Spatial Frequency Channels Research Articles

How fast can a baseball spin before an observer can’t tell the direction of rotation?

In baseball, a pitcher throws a 3” ball toward a batter positioned 60.5 feet away. Shapiro et al (2009, 2011) hypothesized that a batter’s perception of a pitch could be influenced by changes in the ball’s retinal location (i.e., whether the batter views the ball centrally or peripherally). One question concerns whether a curveball spins too fast for a batter to discern the direction of spin. To address this question, we constructed a device in which a microcontroller spins a motor at rates up to 3000 rpm. A baseball rested on a driveshaft extending from the motor; an LCD shutter controlled the presentation duration (set at .6 sec); steady LED lights illuminated the ball. The observer viewed the ball from a distance of 140 cm (ball subtended 3.2 deg) and pressed one of two buttons to indicate the direction of perceived spin. Experiment 1: The ball was presented at rotation rates between 500 and 2500 rpm (40 trials in random order; half the trial were clockwise, half counter clockwise). Experiment 2: Similar set up, but on different sets of trials the observers viewed the ball either centrally or 10 deg in the periphery. Experiment 3: On different sets of trials the ball was positioned on the motor in a “2-seam” or a “4-seam” configuration. For all conditions, at rates up 1200 rpm, observers were typically correct 100% of the time, but near 2000 rpm, observers were typically at chance. Peripheral viewing and spin configuration had little effect on observer’s ability to identify spin direction. Since an MLB curveball spins between 1400 and 1800 rpm, these pitches approach the limit of our perceptual resolution. The minor effect of peripheral viewing and spin configuration suggest, surprisingly, that our ability to perceive spin direction may be mediated by low spatial frequency channels. Meeting abstract presented at VSS 2015

Integration of Multiple Spatial Frequency Channels in Disparity-Sensitive Neurons in the Primary Visual Cortex.

Our visual world is broadband, containing features with a wide range of object scales. On the other hand, single neurons in the primary visual cortex are narrow-band, being tuned narrowly for a specific scale. For robust visual perception, narrow-band information of single neurons must be integrated eventually at some stage. We have examined whether such an integration process begins in the primary visual cortex with respect to binocular processing. The results suggest that a subset of cells appear to combine binocular information across multiple scales. Furthermore, for the majority of these neurons, an optimal parameter of binocular tuning is matched across multiple scales, suggesting the presence of a highly specific neural integration mechanism.

The binocular neural mechanism: disparity coding schemes and population coding

Stereoacuity thresholds measured on disparity pedestals are generally found to increase exponentially as the pedestals move away from horopters. However, Farell, Li, and McKee recently found that for sinusoidal stimuli this threshold function had a dip—a pedestal effect. This paper examines the underlying neural mechanism. We suggest a general disparity coding scheme with two position parameters, which are necessary to account for the phenomenon that the response of the energy model depends on the absolute phase of the stimulus. This scheme was implemented to simulate the responses, calculated from an energy model, of the neurons of a full V1 cortical column. To explain the stereo pedestal effect, we propose a decoding mechanism, which is first processed along the phase dimension and then along the orientation dimension. The final step of the decoding mechanism, probability summation over the outputs of spatial frequency channels, yields the dip, producing a disparity increment threshold function similar to the psychophysical result.

Transcranial direct current stimulation can selectively affect different processing channels in human visual cortex.

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that can modulate cortical activity. Nonetheless, information regarding its functional specificity and the extent by which visual performance can be modulated is still lacking. Here, we used vision as model to address if it differentially affects different cell groups in the stimulated area. We applied tDCS to the occiput and performed a series of visual tests in a sham-controlled repeated-measures design. Achromatic contrast sensitivity was assessed psychophysically during tDCS, with tasks designed to target specific spatial frequency (SF) channels, inferred ON, OFF channels and inferred magnocellular and parvocellular pathways of the visual system. Sweep visual evoked potential (sVEP) for contrast sensitivity and Vernier acuity was recorded before and after tDCS. Anodal tDCS significantly increased thresholds for luminance decrements (OFF) only for the inferred magnocellular thresholds. Although tDCS had no significant effects on Vernier or contrast sVEP thresholds, it modulated suprathreshold amplitudes for both tasks. Cathodal tDCS increased sVEP amplitudes at a low SF, decreased it at a medium, and had no effect at a high SF. Cathodal tDCS increased sVEP phase lags for low and decreased it for high SF (maximum change corresponding to change in apparent latency >6 ms). Cathodal and anodal stimulation decreased amplitudes of sVEP Vernier responses. Exclusive tDCS effects on magnocellular thresholds agree with reports of pathway-specific tDCS effects. The dependence of tDCS effects on SF and contrast levels further suggests that tDCS differentially affects different cell groups in the visual cortex.

Perception of differences in naturalistic dynamic scenes, and a V1-based model.

We investigate whether a computational model of V1 can predict how observers rate perceptual differences between paired movie clips of natural scenes. Observers viewed 198 pairs of movies clips, rating how different the two clips appeared to them on a magnitude scale. Sixty-six of the movie pairs were naturalistic and those remaining were low-pass or high-pass spatially filtered versions of those originals. We examined three ways of comparing a movie pair. The Spatial Model compared corresponding frames between each movie pairwise, combining those differences using Minkowski summation. The Temporal Model compared successive frames within each movie, summed those differences for each movie, and then compared the overall differences between the paired movies. The Ordered-Temporal Model combined elements from both models, and yielded the single strongest predictions of observers' ratings. We modeled naturalistic sustained and transient impulse functions and compared frames directly with no temporal filtering. Overall, modeling naturalistic temporal filtering improved the models' performance; in particular, the predictions of the ratings for low-pass spatially filtered movies were much improved by employing a transient impulse function. The correlations between model predictions and observers' ratings rose from 0.507 without temporal filtering to 0.759 (p = 0.01%) when realistic impulses were included. The sustained impulse function and the Spatial Model carried more weight in ratings for normal and high-pass movies, whereas the transient impulse function with the Ordered-Temporal Model was most important for spatially low-pass movies. This is consistent with models in which high spatial frequency channels with sustained responses primarily code for spatial details in movies, while low spatial frequency channels with transient responses code for dynamic events.

Metamorphopsia and letter recognition.

Acuity is the most commonly used measure of visual function, and reductions in acuity are associated with most eye diseases. Metamorphopsia--a perceived distortion of visual space--is another common symptom of visual impairment and is currently assessed qualitatively using Amsler (1953) charts. In order to quantify the impact of metamorphopsia on acuity, we measured the effect of physical spatial distortion on letter recognition. Following earlier work showing that letter recognition is tuned to specific spatial frequency (SF) channels, we hypothesized that the effect of distortion might depend on the spatial scale of visual distortion just as it depends on the spatial scale of masking noise. Six normally sighted observers completed a 26 alternate forced choice (AFC) Sloan letter identification task at five different viewing distances, and the letters underwent different levels of spatial distortion. Distortion was controlled using spatially band-pass filtered noise that spatially remapped pixel locations. Noise was varied over five spatial frequencies and five magnitudes. Performance was modeled with logistic regression and worsened linearly with increasing distortion magnitude and decreasing letter size. We found that retinal SF affects distortion at midrange frequencies and can be explained with the tuning of a basic contrast sensitivity function, while object-centered distortion SF follows a similar pattern of letter object recognition sensitivity and is tuned to approximately three cycles per letter (CPL). The interaction between letter size and distortion makes acuity an unreliable outcome for metamorphopsia assessment.

Development and aging of visual hemifield asymmetries in contrast sensitivity

The relation of development and aging with models of visual anisotropies and their influence on low-level visual processing remain to be established. Our main goal was to explore visual performance asymmetries in development and normal aging using low-level contrast sensitivity behavioral tasks [probing two distinct spatiotemporal frequency channels, (a) intermediate spatial and null temporal frequency (3.5 cycles per degree (cpd) and 0 Hz); and (b) low spatial and high temporal frequency (0.25 cpd undergoing 25 Hz counterphase flicker)]. Different patterns of functional asymmetries were investigated within four (two neurodevelopmental and two adult) age groups (N = 258 participants; 8-65 years). We found a left visual hemifield/right hemisphere advantage for only the intermediate spatial frequency channel that was present early in life and remained stable throughout adulthood. In contrast, inferior/superior visual hemifield asymmetries, with a direct ecological meaning, were found for both spatiotemporal frequency channels. This inferior visual hemifield advantage emerged early in life and persisted throughout aging. These findings show that both right hemispheric and dorsal retinotopic patterns of dominance in low-level vision emerge early in childhood, maintaining during aging.

Evidence for adjustable bandwidth orientation channels.

The standard model of early vision claims that orientation and spatial frequency are encoded with multiple, quasi-independent channels that have fixed spatial frequency and orientation bandwidths. The standard model was developed using detection and discrimination data collected from experiments that used deterministic patterns such as Gabor patches and gratings used as stimuli. However, detection data from experiments using noise as a stimulus suggests that the visual system may use adjustable-bandwidth, rather than fixed-bandwidth, channels. In our previous work, we used classification images as a key piece of evidence against the hypothesis that pattern detection is based on the responses of channels with an adjustable spatial frequency bandwidth. Here we tested the hypothesis that channels with adjustable orientation bandwidths are used to detect two-dimensional, filtered noise targets that varied in orientation bandwidth and were presented in white noise. Consistent with our previous work that examined spatial frequency bandwidth, we found that detection thresholds were consistent with the hypothesis that observers sum information across a broad range of orientations nearly optimally: absolute efficiency for stimulus detection was 20–30% and approximately constant across a wide range of orientation bandwidths. Unlike what we found with spatial frequency bandwidth, the results of our classification image experiment were consistent with the hypothesis that the orientation bandwidth of internal filters were adjustable. Thus, for orientation summation, both detection thresholds and classification images support the adjustable channels hypothesis. Classification images also revealed hallmarks of inhibition or suppression from uninformative spatial frequencies and/or orientations. This work highlights the limitations of the standard model of summation for orientation. The standard model of orientation summation and tuning was chiefly developed with narrow-band stimuli that were not presented in noise, stimuli that are arguably less naturalistic than the variable bandwidth stimuli presented in noise used in our experiments. Finally, the disagreement between the results from our experiments on spatial frequency summation with the data presented in this paper suggests that orientation may be encoded more flexibly than spatial frequency channels.

An Impulse Response Model and $Q$ -Factor Estimation for Vehicle Cavities

To facilitate efficient communications (i.e., minimum power consumption and maximum information throughput) in vehicle cavities, it is necessary to fully understand the underlying physics of the propagation process. This can be characterized as a statistical model of the channel impulse response, which we derive from a general starting point. The impulse response model is useful in its own right for ultrawideband pulse radio communications, channel simulations, and time-of-arrival positioning systems, and it also allows us to verify the generally accepted property that the energy retained in the cavity exponentially decays with time after an impulse input. This property can be characterized as a cavity Q-factor, and we investigate methods of Q-factor estimation in vehicle cavities, using only a limited amount of data, such as would typically be available to a deployed in-vehicle wireless network. We find that the most reliable approach utilizes an inverse-discrete-Fourier-transform-based method, which finds the maximum-likelihood instantaneous Q-factor, given measured data across various spatial links and frequency channels.

Aftereffect of perceived regularity

Regularity is a ubiquitous feature of the visual world. We demonstrate that regularity is an adaptable visual dimension: The perceived regularity of a pattern is reduced following adaptation to a pattern with a similar or greater degree of regularity. Stimuli consisted of 7×7 element arrays arranged on square grids presented in a circular aperture. The position of each element was randomly jittered from its baseline position by an amount that determined its degree of irregularity. The elements of the pattern consisted of dark Gaussian blobs (GBs), difference of Gaussians (DOGs), or random binary patterns (RBPs). Observers adapted for 60 s to either a single pattern or a pair of patterns with particular regularities, and the perceived regularities of subsequently presented test patterns were measured using a conventional staircase matching procedure. We found that the regularity aftereffect (RAE) was unidirectional: Adaptation only caused test patterns to appear less regular. We also found that RAEs transferred from GB adaptors to both DOG and RBP test patterns and from DOG and RBP adaptors to GB patterns. We suggest that regularity is coded by the peakedness in the distribution of spatial-frequency channel responses across scale, and that the RAE is a result of a flattening of this distribution by adaptation. Thus, the RAE may be a consequence of contrast normalization, and an example of norm-based coding where irregularity is the norm.

Long-term results of half-dose photodynamic therapy for chronic central serous chorioretinopathy with contrast sensitivity changes

To evaluate the efficacy and safety of photodynamic therapy (PDT) with half-dose verteporfin in patients with chronic central serous chorioretinopathy (CCSC) and retinal functional changes, by functional acuity contrast test (FACT). In this study, 27 eyes of 24 patients with CCSC were treated with PDT with half-dose verteporfin. Best-corrected visual acuity (BCVA), central foveal thickness (CFT) and resolution of subretinal fluid on optical coherence tomography (OCT), and leakage on fluorescein angiography (FA) and indocyanine green angiography (ICGA) were assessed. Contrast sensitivity test was performed at baseline and at 12th month for investigating retinal functional changes. The mean follow-up period was 25.33 ± 11.08 months. The mean age was 43.7 ± 8.6 years. Seventeen patients were male (70.8%) and seven patients were female (29.2%). Post PDT at 1st, 3rd, 6th, 12th month and at last follow-up, BCVA were significantly improved compared with the baseline BCVA (P<0.001), and CFT post PDT were significantly thinner than the baseline measurement (P<0.001). There was significant difference between pre- and post-PDT 12th month contrast sensitivities at all five different spatial frequency channels (P<0.01). Half-dose PDT is an effective and safe method in the treatment of CCSC with anatomical and functional success. The measurement of contrast sensitivity by FACT can be useful for evaluating the functional effectiveness of half-dose PDT for CCSC.

Comparison of photoacoustic image reconstruction algorithms using the channelized Hotelling observer

We demonstrate the use of task-based image-quality metrics to compare various photoacoustic image-reconstruction algorithms, including a method based on the pseudoinverse of the system matrix, simple backprojection, filtered backprojection, and a method based on the Fourier transform. We use a three-dimensional forward model with a linear transducer array to simulate a photoacoustic imaging system. The reconstructed images correspond with two-dimensional slices of the object and are 128×128 pixels. In order to compare the algorithms, we use channelized Hotelling observers that predict the detection ability of human observers. We use two sets of channels: constant Q and difference of Gaussian spatial frequency channels. We look at three tasks, identification of a point source in a uniform background, identification of a 0.5-mm cube in a uniform background, and identification of a point source in a lumpy background. For the lumpy background task, which is the most realistic of the tasks, the method based on the pseudoinverse performs best according to both sets of channels.

Aging of Low and High Level Vision: From Chromatic and Achromatic Contrast Sensitivity to Local and 3D Object Motion Perception

The influence of normal aging in early, intermediate and high-level visual processing is still poorly understood. We have addressed this important issue in a large cohort of 653 subjects divided into five distinct age groups, [20;30[, [30;40[, [40;50[, [50;60[and [60;[. We applied a broad range of psychophysical tests, testing distinct levels of the visual hierarchy, from local processing to global integration, using simple gratings (spatial contrast sensitivity -CS- using high temporal/low spatial frequency or intermediate spatial frequency static gratings), color CS using Landolt patches, moving dot stimuli (Local Speed Discrimination) and dot patterns defining 3D objects (3D Structure from Motion, 3D SFM). Aging data were fitted with linear or quadratic regression models, using the adjusted coefficient of determination (R2 a) to quantify the effect of aging. A significant effect of age was found on all visual channels tested, except for the red-green chromatic channel. The high temporal low spatial frequency contrast sensitivity channel showed a mean sensitivity loss of 0.75 dB per decade (R2 a = 0.17, p<0.001), while the lower intermediate spatial frequency channel showed a more pronounced decrease, around 2.35 dB (R2 a = 0.55, p<0.001). Concerning low-level motion perception, speed discrimination decreased 2.71°/s (R2 a = 0.18, p<0.001) and 3.15°/s (R2 a = 0.13, p<0.001) only for short presentations for horizontal and oblique meridians, respectively. The 3D SFM task, requiring high-level integration across dorsal and ventral streams, showed the strongest (quadratic) decrease of motion coherence perception with age, especially when the task was temporally constrained (R2 a = 0.54, p<0.001). These findings show that visual channels are influenced by aging into different extent, with time presenting a critical role, and high-level dorso-ventral dominance of deterioration, which accelerates with aging, in contrast to the other channels that show a linear pattern of deterioration.

P2-32: Influence of Spatial Frequency Information on Face Gender with Different Expressions

Visual image contains broadband information and is processed by different neural channels that are tuned to different spatial frequencies. Here we investigated whether or not our ability in gender identification on emotional faces was influenced by this early visual processing. Four types of emotional (happy, anger, sad, and fear) faces were used, and all of the stimuli were processed by spatial frequency analysis. Spatial frequency content in the original faces was filtered by using a high-pass filter (cut-off frequency was 24 cycles/image) for the HSF stimuli, and a low-pass filter (cut-off frequency was 6 cycles/image) for the LSF stimuli. Participants needed to identify the gender of the faces. The results showed that the participants responded faster and had higher accuracy to LSF faces than to HSF ones. They also responded faster and had higher accuracy to male faces than to female faces. Further analysis revealed that the identification of an angry man and a happy woman had advantage among combinations of genders and emotions in LSF conditions. However, this advantage was not manifested in HSF conditions. We concluded that the identification of gender with different emotions may rely on the processing of low spatial frequency channels.

Modeling of the Passband of the Spatial Frequency Channels of the Visual System

A model of the passband of the spatial frequency channels of the visual system is described. Results of experimental testing of the model are presented.

Physiological evidence for impairment in autosomal dominant optic atrophy at the pre-ganglion level

Functional studies in patients with autosomal dominant optic atrophy (ADOA) are usually confined to analysis of physiological and clinical impact at the ganglion cell (GG) and post GC levels. Here we aimed to investigate the impact of the disease at a pre-GC level and its correlation with GC/post-GC related measures. Visual function was assessed in a population of 22 subjects (44 eyes) from 13 families with ADOA submitted to OPA1 mutation analysis. Quantitative psychophysical methods were used to assess konio and parvocellular chromatic pathways (Cambridge Colour Test) and distinct achromatic spatial frequency channels (Metropsis Contrast Sensitivity Test). Preganglionic and GC measures were assessed with the Multifocal Electroretinogram (mfERG) and Pattern Electroretinogram (PERG) respectively. Global Pattern and Multifocal VEP (visual evoked potentials) were used to assess retinocortical processing, in order to characterize impaired processing at the post GC level. Perimetric sensitivity, retinal and ganglion cell nerve fibre layer (RNFL) thickness measurements were also obtained. Chromatic thresholds were significantly increased for protan, deutan and tritan axes (p < < 0.001 for all comparisons) and achromatic contrast sensitivity (CS) was reduced for all studied six spatial frequency channels (p < < 0.001). We observed significant decreases in peripapillary (p ≤ 0.0008), macular (ring2: p = 0.02; ring 3: p < 0.0001) RNFL, as well as in overall retinal thickness (p < 0.0001 in all regions, except the central one). Interestingly, we found significant decreases in pre-ganglionic multifocal ERG response amplitudes (P1-wave: p ≤ 0.005) that were correlated with retinal thickness (ring 2: r = 0.512; p = 0.026/ring 3: r = 0.583; p = 0.011) and visual acuity (r = 0.458; p = 0.03, central ring 1). Reductions in GC and optic nerve responses amplitude (PERG: p < 0.0001, P50 and N95 components; Pattern VEP: p < 0.0001, P100) were accompanied by abnormalities of the MfVEP, primarily in central locations (ring 1: p = 0.0007; ring 2: p = 0.012). In the ADOA model of ganglion cell damage, parvo-, konio- and magnocellular pathways are concomitantly affected. Structural changes and physiological impairment also occurs at a preganglionic level, suggesting a retrograde damage mechanism with a significant clinical impact on visual function, as shown by correlation analysis. Cortical impairment is only moderately explained by the retinal phenotype, suggesting additional damage mechanisms at the cortical level.

A Rapid Method for Determining the Passband of Spatial Frequency Channels of the Visual System

A rapid method for determining passband in Hz of spatial frequency channels of the visual system is described. The method consists of simultaneous presentation of flickering frequencies of eight pairs of light sources. It was shown in tests in 10 patients that the use of eight pairs of light sources was 37-68 % more effective than using two pairs of light sources (single experimental series).

Attention-free integration of spatial frequency-based information in natural scenes

The integration of visual image information provided by low and high spatial frequency channels is critical to rapid perception of natural scenes. However, little is known about the role of attention in integrating this information. In two experiments, using attention-demanding tasks, we examined the advantage of integration, i.e. the superior categorization accuracies for images, using a wide range of different spatial frequencies. In Experiment 1, a spatially central to-be-identified letter and a peripheral filtered image of a natural scene appeared simultaneously. In Experiment 2, the letter and the image were presented sequentially at the same spatial location. In both experiments results consistently showed an advantage of integration in categorization behavior that was not influenced by attention-demanding tasks. This finding suggests that the integration of frequency-based information in natural scenes is attention-free.

Spatial frequency bandwidth of surround suppression tuning curves.

The contrast detection threshold of a grating located in the periphery is increased if a surrounding grating of the same frequency and orientation is present. This inhibition between center and surround has been termed surround suppression. In this work we measured the spatial frequency bandwidth of surround suppression in the periphery for different spatial frequencies (0.5, 1.1, 3, and 5 cycles/deg) of a sinusoidal grating (target) surrounded by a grating with different spatial frequencies (surround). Using a Bayesian adaptive staircase, we measured contrast detection thresholds in an 8AFC detection task in which the target (grating with a 2.3-deg Butterworth window) could appear in one of eight possible positions at 4° eccentricity. The target was surrounded by a grating (with a 18° Butterworth window) with the same or an orthogonal orientation. In each session we fixed the spatial frequency of the target and changed the spatial frequency and the orientation of the surround. When the surround was orthogonal to the target, the thresholds were similar to those obtained without surround, independent of the surrounding spatial frequency. However, when the target and surround had the same orientation and spatial frequency, the contrast threshold was increased by a factor ranging from 3 to 6 across subjects. This suppression reduced rapidly as the spatial frequency of the surround moved away from that of the target. The bandwidth of the suppressive effect depended on spatial frequency, declining from 2.9 octaves at 0.5 c/deg to 1 octave for frequencies above 3 c/deg. This is consistent with the bandwidth of individual simple cells in visual cortex and of spatial frequency channels measured psychophysically, both of which decline with increasing spatial frequency. This suggests that surround suppression may be due to relatively precise inhibition by cells with the same tuning as the target.

Variance analysis of contrast sensitivity for amblyopia and normal eye based on spatial frequency channels

Background Contrast sensitivity (CS) of amblyopia has been extensively studied,but its relationship with spatial frequency channels needs further research. Objective The purpose of this study was to investigate the reasons of the CS deficits in amblyopia through comparing the differences in CS between amblyopic and normal eyes from the point of view of spatial frequency channels. Methods The CS values of 166 normal children eyes and 143 amblyopic children abnormal eyes were measured by adopting OPTEC 6500.Then,spatial frequency channels' tuning curves were derived via principal component analysis and non-orthogonal rotation.Also,numbers and bandwidths of channels were calculated using the method of full width at half maximum ( FWHM ).All of these were used to analyze the variations of CS between amblyopia and normal eyes by comparing the numbers and the bandwidths of the channels.The reliability of spatial frequency channel was verified by a cross-validation study of 43 amblyopic children. Result At spatial frequency of 1.5,3.0,6.0,12.0,18.0 cpd,the mean of CS of amblyopia were 36.35±21.40,50.33 ± 33.46,46.88 ± 41.72,16.24 ± 17.26,4.67 ± 5.79,and the mean of CS of normal eyes were 49.49±24.69,87.23±40.87,93.18±51.99.36.63±24.72,15.70±13.87,with the rank-sum test results were H =27.83,66.61,68.34,78.23,89.88,P＜0.05.There existed three spatial frequency channels in both amblyopia and normal eyes.At the peaks of 3.0,6.0 and 12.0 cpd,the bandwidths of normal eyes were 1.03,1.02 and 0.99 octaves,and the bandwidths of amblyopia were 1.04,1.01 and 0.73 octaves. Conclusions The reduction in bandwidths of the corresponding spatial frequency channels may cause the CS deficits in amblyopia. Key words: Contrast sensitivity; Spatial frequency channel; Variance analysis; Amblyopia