Gradient Cues Research Articles

Depth Estimation from a Hierarchical Baseline Stereo with a Developed Light Field Camera

This paper presents a hierarchical baseline stereo-matching framework for depth estimation using a novelly developed light field camera. The imaging process of a micro-lens array-based light field camera is derived. A macro-pixel map is constructed by treating each micro-lens as one macro-pixel in the light field’s raw image. For each macro-pixel, a feature vector is represented by leveraging texture and gradient cues over the surrounding ring of neighboring macro-pixels. Next, the micro-lenses containing edges are detected on the macro-pixel map. Hierarchical baseline stereo-matching is performed by macro-pixel-wise coarse matching and pixel-wise fine matching, effectively eliminating matching ambiguities. Finally, a post-processing step is applied to improve accuracy. The lab-designed light field camera’s imaging performance is evaluated in terms of accuracy and processing speed by capturing real-world scenes under studio lighting conditions. And an experiment using rendered synthetic samples is conducted for quantitative evaluation, showing that depth maps with local details can be accurately recovered.

Naive songbirds show seasonally appropriate spring orientation in the laboratory despite having never completed first migration

The role of inherited orientation programmes in determining the outbound migratory routes of birds is increasingly well understood, though less is known about the influence of inherited information on return migration. Previous studies suggest that spatial gradient cues learnt through experience could be of considerable importance when relocating the natal site, though such cues could, in principle, augment rather than replace inherited migratory information. Here, we show that juvenile Eurasian blackcaps ( Sylvia atricapilla ) that have never left northwest Europe (i.e. never had the opportunity to learn navigational information on a continental scale) show significant spring orientation in a direction near-identical to that expected based on ringing recoveries from free-flying individuals. We suggest that this is probably indicative of birds inheriting an orientation programme for spring as well as autumn migration and speculate that, as long as the birds are not displaced far from their normal migration route, the use of inherited spring migratory trajectories might make uni-coordinate ‘stop signs’ sufficiently accurate for the long-distance targeting of their breeding sites.

Cells use molecular working memory to navigate in changing chemoattractant fields.

In order to migrate over large distances, cells within tissues and organisms rely on sensing local gradient cues which are irregular, conflicting, and changing over time and space. The mechanism how they generate persistent directional migration when signals are disrupted, while still remaining adaptive to signal's localization changes remain unknown. Here, we find that single cells utilize a molecular mechanism akin to a working memory to satisfy these two opposing demands. We derive theoretically that this is characteristic for receptor networks maintained away from steady states. Time-resolved live-cell imaging of Epidermal growth factor receptor (EGFR) phosphorylation dynamics shows that cells transiently memorize position of encountered signals via slow-escaping remnant of the polarized signaling state, a dynamical 'ghost', driving memory-guided persistent directional migration. The metastability of this state further enables migrational adaptation when encountering new signals. We thus identify basic mechanism of real-time computations underlying cellular navigation in changing chemoattractant fields.

How might magnetic secular variation impact avian philopatry?

A tendency to return to the natal/breeding site, ‘philopatry’, is widespread amongst migratory birds. It has been suggested that a magnetic ‘map’ could underpin such movements, though it is unclear how a magnetic map might be impacted by gradual drift in the Earth’s magnetic field (‘secular variation’). Here, using the International Geomagnetic Reference Field, we quantified how secular variation translates to movement in the implied positions at which combinations of different magnetic cues (inclination, declination and intensity) intersect, noting that the magnitude of such movements is determined by the magnitude of the movements of each of the two isolines, and the angle between their movement vectors. We propose that magnetic parameters varying in a near-parallel arrangement are unlikely to be used as a bi-coordinate map during philopatry, but that birds could use near-orthogonal magnetic gradient cues as a bi-coordinate map if augmented with navigation using more local cues. We further suggest that uni-coordinate magnetic information could also provide a philopatry mechanism that is substantially less impacted by secular variation than a bi-coordinate ‘map’. We propose that between-year shifts in the position of magnetic coordinates might provide a priori predictions for changes in the breeding sites of migratory birds.

Sniff-synchronized, gradient-guided olfactory search by freely moving mice.

For many organisms, searching for relevant targets such as food or mates entails active, strategic sampling of the environment. Finding odorous targets may be the most ancient search problem that motile organisms evolved to solve. While chemosensory navigation has been well characterized in microorganisms and invertebrates, spatial olfaction in vertebrates is poorly understood. We have established an olfactory search assay in which freely moving mice navigate noisy concentration gradients of airborne odor. Mice solve this task using concentration gradient cues and do not require stereo olfaction for performance. During task performance, respiration and nose movement are synchronized with tens of milliseconds precision. This synchrony is present during trials and largely absent during inter-trial intervals, suggesting that sniff-synchronized nose movement is a strategic behavioral state rather than simply a constant accompaniment to fast breathing. To reveal the spatiotemporal structure of these active sensing movements, we used machine learning methods to parse motion trajectories into elementary movement motifs. Motifs fall into two clusters, which correspond to investigation and approach states. Investigation motifs lock precisely to sniffing, such that the individual motifs preferentially occur at specific phases of the sniff cycle. The allocentric structure of investigation and approach indicates an advantage to sampling both sides of the sharpest part of the odor gradient, consistent with a serial-sniff strategy for gradient sensing. This work clarifies sensorimotor strategies for mouse olfactory search and guides ongoing work into the underlying neural mechanisms.

Fabrication of 3D Scaffolds Displaying Biochemical Gradients along Longitudinally Oriented Microchannels for Neural Tissue Engineering.

Biochemical and physical guidance cues are both pivotal for axonal guidance and nerve regeneration. However, fabrication of a platform that can integrate biochemical gradients and topographical guidance cues remains challenging, especially in a three-dimensional (3D) scaffold that closely mimics in vivo axonal outgrowth conditions and ready to be used for in vivo nerve repair. In this study, a new method was introduced to construct 3D scaffolds displaying continuous biochemical gradients along longitudinally oriented microchannels by combining the modified 3D printing and directional freezing techniques. Fluorescence analysis and ELISA results demonstrated that a continuous biochemical gradient was formed, and scanning electron microscopy revealed a longitudinally oriented microstructure. Dorsal root ganglia explants seeded on the longitudinal sections of the newly developed scaffold (scaffold with nerve growth factor gradient along oriented microstructure, G-NGF + OS) showed that 81.3 ± 4.5% of neurites oriented within ±10°, 0.3 ± 0.1 of guidance ratio, and 1.5-fold of the average length of neurites on the high-nerve growth factor (NGF) concentration side compared to that on the low-NGF concentration side, which were significantly higher than those in the other groups. In addition, the G-NGF + OS scaffold was used to repair a 15 mm sciatic nerve defect in rats. Immunofluorescence staining, Fluoro-Gold retrograde tracing, and transmission electron microscopy results confirmed that the G-NGF + OS scaffold enhanced nerve regeneration to the distal target and promoted myelination of regenerated axons. More importantly, the sciatic functional index and the von Frey test demonstrated that the G-NGF + OS scaffold accelerated sensory and motor functional recovery. These results provide new insights into the importance of combining topographical guidance cues with bioactive molecule gradient cues for neural tissue engineering. The 3D scaffold displaying biochemical gradients along longitudinally oriented microchannels represents a promising platform for the development of advanced devices for severe nervous system injuries.

Natal imprinting to the Earth’s magnetic field in a pelagic seabird

In migratory animals for whom post-natal care is limited, it is essential that there are inherited mechanisms whereby an individual can navigate-first, to the terminus of their migration, and second, back to a suitable breeding site. In birds, empirical evidence suggests that orientation on first migration is controlled by an inherited navigational vector, a direction and a distance in which to move (the "clock and compass" model) [1-5]. The mechanism and information that underlie the return to the natal breeding site are, however, almost entirely unknown. A potential solution to this problem would be for an animal to learn the values for spatially and temporally stable gradient cues that specifically indicate the location of the natal site [6-16]. One potential cue for latitude is magnetic inclination. Here, we use ringing recoveries made over the last 80 years to investigate whether magnetic inclination might be used as a navigational cue to control the latitude of recruitment in a trans-global migrant, the Manx shearwater (Puffinus puffinus). We find that small changes in inclination between when a bird fledges and when it returns from first migration correlate with probabilistic changes in latitude at recruitment, in doing so quantitatively fulfilling a priori predictions as to the magnitude and direction of latitudinal shift. This, we believe, suggests that (1) natal magnetic inclination is learnt prior to fledging and (2) is used to provide latitudinal information when making the first return trip from the wintering grounds.

Electric field-driven building blocks for introducing multiple gradients to hydrogels

Gradient biomaterials are considered as preferable matrices for tissue engineering due to better simulation of native tissues. The introduction of gradient cues usually needs special equipment and complex process but is only effective to limited biomaterials. Incorporation of multiple gradients in the hydrogels remains challenges. Here, beta-sheet rich silk nanofibers (BSNF) were used as building blocks to introduce multiple gradients into different hydrogel systems through the joint action of crosslinking and electric field. The blocks migrated to the anode along the electric field and gradually stagnated due to the solution-hydrogel transition of the systems, finally achieving gradient distribution of the blocks in the formed hydrogels. The gradient distribution of the blocks could be tuned easily through changing different factors such as solution viscosity, which resulted in highly tunable gradient of mechanical cues. The blocks were also aligned under the electric field, endowing orientation gradient simultaneously. Different cargos could be loaded on the blocks and form gradient cues through the same crosslinking-electric field strategy. The building blocks could be introduced to various hydrogels such as Gelatin and NIPAM, indicating the universality. Complex niches with multiple gradient cues could be achieved through the strategy. Silk-based hydrogels with suitable mechanical gradients were fabricated to control the osteogenesis and chondrogenesis. Chondrogenic-osteogenic gradient transition was obtained, which stimulated the ectopic osteochondral tissue regeneration in vivo. The versatility and highly controllability of the strategy as well as multifunction of the building blocks reveal the applicability in complex tissue engineering and various interfacial tissues.

Learning full-reference quality-guided discriminative gradient cues for lane detection based on neural networks

Learning an intelligent lane detection system is significant to autonomous vehicles, which is a crucial module to smart cars. Although conventional approaches have achieved impressive performance, they suffer from the following limitations: (1) lane perception are confronted with different weather conditions and varied illumination. Existing methods lack a unified framework for characterizing different sceneries and (2) the inefficiency of utilizing images due to the potential label noise. To solve these limitations, we propose a lane detection framework towards autonomous vehicles by learning a full-reference quality-aware discriminative gradient deep model, where two types of deep networks are proposed. More specifically, we first design a gradient-guided deep convolutional network to detect the presence of lane, since the gradient value of lane edge is larger than that of other regions. We leverage full-reference image quality assessment (FR-IQA) method to discover more discriminative gradient cues, and geometric attributes are exploited simultaneously. Subsequently, a recurrent neural layer is designed to represent the spatial distribution of detected lanes whose visual cues are difficult to explicitly define. Noticeably, we only utilize a small proportion of the labeled images, whereas the noisy features are abandoned using sparsity penalty. Extensive experiments have demonstrated the effectiveness of our proposed method.

Selective Adhesion and Directional Migration of Endothelial Cells Guided by Cys-Ala-Gly Peptide Density Gradient on Antifouling Polymer Brushes.

Selective adhesion and directional migration of endothelial cells (ECs) on biomaterials is critical to realize the rapid endothelialization. In this study, a Cys-Ala-Gly (CAG) peptide density gradient is generated on homogeneous cell-resisting poly(2-hydroxyethyl methacrylate-co-glycidyl methacrylate) brushes by immersing the brushes in a complementary gradient solution of CAG and competitive mercapto-terminated methoxyl poly(ethylene glycol). The adhesion and spreading of smooth muscle cells (SMCs) is impaired effectively on the gradient surface. About six folds of adherent ECs over SMCs are achieved at the position (10 mm) of highest CAG density on the gradient surface in a co-culture condition. Due to the gradient cues, ECs migrate fastest with the best directionality of 86.7% at the middle of the gradient, leading to the maximum net displacement as well.

Robust Lane Detection and Tracking for Real-Time Applications

An effective lane-detection algorithm is a fundamental component of an advanced driver assistant system, as it provides important information that supports driving safety. The challenges faced by the lane detection and tracking algorithm include the lack of clarity of lane markings, poor visibility due to bad weather, illumination and light reflection, shadows, and dense road-based instructions. In this paper, a robust and real-time vision-based lane detection algorithm with an efficient region of interest is proposed to reduce the high noise level and the calculation time. The proposed algorithm also processes a gradient cue and a color cue together and a line clustering with scan-line tests to verify the characteristics of the lane markings. It removes any false lane markings and tracks the real lane markings using the accumulated statistical data. The experiment results show that the proposed algorithm gives accurate results and fulfills the real-time operation requirement on embedded systems with low computing power.

Biochemical and Mechanical Gradients Synergize To Enhance Cartilage Zonal Organization in 3D.

Articular cartilage is characterized by zonal organizations containing dual gradients of biochemical cues and mechanical cues. However, how biochemical gradient interacts with the mechanical gradient to drive the cartilage zonal development remains largely unknown. Here, we report the development of a dual-gradient hydrogel platform as a 3D niche to elucidate the relative contributions of biochemical and mechanical niche gradients in modulating zonal-specific chondrocyte responses and cartilage zonal organization. Chondroitin sulfate (CS), a major constituent of cartilage extracellular matrix, was chosen as the biochemical cue. Poly(ethylene glycol), a bioinert polymer, was used to create the stiffness gradient. Dual-gradient hydrogels upregulated cartilage marker expressions and increased chondrocyte proliferation and collagen deposition in a zonal-dependent manner. Hydrogels with CS gradient alone exhibited poor mechanical strength and degraded prematurely after 1 week of culture. While CS gradient alone did not support long-term culture, adding CS gradient to mechanical-gradient hydrogels substantially enhanced cell proliferation, glycosaminoglycan production, and collagen deposition compared to mechanical-gradient hydrogels alone. These results suggest that biochemical and mechanical gradient cues synergize to enhance cartilage zonal organization by chondrocytes in 3D. Together, our results validate the potential of dual-gradient hydrogels as a 3D cell niche for cartilage regeneration with zonal organization and may be used to recreate other tissue interfaces.

Dual Cross-Linked Biofunctional and Self-Healing Networks to Generate User-Defined Modular Gradient Hydrogel Constructs.

Gradient hydrogels have been developed to mimic the spatiotemporal differences of multiple gradient cues in tissues. Current approaches used to generate such hydrogels are restricted to a single gradient shape and distribution. Here, a hydrogel is designed that includes two chemical cross-linking networks, biofunctional, and self-healing networks, enabling the customizable formation of modular gradient hydrogel construct with various gradient distributions and flexible shapes. The biofunctional networks are formed via Michael addition between the acrylates of oxidized acrylated hyaluronic acid (OAHA) and the dithiol of matrix metalloproteinase (MMP)-sensitive cross-linker and RGD peptides. The self-healing networks are formed via dynamic Schiff base reaction between N-carboxyethyl chitosan (CEC) and OAHA, which drives the modular gradient units to self-heal into an integral modular gradient hydrogel. The CEC-OAHA-MMP hydrogel exhibits excellent flowability at 37 °C under shear stress, enabling its injection to generate gradient distributions and shapes. Furthermore, encapsulated sarcoma cells respond to the gradient cues of RGD peptides and MMP-sensitive cross-linkers in the hydrogel. With these superior properties, the dual cross-linked CEC-OAHA-MMP hydrogel holds significant potential for generating customizable gradient hydrogel constructs, to study and guide cellular responses to their microenvironment such as in tumor mimicking, tissue engineering, and stem cell differentiation and morphogenesis.

Computational estimation of scene structure through texture gradient cues

Analyzing the depth structure implied in two-dimensional images is one of the most active research areas in computer vision. Here, we propose a method of utilizing texture within an image to derive its depth structure. Though most approaches for deriving depth from a single still image utilize luminance edges and shading to estimate scene structure, relatively little work has been done to utilize the abundant texture information in images. Our new approach begins by analyzing the two cues of local spatial frequency and orientation distributions of the textures within an image, which are used to compute the local slant information across the image. The slant and frequency information are merged to create a unified depth map, providing an important channel for image structure information that can be combined with other available cues. The capabilities of the algorithm are illustrated for a variety of images of planar and curved surfaces under perspective projection, in most of which the depth structure is effortlessly perceived by human observers. Since these operations are readily implementable in neural hardware in early visual cortex, they therefore represent a model of the human perception of the depth structure of images from texture gradient cues.

Adaptive Greedy Dictionary Selection for Web Media Summarization.

Initializing an effective dictionary is an indispensable step for sparse representation. In this paper, we focus on the dictionary selection problem with the objective to select a compact subset of basis from original training data instead of learning a new dictionary matrix as dictionary learning models do. We first design a new dictionary selection model via l2,0 norm. For model optimization, we propose two methods: one is the standard forward-backward greedy algorithm, which is not suitable for large-scale problems; the other is based on the gradient cues at each forward iteration and speeds up the process dramatically. In comparison with the state-of-the-art dictionary selection models, our model is not only more effective and efficient, but also can control the sparsity. To evaluate the performance of our new model, we select two practical web media summarization problems: 1) we build a new data set consisting of around 500 users, 3000 albums, and 1 million images, and achieve effective assisted albuming based on our model and 2) by formulating the video summarization problem as a dictionary selection issue, we employ our model to extract keyframes from a video sequence in a more flexible way. Generally, our model outperforms the state-of-the-art methods in both these two tasks.

Processing and surface modification of polymer nanofibers for biological scaffolds: a review.

Polymeric fibrous constructs possess high surface area-to-volume ratios when compared with solid substrates and are quite commonly used as tissue engineering and cell growth scaffolds. An overview of important design and material considerations for fibrous scaffolds as well as an outline of both established and emerging solution- and melt-based fabrication techniques is provided. Innovative post-process surface modification avenues using "click" chemistry with both single and dual active cues as well as gradient cues, which maintain the fibrous structure are described. By combining process parameters with post-process surface modification, researchers have been able to selectively tune cellular response after seeding and culturing on fibrous constructs.

Preparation of gelatin density gradient on poly(ε-caprolactone) membrane and its influence on adhesion and migration of endothelial cells

Directional migration of endothelial cells (ECs) can be achieved by gradient cues in vitro, which mimics the corresponding biological events in vivo. Currently, most of the gradients have been prepared on model surfaces which are too simple compared to real degradable biomaterials. In this study, the amino group density gradient was prepared on poly(ε-caprolactone) (PCL) membrane surface by a gradient aminolysis method, which was transferred into gelatin density gradient by covalent linking with glutaraldehyde. The resulted gelatin density gradient ranged from 0.49 to 1.57μg/cm(2) on the PCL membrane. The adhesion, orientation and migration of ECs on the PCL membrane with the gelatin density gradient were studied. The ECs showed preferred orientation and directional migration toward the gradient direction with enhanced gelatin density at proper position (gelatin density), forwarding a new step toward the preparation of applicable gradient biomaterials in tissue regeneration.

A Developmental Approach to Predicting Neuronal Connectivity from Small Biological Datasets: A Gradient-Based Neuron Growth Model

Relating structure and function of neuronal circuits is a challenging problem. It requires demonstrating how dynamical patterns of spiking activity lead to functions like cognitive behaviour and identifying the neurons and connections that lead to appropriate activity of a circuit. We apply a “developmental approach” to define the connectome of a simple nervous system, where connections between neurons are not prescribed but appear as a result of neuron growth. A gradient based mathematical model of two-dimensional axon growth from rows of undifferentiated neurons is derived for the different types of neurons in the brainstem and spinal cord of young tadpoles of the frog Xenopus. Model parameters define a two-dimensional CNS growth environment with three gradient cues and the specific responsiveness of the axons of each neuron type to these cues. The model is described by a nonlinear system of three difference equations; it includes a random variable, and takes specific neuron characteristics into account. Anatomical measurements are first used to position cell bodies in rows and define axon origins. Then a generalization procedure allows information on the axons of individual neurons from small anatomical datasets to be used to generate larger artificial datasets. To specify parameters in the axon growth model we use a stochastic optimization procedure, derive a cost function and find the optimal parameters for each type of neuron. Our biologically realistic model of axon growth starts from axon outgrowth from the cell body and generates multiple axons for each different neuron type with statistical properties matching those of real axons. We illustrate how the axon growth model works for neurons with axons which grow to the same and the opposite side of the CNS. We then show how, by adding a simple specification for dendrite morphology, our model “developmental approach” allows us to generate biologically-realistic connectomes.

Directional migration of vascular smooth muscle cells guided by synergetic surface gradient and chemical pattern of poly(ethylene glycol) brushes

Directional migration of cells in vitro can mimic corresponding biological events in vivo, which provides a way to determine the cascade responses in the tissue regeneration process and develop novel criteria for the design of tissue-inductive biomaterials. In this work, a density gradient of methoxy poly(ethylene glycol) brushes (from 0.37 to 0.95 chains/nm2) on plain and striped pattern surfaces was fabricated, using a dynamically controlled reaction process, and characterized by X-ray photoelectron spectroscopy and quartz crystal microbalance with dissipation. Adhesion and migration behavior of vascular smooth muscle cells were studied on the gradient and gradient-patterned surfaces. The vascular smooth muscle cells exhibited preferential orientation and enhanced directional migration on the gradient surface toward the lower end of the methoxy poly(ethylene glycol) density. By introducing methoxy poly(ethylene glycol) chemical striped patterns in parallel with the gradient direction on the surface, the extent of cell orientation and directional migration were significantly improved. Due to the synergetic effects of surface methoxy poly(ethylene glycol) striped patterns and gradient cues, almost all cells were oriented, and 67% of the cells were observed to move unidirectionally.

On the Role of Vertical Texture Cues in Height Perception

When people judge height from the top looking down they tend to overestimate vertical distance. Initial findings suggest that this perceptual bias may be in part due to the experienced fear of falling. However, previous studies did not control for potentially relevant optical invariants, especially vertical texture gradient cues, that may inform such perceptual judgments. In this study, we examined to what degree texture gradient cues may account for the perceptual bias in height perception. To this end, in 2 different conditions in which vertical texture gradient cues were either present or absent, participants provided distance and size judgments from an elevated position (5.68 m) and from the ground. Results revealed a systematic overestimation of vertical extent and object size when judged from a height. Yet, the absence or presence of texture gradient cues did not differentially influence these judgments. Finally, although fear levels were reported to be larger at the elevated position than on the ground, fear levels did not correlate with the perceptual bias, thereby demanding further research into the mechanisms underlying the perceptual bias in height perception.