Low Spatial Frequencies Research Articles

Probing Three-dimensional Magnetic Fields. III. Synchrotron Emission and Machine Learning

Synchrotron observation serves as a tool for studying magnetic fields in the interstellar medium and intracluster medium, yet its ability to unveil three-dimensional (3D) magnetic fields, meaning probing the field’s plane-of-the-sky (POS) orientation, inclination angle relative to the line of sight, and magnetization from one observational data, remains largely underexplored. Inspired by the latest insights into anisotropic magnetohydrodynamic (MHD) turbulence, we found that synchrotron emission’s intensity structures inherently reflect this anisotropy, providing crucial information to aid in 3D magnetic field studies: (i) the structure’s elongation gives the magnetic field’s POS orientation and (ii) the structure’s anisotropy degree and topology reveal the inclination angle and magnetization. Capitalizing on this foundation, we integrate a machine learning approach—convolutional neural network (CNN)—to extract this latent information, thereby facilitating the exploration of 3D magnetic fields. The model is trained on synthetic synchrotron emission maps, derived from 3D MHD turbulence simulations encompassing a range of sub-Alfvénic to super-Alfvénic conditions. We show that the CNN is physically interpretable and the CNN is capable of obtaining the POS orientation, inclination angle, and magnetization. Additionally, we test the CNN against the noise effect and the missing low-spatial frequency. We show that this CNN-based approach maintains a high degree of robustness even when only high-spatial frequencies are maintained. This renders the method particularly suitable for application to interferometric data lacking single-dish measurements. We applied this trained CNN to the synchrotron observations of a diffuse region. The CNN-predicted POS magnetic field orientation shows a statistical agreement with that derived from synchrotron polarization.

Comparison of the quantitative contrast sensitivity function between early keratoconus and normal eyes

PurposeTo compare the characteristics of the quantitative contrast sensitivity function (qCSF) in eyes with early keratoconus (Early-KC) and normal control (NC) eyes and investigate the associated factors.DesignA cross-sectional study.MethodsThis study included 43 eyes of 43 patients with Early-KC (including subclinical keratoconus [SKC] and forme fruste keratoconus [FFKC]) and 77 NC eyes of 77 participants with corrected distance visual acuity (CDVA) all ≥ 20/20. Contrast sensitivity (CS) was assessed using the qCSF tests. Subgroup analysis was performed according to keratoconus type(SKC and FFKC) and astigmatism(cylindrical refraction >-1.0D or ≤-1.0D).ResultsSex ratio, spherical refraction, and spherical equivalent (SE) varied significantly between the two groups (all P < 0.01). The area under log CSF (AULCSF), CSF Acuity, and CS at low (1.0 and 1.5 cycles per degree [cpd]) and high (12.0 and 18.0 cpd) spatial frequencies decreased significantly in the Early-KC group than that in the NC group (all P < 0.05). The subgroup analysis revealed a similar decrease in the SKC group (all P < 0.05). AULCSF, CSF Acuity, and CS at high spatial frequencies of patients with cylindrical refraction ≤-1.0D in the Early-KC group decreased significantly (all P < 0.05) than those in the NC group. The index of vertical asymmetry and index of height decentration correlated negatively with CS at 1.5 cpd (r= -0.321 and -0.306; both P < 0.05).ConclusionsCS decreased significantly at low and high spatial frequencies in Early-KC, though with normal CDVA. The qCSF test can sensitively reflect visual performance in early keratoconus.

The early visual processing of faces in a basic classification task: An ERP study of spatial frequency, other-race and other species effect

ABSTRACT This study investigates the neural processing of other-race and other-species faces, using spatial frequencies (SFs) to probe into early visual processing mechanisms. We conducted a basic classification task to reveal fundamental aspects of facial processing without the complexity of higher-order recognition tasks. This approach utilized SFs set at low (LSF, 0.53 cycles per degree [cpd]), broadband (BB), and high (HSF, 2.1 cpd) SFs to examine their impact on the both P100 and N170 components of event-related potentials. Our findings revealed a significant latency difference in the N170 component under HSF conditions, indicating race-specific efficiency in processing fine details between Asian and Caucasian faces. Furthermore, N170 amplitudes were consistently larger for human than monkey faces, with this difference more pronounced in LSF and BB conditions. This finding indicates that the other-species effect in early visual processing is more pronounced in holistic processing (LSF and BB conditions) rather than in fine-detail processing.

Energy–dependent femtosecond LIPSS on germanium and application in explosives sensing

Abstract In this study, we fabricated laser-induced periodic surface structures (LIPSS) on a germanium surface through laser ablation in air using axicon and femtosecond (fs) pulses. This novel approach permitted the nanoscale material processing outcome refinement via an fs Bessel beam. Our investigations aimed at systematically understanding the formation of periodic structures under various experimental conditions, such as (i) different pulse energies ranging from 50 µJ to 1000 µJ at a constant scan speed and (ii) constant energy with different scan speeds (0.1–3 mm s−1). By adjusting the fluences and scan speeds, we were able to identify the parametric space and alter the periodicity of the low-spatial frequency LIPSS and high-spatial frequency LIPSS on germanium, which were analyzed using field emission scanning electron microscopy. An optimal LIPSS formation over a large area of germanium was achieved at an input energy of 250 µJ and a scan speed of 0.75 mm s−1. Additionally, we measured the contact angles of the Ge nanostructures (GeNSs) to demonstrate their hydrophobic nature and non-wetting properties, providing insights into the behavior of LIPSS. Subsequently, the GeNSs were coated with a ∼15 nm thick gold (Au) film using a thermal deposition method. Utilizing these, the surface-enhanced Raman spectroscopy (SERS) technique detected diverse analytes, such as tetryl (an explosive) at a concentration of 50 µM and thiram (a pesticide) at 500 nM. The SERS enhancement factors for tetryl and thiram molecules on GeNSs coated with a 15 nm-thick Au layer were determined to be 2.5 × 104 and 4.2 × 105, respectively.

Comparison of arm cavity optical losses for the two wavelengths of the Einstein telescope gravitational wave detector

Abstract A new generation of gravitational wave detectors is currently being designed with the likely use of a different laser wavelength compared to current instruments. The estimation of the optical losses for this new wavelength is particularly relevant to derive the detector sensitivity and also to anticipate the optical performances of future instruments. In this article, we measured the absorption and angle-resolved scattering of several mirror samples in order to compare optical losses at a wavelength of 1064 and 1550 nm. In addition, we have carried out simulations of the Einstein Telescope arm cavities at 1064 and 1550 nm taking into account losses due to surface low-spatial frequency flatness. Our results suggest that optical losses as measured at 1064 nm are about twice as large as those at 1550 nm as predicted with a simple model.

Microsaccadic suppression of peripheral perceptual detection performance as a function of foveated visual image appearance.

Microsaccades are known to be associated with a deficit in perceptual detection performance for brief probe flashes presented in their temporal vicinity. However, it is still not clear how such a deficit might depend on the visual environment across which microsaccades are generated. Here, and motivated by studies demonstrating an interaction between visual background image appearance and perceptual suppression strength associated with large saccades, we probed peripheral perceptual detection performance of human subjects while they generated microsaccades over three different visual backgrounds. Subjects fixated near the center of a low spatial frequency grating, a high spatial frequency grating, or a small white fixation spot over an otherwise gray background. When a computer process detected a microsaccade, it presented a brief peripheral probe flash at one of four locations (over a uniform gray background) and at different times. After collecting full psychometric curves, we found that both perceptual detection thresholds and slopes of psychometric curves were impaired for peripheral flashes in the immediate temporal vicinity of microsaccades, and they recovered with later flash times. Importantly, the threshold elevations, but not the psychometric slope reductions, were stronger for the white fixation spot than for either of the two gratings. Thus, like with larger saccades, microsaccadic suppression strength can show a certain degree of image dependence. However, unlike with larger saccades, stronger microsaccadic suppression did not occur with low spatial frequency textures. This observation might reflect the different spatiotemporal retinal transients associated with the small microsaccades in our study versus larger saccades.

Fabrication of grooves on 4H–SiC using femtosecond laser vector beam

Femtosecond lasers are widely acknowledged for their capability to groove and generate laser-induced periodic surface structures (LIPSS) on various materials. LIPSS structures include LSFL (low spatial frequency LIPSS) and HSFL (high spatial frequency LIPSS). In this study, a segmented waveplate (SWP) was used to transform a linearly polarized femtosecond laser Gaussian beam with a wavelength of 515 nm into a vector beam with a donut-shaped energy peak distribution and polarization of azimuthal and radial. Grooving was conducted on three different SiC surface conditions: polished and pre-processed (LSFL and HSFL), using linear, azimuthal, and radial polarization. This study investigated the differences in microstructure morphology and compared the ablation depths of the grooves. It was found that grooving on pre-processed LSFL 4H–SiC surfaces resulted in grooves with higher ablation depths than those produced on polished surfaces. Additionally, using azimuthal polarization on the HSFL surface could create fine HSFL structures (with a period of approximately 100 nm) between the initial periodic structures (with a period of approximately 250 nm) formed during the pre-processing.

Optimization of 3D imaging time reduction by assessing spatial resolution in the slice selective direction using the ladder method

PurposeAssessing spatial resolution in MRI is challenging due to non-linearity. Despite the widespread use of 3D imaging in clinical practice for lesion detection and multi-planar reconstruction (MPR), the extended acquisition time poses a shortcoming. To address this, the “Slice resolution” parameter is utilized; however, its impact on MPR images is unclear. This study aims to assess spatial resolution using the ladder method, investigate the effects of diverse slice resolution settings in various imaging sequences, and propose optimal conditions. MethodsImages were acquired using various 3D imaging sequences—SPACE T1WI, SPACE T2WI, and VIBE T1WI—with different slice resolutions. Axial cross-section images were acquired and reconstructed into coronal cross-sections. The ladder method was employed for objective evaluation, including spatial frequency analysis. Additionally, visual evaluation was conducted and compared with ladder method results. ResultsFor three imaging sequences, the evaluated value of ladder method remained relatively constant from 100 % to 80 % slice resolution. However, the evaluated value decreased in low-spatial frequency for slice resolution below 70 %. ConclusionsResults from both ladder method and visual evaluations indicated image quality remained stable when the slice resolution was decreased to 80 %, potentially enabling a 20 % reduction in imaging time while preserving resolution in other cross-sections reconstructed by MPR.

Focusing on mixed narrow band stimuli: Implications for mechanisms of accommodation and displays.

The eye has considerable chromatic aberration, meaning that the accommodative demand varies with wavelength. Given this, how does the eye accommodate to light of differing spectral content? Previous work is not conclusive but, in general, the eye focuses in the center of the visible spectrum for broadband light, and it focuses at a distance appropriate for individual wavelengths for narrowband light. For stimuli containing two colors, there are also mixed reports. This is the second of a series of two papers where we investigate accommodation in relation to chromatic aberrationFernandez-Alonso, Finch, Love, and Read (2024). In this paper, for the first time, we measure how the eye accommodates to images containing two narrowband wavelengths, with varying relative luminance under monocular conditions. We find that the eye tends to accommodate between the two extremes, weighted by the relative luminance. At first sight, this seems reasonable, but we show that image quality would be maximized if the eye instead accommodated on the more luminous wavelength. Next we explore several hypotheses as to what signal the eye might be using to drive accommodation and compare these with the experimental data. We show that the data is best explained if the eye seeks to maximize contrast at low spatial frequencies. We consider the implication of these results for both the mechanism behind accommodation, and for modern displays containing narrowband illuminants.

Improvement of acuity following motion adaptation: the role of low spatial frequencies

Improvement of acuity following motion adaptation: the role of low spatial frequencies

Motion onset VEPs can see through the blur

Motion-onset visual evoked potentials (MO VEPs) are robust to dioptric blur when low contrast and low spatial frequency patterns are used for stimulation. To reveal mechanisms of MO VEPs robustness, we studied whether the resistance to defocus persists even when using a high-contrast checkerboard using digital defocus in the emmetropic eyes of 13 subjects (males 20–60 years). We compared the dominant components of MO VEPs to pattern-reversal VEPs (PR VEP), which are sensitive to the blur. For stimulation, we used checkerboard patterns with 15´ and 60´ checks. To defocus the checkerboard, we rendered it with a second-order Zernike polynomial (Z20\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${Z}_{2}^{0}$$\\end{document}) with an equivalent defocus of 0, 2, or 4 D. For PR VEP, the checkerboards were reversed in terms of their contrast. To evoke MO VEP, the checkerboard of 60´ checks moved for 200 ms with a speed of 5 or 10 deg/s in the cardinal directions. The MO VEP did not change in peak time (P ≥ 0.0747) or interpeak amplitude (P > 0.0772) with digital blur. In contrast, for PR VEP, the results showed a decrease in interpeak amplitude (P ≤ 6.65ˑ10-4) and an increase in peak time (P ≤ 0.0385). Thus, we demonstrated that MO VEPs evoked by checkerboard, structure containing high spatial content, can be robust to defocus.

Clinical Outcomes after Implantation of a Novel Binocular Complementary Extended Depth-of-Focus Intraocular Lens Compared to Bilateral Low Near-Add Multifocal Intraocular Lenses

Introduction: Nowadays, patients expect to be less spectacle dependent at all distances after cataract surgery. However, all intraocular lens (IOL) models that currently offer this function can also have negative visual effects. Aim of this study was therefore to compare the visual function performance of a novel and a conventional IOL solution for multifocality. Methods: Patients scheduled for bilateral cataract surgery were randomized to receive one of two different IOL combinations: either a binocular complementary extended depth-of-focus IOL set or a low near-add MIOL. Patients had visual acuity (VA) assessment at distance, intermediate, and near as well as evaluation of contrast sensitivity, halometry, and reading performance. Results: In total, 56 eyes of 28 patients were enrolled. At 6 months, there were no statistically significant differences in binocular VA between the ARTIS SYMBIOSE and the AT LARA. Contrast sensitivity, measured binocularly, at 1.5 cycles per degree under photopic conditions without glare was 1.54 logCS with the ARTIS SYMBIOSE and 1.43 logCS with the AT LARA (p = 0.046), under mesopic conditions with glare at 1.5 and 3 cycles per degree 1.31 logCS and 1.28 logCS with the ARTIS SYMBIOSE, respectively, compared to 0.58 logCS and 0.51 logCS with the AT LARA (p = 0.002; p = 0.006). Binocular halos and reading performance between both groups were similar. Conclusion: There were no significant differences between both groups in VA at different distances, reading ability, or halometry. The contrast sensitivity at low spatial frequencies was better in the ARTIS SYMBIOSE group under photopic conditions without glare as well as mesopic conditions with glare, which could potentially improve quality of vision with these IOLs, especially under mesopic conditions.

Fuzzy ripple artifact in high resolution fMRI: identification, cause, and mitigation.

High resolution fMRI is a rapidly growing research field focused on capturing functional signal changes across cortical layers. However, the data acquisition is limited by low spatial frequency EPI artifacts; termed here as Fuzzy Ripples. These artifacts limit the practical applicability of acquisition protocols with higher spatial resolution, faster acquisition speed, and they challenge imaging in lower brain areas. We characterize Fuzzy Ripple artifacts across commonly used sequences and distinguish them from conventional EPI Nyquist ghosts, off-resonance effects, and GRAPPA artifacts. To investigate their origin, we employ dual polarity readouts. Our findings indicate that Fuzzy Ripples are primarily caused by readout-specific imperfections in k-space trajectories, which can be exacerbated by inductive coupling between third-order shims and readout gradients. We also find that these artifacts can be mitigated through complex-valued averaging of dual polarity EPI or by disconnecting the third-order shim coils. The proposed mitigation strategies allow overcoming current limitations in layer-fMRI protocols: (1)Achieving resolutions beyond 0.8mm is feasible, and even at 3T, we achieved 0.53mm voxel functional connectivity mapping.(2)Sub-millimeter sampling acceleration can be increased to allow sub-second TRs and laminar whole brain protocols with up to GRAPPA 8.(3)Sub-millimeter fMRI is achievable in lower brain areas, including the cerebellum.

A quantitative approach to reflectance transformation imaging in profilometric applications

Reflectance transformation imaging (RTI) is a technique employed to assess both the intensity and directional properties of light reflected from an object, aiming to visualize an object under different incident light directions. This technique has quickly become a commonly used approach for the documentation, acquisition and deciphering for cultural heritage objects, because it enables to enhance and highlight image details. The output of this mathematical synthesis technique, which elaborates illumination information, is contained in a pseudocolour image called normal map. Starting from this image, this technique allows to obtain the reconstruction of the outline of semi-flat objects by integrating the normal map given by the RTI, i.e. normal integration. The technique of normal integration has been known for many years, being widely used in graphical modelling across various fields of 3D imaging. In the context of cultural heritage, this particular form of 3D modelling has already found application in the reconstruction of semi-flat objects, albeit with some limitations in the accurate portraying of low spatial frequencies. However, quantitative and systematic studies on the efficiency of 3D rendering of objects using RTI and normal integration have not yet been conducted. In the context of this research, the quantitative evaluation of the accuracy of the RTI technique with normal integration for profilometric applications on semi-flat objects is proposed and a study on its fidelity in reproducing the considered artefact is conducted. For this purpose, ad hoc profilometric targets were designed, realized and analysed to quantitatively compare the results of RTI and normal integration with different standardized techniques: micro-photogrammetry, laser scanning and optical profilometry.Graphical abstract

The effect of aging on the dual-route model of emotion processing applied to memory recognition.

Emotional faces are automatically processed in the human brain through a cortical route (conscious processing based on high spatial frequencies, HSF) and a subcortical route (subliminal processing based on low spatial frequencies, LSF). How each route contributes to emotional face recognition is still debated, and little is known about this process in aging. Here, 147 younger adults (YA) and 137 older adults (OA) were passively presented with neutral, happy, and angry faces, shown as (a) unfiltered, (b) filtered at LSF, and (c) hybrid (emotional LSF superimposed to the neutral HSF of the same face). In a succeeding recognition phase, the same faces and new faces were shown as unfiltered, and participants were asked whether each face had been already presented in the encoding phase. Despite the better performance by YA compared with OA for neutral faces presented as unfiltered (cortical route), the performance of OA was better than that of YA for angry faces presented as hybrid and for happy faces presented at LSF and as hybrid. We conclude that the activity of the subcortical route during the encoding phase facilitates emotional recognition in aging. Results are discussed in accordance with the dual-route model. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Binocular Visual Deficits at Low to High Spatial Frequency in Intermittent Exotropia After Surgery.

To investigate binocular visual deficits at low to high spatial frequencies in patients with intermittent exotropia (IXT) after surgical correction, using the binocular orientation combination task. Thirteen patients whose IXT has been aligned surgically (17 ± 4.8 years old; 7 females) and 13 normal individuals (21.8 ± 2.5 years old; 6 females) were recruited. All participants had normal or corrected-to-normal visual acuity. The IXT patients had undergone surgery at least one month prior to the study and achieved successful eye alignment post-surgery. We measured participants' balance points (BPs), defined as the interocular contrast ratio (nondominant eye/dominant eye) when both eyes contributed equally to binocular combination, using the binocular orientation combination task at three spatial frequencies (0.5, 4.0, and 8.0 cycles/degree). The absolute values of log10(BP) (i.e., |logBP|) and the area under of the |logBP| versus spatial frequency curve were used to quantify the extent of binocular imbalance. Surgery aligned the eye position of patients with IXT, with a postoperative exodeviation of -4.92 ± 4.29 prism diopters at distance. Participants' |logBP| values showed significant differences between groups, F(1,24) = 9.175, P = 0.006, and across spatial frequencies, F(2,48) = 7.127, P = 0.002. However, the interaction between group and spatial frequency was not significant, F(2,48) = 0.379, P = 0.687. Patients whose IXT has been alighted surgically experience binocular imbalance across a wide range of spatial frequencies, with greater binocular imbalance occurring at high spatial frequencies than low spatial frequencies.

Tracing Magnetic Fields with the Gradient Technique: Spatial Filtering Effect and Use of Interferometers

Probing magnetic fields in astrophysical environments is both important and challenging. The Gradient Technique (GT) is a new tool for tracing magnetic fields, rooted in the properties of magnetohydrodynamic (MHD) turbulence and turbulent magnetic reconnection. In this work, we examine the performance of GT when applied to synthetic synchrotron emission and spectroscopic data obtained from sub-Alfvénic and trans-Alfvénic MHD simulations. We demonstrate the improved accuracy of GT in tracing magnetic fields in the absence of low spatial frequencies. Additionally, we apply a low-spatial-frequency filter to a diffuse neutral hydrogen region selected from the GALFA-H i survey. Our results show an increased alignment between the magnetic fields inferred from GT and the Planck 353 GHz polarization measurements.

The Relationship between Clinical and Psychophysical Assessments of Visual Perceptual Disturbances in Individuals at Clinical High Risk for Psychosis: A Preliminary Study.

This study investigated relations between a measure of early-stage visual function and self-reported visual anomalies in individuals at clinical high risk for psychosis (CHR-P). Eleven individuals at CHR identified via the Structured Interview for Psychosis-Risk Syndromes (SIPS) were recruited from a CHR-P research program in NYC. The sample was ~36% female, ranging from 16 to 33 years old (M = 23.90, SD = 6.14). Participants completed a contrast sensitivity task on an iPad with five spatial frequencies (0.41-13 cycles/degree) and completed the self-report Audio-Visual Abnormalities Questionnaire. Higher contrast sensitivity (better performance) to low spatial frequencies was associated with higher perceptual (r = 0.616, p = 0.044) and visual disturbances (r = 0.667, p = 0.025); lower contrast sensitivity to a middle spatial frequency was also associated with higher perceptual (r = -0.604, p = 0.049) and visual disturbances (r = -0.606, p = 0.048). This relation between the questionnaire and contrast sensitivity to low spatial frequency may be indicative of a reduction in lateral inhibition and "flooding" of environmental stimuli. The association with middle spatial frequencies, which play a critical role in face processing, may result in a range of perceptual abnormalities. These findings demonstrate that self-reported perceptual anomalies occur in these individuals and are linked to performance on a measure of early visual processing.

Lower indoor spatial frequency increases the risk of myopia in children

Background/aimsAnimal models have shown that the absence of high-frequency visual information can precipitate the onset of myopia, but this relationship remains unclear in humans. This study aims to explore the...

Cryo-EM phase-plate images reveal unexpected levels of apparent specimen damage.

Apoferritin (apoF) is commonly used as a test specimen in single-particle electron cryo-microscopy (cryo-EM), since it consistently produces density maps that go to 3 Å resolution or higher. When we imaged apoF with a laser phase plate (LPP), however, we observed more severe particle-to-particle variation in the images than we had previously thought to exist. Similarly, we found that images of ribulose bisphosphate carboxylase/oxygenase (rubisco) also exhibited a much greater amount of heterogeneity than expected. By comparison to simulations of images, we verified that the heterogeneity is not explained by the known features of the LPP, shot noise, or differences in particle orientation. We also demonstrate that our specimens are comparable to those previously used in the literature, based on using the final-reconstruction resolution as the metric for evaluation. All of this leads us to the hypothesis that the heterogeneity is due to damage that has occurred either during purification of the specimen or during preparation of the grids. It is not, however, our goal to explain the causes of heterogeneity; rather, we report that using the LPP has made the apparent damage too obvious to be ignored. In hindsight, similar heterogeneity can be seen in images of apoF and the 20S proteasome which others had recorded with a Volta phase plate. We therefore conclude that the increased contrast of phase-plate images (at low spatial frequencies) should also make it possible to visualize, on a single-particle basis, various forms of biologically functional heterogeneity in structure that had previously gone unnoticed.