Orientation Contrast Research Articles

Contrast response in a comprehensive network model of macaque V1.

The response to contrast is one of the most important functions of the macaque primary visual cortex, V1, but up to now there has not been an adequate theory for it. To fill this gap in our understanding of cortical function, we built and analyzed a new large-scale, biologically constrained model of the input layer, 4Cα, of macaque V1. We called the new model CSY2. We challenged CSY2 with a three-parameter family of visual stimuli that varied in contrast, orientation, and spatial frequency. CSY2 accurately simulated experimental data and made many new predictions. It accounted for 1) the shapes of firing-rate-versus-contrast functions, 2) orientation and spatial frequency tuning versus contrast, and 3) the approximate contrast-invariance of cortical activity maps. Post-analysis revealed that the mechanisms that were needed to produce the successful simulations of contrast response included strong recurrent excitation and inhibition that find dynamic equilibria across the cortical surface, dynamic feedback between L6 and L4, and synaptic dynamics like inhibitory synaptic depression.

Connecting soft x-ray anisotropy with local order in conjugated polymers

Abstract

Multimodal x-ray and electron microscopy of the Allende meteorite.

Multimodal microscopy that combines complementary nanoscale imaging techniques is critical for extracting comprehensive chemical, structural, and functional information, particularly for heterogeneous samples. X-ray microscopy can achieve high-resolution imaging of bulk materials with chemical, magnetic, electronic, and bond orientation contrast, while electron microscopy provides atomic-scale spatial resolution with quantitative elemental composition. Here, we combine x-ray ptychography and scanning transmission x-ray spectromicroscopy with three-dimensional energy-dispersive spectroscopy and electron tomography to perform structural and chemical mapping of an Allende meteorite particle with 15-nm spatial resolution. We use textural and quantitative elemental information to infer the mineral composition and discuss potential processes that occurred before or after accretion. We anticipate that correlative x-ray and electron microscopy overcome the limitations of individual imaging modalities and open up a route to future multiscale nondestructive microscopies of complex functional materials and biological systems.

Salient object detection via reciprocal function filter

Salient object detection (SOD) has been attracting a lot of interest, and recently many computational models have been developed. Here, the authors formulate a SOD model, in which saliency map is computed as a combination of the colour, its distribution-based saliency and orientation saliency. Similar to traditional SODs, the proposed method is based on super-pixel segmentation and super-pixel utilises both colour and its distribution-based saliency to generate a coarse saliency map. However, distinct from traditional SODs, authors further use orientation contrast to optimise the coarse saliency map to obtain an improved saliency map. Authors’ contributions are twofold. First, the authors combine colour uniqueness and its distribution with local orientation information (LOI) used in Itti's model to effectively improve profiles of salient regions. Second, a reciprocal function is defined to substitute the Gabor function used in LOI, and the authors have proved that the substitution could detect relatively homogeneous and uniform regions at the boundary of salient object, whereas it is what the traditional models lack. Authors’ approach significantly outperforms state-of-the-art methods on four benchmark datasets, while the authors demonstrate that the proposed method runs as fast as most existing algorithms.

Real-time magnetic resonance imaging \u2013 guided coronary intervention in a porcine model

X-ray fluoroscopy is the gold standard for coronary diagnostics and intervention. Magnetic resonance imaging is a radiation-free alternative to x-ray with excellent soft tissue contrast in arbitrary slice orientation. Here, we assessed real-time MRI-guided coronary interventions from femoral access using newly designed MRI technologies. Six Goettingen minipigs were used to investigate coronary intervention using real-time MRI. Catheters were custom-designed and equipped with an active receive tip-coil to improve visibility and navigation capabilities. Using modified standard clinical 5 F catheters, intubation of the left coronary ostium was successful in all animals. For the purpose of MR-guided coronary interventions, a custom-designed 8 F catheter was used. In spite of the large catheter size, and therefore limited steerability, intubation of the left coronary ostium was successful in 3 of 6 animals within seconds. Thereafter, real-time guided implantation of a non-metallic vascular scaffold into coronary arteries was possible. This study demonstrates that real-time MRI-guided coronary catheterization and intervention via femoral access is possible without the use of any contrast agents or radiation, including placement of non-metallic vascular scaffolds into coronary arteries. Further development, especially in catheter and guidewire technology, will be required to drive forward routine MR-guided coronary interventions as an alternative to x-ray fluoroscopy.

Strength of forest edge effects on litter‐dwelling macro‐arthropods across Europe is influenced by forest age and edge properties

AbstractAimForests are highly fragmented across Western Europe, making forest edges important features in many agricultural landscapes. Forest edges are subject to strong abiotic gradients altering the forest environment and resulting in strong biotic gradients. This has the potential to change the forest's capacity to provide multiple ecosystem services such as nutrient cycling, carbon sequestration and natural pest control. Soil organisms play a key role in this perspective; however, these taxa are rarely considered in forest edge research.LocationA latitudinal gradient of 2,000 km across Western Europe.MethodsWe sampled six dominant taxa of litter‐dwelling macro‐arthropods (carabid beetles, spiders, harvestmen, centipedes, millipedes and woodlice) in forest edges and interiors of 192 forest fragments in 12 agricultural landscapes. We related their abundance and community composition to distance from the edge and the interaction with forest age, edge orientation and edge contrast (contrast between land use types at either side of the edge).ResultsThree out of six macro‐arthropod taxa have higher activity‐density in forest edges compared to forest interiors. The abundance patterns along forest edge‐to‐interior gradients interacted with forest age. Forest age and edge orientation also influenced within‐fragment compositional variation along the forest edge‐to‐interior gradient. Edge contrast influenced abundance gradients of generalist predators. In general, older forest fragments, south‐oriented edges and edges along structurally more continuous land use (lower contrast between forest and adjacent land use) resulted in stronger edge‐to‐interior gradients while recent forests, north‐oriented edges and sharp land use edges induced similarity between forest edge and interior along the forest edge‐to‐interior gradients in terms of species activity‐density and composition.Main conclusionsEdge effects on litter‐dwelling macro‐arthropods are anticipated to feedback on important ecosystem services such as nutrient cycling, carbon sequestration and natural pest control from small forest fragments.

Figure-ground perception in the awake mouse and neuronal activity elicited by figure-ground stimuli in primary visual cortex

Figure-ground segregation is the process by which the visual system identifies image elements of figures and segregates them from the background. Previous studies examined figure-ground segregation in the visual cortex of monkeys where figures elicit stronger neuronal responses than backgrounds. It was demonstrated in anesthetized mice that neurons in the primary visual cortex (V1) of mice are sensitive to orientation contrast, but it is unknown whether mice can perceptually segregate figures from a background. Here, we examined figure-ground perception of mice and found that mice can detect figures defined by an orientation that differs from the background while the figure size, position or phase varied. Electrophysiological recordings in V1 of awake mice revealed that the responses elicited by figures were stronger than those elicited by the background and even stronger at the edge between figure and background. A figural response could even be evoked in the absence of a stimulus in the V1 receptive field. Current-source-density analysis suggested that the extra activity was caused by synaptic inputs into layer 2/3. We conclude that the neuronal mechanisms of figure-ground segregation in mice are similar to those in primates, enabling investigation with the powerful techniques for circuit analysis now available in mice.

Orientation Tuning and Contrast Dependence of Continuous Flash Suppression in Amblyopia and Normal Vision.

Suppression in amblyopia may be an unequal form of normal interocular suppression or a distinct pathophysiology. To explore this issue, we examined the orientation tuning and contrast dependence of continuous flash suppression (CFS) in adults with amblyopia and visually normal controls. Nine patients (mean age, 26.9 ± SD 4.7 years) and 11 controls (mean age, 24.8 ± SD 5.3 years) participated. In the CFS paradigm, spatially one-dimensional noise refreshing at 10 Hz was displayed in one eye to induce suppression of the other eye, and suppression strength was measured by using a grating contrast increment detection task. In experiment 1, noise contrast was fixed and the orientation difference between the noise and the grating was varied. In experiment 2, noise and grating orientations were identical and noise contrast was varied. Suppression patterns varied in both groups. In experiment 1, controls showed consistently orientation-tuned CFS (mean half-height bandwidth, 35.8° ± SD 21.5°) with near-equal strength between eyes. Five of nine patients with amblyopia exhibited orientation-independent CFS. Eight patients had markedly unequal suppression between eyes. Experiment 2 found that increasing the noise contrast to the amblyopic eye may produce suppression of the fellow eye, but suppression remained unequal between eyes. Our data revealed that orientation specificity in CFS was very broad or absent in some patients with amblyopia, which could not be predicted by clinical measures. Suppression was unbalanced across the entire contrast range for most patients. This suggests that abnormal early visual experience disrupts the development of interocular suppression mechanisms.

Automatic Detection of Orientation Contrast Occurs at Early but Not Earliest Stages of Visual Cortical Processing in Humans.

Orientation contrast is formed when some elements orient differently from their surroundings. Although orientation contrast can be processed in the absence of top-down attention, the underlying neural mechanism for this automatic processing in humans is controversial. In particular, whether automatic detection of orientation contrast occurs at the initial feedforward stage in the primary visual cortex (i.e., V1) remains unclear. Here, we used event-related potentials (ERPs) to examine the automatic processing of orientation contrast in humans. In three experiments, participants completed a task at fixation while orientation contrasts were presented in the periphery, either in the upper visual field (UVF) or the lower visual field (LVF). All experiments showed significant positive potentials evoked by orientation contrasts over occipital areas within 100 ms after stimulus onset. These contrast effects occurred 10–20 ms later than the C1 components evoked by identically located abrupt onset stimuli which indexes the initial feedforward activity in V1. Compared with those in the UVF, orientation contrasts in the LVF evoked earlier and stronger activities, probably reflecting a LVF advantage in processing of orientation contrast. Even when orientation contrasts were rendered almost invisible by backward masking (in Experiment 2), the early contrast effect in the LVF was not disrupted. These findings imply that automatic processing of orientation contrast could occur at early visual cortical processing stages, but was slightly later than the initial feedforward processing in human V1; such automatic processing may involve either recurrent processing in V1 or feedforward processing in early extrastriate visual cortex.Highlights -We examined the earliest automatic processing of orientation contrast in humans with ERPs.-Significant orientation contrast effect started within 100 ms in early visual areas.-The earliest orientation contrast effect occurred later than the C1 evoked by abrupt onset stimuli.-The earliest orientation contrast effect was independent of top-down attention and awareness.-Automatic detection of orientation contrast arises slightly after the initial feedforward processing in V1.

Bottom-up saliency and top-down learning in the primary visual cortex of monkeys

Early sensory cortex is better known for representing sensory inputs but less for the effect of its responses on behavior. Here we explore the behavioral correlates of neuronal responses in primary visual cortex (V1) in a task to detect a uniquely oriented bar-the orientation singleton-in a background of uniformly oriented bars. This singleton is salient or inconspicuous when the orientation contrast between the singleton and background bars is sufficiently large or small, respectively. Using implanted microelectrodes, we measured V1 activities while monkeys were trained to quickly saccade to the singleton. A neuron's responses to the singleton within its receptive field had an early and a late component, both increased with the orientation contrast. The early component started from the outset of neuronal responses; it remained unchanged before and after training on the singleton detection. The late component started ∼40 ms after the early one; it emerged and evolved with practicing the detection task. Training increased the behavioral accuracy and speed of singleton detection and increased the amount of information in the late response component about a singleton's presence or absence. Furthermore, for a given singleton, faster detection performance was associated with higher V1 responses; training increased this behavioral-neural correlate in the early V1 responses but decreased it in the late V1 responses. Therefore, V1's early responses are directly linked with behavior and represent the bottom-up saliency signals. Learning strengthens this link, likely serving as the basis for making the detection task more reflexive and less top-down driven.

Crystallographic texture can be rapidly determined by electrochemical surface analytics

Orientation affects application-defining properties of crystalline materials. Hence, information in this regard is highly-prized. We show that electrochemical jet processing (EJP), when coupled with accurate metrological appraisal, can characterise crystallographic texture. Implementation of this technique allows localised dissolution to be anisotropic and dependent on etch-rate selectivity, defined by the crystallography. EJP therefore, generates complex, but characteristic topographies. Through rapid surface processing and analysis, textural information can be elucidated. In this study, samples of polycrystalline Al and Ni have been subjected to EJP, and the resulting surfaces analysed to generate three-colour orientation contrast maps. Comparison of raw data acquired through our method with prior electron back-scatter diffraction data shows broad correlation and assignment (68% on a pixel-by-pixel basis), showcasing rapid large-area analysis at high efficiency.

Bidirectional encoding of motion contrast in the mouse superior colliculus.

Detection of salient objects in the visual scene is a vital aspect of an animal's interactions with its environment. Here, we show that neurons in the mouse superior colliculus (SC) encode visual saliency by detecting motion contrast between stimulus center and surround. Excitatory neurons in the most superficial lamina of the SC are contextually modulated, monotonically increasing their response from suppression by the same-direction surround to maximal potentiation by an oppositely-moving surround. The degree of this potentiation declines with depth in the SC. Inhibitory neurons are suppressed by any surround at all depths. These response modulations in both neuronal populations are much more prominent to direction contrast than to phase, temporal frequency, or static orientation contrast, suggesting feature-specific saliency encoding in the mouse SC. Together, our findings provide evidence supporting locally generated feature representations in the SC, and lay the foundations towards a mechanistic and evolutionary understanding of their emergence.

Space rocks and optimising scanning electron channelling contrast

Forescatter electron imaging is a popular microscopy technique, especially for scanning electron microscopes equipped with an electron backscatter diffraction detector. In principal, this method enables qualitative imaging of microstructure but quantitative assessment can be limited due to limited information about the contrast afforded. In this work, we explore forescatter electron imaging and demonstrate that imaging can be optimised for topographic, phase, and subtle orientation contrast imaging through appropriate sample and detector positioning. We demonstrate the relationship between imaging modes using systematic variation in detector positioning and compare this with pseudo-forescatter electron images, obtained from image analysis of diffraction patterns, to explore and confirm image contrast modes. We demonstrate these contrast mechanisms on a map obtained from a sample of the Gibeon meteorite.

Multitask saliency detection model for synthetic aperture radar (SAR) image and its application in SAR and optical image fusion

Saliency detection in synthetic aperture radar (SAR) images is a difficult problem. This paper proposed a multitask saliency detection (MSD) model for the saliency detection task of SAR images. We extract four features of the SAR image, which include the intensity, orientation, uniqueness, and global contrast, as the input of the MSD model. The saliency map is generated by the multitask sparsity pursuit, which integrates the multiple features collaboratively. Detection of different scale features is also taken into consideration. Subjective and objective evaluation of the MSD model verifies its effectiveness. Based on the saliency maps obtained by the MSD model, we apply the saliency map of the SAR image to the SAR and color optical image fusion. The experimental results of real data show that the saliency map obtained by the MSD model helps to improve the fusion effect, and the salient areas in the SAR image can be highlighted in the fusion results.

The Lifetime of Salience Extends Beyond the Initial Saccade.

Several models of selection in search predict that saccades are biased toward conspicuous objects (also referred to as salient objects). Indeed, it has been demonstrated that initial saccades are biased toward the most conspicuous candidate. However, in a recent study, no such bias was found for the second saccade, and it was concluded that the attraction of conspicuous elements is limited to only short-latency initial saccades. This conclusion is based on only a single feature manipulation (orientation contrast) and conflicts with the prediction of influential salience models. Here, we investigate whether this result can be generalized beyond the domain of orientation. In displays containing three luminance annuli (Experiment 1), we find a considerable bias toward the most conspicuous candidate for the second saccade. In Experiment 1, the target could not be discriminated peripherally. When we made the target peripherally discriminable, the second saccade was no longer biased toward the more conspicuous candidate (Experiment 2). Thus, conspicuity plays a role in saccadic selection beyond the initial saccade. Whether second saccades are biased toward conspicuous objects appears to depend on the type of feature contrast underlying the conspicuity and the peripheral discriminability of target properties.

Penetration Depth and Defect Image Contrast Formation in Grazing-Incidence X-ray Topography of 4H-SiC Wafers

Synchrotron x-ray topography in grazing-incidence geometry is useful for discerning defects at different depths below the crystal surface, particularly for 4H-SiC epitaxial wafers. However, the penetration depths measured from x-ray topographs are much larger than theoretical values. To interpret this discrepancy, we have simulated the topographic contrast of dislocations based on two of the most basic contrast formation mechanisms, viz. orientation and kinematical contrast. Orientation contrast considers merely displacement fields associated with dislocations, while kinematical contrast considers also diffraction volume, defined as the effective misorientation around dislocations and the rocking curve width for given diffraction vector. Ray-tracing simulation was carried out to visualize dislocation contrast for both models, taking into account photoelectric absorption of the x-ray beam inside the crystal. The results show that orientation contrast plays the key role in determining both the contrast and x-ray penetration depth for different types of dislocation.

Tensile properties and microstructure of a cryomilled nanograined Al-Mg alloy near the AA5083 composition

In an effort to develop high strength aluminum alloys, a near-AA5083 Al-Mg alloy powder was cryomilled in liquid nitrogen and consolidated by vacuum hot-pressing (VHP). The composition of the Al-Mg alloy was designed to minimize intermetallic particles that often lead to premature fracture especially when flow strength is high. The as-VHP material had poor ductility and therefore was extruded under different conditions to obtain a good combination of tensile strength and ductility. The microstructure of samples were characterized using X-ray diffraction, EBSD, FIB induced secondary electron orientation contrast imaging, and TEM techniques, which together provided unique assessment at multiple length scales. The samples exhibited a wide range of grain sizes that could be binned into three different groups: i) grain sizes of 50–100nm, (ii) sizes in the 100–300nm range, and (iii) elongated larger grains with widths in the range 0.5–2µm. Room temperature tensile tests with cylindrical dog-bone geometry indicated a high ultimate strength of 730–770MPa and reproducible elongation to failure of about 3–4%. This combination of strength and ductility in the material are some of the best that have been reported for alloys close to the weldable and corrosion resistant AA5083 composition, and likely a result of the multi-scale microstructure resulting from the processing route.

A second-order orientation-contrast stimulus for population-receptive-field-based retinotopic mapping

Visual field or retinotopic mapping is one of the most frequently used paradigms in fMRI. It uses activity evoked by position-varying high luminance contrast visual patterns presented throughout the visual field for determining the spatial organization of cortical visual areas. While the advantage of using high luminance contrast is that it tends to drive a wide range of neural populations – thus resulting in high signal-to-noise BOLD responses – this may also be a limitation, especially for approaches that attempt to squeeze more information out of the BOLD response, such as population receptive field (pRF) mapping. In that case, more selective stimulation of a subset of neurons – despite reduced signals – could result in better characterization of pRF properties. Here, we used a second-order stimulus based on local differences in orientation texture – to which we refer as orientation contrast – to perform retinotopic mapping. Participants in our experiment viewed arrays of Gabor patches composed of a foreground (a bar) and a background. These could only be distinguished on the basis of a difference in patch orientation. In our analyses, we compare the pRF properties obtained using this new orientation contrast-based retinotopy (OCR) to those obtained using classic luminance contrast-based retinotopy (LCR). Specifically, in higher order cortical visual areas such as LO, our novel approach resulted in non-trivial reductions in estimated population receptive field size of around 30%. A set of control experiments confirms that the most plausible cause for this reduction is that OCR mainly drives neurons sensitive to orientation contrast. We discuss how OCR – by limiting receptive field scatter and reducing BOLD displacement – may result in more accurate pRF localization as well. Estimation of neuronal properties is crucial for interpreting cortical function. Therefore, we conclude that using our approach, it is possible to selectively target particular neuronal populations, opening the way to use pRF modeling to dissect the response properties of more clearly-defined neuronal populations in different visual areas.

Measuring and modeling salience with the theory of visual attention.

For almost three decades, the theory of visual attention (TVA) has been successful in mathematically describing and explaining a wide variety of phenomena in visual selection and recognition with high quantitative precision. Interestingly, the influence of feature contrast on attention has been included in TVA only recently, although it has been extensively studied outside the TVA framework. The present approach further develops this extension of TVA's scope by measuring and modeling salience. An empirical measure of salience is achieved by linking different (orientation and luminance) contrasts to a TVA parameter. In the modeling part, the function relating feature contrasts to salience is described mathematically and tested against alternatives by Bayesian model comparison. This model comparison reveals that the power function is an appropriate model of salience growth in the dimensions of orientation and luminance contrast. Furthermore, if contrasts from the two dimensions are combined, salience adds up additively.

On the accuracy of grain boundary character determination by pseudo-3D EBSD

Pseudo-3D orientation microscopy based on electron backscatter diffraction (EBSD) is a simple way to characterize all the 5 rotational parameters of grain boundaries. It is based on the observation of grain boundaries across a sharp edge of a sample using EBSD to determine the crystal orientation and orientation contrast from backscattered electrons to observe the grain boundary traces on the surface. In this paper we discuss the possible errors and the accuracy of these kind of measurements. It is found that the boundary misorientation can be measured better than 0.5°, while the boundary plane can be determined to better than 1°. A coherent twin grain boundary with well-known crystallographic parameters in an austenitic stainless steel with fcc crystal structure was taken as reference.