Chromatic Gratings Research Articles

Object color knowledge representation occurs in the macaque brain despite the absence of a developed language system.

Animals guide their behaviors through internal representations of the world in the brain. We aimed to understand how the macaque brain stores such general world knowledge, focusing on object color knowledge. Three functional magnetic resonance imaging (fMRI) experiments were conducted in macaque monkeys: viewing chromatic and achromatic gratings, viewing grayscale images of their familiar fruits and vegetables (e.g., grayscale strawberry), and viewing true- and false-colored objects (e.g., red strawberry and green strawberry). We observed robust object knowledge representations in the color patches, especially the one located around TEO: the activity patterns could classify grayscale pictures of objects based on their memory color and response patterns in these regions could translate between chromatic grating viewing and grayscale object viewing (e.g., red grating-grayscale images of strawberry), such that classifiers trained by viewing chromatic gratings could successfully classify grayscale object images according to their memory colors. Our results showed direct positive evidence of object color memory in macaque monkeys. These results indicate the perceptually grounded knowledge representation as a conservative memory mechanism and open a new avenue to study this particular (semantic) memory representation with macaque models.

Color-binding errors induced by modulating effects of the preceding stimulus on onset rivalry.

Onset rivalry can be modulated by a preceding stimulus with features similar to rivalrous test stimuli. In this study, we used this modulating effect to investigate the integration of color and orientation during onset rivalry using equiluminant chromatic gratings. Specifically, we explored whether this modulating effect leads to a decoupling of color and orientation in chromatic gratings, resulting in a percept distinct from either of the rivalrous gratings. The results demonstrated that color-binding errors can be observed in a form where rivalrous green-gray clockwise and red-gray counterclockwise gratings yield the percept of a bichromatic, red-green grating with either clockwise or counterclockwise orientation. These errors were observed under a brief test duration (30 ms), with both monocular and binocular presentations of the preceding stimulus. The specific color and orientation combination of the preceding stimulus was not critical for inducing color-binding errors, provided it was composed of the test color and orientation. We also found a notable covariant relationship between the perception of color-binding errors and exclusive dominance, where the perceived orientation in color-binding errors generally matched that in exclusive dominance. This finding suggests that the mechanisms underlying color-binding errors may be related to, or partially overlap with, those determining exclusive dominance. These errors can be explained by the decoupling of color and orientation in the representation of the suppressed grating, with the color binding to the dominant grating, resulting in an erroneously perceived bichromatic grating.

Orientation selectivity mapping in the visual cortex

The orientation map is one of the most well-studied functional maps of the visual cortex. However, results from the literature are of different qualities. Clear boundaries among different orientation domains and blurred uncertain distinctions were shown in different studies. These unclear imaging results will lead to an inaccuracy in depicting cortical structures, and the lack of consideration in experimental design will also lead to biased depictions of the cortical features. How we accurately define orientation domains will impact the entire field of research. In this study, we test how spatial frequency (SF), stimulus size, location, chromatic, and data processing methods affect the orientation functional maps (including a large area of dorsal V4, and parts of dorsal V1) acquired by intrinsic signal optical imaging. Our results indicate that, for large imaging fields, large grating stimuli with mixed SF components should be considered to acquire the orientation map. A diffusion model image enhancement based on the difference map could further improve the map quality. In addition, the similar outcomes of achromatic and chromatic gratings indicate two alternative types of afferents from LGN, pooling in V1 to generate cue-invariant orientation selectivity.

Riemannian color difference metric for spatial sinusoidal color variations

Several studies report on the sensitivity of human vision to static spatial sinusoidal achromatic and chromatic contrast variations. However, a Riemannian color difference metric, which includes the spatial and colorimetric properties of sinusoidal gratings, is lacking. Such a metric is important for various applications. Here we report on the development of a new Riemannian metric, for the prediction of detection ellipsoids in color space, for spatial sinusoidal gratings as a function of the grating’s size, spatial frequency, luminance and chromaticity. The metric is based on measurements and models of achromatic and isoluminous chromatic contrast sensitivity functions available in literature, and the Riemannian metric for split fields which we reported earlier. We find adequate agreement with various data sets of experimental achromatic and isoluminous chromatic contrast sensitivity functions and with experimentally determined threshold ellipses of isoluminous chromatic Gabor gratings.

Dispersion-engineered metasurfaces reaching broadband 90% relative diffraction efficiency

Dispersion results from the variation of index of refraction as well as electric field confinement in sub-wavelength structures. It usually results in efficiency decrease in metasurface components leading to troublesome scattering into unwanted directions. In this letter, by dispersion engineering, we report a set of eight nanostructures whose dispersion properties are nearly identical to each other while being capable of providing 0 to 2π full-phase coverage. Our nanostructure set enables broadband and polarization-insensitive metasurface components reaching 90% relative diffraction efficiency (normalized to the power of transmitted light) from 450 nm to 700 nm in wavelength. Relative diffraction efficiency is important at a system level – in addition to diffraction efficiency (normalized to the power of incident light) – as it considers only the transmitted optical power that can affect the signal to noise ratio. We first illustrate our design principle by a chromatic dispersion-engineered metasurface grating, then show that other metasurface components such as chromatic metalenses can also be implemented by the same set of nanostructures with significantly improved relative diffraction efficiency.

Parameters of Optokinetic Nystagmus Are Influenced by the Nature of a Visual Stimulus

Studies on contrast sensitivity (CS) testing using optokinetic nystagmus (OKN) proposed adjusting the stimulus presentation duration based on its contrast, to increase the time efficiency of such measurement. Furthermore, stimulus-specific limits of the least OKN gain might reduce false negatives in OKN detection procedures. Therefore, we aimed to test the effects of various stimulus characteristics on OKN and to propose the stimulus-specific limits for the OKN gain and stimulus presentation duration. We tested the effect of contrast (C), spatial frequency (SF), and color on selected parameters of robust OKN response, namely its onset and offset time, amplitude, and gain. The right eyes of fifteen emmetropes were tracked with an infrared eye tracker during monocular observations of sinusoidal gratings moving over the horizontal plane with a velocity of (21∘/s). The available contrast levels were C: 0.5%, 2.0%, 8.2%, 16.5%, 33.0%, and 55.5% presented in a random order for ten times in all measurements of SF: 0.12, 0.25, 0.5, and 1.00 cycles per degree and grating type: luminance, red-green, and blue-yellow. This study showed a significant effect of the stimulus characteristics on the OKN onset, offset and gain. The effect of SF was insignificant in OKN amplitude; however, it indicated significance for the C and grating type. Furthermore, the OKN gain and offset limits were proposed as functions of contrast for the luminance and chromatic gratings. This study concludes the characteristics of a visual stimulus have an effect on the OKN gain and onset and offset time, yet do not affect the eye-movement amplitude considerably. Moreover, the proposed limits are expected to improve the time efficiency and eye-movement detection in OKN-based contrast sensitivity measurements.

Direct observation of dynamic thermal lensing in nanoscale surfaces/interfaces of end-pumped Nd:YVO4 laser

This work describes non-invasive observations of dynamic thermal lensing in an end-pumped Nd:YVO4 laser. Measurements were obtained with a nanoscale optical ruler created using chromatic aberrations, pinholes, dispersive gratings, and position-sensitive detectors. This study reports on variations in thermally induced oscillations and on underdamping behavior in the regime of high pump power. Dynamic data related to the thermal behavior can be used to study the balance between driving forces (pumping and thermal stresses) and damping losses (cooling and output coupling). The non-invasive nature of the proposed scheme along with its sub-100-nm axial resolution, sub-millisecond time resolution, and simple configuration makes it widely applicable for studying heat-related phenomena within laser crystals. The preliminary results show that such non-invasive methods could be used to detect thermally induced nanoscale deformations in integrated circuits, high-power optoelectronic devices, and optical devices with high-power illumination.

Effect of cone spectral topography on chromatic detection sensitivity.

The spatial and spectral topography of the cone mosaic set the limits for detection and discrimination of chromatic sinewave gratings. Here, we sought to compare the spatial characteristics of mechanisms mediating hue perception against those mediating chromatic detection in individuals with known spectral topography and with optical aberrations removed with adaptive optics. Chromatic detection sensitivity in general exceeded previous measurements and decreased monotonically for increasingly skewed cone spectral compositions. The spatial grain of hue perception was significantly coarser than chromatic detection, consistent with separate neural mechanisms for color vision operating at different spatial scales.

Perceptual resolution of ambiguous neural representations for form and chromaticity.

A coherent percept of our visual world is important for functioning. Ambiguities, however, are implicit in visual neural representations and must be resolved for stable perception of objects and scenes. Grouping processes can link multiple neurally ambiguous fragments across the visual field. Experiments here determined how multiple visual features of each fragment contribute to perceptual resolution of ambiguity by grouping. Chromatic interocular-switch rivalry, a technique for presenting competing dichoptic images, was used to induce ambiguous neural representations for equiluminant chromatic discs and gratings. Two dichoptic stimuli were presented simultaneously to measure the amount of time they both appeared the same in at least one feature domain. The two stimuli were grouped when they appeared to share ambiguous features such as color, orientation, and spatial frequency more often than chance. Experiments here tested whether unshared and unambiguous features impeded grouping of the ambiguous components. Overall, the results show that grouping can be driven by neural ambiguity that is common for fragments across the visual field, even when the fragments also have other unshared, unambiguous features.

High acuity vision corrected for chromatic and monochromatic aberrations is associated with color discrimination without red-green or blue-yellow sensations

From the earliest measurements of contrast sensitivity for chromatic gratings, it has been reported that isoluminant red-green gratings appear achromatic at high spatial frequencies. To better understand this phenomenon, we used a forced-choice paradigm to measure contrast sensitivity functions for red-green and S-cone isolating sinusoidal chromatic gratings. For all red-green grating experiments, monochromatic aberrations were compensated via adaptive optics. Longitudinal and transverse chromatic aberration were compensated by a specialized Badal system and a Vernier alignment task, respectively. We measured responses to red-green gratings under two conditions. In the first condition, which is comparable to the measurement of conventional chromatic contrast sensitivity, we measured contrast thresholds for detecting the grating pattern from uniform-field distractor targets. In the second condition, for a series of spatial frequencies, subjects were required to pick out a red-green grating from isochromatic grating distractors. Subjects were able to detect red-green gratings with a much higher spatial frequency cut-off (>30 c/deg) than for S-cone isolating gratings. Subjects were able to discriminate colored from isochromatic gratings with a high spatial frequency cutoff of only 10–12 c/deg, which was more similar to that found for S-cone gratings. Thus, subjects were able to detect changing red-green wavelength content at high spatial frequencies without perceiving hue sensations. The results are consistent with the unconventional hypothesis that achromatic and hue sensations are separated at the level of two subpopulations of midget ganglion cells, one serving high acuity black-white vision and a second more sparse mosaic serving hue perception at a lower spatial resolution.

Visual evoked cortical potential elicited by pseudoisochromatic stimulus.

Visual evoked cortical potentials (VECPs) are useful for investigating the mechanisms and dysfunctions of color vision. Chromatic sinusoidal gratings are generally used to elicit VECPs, but they require long psychophysical measurements to match the perceptual luminance between their stripes. An alternative method is to use pseudoisochromatic stimuli, which makes use of luminance noise to mask luminance clues and force the target perception to be dependent on chromatic contrast. In this study, we compared VECPs generated by sinusoidal gratings and pseudoisochromatic gratings. Contrary to chromatic sinusoidal gratings, pseudoisochromatic stimuli do not require the use of previous methods to find the equiluminance of the stimulus. Normal trichromats were recruited to be tested with red-green chromatic sinusoidal gratings and pseudoisochromatic gratings presented by pattern onset-offset and pattern reversal modes in five spatial frequencies. In addition, we also tested four different chromatic contrast pairs in pattern onset-offset mode presentation in five trichromats and one colorblind subject (deuteranope). Pattern onset-offset VECPs elicited by sinusoidal gratings had a larger amplitude than those obtained with pseudoisochromatic stimuli, whereas pattern reversal VECPs elicited by pseudoisochromatic gratings had similar amplitudes compared to those elicited by sinusoidal gratings. We found no difference between the VECP amplitudes elicited by sinusoidal and pseudoisochromatic gratings containing different chromatic contrast. Color-blind subjects displayed absent or small responses to the stimuli. Pseudoisochromatic stimulus can be an alternative stimulus to generate VECPs dominated by the chromatic mechanism.

Spatio-chromatic sensitivity explained by post-receptoral contrast.

We measured and modeled visibility thresholds of spatial chromatic sine-wave gratings at isoluminance. In two experiments we manipulated the base color, direction of chromatic modulation, spatial frequency, the number of cycles in the grating, and grating orientation. In Experiment 1 (18 participants) we studied four chromatic modulation directions around three base colors, for spatial frequencies 0.15-5 cycles/deg. Results show that the location, size and orientation of fitted ellipses through the observer-averaged thresholds varied with spatial frequency and base color. As expected, visibility threshold decreased with decreasing spatial frequency, except for the lowest spatial frequency, for which the number of cycles was only three. In Experiment 2 (27 participants) we investigated the effect of the number of cycles at spatial frequencies down to 0.025 cycles/deg. This showed that the threshold elevation at 0.15 cycles/deg in Experiment 1 was at least partly explained by the small number of cycles. We developed two types of chromatic detection models and fitted these to the threshold data. Both models incorporate probability summation across spatially weighted chromatic contrast signals, but differ in the stage at which the contrast signal is calculated. In one, chromatic contrast is determined at the cone receptor level, the dominant procedure in literature. In the other model, it is determined at a postreceptoral level, that is, after cone signals have been transformed into chromatic-opponent channels. We applied Akaike's Information Criterion to compare the performance of the models and calculated their relative probabilities and evidence ratios. We found evidence in favor of the second model and conclude that postreceptoral contrast is the most accurate determinant for chromatic contrast sensitivity.

The Importance of Spatial Visual Scene Parameters in Predicting Optimal Cone Sensitivities in Routinely Trichromatic Frugivorous Old-World Primates.

Computational models that predict the spectral sensitivities of primate cone photoreceptors have focussed only on the spectral, not spatial, dimensions. On the ecologically valid task of foraging for fruit, such models predict the M-cone (“green”) peak spectral sensitivity 10–20 nm further from the L-cone (“red”) sensitivity peak than it is in nature and assume their separation is limited by other visual constraints, such as the requirement of high-acuity spatial vision for closer M and L peak sensitivities. We explore the possibility that a spatio-chromatic analysis can better predict cone spectral tuning without appealing to other visual constraints. We build a computational model of the primate retina and simulate chromatic gratings of varying spatial frequencies using measured spectra. We then implement the case study of foveal processing in routinely trichromatic primates for the task of discriminating fruit and leaf spectra. We perform an exhaustive search for the configurations of M and L cone spectral sensitivities that optimally distinguish the colour patterns within these spectral images. Under such conditions, the model suggests that: (1) a long-wavelength limit is required to constrain the L cone spectral sensitivity to its natural position; (2) the optimal M cone peak spectral sensitivity occurs at ~525 nm, close to the observed position in nature (~535 nm); (3) spatial frequency has a small effect upon the spectral tuning of the cones; (4) a selective pressure toward less correlated M and L spectral sensitivities is provided by the need to reduce noise caused by the luminance variation that occurs in natural scenes.

Chromaticity separation and the alpha response

Chromatic gratings can be uncomfortable to view and can evoke a large haemodynamic response. Both the discomfort and the amplitude of the haemodynamic response increase monotonically with the perceptual difference in the colour of the component bars of the grating, as registered by the separation in their chromaticity in the CIE 1976 UCS diagram. Individuals with photosensitive epilepsy exhibit epileptiform EEG activity in response to flickering light of alternate colours. The probability of the epileptiform response again increases monotonically with the separation of the colours in the CIE UCS diagram. We investigated whether alpha power, which is known to reflect the excitation of large populations of neurons, is similarly affected by the separation in chromaticity. Chromatic square-wave gratings with bars that differed in CIE UCS chromaticity were presented, together with a central fixation cross. In 18 non-clinical participants, alpha responses were recorded over the visual cortex (O1, Oz, O2, PO3, POz, PO4, P1, P2) and compared to responses in prefrontal cortex (F1, F2). Gratings comprised bars of two alternate colours that either had a small difference in chromaticity (mean CIE UCS separation of 0.03), a medium difference (mean separation of 0.19), or a large difference (mean separation of 0.43). The colour pairs had chromaticities that lay on the red-green, red-blue, or blue-green borders of the screen gamut. Regardless of the hue, the larger the separation in chromaticity, the greater the alpha desynchronization and the lower the alpha power (p = 0.004), but only in posterior electrodes (p < 0.001). Together this indicates that differences in colour evoke a cortical excitation that increases monotonically with the colour difference. In this respect the alpha response resembles the haemodynamic response.

Enhanced Fine-Form Perception Does Not Contribute to Gestalt Face Perception in Autism Spectrum Disorder.

Individuals with autism spectrum disorder (ASD) show superior performance in processing fine detail, but often exhibit impaired gestalt face perception. The ventral visual stream from the primary visual cortex (V1) to the fusiform gyrus (V4) plays an important role in form (including faces) and color perception. The aim of this study was to investigate how the ventral stream is functionally altered in ASD. Visual evoked potentials were recorded in high-functioning ASD adults (n = 14) and typically developing (TD) adults (n = 14). We used three types of visual stimuli as follows: isoluminant chromatic (red/green, RG) gratings, high-contrast achromatic (black/white, BW) gratings with high spatial frequency (HSF, 5.3 cycles/degree), and face (neutral, happy, and angry faces) stimuli. Compared with TD controls, ASD adults exhibited longer N1 latency for RG, shorter N1 latency for BW, and shorter P1 latency, but prolonged N170 latency, for face stimuli. Moreover, a greater difference in latency between P1 and N170, or between N1 for BW and N170 (i.e., the prolongation of cortico-cortical conduction time between V1 and V4) was observed in ASD adults. These findings indicate that ASD adults have enhanced fine-form (local HSF) processing, but impaired color processing at V1. In addition, they exhibit impaired gestalt face processing due to deficits in integration of multiple local HSF facial information at V4. Thus, altered ventral stream function may contribute to abnormal social processing in ASD.

Research on medical applications of contrast sensitivity function to red–green gratings in 3D space

Contrast sensitivity function (CSF), which is a quick and valid index to measure human visual performance, can be applied into medical monitoring and various retinal disease diagnosis. However, there is a lack of researches on CSF in three-dimensional (3D) space. In this paper, CSF of human color vision to red–green chromatic gratings in 3D space is fully investigated. Based on the typical chromatic contrast sensitivity test system, in which the screen is parallel to human face, four inclined planes in 3D space are taken into consideration. Contrast thresholds for chromatic gratings of inclined planes are measured within 20 individual subjects. Experimental results show that the contrast sensitivity of each inclined plane is a low-pass function for red–green gratings. In order to fully utilize chromatic contrast sensitivity in 3D space, a visual model of red–green contrast sensitivity in 3D space is well fitted. Simulation results demonstrate that the proposed model achieves high consistency with human chromatic visual characteristics.

A Potential VEP Biomarker for Mild Cognitive Impairment: Evidence from Selective Visual Deficit of Higher-Level Dorsal Pathway.

Visual dysfunctions are common in Alzheimer's disease (AD). Our aim was to establish a neurophysiological biomarker for amnestic mild cognitive impairment (aMCI). Visual evoked potentials (VEPs) were recorded in aMCI patients who later developed AD (n = 15) and in healthy older (n = 15) and younger controls (n = 15). Visual stimuli were optimized to separately activate lower and higher levels of the ventral and dorsal streams. We compared VEP parameters across the three groups of participants and conducted a linear correlation analysis between VEPs and data from neuropsychological tests. We then used a receiver operating characteristic (ROC) analysis to discriminate those with aMCI from those who were healthy older adults. The latency and phase of VEPs to lower-level stimuli (chromatic and achromatic gratings) were significantly affected by age but not by cognitive decline. Conversely, VEP latencies for higher-ventral (faces and kanji-words) and dorsal (kana-words and optic flow motion) stimuli were not affected by age, but they were significantly prolonged in aMCI patients. Interestingly, VEPs for higher-dorsal stimuli were related to outcomes of neuropsychological tests. Furthermore, the ROC analysis showed that the highest areas under the curve were obtained for VEP latencies in response to higher-dorsal stimuli. These results suggest aMCI-related functional impairment specific to higher-level visual processing. Further, dysfunction in the higher-level of the dorsal stream could be an early indicator of cognitive decline. Therefore, we conclude that VEPs associated with higher-level dorsal stream activity can be a sensitive biomarker for early detection of aMCI.

Binocular rivalry from invisible patterns

Binocular rivalry arises when incompatible images are presented to the two eyes. If the two eyes' conflicting features are invisible, leading to identical perceptual interpretations, does rivalry competition still occur? Here we investigated whether binocular rivalry can be induced from conflicting but invisible spatial patterns. A chromatic grating counterphase flickering at 30 Hz appeared uniform, but produced significant tilt aftereffect and orientation-selective adaptation. The invisible pattern also generated significant BOLD activities in the early visual cortex, with minimal response in the parietal and frontal cortical areas. Compared with perceptually matched uniform stimuli, a monocularly presented invisible chromatic grating enhanced the rivalry competition with a low-contrast visible grating presented to the other eye. Furthermore, switching from a uniform field to a perceptually matched invisible chromatic grating produced interocular suppression at approximately 200 ms after onset of the invisible grating. Experiments using briefly presented monocular probes revealed evidence for sustained rivalry competition between two invisible gratings during continuous dichoptic presentations. These findings indicate that even without visible interocular conflict, and with minimal engagement of frontoparietal cortex and consciousness related top-down feedback, perceptually identical patterns with invisible conflict features produce rivalry competition in the early visual cortex.

Speed and the coherence of superimposed chromatic gratings

On the basis of measurements of the perceived coherence of superimposed drifting gratings, Krauskopf and Farell (1990) proposed that motion is analysed independently in different chromatic channels. They found that two gratings appeared to slip if each modulated one of the two ‘cardinal’ color mechanisms S/(L+M) and L/(L+M). If the gratings were defined along intermediate color directions, observers reported a plaid, moving coherently. We hypothesised that slippage might occur in chromatic gratings if the motion signal from the S/(L+M) channel is weak and equivalent to a lower speed. We asked observers to judge coherence in two conditions. In one, S/(L+M) and L/(L+M) gratings were physically the same speed. In the other, the two gratings had perceptually matched speeds. We found that the relative incoherence of cardinal gratings is the same whether gratings are physically or perceptually matched in speed. Thus our hypothesis was firmly contradicted. In a control condition, observers were asked to judge the coherence of stationary gratings. Interestingly, the difference in judged coherence between cardinal and intermediate gratings remained as strong as it was when the gratings moved. Our results suggest a possible alternative interpretation of Krauskopf and Farell’s result: the processes of object segregation may precede the analysis of the motion of chromatic gratings, and the same grouping signals may prompt object segregation in the stationary and moving cases.

Method for the measurement of chromatic contrast

The contrast of simple achromatic periodic patterns such as square-wave gratings is well defined and agrees with the Michelson contrast, but this is not so for chromatic contrast. It would be desirable to be able to determine the contrast of two colors for use in a general legibility metric. A method for determining the contrast of chromatic square-wave gratings, which is based on the CIE 1976 color difference formula, is proposed. In order to prove the feasibility, we first investigated the contrast sensitivity function of an achromatic grating using Michelson contrast for comparison through sets of psychophysical experiments keeping the mean luminance constant at 22.54 cd/m2. Further experiments compared the curve of contrast sensitivity function for red, green, and red-green chromatic square-wave grating in terms of the proposed formula and conventional formula in LMS cone contrast space, which is frequently used in calculating the contrast of color gratings. The results revealed that proposed method is helpful in calculating the contrast of chromatic square-wave gratings and reveals important physical meaning in measuring the contrast of chromatic gratings.