Similar Spatial Frequency Research Articles

Modal Decomposition of Wake Flow Behind a Circular Cylinder

This study demonstrates the effectiveness of Proper Orthogonal Decomposition (POD) as a method for analyzing flow structures and as a data reduction tool for both numerical simulations and experimental techniques. The reduced data can be integrated to data driven inference of partial differential equations which can lead to reduced order modeling. By focusing on the flow around a cylinder, we show that POD efficiently identifies the most energetic modes, allowing for the flow's reconstruction with a limited dataset. Our findings indicate that the first 10 POD modes appear in pairs with similar spatial patterns, frequency spectra, and energy contributions. These modes encapsulate nearly 92% of the total energy, highlighting the method's precision in capturing essential dynamics. Specifically, modes 1 and 2 correspond to streamwise vortex shedding, while modes 3 and 4 relate to the lateral flow motion. The reconstructed flow, based on these dominant modes, closely matches numerical analysis results, validating POD's utility in developing reduced order models and enhancing our understanding of fluid-structure interactions.

Learning spatial frequency identification through reweighted decoding.

Perceptual learning, the improvement of perceptual judgments with practice, occurs in many visual tasks. There are, however, relatively fewer studies examining perceptual learning in spatial frequency judgments. In addition, perceptual learning has generally been studied in two-alternative tasks, occasionally in n-alternative tasks, and infrequently in identification. Recently, perceptual learning was found in an orientation identification task (eight-alternatives) and was well accounted for by a new identification integrated reweighting theory (I-IRT) (Liu et al., submitted). Here, we examined perceptual learning in a similar eight-alternative spatial frequency absolute identification task in two different training protocols, finding learning in the majority but not all observers. We fit the I-IRT to the spatial frequency learning data and discuss possible model explanations for variations in learning.

Statistical Tool Size Study for Computer-Controlled Optical Surfacing

Over the past few decades, computer-controlled optical surfacing (CCOS) systems have become more deterministic. A target surface profile can be predictably achieved with a combination of tools of different sizes. However, deciding the optimal set of tool sizes that will achieve the target residual error in the shortest run time is difficult, and no general guidance has been proposed in the literature. In this paper, we present a computer-assisted study on choosing the proper tool size for a given surface error map. First, we propose that the characteristic frequency ratio (CFR) can be used as a general measure of the correction capability of a tool over a surface map. Second, the performance of different CFRs is quantitatively studied with a computer simulation by applying them to guide the tool size selection for polishing a large number of randomly generated surface maps with similar initial spatial frequencies and root mean square errors. Finally, we find that CFR = 0.75 achieves the most stable trade-off between the total run time and the number of iterations and thus can be used as a general criterion in tool size selection for CCOS processes. To the best of our knowledge, the CFR is the first criterion that ties tool size selection to overall efficiency.

A simultaneous EEG-fMRI study of thalamic load-dependent working memory delay period activity.

Working memory (WM) is an essential component of executive functions which depend on maintaining task-related information online for brief periods in both the presence and absence of interfering stimuli. Active maintenance occurs during the WM delay period, the time between stimulus encoding and subsequent retrieval. Previous studies have extensively documented prefrontal and posterior parietal cortex activity during the WM delay period, but the role of subcortical structures including the thalamus remains to be fully elucidated, especially in humans. Using a simultaneous electroencephalogram (EEG)-functional magnetic resonance imaging (fMRI) approach, we investigated the role of the thalamus during the WM delay period in a modified Sternberg paradigm following low and high memory load encoding of naturalistic scenes. During the delay, participants passively viewed scrambled scenes containing similar color and spatial frequency to serve as a perceptual baseline. Individual source estimation was weighted by the location of the thalamic fMRI signal relative to the WM delay period onset. The effects memory load on maintenance were observed bilaterally in thalamus with higher EEG source amplitudes in the low compared to high load condition occurring 160-390 ms after the onset of the delay period. The main finding that thalamic activation was elevated during the low compared to high condition despite similar duration of perceptual input and upcoming motor requirements suggests a capacity-limited role for sensory filtering of the thalamus during consolidation of stimuli into WM, where the highest activity occurs when fewer stimuli need to be maintained in the presence of interfering perceptual stimuli during the delay. The results are discussed in the context of theories regarding the role of the thalamus in sensory gating during working memory.

Brain Connectivity: A Journal of Clinical Neurology, Neuroscience, & Neuroimaging Advancing the Field of Neurology

<i>Brain Connectivity: A Journal of Clinical Neurology, Neuroscience, &amp; Neuroimaging</i> Advancing the Field of Neurology

Interhemispheric Connectivity Supports Load-Dependent Working Memory Maintenance for Complex Visual Stimuli.

Abstract Introduction: One manipulation used to study the neural basis of working memory (WM) is to vary the information load at encoding, then measure activity and connectivity during maintenance in the delay period. A hallmark finding is increased delay activity and connectivity between frontoparietal brain regions with increased load. Most WM studies, however, employ simple stimuli during encoding and unfilled intervals during the delay. In this study, we asked how delay period activity and connectivity change during low and high load maintenance of complex stimuli. Methods: Twenty-two participants completed a modified Sternberg WM task with two or five naturalistic scenes as stimuli during scalp electroencephalography (EEG). On each trial, the delay was filled with phase-scrambled scenes to provide a visual perceptual control with similar color and spatial frequency as presented during encoding. Functional connectivity during the delay was assessed by the phase-locking value (PLV). Results: Results showed reduced theta/alpha delay activity amplitude during high compared with low WM load across frontal, central, and parietal sources. A network with higher connectivity during low load consisted of increased PLV between (1) left frontal and right posterior temporal sources in the theta/alpha bands, (2) right anterior temporal and left central sources in the alpha and lower beta bands, and (3) left anterior temporal and posterior temporal sources in the theta, alpha, and lower beta bands. Discussion: The findings suggest a role for interhemispheric connectivity during WM maintenance of complex stimuli with load modulation when limited attentional resources are essential for filtering. Impact statement The patterns of brain connectivity subserving working memory (WM) have largely been investigated to date using simple stimuli, including letters, digits, and shapes and during unfilled WM delay intervals. Fewer studies describe functional connectivity changes during the maintenance of more naturalistic stimuli in the presence of distractors. In the present study, we employed a scene-based WM task during electroencephalography in healthy humans and found that during low-load WM maintenance with distractors increased interhemispheric connectivity in frontotemporal networks. These findings suggest a role for increased interhemispheric connectivity during maintenance of complex stimuli when attentional resources are essential for filtering.

Contrast adaptation of flankers reduces collinear facilitation and inhibition

Increase (facilitation) or decrease (inhibition) of contrast sensitivity for a Gabor patch presented between two collinear flankers is a well-studied contextual modulation phenomenon. It has been suggested that this effect has its neural bases in the primary visual cortex, specifically the horizontal connections between hypercolumns with similar orientation and spatial frequency selectivity. Another typical phenomenon dependent on early visual areas is contrast adaptation, in which the neural response to a contrast stimulus is decreased after exposure. Here, we investigated the effect of contrast adaptation of the flankers on the magnitude of collinear modulation by testing whether contrast adaptation reduced collinear facilitation and collinear inhibition. Results showed dissociation in the effect of collinear flanker adaptation, which increased contrast thresholds for the target in the facilitatory configuration and reduced them in the inhibitory configuration. Moreover, the effect was specific for the collinear configuration, since contrast adaptation of orthogonal flankers did not affect the contrast of the target, pointing towards the involvement of early visual units specific for orientation. Surprisingly, the same pattern of results was also confirmed when the inhibitory configuration was tested with low-contrast flankers, indicating that the effect of adaptation does not depend on a decrease in perceived contrast of the flankers. Taken together, these results suggest that contrast adaptation disrupts collinear modulation and that contrast thresholds can be affected by adapting portions of the visual field outside the receptive field of the units processing the contrast of the target (i.e., the flankers).

Variations in photoreceptor throughput to mouse visual cortex and the unique effects on tuning

Visual input to primary visual cortex (V1) depends on highly adaptive filtering in the retina. In turn, isolation of V1 computations requires experimental control of retinal adaptation to infer its spatio-temporal-chromatic output. Here, we measure the balance of input to mouse V1, in the anesthetized setup, from the three main photoreceptor opsins—M-opsin, S-opsin, and rhodopsin—as a function of two stimulus dimensions. The first dimension is the level of light adaptation within the mesopic range, which governs the balance of rod and cone inputs to cortex. The second stimulus dimension is retinotopic position, which governs the balance of S- and M-cone opsin input due to the opsin expression gradient in the retina. The fitted model predicts opsin input under arbitrary lighting environments, which provides a much-needed handle on in-vivo studies of the mouse visual system. We use it here to reveal that V1 is rod-mediated in common laboratory settings yet cone-mediated in natural daylight. Next, we compare functional properties of V1 under rod and cone-mediated inputs. The results show that cone-mediated V1 responds to 2.5-fold higher temporal frequencies than rod-mediated V1. Furthermore, cone-mediated V1 has smaller receptive fields, yet similar spatial frequency tuning. V1 responses in rod-deficient (Gnat1−/−) mice confirm that the effects are due to differences in photoreceptor opsin contribution.

Does rehearsal matter? Left anterior temporal alpha and theta band changes correlate with the beneficial effects of rehearsal on working memory

Rehearsal during working memory (WM) maintenance is assumed to facilitate retrieval. Less is known about how rehearsal modulates WM delay activity. In the present study, 44 participants completed a Sternberg Task with either intact novel scenes or phase-scrambled scenes, which had similar color and spatial frequency but lacked semantic content. During the rehearsal condition participants generated a descriptive label during encoding and covertly rehearsed during the delay period. During the suppression condition participants did not generate a label during encoding and suppressed (repeated “the”) during the delay period. This was easy in the former (novel scenes) but more difficult in the later condition (phase-scrambled scenes) where scenes lacked semantic content. Behavioral performance and EEG delay activity was analyzed as a function of maintenance strategy. Performance during WM revealed a benefit of rehearsal for phase-scrambled but not intact scenes. Examination of the absolute amplitude revealed three underlying sources of activity for rehearsal, including the left anterior temporal (ATL) and left and midline parietal regions. Increases in alpha and theta activity in ATL were correlated with improvement in performance on WM with rehearsal only when labeling was not automatic (e.g., phase-scrambled scenes), which may reflect differences in labeling and rehearsal (i.e., semantic associations vs. shallow labels). We conclude that rehearsal only benefits memory for visual stimuli that lack semantic information, and that this is correlated with changes in alpha and theta rhythms.

X-ray image quality and system exposure parameters for a hybrid Angio-MR system

This study investigates the exposure parameters and required x-ray tube output when performing neurological procedures on a hybrid Angio-MR concept system proposed by Siemens Healthineers. The x-ray part of this system uses a longer source to detector distance than conventional (C-arm) systems and will have a fixed amount of filtration. Additionally, as the x-ray source is situated inside a magnetic field, the focal spot size and shape may be slightly distorted. In order to compare the Angio-MR system to a typical C-arm system, the exposure parameters of 60 thrombectomy procedures, performed in our hospital over the course of one year, were investigated in detail and a set of median values was determined. An analytical simulation platform was then developed to calculate the required tube voltage, tube current and pulse length to reach similar spatial frequency dependent signal difference to noise ratio (SDNR(u)) values as a conventional C-arm angiography system. These simulations were performed for a variety of focal spot sizes for the Angio-MR system. Results show that a standard current x-ray tube has sufficient power to reach similar SDNR(u) values as obtained in a conventional system if the focal spot size between both systems is comparable.

Local organization of spatial frequency tuning dynamics in the cat visual areas 17 and 18.

Spatial frequency (SF) is a prominent feature to which most neurons in cat areas 17 and 18 (area 17/18) exhibit tuning selectivity. Previous studies have shown that neurons with similar SF tunings are locally clustered into SF preference domains. However, the functional organization of SF tuning remains not fully understood. Neurons in these areas show a variety of SF tuning dynamics; however, it is unknown how neurons with diverse dynamics are locally organized to form the population dynamics of the domains. The laminar organization of SF dynamics is also unknown, knowledge of which may be useful for determining how SF tuning dynamics of cat area 17/18 neurons arise in cortical circuits. To address these issues, we recorded the activities of multiple neurons in the cat area 17/18 using microelectrode arrays and characterized the time courses of the SF tunings of these neurons by a subspace reverse correlation. A wide range of SF dynamics was already present in the input layer, suggesting that intracortical mechanisms contribute to generating SF dynamics inside this layer but do not further shape it outside this layer. Local neuronal pools with similar SF tunings contained diverse SF dynamics. The average preferred SF of a pool similarly increased with response time. Moreover, the range of single-neuron preferred SFs in a pool tended to increase with time. Our results suggest that, in the presence of organized tuning diversity within an SF domain, the cortical SF organization remains stable during response time in cat area 17/18.NEW & NOTEWORTHY In cat area 17/18, we found that a local pool of neurons with similar spatial frequency (SF) tunings shows diverse but organized dynamics. Our results suggest that, in the presence of organized tuning diversity within an SF domain, the cortical SF organization remains stable over response time in these areas. Laminar analysis suggests that intracortical mechanisms contribute to generating SF dynamics inside the input layer but do not further shape it outside this layer.

Short-latency ocular-following responses: Weighted nonlinear summation predicts the outcome of a competition between two sine wave gratings moving in opposite directions.

We recorded horizontal ocular-following responses to pairs of superimposed vertical sine wave gratings moving in opposite directions in human subjects. This configuration elicits a nonlinear interaction: when the relative contrast of the gratings is changed, the response transitions abruptly between the responses elicited by either grating alone. We explore this interaction in pairs of gratings that differ in spatial and temporal frequency and show that all cases can be described as a weighted sum of the responses to each grating presented alone, where the weights are a nonlinear function of stimulus contrast: a nonlinear weighed summation model. The weights depended on the spatial and temporal frequency of the component grating. In many cases the dominant component was not the one that produced the strongest response when presented alone, implying that the neuronal circuits assigning weights precede the stages at which motor responses to visual motion are generated. When the stimulus area was reduced, the relationship between spatial frequency and weight shifted to higher frequencies. This finding may reflect a contribution from surround suppression. The nonlinear interaction is strongest when the two components have similar spatial frequencies, suggesting that the nonlinearity may reflect interactions within single spatial frequency channels. This framework can be extended to stimuli composed of more than two components: our model was able to predict the responses to stimuli composed of three gratings. That this relatively simple model successfully captures the ocular-following responses over a wide range of spatial/temporal frequency and contrast parameters suggests that these interactions reflect a simple mechanism.

Relationship of Depth Adaptation Between Disparity-Specified Plaids and Their Components.

In the luminance domain, studies show that perceived contrasts of plaids are a nonlinear summation of their components. In the disparity domain, perceived depth has been studied by using a depth adaptation paradigm with simple surfaces; however, the relationship between depth adaptation between plaids and their components has not been investigated. To clarify this, combinations of disparity-defined horizontal corrugation (marked as horizontal) and disparity-defined plaids as adaptor-probe pairs were used. Three experiments were performed: The first two compared the aftereffects between horizontal-horizontal and plaid-horizontal pairs (Comparison 1) and between horizontal-plaid and plaid-plaid pairs (Comparison 2). Experiments 1 and 2 controlled the plaids to have the same and doubled peak-to-trough amplitudes as the horizontal corrugation, respectively. In Comparison 1, the horizontal or horizontally oriented component of the plaids was adapted. In Comparison 2, the plaid adaptor or horizontally oriented component of the plaid test stimuli was adapted. Thus, depth adaptation may be linked to cyclopean-oriented depth-from-disparity bandpass filters. The depth adaptation degree was determined by the adaptation of amplitudes of the similar oriented channels between the adaptation and test stimuli. Experiment 3 compared the aftereffects between noise-horizontal and horizontal-horizontal pairs. Since the noise adaptor contained multispatial frequency channels, only the channels with similar spatial frequencies as the horizontal corrugation were adapted, thus causing smaller depth aftereffects.

Experience-Dependent Development of Feature-Selective Synchronization in the Primary Visual Cortex

Early visual experience is essential for the maturation of visual functions in which the primary visual cortex plays crucial roles. The extraction of visual features based on response selectivity of individual neurons, a fundamental process in the cortex, is basically established by eye opening in rodents, suggesting that visual experience is required for the development of neural functions other than feature extraction. Here, we show that synchronized firing, which is important for visual information processing, occurs selectively in adjacent neurons sharing similar orientation or spatial frequency preferences in layers 2-4 (upper layer) of rat visual cortex. This feature-selective spike synchrony was rudimentary when the eyes opened and became prominent during the first few weeks after eye opening only in the presence of pattern vision. In contrast, synchronization in layers 5-6 (lower layer) was almost independent of orientation similarity and more weakly dependent on spatial frequency similarity compared with upper layer synchrony. Lower layer synchronization was strengthened during development after eye opening independently of visual experience as a whole. However, the feature selectivity of synchronization was regulated by visual inputs, whereas the inputs without contours were sufficient for this regulation. Therefore, we speculate that feature-selective synchronization in the upper layer may convey detailed information on visual objects to the higher-order cortex, whereas weakly feature-selective synchronization in the lower layer may covey rather rough visual information to the subcortical areas or higher-order cortex. A major role of visual experience may be to establish the specific neural circuits underlying highly feature-selective synchronization.SIGNIFICANCE STATEMENT The neuronal mechanisms underlying experience-dependent improvement of visual functions still remain unresolved. In this study, we investigated whether early visual experience contributes to the development of synchronized neural firing in the primary visual cortex, which plays important roles in visual information processing. We found that synchronized firing depends more remarkably on the similarity of preferred visual stimuli in the upper than lower layer neurons. Pattern vision during development was required for the establishment of spike synchrony in the upper but not the lower layer. These findings provide a new view regarding the role of sensory experience in the functional development of the cortex and the differences in the modes of information processing in the upper and lower cortical layers.

Insight into aspheric misfit with hard tools: mapping the island of low mid-spatial frequencies

This paper addresses computer numerical control (CNC) polishing of aspheric or freeform optics. Prior CNC grinding of the asphere tends to produce mid-spatial frequencies (MSFs) at some level. Precessions polishing can rectify these, but the very ability of the bonnet tooling to adapt to the local asphere enables it to do so, at least in part, to similar spatial frequencies in the MSFs. To accelerate smoothing, hard tools can, in principle, be used, but aspheric misfit is often assumed to preclude this. In this paper, we explore new insight into the role of abrasive particle size in accommodating misfit. First, we report on a glass-bending rig to produce a continuous range of complex surfaces, while withstanding process forces. Then, we describe how this was used to evaluate the triangle of misfit, abrasive size, and MSFs produced for hard rotating tools. This has revealed a regime in which such tools can be used without introducing significant new MSFs, as evidenced by manufacture of prototype off-axis aspheric segments for the European Extremely Large Telescope project.

The Two-Dimensional Gabor Function Adapted to Natural Image Statistics: A Model of Simple-Cell Receptive Fields and Sparse Structure in Images.

The two-dimensional Gabor function is adapted to natural image statistics, leading to a tractable probabilistic generative model that can be used to model simple cell receptive field profiles, or generate basis functions for sparse coding applications. Learning is found to be most pronounced in three Gabor function parameters representing the size and spatial frequency of the two-dimensional Gabor function and characterized by a nonuniform probability distribution with heavy tails. All three parameters are found to be strongly correlated, resulting in a basis of multiscale Gabor functions with similar aspect ratios and size-dependent spatial frequencies. A key finding is that the distribution of receptive-field sizes is scale invariant over a wide range of values, so there is no characteristic receptive field size selected by natural image statistics. The Gabor function aspect ratio is found to be approximately conserved by the learning rules and is therefore not well determined by natural image statistics. This allows for three distinct solutions: a basis of Gabor functions with sharp orientation resolution at the expense of spatial-frequency resolution, a basis of Gabor functions with sharp spatial-frequency resolution at the expense of orientation resolution, or a basis with unit aspect ratio. Arbitrary mixtures of all three cases are also possible. Two parameters controlling the shape of the marginal distributions in a probabilistic generative model fully account for all three solutions. The best-performing probabilistic generative model for sparse coding applications is found to be a gaussian copula with Pareto marginal probability density functions.

Combining 1-D components to extract pattern information: It is about more than component similarity.

At least under some conditions, plaid stimuli are processed by combining information first extracted in orientation and scale-selective channels. The rules that govern this combination across channels are only partially understood. Although the available data suggests that only components having similar spatial frequency and contrast are combined, the extent to which this holds has not been firmly established. To address this question, we measured, in human subjects, the short-latency reflexive vergence eye movements induced by stereo plaids in which spatial frequency and contrast of the components are independently varied. We found that, although similarity in component spatial frequency and contrast matter, they interact in a nonseparable way. One way in which this relationship might arise is if the internal estimate of contrast is not a faithful representation of stimulus contrast but is instead spatial frequency–dependent (with higher spatial frequencies being boosted). We propose that such weighting might have been put in place by a mechanism that, in an effort of achieve contrast constancy and/or coding efficiency, regulates the gain of detectors in early visual cortex to equalize their long-term average response to natural images.

Rotation invariant texture classification using extended wavelet channel combining and LL channel filter bank

In this paper, a simple and effective method for rotation invariant texture classification is proposed. The suggested feature vector is small as compared to other state-of-the-art methods. The proposed method is the development of wavelet combining channels approaches that have been introduced for extraction of rotation invariant features. These methods have two big problems with textures with strong directionality and textures with similar spatial frequency and different directionality. Therefore, these methods have not attracted much attention for texture classification. Our method introduces several extensions on the wavelet combining channels idea: first, it changes the combining scheme, second, it applies a rescaling scheme, third, it changes the method of calculating the energy features and forth, it uses low horizontal and low vertical frequency (LL) channel features in each level of decomposition. We also apply a filter bank on the first level of decomposition for extracting the edge features and different texture features. To illustrate the impact of these developments, we applied the proposed method on four different databases, from the Outex, Brodatz, and VisTex, consisting of 24, 25, 54, and 87 different classes of textures, respectively. The results show that the proposed method has comparable and even better performance than the other state-of-the-art methods. The results show that by increasing the number of classes from 25 to 87, maximum classification accuracy has dropped by only 2.4%. For more comparisons, the method is applied on two different illumination textures and it obtains an average of almost 10.9% increase in accuracy than the traditional wavelet channel combining method.

The effect of viewing distance on responses to the pattern glare test

BackgroundWe investigated whether symptoms of pattern glare were affected by viewing distance, as distinct from spatial frequency, because of an association between symptoms and anomalies of accommodation and vergence.MethodsOne hundred young adults viewed gratings with spatial frequencies of 0.3, 2.3 and 9.4-cycles per degree (cpd) at four test distances (0.4, 0.8, 1.6 and 3.2 metres). Participants were asked to grade the presence of 15 symptoms of visual perceptual distortions and discomfort, on a scale from zero (no symptoms) to 10 (maximum perceptual and somatic symptoms).ResultsThe viewing distance did not affect the nature and strength of symptoms, when viewing gratings with similar spatial frequencies. The symptoms increased with spatial frequency (p < 0.008 for all comparisons).ConclusionsThe symptoms from the Pattern Glare Test do not appear to be modulated by the changes in accommodation and vergence associated with viewing distance, at least in an unselected sample of students. The highest spatial frequency of the current Pattern Glare Test was 9.4-cpd at 0.4 metre and this is insufficiently high to measure the reduction in symptoms at high spatial frequencies. If assessing relative aversion to gratings of different spatial frequencies, it may be useful to increase the testing distance to 0.6 metre so as to increase the spatial frequency of the third grating to 14.2-cpd.

Contrast adaptation is spatial frequency specific in mouse primary visual cortex

Contrast adaptation, generated by prolonged viewing of a high contrast spatial pattern, is known to reduce perceptual sensitivity to subsequently presented stimuli of similar spatial frequency (SF). Neural correlates of this pattern-specific contrast adaptation have been described in several classic studies in cat primary visual cortex (V1). These results have also recently been extended to mice, which is a genetically manipulable animal model. Here we attempt to parse the potential mechanisms contributing to this phenomenon by determining whether the SF specificity of contrast adaptation observed in mouse V1 neurons depends on the spike rate elicited by the adapting gratings. We found that adapting stimuli that drove a neuron more strongly generally produced more adaptation, implicating an intrinsic or fatigue-like process. Importantly, we also observed that slightly stronger contrast adaptation was produced when the adapting SF matched the test SF even when matched and nonmatched adapting gratings elicited similar spike rates indicating extrinsic or network processes contribute as well.