Interplane bulk motion analysis and removal based on normalized cross‐correlation in optical coherence tomography angiography

Bulk motion seriously degrades the image quality of optical coherence tomography angiography (OCTA). Conventional correction methods focus on in‐plane displacement, while the bulk motion component perpendicular to B‐scans also introduces noise. This work first presents an evaluation of this component using a specific scan protocol and an approximate expression derived from peak‐normalized cross‐correlation values, and then quantitatively assesses how interplane bulk motion noise reduce the sensitivity of cross‐sectional angiograms. Finally, we developed a repetitive bulk motion correction method based on the estimated displacements and redundant volume scans. The correction does not require registration and angiogram reconstruction of low flow sensitivity frames, and the results of in vivo mice skin OCTA imaging experiments show that the proposed method can effectively reduce bulk motion noise caused by cardiac and respiratory motion and occasional shaking, and improve OCTA image quality, which has practical significance for clinical OCTA diagnosis and analysis.     

Cross‐scanning optical coherence tomography angiography for eye motion correction

We propose a cross‐scanning optical coherence tomography (CS‐OCT) system to correct eye motion artifacts in OCT angiography images. This system employs a dual‐illumination configuration with two orthogonally polarized beams, each of which simultaneously perform raster scanning in perpendicular direction with each other over the same area. In the reference arm, a polarization delay unit is used to acquire the two orthogonally polarized interferograms with a single photo detector by introducing different optical delay lines. The two cross‐scanned volume data are affected by the same eye motion but in two orthogonal directions. We developed a motion correction algorithm, which removes artifacts in the slow axis of each angiogram using the other and merges them through a nonrigid registration algorithm. In this manner, we obtained a motion‐corrected angiogram within a single volume scanning time without additional eye‐tracking devices.     

Retinal cross-section motion correction in three-dimensional retinal optical coherence tomography

Motion correction is an important issue in ophthalmic optical coherence tomography (OCT), and can improve the ability of data sets to reflect the physiological structures of tissues and make visualization and subsequent analysis easier. In this study, we present a novel method to correct the cross-sectional motion artifacts in retinal OCT volumes. Motion along the x-direction (fast-scan direction) is corrected through the normalized cross-correlation algorithm, while axial motion compensation is performed using the polynomial fitting method on the inner segment/outer segment (IS/OS) layer segmented by the shortest path faster algorithm (SPFA). The results of volunteers with central serous chorioretinopathy demonstrate that the proposed method effectively corrects motion artifacts in OCT volumes and may have potential application value in the evaluation of ophthalmic diseases such as diabetic retinopathy, glaucoma and age-related macular degeneration.     

Deep-learning-based motion correction in optical coherence tomography angiography

Optical coherence tomography angiography (OCTA) is a widely applied tool to image microvascular networks with high spatial resolution and sensitivity. Due to limited imaging speed, the artifacts caused by tissue motion can severely compromise visualization of the microvascular networks and quantification of OCTA images. In this article, we propose a deep-learning-based framework to effectively correct motion artifacts and retrieve microvascular architectures. This method comprised two deep neural networks in which the first subnet was applied to distinguish motion corrupted B-scan images from a volumetric dataset. Based on the classification results, the artifacts could be removed from the en face maximum-intensity-projection (MIP) OCTA image. To restore the disturbed vasculature induced by artifact removal, the second subnet, an inpainting neural network, was utilized to reconnect the broken vascular networks. We applied the method to postprocess OCTA images of the microvascular networks in mouse cortex in vivo. Both image comparison and quantitative analysis show that the proposed method can significantly improve OCTA image by efficiently recovering microvasculature from the overwhelming motion artifacts.     

The influence of footwear on foot motion during walking and running

There are evidences to suggest that wearing footwear constrains the natural barefoot motion during locomotion. Unlike prior studies that deduced foot motions from shoe sole displacement parameters, the aim of this study was to examine the effect of footwear motion on forefoot to rearfoot relative motion during walking and running. The use of a multi-segment foot model allowed accurate both shoe sole and foot motions (barefoot and shod) to be quantified. Two pairs of identical sandals with different midsole hardness were used. Ten healthy male subjects walked and ran in each of the shod condition.The results showed that for barefoot locomotion there was more eversion of the forefoot and it occurred faster than for shod locomotion. In this later condition, the range of eversion was reduced by 20% and the rate of eversion in late stance by 60% in comparison to the barefoot condition. The sole constrained both the torsional (eversion/inversion) and adduction range of motion of the foot. Interestingly, during the push-off phase of barefoot locomotion the rate and direction of forefoot torsion varied between individuals. However, most subjects displayed a forefoot inversion direction of motion while shod. Therefore, this experiment showed that the shoes not only restricted the natural motion of the barefoot but also appeared to impose a specific foot motion pattern on individuals during the push-off phase. These findings have implications for the matching of footwear design characteristics to individual natural foot function.     

Visual detection of paradoxical motion in flies

Summary From psychophysics it is known that humans easily perceive motion in Fourier-stimuli in which dots are displaced coherently into one direction. Furthermore, motion can be extracted from Drift-balanced stimuli in which the dots on average have no distinct direction of motion, or even in paradox -motion stimuli where the dots are displaced opposite to the perceived direction of motion. Whereas Fourier-motion can be explained by very basic motion detectors and nonlinear preprocessing of the input can account for the detection of Drift-balanced motion, a hierarchical model with two layers of motion detectors was proposed to explain the perception of -motion. The well described visual system of the fly allows to investigate whether these complex motion stimuli can be detected in a comparatively simple brain.The detection of such motion stimuli was analyzed for various random-dot cinematograms with extracellular recordings from the motion-sensitive Hl-neuron in the third visual ganglion of the blowfly Calliphora erythrocephala. The results were compared to computer-simulations of a hierarchical model of motion detector networks.For Fourier- and Drift-balanced motion stimuli, the Hl-neuron responds directionally selective to the moving object, whereas for -motion stimuli, the preferred direction is given by the dot displacement. Assuming nonlinear preprocessing of the detector input, such as a half-wave rectification, elementary motion detectors of the correlation type can account for these results.Abbreviations EMD elementary motion detector     

Asynchronous processing in vision: color leads motion

It has been demonstrated that subjects do not report changes in color and direction of motion as being co-incidental when they occur synchronously. Instead, for the changes to be reported as being synchronous, changes in direction of motion must precede changes in color. To explain this observation, some researchers have suggested that the neural processing of color and motion is asynchronous. This interpretation has been criticized on the basis that processing time may not correlate directly and invariantly with perceived time of occurrence. Here we examine this possibility by making use of the color-contingent motion aftereffect. By correlating color states disproportionately with two directions of motion, we produced and measured color-contingent motion aftereffects as a function of the range of physical correlations. The aftereffects observed are consistent with the perceptual correlation between color and motion being different from the physical correlation. These findings demonstrate asynchronous processing for different stimulus attributes, with color being processed more quickly than motion. This suggests that the time course of perceptual experience correlates directly with that of neural activity.     

The segregation and integration of colour in motion processing revealed by motion after-effects

Analysis of the colour and motion of objects is widely believed to take place within segregated processing pathways in the primate visual system. However, it is apparent that this segregation cannot remain absolute and that there must be some capacity for integration across these sub-modalities. In this study, we have assessed the extent to which colour constitutes a separable entity in human motion processing by measuring the chromatic selectivity of two kinds of after-effect resulting from motion adaptation. First, the traditional motion after-effect, where prolonged inspection of a unidirectional moving stimulus results in illusory motion in the opposite direction, was found to exhibit a high degree of chromatic selectivity. The second type of after-effect, in which motion adaptation induces misperceptions in the spatial position of stationary objects, was completely insensitive to chromatic composition. This dissociation between the chromatic selectivities of these after-effects shows that chromatic inputs remain segregated at early stages of motion analysis, while at higher levels of cortical processing there is integration across chromatic, as well as achromatic inputs, to produce a unified perceptual output.     

A motion direction map in macaque V2

In mammals, the perception of motion starts with direction-selective neurons in the visual cortex. Despite numerous studies in monkey primary and second visual cortex (V1 and V2), there has been no evidence of direction maps in these areas. In the present study, we used optical imaging methods to study the organization of motion response in macaque V1 and V2. In contrast to the findings in other mammals (e.g., cats and ferrets), we found no direction maps in macaque V1. Robust direction maps, however, were found in V2 thick/pale stripes and avoided thin stripes. In many cases direction maps were located within thick stripes and exhibited pinwheel or linear organizations. The presence of motion maps in V2 points to a newfound prominence of V2 in motion processing, for contributing to motion perception in the dorsal pathway and/or for motion cue-dependent form perception in the ventral pathway.     

Induced motion at texture-defined motion boundaries

When a static textured background is covered and uncovered by a moving bar of the same mean luminance we can clearly see the motion of the bar. Texture-defined motion provides an example of a naturally occurring second-order motion. Second-order motion sequences defeat standard spatio-temporal energy models of motion perception. It has been proposed that second-order stimuli are analysed by separate systems, operating in parallel with luminance-defined motion processing, which incorporate identifiable pre-processing stages that make second-order patterns visible to standard techniques. However, the proposal of multiple paths to motion analysis remains controversial. Here we describe the behaviour of a model that recovers both luminance-defined and an important class of texture-defined motion. The model also accounts for the induced motion that is seen in some texture-defined motion sequences. We measured the perceived direction and speed of both the contrast envelope and induced motion in the case of a contrast modulation of static noise textures. Significantly, the model predicts the perceived speed of the induced motion seen at second-order texture boundaries. The induced motion investigated here appears distinct from classical induced effects resulting from motion contrast or the movement of a reference frame.     

A model for direction selectivity in threshold motion perception

Thresholds were measured for a moving line superimposed on moving sinusoidal gratings. When line and grating moved in the same direction significant subthreshold summation was observed over a range of spatial frequencies. For motion of the line and grating in opposite directions, summation was never observed. This supports the hypothesis that direction selective mechanisms are responsible for motion perception at threshold. Further analysis of the data produced estimates of the spatial frequency tuning of these mechanisms. A quantitative model is proposed to interpret the data, and it is suggested that flickering gratings are not decomposed into their moving components by the visual system.     

End-stopping and the aperture problem: two-dimensional motion signals in macaque V1

Our perception of fine visual detail relies on small receptive fields at early stages of visual processing. However, small receptive fields tend to confound the orientation and velocity of moving edges, leading to ambiguous or inaccurate motion measurements (the aperture problem). Thus, it is often assumed that neurons in primary visual cortex (V1) carry only ambiguous motion information. Here we show that a subpopulation of V1 neurons is capable of signaling motion direction in a manner that is independent of contour orientation. Specifically, end-stopped V1 neurons obtain accurate motion measurements by responding only to the endpoints of long contours, a strategy which renders them largely immune to the aperture problem. Furthermore, the time course of end-stopping is similar to the time course of motion integration by MT neurons. These results suggest that cortical neurons might represent object motion by responding selectively to two-dimensional discontinuities in the visual scene.     

Back Cover Image,Volume 117, Number 2, February 2020

During the manufacturing process, solutions of protein-based drugs are exposed to hydrodynamic forces, which can potentially affect protein stability and aggregation. Despite being an area of extensive investigation, the effect of hydrodynamic flow on protein aggregation is still controversial. In this study, we designed an experimental setup that allowed us to investigate flow- and interface-induced protein aggregation of two model immunoglobulins in the presence of well-defined flow stresses and solid–liquid interfaces. Within the range of shear rates typically encountered in bioprocessing (), we observed that increasing the shear rate by three orders of magnitude had a negligible effect on protein aggregation. By contrast, changes in the materials of the syringe barrels had a dramatic effect on the monomer loss, demonstrating the key role of solid–liquid interfaces in flow-induced aggregation. This finding was confirmed by the observed inverse dependence of the aggregation rate on the initial protein concentration, which is inconsistent with mechanisms of protein aggregation in bulk solution. Overall, our results reveal the presence of a synergistic effect of interfaces and hydrodynamic flow in flow-induced protein aggregation, which arises from the formation of protein particles or films on interfaces followed by displacement by flow or mechanical scraping.     

Influence of Correspondence Noise and Spatial Scaling on the Upper Limit for Spatial Displacement in Fully-Coherent Random-Dot Kinematogram Stimuli

Correspondence noise is a major factor limiting direction discrimination performance in random-dot kinematograms [1]. In the current study we investigated the influence of correspondence noise on Dmax, which is the upper limit for the spatial displacement of the dots for which coherent motion is still perceived. Human direction discrimination performance was measured, using 2-frame kinematograms having leftward/rightward motion, over a 200-fold range of dot-densities and a four-fold range of dot displacements. From this data Dmax was estimated for the different dot densities tested. A model was proposed to evaluate the correspondence noise in the stimulus. This model summed the outputs of a set of elementary Reichardt-type local detectors that had receptive fields tiling the stimulus and were tuned to the two directions of motion in the stimulus. A key assumption of the model was that the local detectors would have the radius of their catchment areas scaled with the displacement that they were tuned to detect; the scaling factor k linking the radius to the displacement was the only free parameter in the model and a single value of k was used to fit all of the psychophysical data collected. This minimal, correspondence-noise based model was able to account for 91% of the variability in the human performance across all of the conditions tested. The results highlight the importance of correspondence noise in constraining the largest displacement that can be detected.     

Visual motion interferes with lexical decision on motion words

Embodied theories of cognition propose that neural substrates used in experiencing the referent of a word, for example perceiving upward motion, should be engaged in weaker form when that word, for example 'rise', is comprehended [1-3]. This claim has been broadly supported in the motor domain (for example [4,5]), whilst evidence is supportive, but less clear cut, for perception (for example [6-8]). Motivated by the finding that the perception of irrelevant background motion at near-threshold, but not supra-threshold, levels interferes with task execution [9], we assessed whether interference from near-threshold background motion was modulated by its congruence with the meaning of words (semantic content) when participants completed a lexical decision task (deciding if a string of letters is a real word or not). Reaction times for motion words, such as 'rise' or 'fall', were slower when the direction of visual motion and the 'motion' of the word were incongruent - but only when the visual motion was at near-threshold levels (supporting [9]). When motion was supra-threshold, the distribution of error rates, not reaction times, implicated low-level motion processing in the semantic processing of motion words. As the perception of near-threshold signals is not likely to be influenced by strategies [9], our results support a close contact between semantic information and perceptual systems.     

Saccadic suppression precedes visual motion analysis.

There is now good evidence that perception of motion is strongly suppressed during saccades (rapid shifts of gaze), presumably to blunt the disturbing sense of motion that saccades would otherwise elicit. Other aspects of vision, such as contrast detection of high-frequency or equiluminant gratings, are virtually unaffected by saccades [1] [2] [3] [4] [5]. This has led to the suggestion that saccades may suppress selectively the magnocellular pathway (which is strongly implicated in motion perception), leaving the parvocellular pathway unaffected [5] [6]. Here, we investigate the neural level at which perception of motion is suppressed. We used a simple technique in which an impression of motion is generated from only two frames, allowing precise control over the stimulus [7] [8]. One frame has a certain fixed contrast, whereas the contrast of the other (the test frame) is varied to determine the threshold for motion discrimination (that is, the lowest test-frame contrast level at which the direction of motion can be correctly guessed). Contrast thresholds of the test depended strongly and non-monotonically on the contrast of the fixed-contrast frame, with a minimum at medium contrast. To study the effect of saccadic suppression, we triggered the two-frame sequence by a voluntary saccade. Thresholds during saccades increased in a way that suggested that saccadic suppression precedes motion analysis: when the test frame was first in the motion sequence there was a general depression of sensitivity, whereas when it was second, the contrast response curve was shifted to a higher contrast range, sometimes even resulting in higher sensitivity than without a saccade. The dependence on presentation order suggests that saccadic suppression occurs at an early stage of visual processing, on the single frames themselves rather than on the combined motion signal. As motion detection itself is thought to occur at an early stage, saccadic suppression must take place at a very early phenomenon.     

The hierarchy of directional interactions in visual motion processing

It is well known that context influences our perception of visual motion direction. For example, spatial and temporal context manipulations can be used to induce two well-known motion illusions: direction repulsion and the direction after-effect (DAE). Both result in inaccurate perception of direction when a moving pattern is either superimposed on (direction repulsion), or presented following adaptation to (DAE), another pattern moving in a different direction. Remarkable similarities in tuning characteristics suggest that common processes underlie the two illusions. What is not clear, however, is whether the processes driving the two illusions are expressions of the same or different neural substrates. Here we report two experiments demonstrating that direction repulsion and the DAE are, in fact, expressions of different neural substrates. Our strategy was to use each of the illusions to create a distorted perceptual representation upon which the mechanisms generating the other illusion could potentially operate. We found that the processes mediating direction repulsion did indeed access the distorted perceptual representation induced by the DAE. Conversely, the DAE was unaffected by direction repulsion. Thus parallels in perceptual phenomenology do not necessarily imply common neural substrates. Our results also demonstrate that the neural processes driving the DAE occur at an earlier stage of motion processing than those underlying direction repulsion.     

Automatic imaging of Drosophila embryos with light sheet fluorescence microscopy on chip

We present a microscope on chip for automated imaging of Drosophila embryos by light sheet fluorescence microscopy. This integrated device, constituted by both optical and microfluidic components, allows the automatic acquisition of a 3D stack of images for specimens diluted in a liquid suspension. The device has been fully optimized to address the challenges related to the specimens under investigation. Indeed, the thickness and the high ellipticity of Drosophila embryos can degrade the image quality. In this regard, optical and fluidic optimization has been carried out to implement dual-sided illumination and automatic sample orientation. In addition, we highlight the dual color investigation capabilities of this device, by processing two sample populations encoding different fluorescent proteins. This work was made possible by the versatility of the used fabrication technique, femtosecond laser micromachining, which allows straightforward fabrication of both optical and fluidic components in glass substrates.