Steven M. Reddy, Nicholas E. Timms, Patrick Trimby, Peter D. Kinny, Craig Buchan, Kevin Blake; Crystal-plastic deformation of zircon: A defect in the assumption of chemical robustness. Geology 2006;; 34 (4): 257–260. doi: https://doi.org/10.1130/G22110.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Orientation contrast imaging and quantitative electron backscatter diffraction analysis of a zircon collected from an Indian Ocean gabbro reveal intragrain crystallographic misorientations (up to 14°) and low-angle orientation boundaries concentrated in the zircon tips. These features represent the formation and migration of dislocations and provide the first evidence of crystal-plastic deformation of zircon under crustal conditions. Panchromatic and wavelength cathodoluminescence (CL), combined with quantitative rare earth element (REE) ion microprobe analyses, demonstrate modification of zircon REE chemistry within the areas of crystal plasticity. These data indicate that the enhanced diffusion of REEs into the zircon is spatially linked to the presence of dislocations that behave as high-diffusivity pathways, increasing bulk diffusion rates and effective diffusion distances in the zircon by several orders of magnitude. In addition, discrete ∼2 μm zones of reduced panchromatic CL correspond exactly to the position of low-angle orientation boundaries and demonstrate a defect dependence on CL signal at high dislocation densities. The presence of deformation-related crystal-plastic microstructures in zircon, and their role in modifying elemental diffusion, questions the commonly made assumption of chemical robustness and has fundamental implications for the interpretation of zircon trace-element and isotopic data. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Background Changes Delay Information Represented in Macaque V1 Neurons

In natural behavioral situations, saccadic eye movements not only introduce new stimuli into V1 receptive fields, they also cause changes in the background. We recorded in awake macaque V1 using a fixation paradigm and compared evoked activity to small stimuli when the background was either static or changing as with a saccade. When a stimulus was shown on a static background, as in most previous experiments, the initial response was orientation selective and contrast was inversely correlated with response latency. When a stimulus was introduced with a background change, V1 neurons showed a qualitatively different temporal response pattern in which information about stimulus orientation and contrast was delayed. The delay in the representation of visual information was found with three different types of background change-luminance increment, luminance decrement, and a pattern change with fixed mean luminance. We also found that with a background change, V1 off responses were suppressed and had a shorter time course compared with the static-background situation. Our results suggest that the distribution of temporal changes across the visual field plays a fundamental role in determining V1 responses. In the static-background condition, temporal change in the visual input occurs only in a small portion of the visual field. In the changing-background condition, and presumably in natural vision, temporal changes are widely distributed. Thus a delayed representation of visual information may be more representative of natural visual situations.

White beam X-ray topographic measurement of spontaneous strain in strontium titanate

We have measured the spontaneous strain of a zone boundary mode phase transition in strontium titanate by using the X-ray topographic contrast produced by the lamellar domain structure. The hill and valley structure of the lattice planes intersecting the domain walls leads to the orientation contrast consisting of pairs of a bright stripe and a dark stripe. The linear relation between the splitting of the pairing stripes and a camera distance gives the temperature dependence of the spontaneous strain, which is in the order of 10−4 at 4.8K, as well as the critical index of order parameter β=0.32.

Interactions between Orientation and Contrast Modulations Suggest Limited Cross-Cue Linkage

Recent studies of texture segmentation and second-order vision have proposed very similar models for the detection of orientation modulation and contrast modulation (OM and CM). From the similarity of the models it is tempting to assume that the two cues might be processed by a single generalised texture mechanism; however, recent results (Kingdom et al, 2003 Visual Neuroscience 2 65-76) have suggested that these cues are detected independently, or at least in a mechanism that is able to maintain an apparent independence between the cues. We tested new combinations of OM and CM and found that CM at 0.4 cycle deg(-1) facilitates the detection of OM at 0.2 cycle deg(-1) when the peaks of contrast align with the extremes of orientation. There is also some evidence of weak facilitation of CM by OM under the same conditions. Further, this facilitation can be predicted by filter--rectify--filter channels optimised for the detection of each cue, adding weight to the argument that texture cues are processed in a single generalised mechanism that nonetheless achieves cue independence or near-independence in many circumstances. We also found that the amount of suprathreshold masking produced by an orientation cue depends on the overall percept formed by that cue.

XRDT Study of Structural Defects of 6H-SiC Crystals

X-Ray Diffraction Topography (XRDT) and Optical Microscopy (OM) are adopted to study extended structural defects in 6H-SiC bulky crystals. Topographs are taken by means of White Beam Synchrotron Radiation Source (WB-SRS-XRDT) and by means of monochromatic radiation (MoKα1) with conventional source (Lang method). All studied samples are characterised by the presence of linear defects, dislocations and microchannels, uniformly distributed in the crystal. Such defects draw a net of independent systems of parallel lines, with different orientation and different contrast widths. Micro-channels are parallel to the c axis, whereas dislocations are perpendicular or nearly parallel to the c axis. The last are unit screw dislocations. It has been concluded that the growth mechanism is driven by screw dislocations and that channels results from the coalescence of parallel dislocations.

Interpretation of orientational contrast in STM images of GaAs(1 1 0) cleavage surface

The electronic structure of GaAs(1 1 0) surface is analyzed using Density Functional Theory (DFT-GGA) in atomic orbital basis (LCAO). The surface orbitals and the corresponding local density of electronic states (LDOS) are calculated for purposes of interpreting STM images. We show how local atomic orbitals of surface atoms are related to tunneling channels for electrons in STM imaging. A destructive interference between orbitals of two neighbouring atoms increases the contrast between the two atoms, and this is reflected in directionality of STM patterns of GaAs(1 1 0) surfaces. We also discuss how the basic formalism of Tersoff–Hamann approach to STM simulation can be reformulated to reveal the role of phase difference between tunneling channels.

Influence of the gravitational vertical on geometric visual illusions

The occurrence of geometric orientation illusions and the perception of ambiguous figures were analyzed in 24 subjects during static body tilt relative to gravity on Earth. Results showed that illusions such as the Rock's diamond/square, the Ponzo illusion, and orientation contrast illusions occurred less frequently, and that depth reversal of ambiguous figures took more time when subjects were lying on their side or supine compared to upright, thus suggesting that the gravitational reference plays a significant role in these “visual” illusions. The structure of images, our representation of the environment, and orientation relative to gravity are all integral parts in interpreting visual images. In a weightless environment where no gravitational reference can be used, it is expected that similar alterations in visual perception will occur.

Contextual Modulation outside of Awareness

Contextual effects are ubiquitous in vision and reveal fundamental principles of sensory coding. Here, we demonstrate that an oriented surround grating can affect the perceived orientation of a central test grating even when backward masking of the surround prevents its orientation from being consciously perceived. The effect survives introduction of a gap between test and surround of over a degree even under masking, suggesting either that contextual information can effectively propagate across early visual cortex in the absence of awareness of the signaled context or that it can proceed undetected to higher processing levels at which such horizontal propagation may not be necessary. The effect under masking also shows partial interocular transfer, demonstrating processing of orientation by binocular neurons in visual cortex in the absence of conscious orientation perception. This pattern of results is consistent with the suggestion that simultaneous orientation contrast is mediated at multiple levels of the visual processing hierarchy, and it supports the view that propagation of signals to and, possibly, back from higher visual areas is necessary for conscious perception.

In Situ Recrystallization Imaging of Polycrystalline Ni-S Alloy by SEM

AbstractImpurity segregation at alloy grain boundaries (GB) depends on many parameters such as GB character and prior thermomechanical history. In this paper we describe an experimental set-up designed to record the individual behaviour of GB during recrystallization of a Nickel-Sulphur alloy. The aim is to correlate segregation level with mobility and disorientation of GB. Therefore a method allowing direct observation of the GB motion as well as disorientation mapping is needed.Studying grain growth and recrystallization of this particular alloy by Scanning Electron Microscopy is possible thanks to real time in-situ imaging. This requires a sensitive crystal orientation contrast and the ability to image hot evolving samples. To heat the samples, a special device designed for the SEM analysis chamber is used. This device is used at 455°C but is able to reach higher temperatures. In order to get the orientation contrast images, as well as to protect the Secondary Electron (SE) detector from heat and light, a channel plate converter has been designed. This experimental set-up, combined with the heating system, allow the observation of the sample during recrystallization and recording small videos providing information such as grain boundary displacement rate. Then, associated with an EBSD analysis we can get the disorientation map of the area of interest and eventually an electrochemical technique is performed to create preferential dissolution at GB. Geometry of the grooves resulting from an electrochemical etching gives an evaluation of the segregation level in GB.

Grain structure and texture of cast iron aluminides

AbstractThe grain structure and texture of cast iron aluminides have been investigated in the scanning electron microscope by orientation contrast imaging and electron back‐scatter diffraction, respectively. Depending on the aluminum concentration the alloys exhibit a fine‐grained chill zone, medium grain size columnar zone and/or coarse‐grained equiaxed zone. The texture is random except in the columnar zone which is characterized by a <100> fibre texture in the direction of heat flow, i.e parallel to the elongated grains. The development of grain structure and texture is discussed with respect to cooling rate, thermal and constitutional undercooling, and order. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Scanning/transmission electron microscopy and dual-beam sample preparation for the analysis of crystalline materials

Scanning electron microscopy, scanning transmission electron microscopy and transmission electron microscopy are powerful tools in the analysis of crystalline materials. In combination with their associated analytical techniques, they roughly cover three length scales: the ‘supra-nm’ scale, the ‘nm’ scale and the ‘sub-nm’ scale, respectively. In this paper, the investigation of crystalline properties is presented, by means of 2D and 3D characterization examples. Techniques such as channeling contrast, electron backscatter diffraction, crystal orientation contrast on thin films and convergent beam electron diffraction are covered. Furthermore, time-efficient and site-specific focused ion beam and dual-beam sample preparation for any of the techniques above are introduced.

Long-range spatial integration across contrast signs: a probabilistic mechanism?

It is widely assumed that the integration of orientation contrast across spatial gaps within the long-range regime is not selective to the contrast sign of the individual stimuli. Probabilistic models of perceptual integration, however, suggest that long-range spatial integration should be, if not selective, at least sensitive to local contrast signs. To clarify this issue, we tested predictions of a model based on conditional probabilistic weights of identical and opposite contrast signs in a simple spatial configuration of two co-linear lines. Contrast detection thresholds of the target line presented either by itself (control condition) or simultaneously with the co-linear inducer (test condition) were measured. The contrast sign of targets and inducers was varied so that all four possible combinations of signs were produced in the test conditions: (1) dark target with dark inducer, (2) dark target with bright inducer, (3) bright target with bright inducer and (4) bright target with dark inducer. The contrast intensity (Weber ratio) of dark and bright inducers was identical. The coaxial distance between target and inducer was constant in each of two experiments, testing for two distances that corresponded to an angular separation within the long-range domain of spatial integration as defined previously. It is found that targets and inducers with identical contrast signs produce significantly stronger facilitating effects on detection than stimuli with opposite signs. The data closely match predictions consistent with those of a probabilistic model of line contrast integration across spatial gaps and contrast signs within the long-range regime.

Microstructure and Texture Development during Recrystallization of Rolled Molybdenum Sheets

Recrystallization is an important tool to adjust the grain size and texture of polycrystalline materials in order to optimize their properties. In the present work recrystallization and the related changes of texture have been studied on rolled molybdenum sheets as a function of temperature and time. The microstructure was investigated by orientation contrast in a scanning electron microscope. The kinetics of recrystallization displayed in a JMAK plot yields Avrami coefficients decreasing with temperature from 2.5 to 1.1. The activation energy amounts to 5 eV, which agrees well with the volume self-diffusion coefficient. The textures of the sheet surface and central layer were measured by X-ray diffraction. The rolling texture in the centre of the sheets is characterized by a strong a-fibre with the rotated cube component {100}<110> dominating. Besides that, there exists a weak g-fibre. In contrast, the surface layer is characterized by a weak cube component. During recrystallization changes in texture are insignificant. With long annealing times all texture components tend to degrade. Conclusions regarding the deep-drawability are drawn.

A Technique for Real-Time, In-Situ SEM Observation of Grain Growth at Elevated Temperatures

We present a novel electron detector and technique permitting orientation contrast images to be generated using back-scattered electron signals. The detector is a modification of the converter plate. It is unaffected by Infra-Red and light photons and invulnerable to the effects of heat. Consequently, the detector is removed as the temperature limiting factor in elevated temperature SEM grain observations. The detector has been successfully applied to an environmental SEM (ESEM) operating in high vacuum mode and has produced good quality video image sequences of hot metal specimens with frame rates between 1 per second and 1 per 30 seconds (largely material dependent). Temperatures attained were up to 850°C, limited by the capability of the hot stage. Although developed for the ESEM the technique is applicable to any SEM provided that adequate measures are taken to manage contamination and heat effects on other microscope components. We have produced images and short, accelerated video sequences of recrystallisation and grain growth phenomena in steel, aluminium, copper and gold by the new technique. The technique has also successfully imaged phase transformations and high temperature behaviour of microelectronic materials.

Crystallographic orientation contrast associated with Ga+ ion channelling for Fe and Cu in focused ion beam method

In this study, Cu and Fe single crystals are used to examine the change in secondary electron intensity associated with Ga(+) ion channelling in a focused ion beam (FIB) system. The single crystals having three different orientations are tilted with respect to the beam incidence and the resulting variation in the secondary electron intensity is measured through the variation in brightness of the crystals. It is shown that intensity minima appear at the beam directions normal to the lower indices of the crystal orientations. The appearance of the intensity minima including the magnitude of the minima is consistent with the prediction based on the event of ion channelling in the crystal and is affected by the crystal structure. The effect of background on the intensity minima is discussed in this study. It is suggested that the presence of the intensity minima may be used to identify a crystal orientation including a crystal structure.

Characterising the dynamics of individual embedded dislocation structures

The 3DXRD concept is extended to the characterisation of elements within the microstructure of deformed metals. Using orientation contrast, the dynamics of individual embedded cells are characterised as a function of annealing with a size-limitation of 100 nm. Applications for studies of recovery and nucleation are demonstrated and discussed.

Optical anisotropy of nanotube suspensions.

A semimacroscopic model of an optically anisotropic nanotube suspension is derived perturbatively from Maxwell's equations in a dielectric medium. We calculate leading-order expressions, valid in the dilute and semidilute limits, for the intrinsic and form contributions to the complex dielectric tensor in terms of the volume fraction, mean orientation, aspect ratio, optical anisotropy, and optical contrast of the nanotubes. The birefringence and dichroism are derived explicitly to leading order in fluctuations, and the connection with depolarized light scattering is established. The results are generalized to include tube deformation.