Chromatic Signals Research Articles

Ocular biometric responses to simulated polychromatic defocus.

Evidence from human studies of ocular accommodation and studies of animals reared in monochromatic conditions suggest that chromatic signals can guide ocular growth. We hypothesized that ocular biometric response in humans can be manipulated by simulating the chromatic contrast differences associated with imposition of optical defocus. The red, green, and blue (RGB) channels of an RGB movie of the natural world were individually incorporated with computational defocus to create two different movie stimuli. The magnitude of defocus incorporated in the red and blue layers was chosen such that, in one case, it simulated +3 D defocus, referred to as color-signed myopic (CSM) defocus, and in another case it simulated -3 D defocus, referred to as color-signed hyperopic (CSH) defocus. Seventeen subjects viewed the reference stimulus (unaltered movie) and at least one of the two color-signed defocus stimuli for ∼1 hour. Axial length (AL) and choroidal thickness (ChT) were measured immediately before and after each session. AL and subfoveal ChT showed no significant change under any of the three conditions. A significant increase in vitreous chamber depth (VCD) was observed following viewing of the CSH stimulus compared with the reference stimulus (0.034 ± 0.03 mm and 0 ± 0.02 mm, respectively; p = 0.018). A significant thinning of the crystalline lens was observed following viewing of the CSH stimulus relative to the CSM stimulus (-0.033 ± 0.03 mm and 0.001 ± 0.03 mm, respectively; p = 0.015). Differences in the effects of CSM and CSH conditions on VCD and lens thickness suggest a directional, modulatory influence of chromatic defocus. On the other hand, ChT responses showed large variability, rendering it an unreliable biomarker for chromatic defocus-driven responses, at least for the conditions of this study.

Binocular integration of chromatic and luminance signals.

Much progress has been made in understanding how the brain combines signals from the two eyes. However, most of this work has involved achromatic (black and white) stimuli, and it is not clear if the same processes apply in color-sensitive pathways. In our first experiment, we measured contrast discrimination ("dipper") functions for four key ocular configurations (monocular, binocular, half-binocular, and dichoptic), for achromatic, isoluminant L-M and isoluminant S-(L+M) sine-wave grating stimuli (L: long-, M: medium-, S: short-wavelength). We find a similar pattern of results across stimuli, implying equivalently strong interocular suppression within each pathway. Our second experiment measured dichoptic masking within and between pathways using the method of constant stimuli. Masking was strongest within-pathway and weakest between S-(L+M) and achromatic mechanisms. Finally, we repeated the dipper experiment using temporal luminance modulations, which produced slightly weaker interocular suppression than for spatially modulated stimuli. We interpret our results in the context of a contemporary two-stage model of binocular contrast gain control, implemented here using a hierarchical Bayesian framework. Posterior distributions of the weight of interocular suppression overlapped with a value of 1 for all dipper data sets, and the model captured well the pattern of thresholds from all three experiments.

Contrary to phylogenetic predictions, crown coloration of the Eared Dove is not regulated by sex hormones

The mechanisms regulating plumage coloration appear to be phylogenetically conserved in avian species: the plesiomorphic character state is estrogen-dependent regulation, while testosterone, luteinizing hormone, and non-hormonal control are derived states. Limited data exist on the underlying regulatory processes of sexual dichromatism in the Eared Dove ( Zenaida auriculata (Des Murs, 1847)). Since the Columbiformes order is close to basal branches, we hypothesized that estrogen and testosterone play a central role in the regulation of crown plumage coloration in the Eared Dove. To test this, we subjected adult males to a forced molt accompanied by an exogenous increase of estradiol and testosterone to determine whether the presence of these hormones during molting modified the spectrophotometric characteristics of the plumage. No significant differences were found between treatments and controls in the colorimetric variables hue, ultraviolet saturation, and brightness. Similarly, the avian visual model showed no perceptible changes in chromatic and achromatic signals in the individual male adults. We also analyzed the effects of estrogen and testosterone on the growth speed of the new feathers. The hormone-treated groups increased in the speed of molting compared to the control group. These results suggest that the Eared Dove is an exception to basal branch models: rather than regulating coloration, sex hormones speed up the growth of the new feathers.

In primary visual cortex fMRI responses to chromatic and achromatic stimuli are interdependent and predict contrast detection thresholds

Chromatic and achromatic signals in primary visual cortex have historically been considered independent of each other but have since shown evidence of interdependence. Here, we investigated the combination of two components of a stimulus; an achromatic dynamically changing check background and a chromatic (L-M or S cone) target grating. We found that combinations of chromatic and achromatic signals in primary visual cortex were interdependent, with the dynamic range of responses to chromatic contrast decreasing as achromatic contrast increased. A contrast detection threshold study also revealed interdependence of background and target, with increasing chromatic contrast detection thresholds as achromatic background contrast increased. A model that incorporated a normalising effect of achromatic contrast on chromatic responses, but not vice versa, best predicted our V1 data as well as behavioural thresholds. Further along the visual hierarchy, the dynamic range of chromatic responses was maintained when compared to achromatic responses, which became increasingly compressive.

A Lateral Flow Assay Based on Streptavidin-biotin Amplification System with Recombinase Polymerase Amplification for Rapid and Quantitative Detection of Salmonella enteritidis.

Salmonella constitutes a prevalent alimentary pathogen, instigating zoonotic afflictions. Consequently, the prompt discernment of Salmonella in sustenance is of cardinal significance. Lateral flow assays utilizing colorimetric methodologies adequately fulfill the prerequisites of point-of-care diagnostics, however, their detection threshold remains elevated, generally permitting only qualitative discernment, an impediment to the preliminary screening of nascent pathogens. In response to this conundrum, we propose a lateral flow diagnostic predicated upon a streptavidin-biotin amplification system with recombinase polymerase amplification engineered for the expeditious and quantitative discernment of Salmonella enteritidis. Trace nucleic acids within a sample undergo exponential amplification via recombinase polymerase amplification to a level discernable, constituting the initial signal amplification. Subsequently, along the test line (T-line) of the lateral flow strip, the chromatic signal undergoes augmentation by securing a greater quantity of AuNPs through the magnification capacity of the streptavidin-biotin mechanism, affecting the second signal amplification. Quantitative results are procured via smartphone capture and transferred to computer software for precise calculation of the targeted quantity. The lateral flow strip exhibits a LOD at 19.41 CFU/mL for cultured S. enteritidis. The RSD of three varying concentrations were respectively 3.74 %, 5.96 %, and 4.25 %.

Invited Session III: Diversity in chromatic processing across the animal kingdom: Comparative connectomics reveals noncanonical wiring for color vision in human foveal retina.

The Old World macaque monkey and New World common marmoset provide fundamental models for human visual processing, yet the human ancestral lineage diverged from these monkey lineages over 25 Mya. We therefore asked whether fine-scale synaptic wiring in the nervous system is preserved across these three primate families, despite long periods of independent evolution. We applied connectomic electron microscopy to the specialized foveal retina where circuits for highest acuity and color vision reside. Synaptic motifs arising from the cone photoreceptor type sensitive to short (S) wavelengths and associated with "blue-yellow" (S-ON and S-OFF) color-coding circuitry were reconstructed. We found that distinctive circuitry arises from S cones for each of the three species. The S cones contacted neighboring L and M (long- and middle-wavelength sensitive) cones in humans, but such contacts were rare or absent in macaques and marmosets. We discovered a major S-OFF pathway in the human retina and established its absence in marmosets. Further, the S-ON and S-OFF chromatic pathways make excitatory-type synaptic contacts with L and M cone types in humans, but not in macaques or marmosets. Our results predict that early-stage chromatic signals are distinct in the human retina and imply that solving the human connectome at the nanoscale level of synaptic wiring will be critical for fully understanding the neural basis of human color vision.

Poster Session II: Attentional modulation of the achromatic and chromatic reversal VEP.

Previous literature has consistently revealed attentional modulation of the Visual Evoked Potential (VEP) response to achromatic pattern reversal stimuli but little to no attentional modulation of the VEP response to chromatic pattern onsets. Magnetic Resonance Imaging (MRI) research, however, has reported modulation of the responses to both achromatic and chromatic pattern reversal stimuli. Numerous methodological differences including mode of presentation, stimulus contrast, and attentional demand, make comparison of these results difficult. In this study, we report the results of experiments using comparable perceptual contrasts, pattern reversals, and a coextensive and highly-demanding, multiple object tracking (MOT) task. Our findings support prior VEP results indicating that although achromatic VEPs are modulated by attention, chromatic VEPs are more robust to attentional modulation, even for highly demanding distractor tasks. We also found that when compared to a non-attentional condition, the attenuation of the VEP when attending to the MOT task was greater in magnitude than the enhancement of the VEP when attending to the VEP stimulus. This supports prior conclusions, that while avoiding active distraction is likely important, insuring an "attentive state" is not always necessary when recording VEPs. Further experiments are underway to investigate why attentional modulation of chromatic signals in early visual cortex are observed in MRI but not VEP recordings.

Two sides of the same wing: ventral scales enhance dorsal wing color in the butterfly Speyeria mormonia.

Biological visual signals are often produced by complex interactions between light-absorbing and light-scattering structures, but for many signals potential interactions between different light-interacting components have yet to be tested. Butterfly wings, for example, are thin enough that their two sides may not be optically isolated. We tested if ventral wing scales of the Mormon fritillary Speyeria mormonia impact the appearances of dorsal orange patches, which are thought to be involved in sexual signaling. Using reflectance spectroscopy, we found ventral scales, either silvered or non-silvered, make dorsal orange patches significantly brighter, with the silvered scales having the greater effect. Computational modeling indicates both types of ventral scales enhance the chromatic perceptual signal of dorsal orange patches, with only the silvered scales also enhancing their achromatic perceptual signal. A lack of optical independence between the two sides of the wings of S. mormonia implies the wing surfaces of butterflies have intertwined signaling functions and evolutionary histories.

Interocular binding of chromatic signals across time

Interocular binding of chromatic signals across time

Comparative connectomics reveals noncanonical wiring for color vision in human foveal retina

The Old World macaque monkey and New World common marmoset provide fundamental models for human visual processing, yet the human ancestral lineage diverged from these monkey lineages over 25 Mya. We therefore asked whether fine-scale synaptic wiring in the nervous system is preserved across these three primate families, despite long periods of independent evolution. We applied connectomic electron microscopy to the specialized foveal retina where circuits for highest acuity and color vision reside. Synaptic motifs arising from the cone photoreceptor type sensitive to short (S) wavelengths and associated with "blue-yellow" (S-ON and S-OFF) color-coding circuitry were reconstructed. We found that distinctive circuitry arises from S cones for each of the three species. The S cones contacted neighboring L and M (long- and middle-wavelength sensitive) cones in humans, but such contacts were rare or absent in macaques and marmosets. We discovered a major S-OFF pathway in the human retina and established its absence in marmosets. Further, the S-ON and S-OFF chromatic pathways make excitatory-type synaptic contacts with L and M cone types in humans, but not in macaques or marmosets. Our results predict that early-stage chromatic signals are distinct in the human retina and imply that solving the human connectome at the nanoscale level of synaptic wiring will be critical for fully understanding the neural basis of human color vision.

Screening for mild anomalous trichromacy using the Ishihara plates test.

The Ishihara plates test is one of the most established and widely used means of identifying color vision deficiencies. However, literature examining the effectiveness of the Ishihara plates test has identified weaknesses, particularly when screening for milder anomalous trichromacy. We constructed a model of the chromatic signals expected to contribute to false negative readings by calculating, for particular anomalous trichromatic observers, the differences in chromaticity between the ground and pseudoisochromatic portions of plates. Predicted signals from five plates were compared for seven editions of the Ishihara plates test, for six observers with three severities of anomalous trichromacy, under eight illuminants. We found significant effects of variation in all of these factors other than edition on the predicted color signals available to read the plates. The impact of edition was tested behaviorally with 35 observers with color vision deficiency and 26 normal trichromats, which corroborated the minimal effect of edition predicted by the model. We found a significant negative relationship between predicted color signals for anomalous trichromats and behavioral false negative plate readings (ρ=-0.46, p=0.005 for deuteranomals, ρ=-0.42, p=0.01 for protanomals), suggesting that residual observer-specific color signals in portions of plates designed to be isochromatic may be contributing to false negative readings, and validating our modeling approach.

Perception of brown with variation in center chromaticity and surround luminance.

Brown is a contrast color that depends on complex combinations of chromatic and achromatic signals. We measured brown perception with variations in chromaticity and luminance in center-surround configurations. In Experiment 1, the dominant wavelength and saturation in terms of S-cone stimulation were tested with five observers in a fixed surround luminance (60c d/m 2). A paired-comparison task required the observer to select the better exemplar of brown in one of two, simultaneously presented, stimuli (1.0° center diameter; annulus of 9.48° outer-diameter). In Experiment 2, the same task was tested with five observers in which surround luminance was varied (from 13.1 to 99.6c d/m 2) for two center chromaticities. The results were a set of win-loss ratios for each stimulus combination and converted to Z-scores. An ANOVA did not reveal a significant main effect of the observer factor but revealed a significant interaction with red/green (a ∗) [but not with the dominant wavelength and the S-cone stimulation (or b ∗)]. Experiment 2 revealed observer variation in interactions with surround luminance and S-cone stimulation. The averaged data plotted in 1976L ∗ a ∗ b ∗ color space indicate that high Z-score values widely distribute in the area of a ∗ from 5 to 28 and b ∗ over 6. The balance of the strength between yellowness and blackness differs among observers owing to the amount of induced blackness required for the best brown.

Gaining the system: limits to compensating colordeficiencies through post-receptoral gainchanges.

Color percepts of anomalous trichromats are often more similar to normal trichromats than predicted from their receptor spectral sensitivities, suggesting that post-receptoral mechanisms can compensate for chromatic losses. The basis for these adjustments and the extent to which they could discount the deficiency are poorly understood. We modeled the patterns of compensation that might result from increasing the gains in post-receptoral neurons to offset their weakened inputs. Individual neurons and the population responses jointly encode luminance and chromatic signals. As a result, they cannot independently adjust for a change in the chromatic inputs, predicting only partial recovery of the chromatic responses and increased responses to achromatic contrast. These analyses constrain the potential sites and mechanisms of compensation for a color loss and characterize the utility and limits of neural gain changes for calibrating color vision.

Luminance Contrast Drives Interactions between Perception and Working Memory.

Visual working memory (WM) enables the use of past sensory experience in guiding behavior. Yet, laboratory tasks commonly evaluate WM in a way that separates it from its sensory bottleneck. To understand how perception interacts with visual memory, we used a delayed shape recognition task to probe how WM may differ for stimuli that bias processing toward different visual pathways. Luminance compared with chromatic signals are more efficient in driving the processing of shapes and may thus also lead to better WM encoding, maintenance, and memory recognition. To evaluate this prediction, we conducted two experiments. In the first psychophysical experiment, we measured contrast thresholds for different WM loads. Luminance contrast was encoded into WM more efficiently than chromatic contrast, even when both sets of stimuli were equated for discriminability. In the second experiment, which also equated stimuli for discriminability, early sensory responses in the EEG that are specific to luminance pathways were modulated by WM load and thus likely reflect the neural substrate of the increased efficiency. Our results cannot be accounted for by simple saliency differences between luminance and color. Rather, they provide evidence for a direct connection between low-level perceptual mechanisms and WM by showing a crucial role of luminance for forming WM representations of shape.

Coding of chromatic spatial contrast by macaque V1 neurons.

Color perception relies on comparisons between adjacent lights, but how the brain performs these comparisons is poorly understood. To elucidate the underlying mechanisms, we recorded spiking responses of individual V1 neurons in macaque monkeys to pairs of stimuli within the classical receptive field (RF). We estimated the spatial-chromatic RF of each neuron and then presented customized colored edges using a closed-loop technique. We found that many double-opponent (DO) cells, which have spatially and chromatically opponent RFs, responded to chromatic contrast as a weighted sum, akin to how other V1 neurons responded to luminance contrast. Yet other neurons integrated chromatic signals nonlinearly, confirming that linear signal integration is not an obligate property of V1 neurons. The functional similarity of cone-opponent DO cells and cone non-opponent simple cells suggests that these two groups may share a common underlying circuitry, promotes the construction of image-computable models for full-color image representation, and sheds new light on V1 complex cells.

Maintaining chromatic dispersion and signal gain performances in a chalcogenide buffer step-index optical fiber.

Aiming at maintaining the chromatic dispersion properties and fiber optical parametric amplification (FOPA) performance when fiber core fluctuation occurs, we propose a buffer step-index optical fiber. The AsSe2 chalcogenide glass is employed as the core material due to its high nonlinearity and broad transmission spectrum. The calculated results in this study show that the chromatic dispersion variation due to the change of core diameter can be greatly suppressed and a continuous and very broad FOPA signal gain spectrum can be obtained and maintained by carefully controlling the core, buffer and cladding properties such as refractive index and diameters. The calculated results in this study showed that by using the proposed 3-cm-long fiber pumped at 5.02 µm, a broad signal gain bandwidth from 3 to 14 µm at about 15 dB is attainable although the fiber core diameter Dc drastically fluctuated from 2 to 5 µm and the buffer diameter Db varies from 8.9 to 9.3 µm. Moreover, when Dc varies in smaller range from 3 to 4 µm, the FOPA signal gain spectra calculated at different fixed values of Db in the range from 8.9 to 9.3 µm are highly maintained. When Db is kept at 9.0 µm and Dc varies from 3 to 4 µm, the calculated FOPA signal gain spectra at different pump wavelengths from 4.98 to 5.02 µm are also nearly identical in the wavelength range from 3 up to 13 µm.

A wave-optical model for chromatic confocal sensing using multimode fibre incoherent illumination

This paper presents a wave-optical model for chromatic confocal signals based on multimode optical fibre data acquisition. It is shown that the emitted light from a multimode fibre can be modelled as a point source grid. The confocal signal can be obtained by incoherent summation of the intensities of the reflected fields in the detector plane. In this manner, this two-dimensional wave-optical model encompasses the temporal incoherence of the light source due to the usage of a broadband light source and the spatial incoherence due to the application of a multimode fibre. In fact, this model enables the modelling of different degrees of coherence. Therefore, to our knowledge, it represents the most thorough modelling tool for chromatic confocal signals. Physical parameters such as the spectral distribution of the polychromatic light source, the lateral distribution of point sources on the grid, and the strength of longitudinal aberrations, and their influences on the chromatic confocal signal have been evaluated in detail. Consideration of all these parameters results in simulated chromatic confocal signals, which almost perfectly match the experimentally recorded chromatic confocal signals. The results obtained will be very useful for the development of a deeper understanding of chromatic confocal sensing, to optimise the setup parameters with regard to a particular application and to reinterpret the results, possibly in combination with artificial intelligence algorithms, in order to increase the measurement precision and to reduce the demands imposed on the manufacturing quality of the optical elements.

Structural Differences Across Multiple Visual Cortical Regions in the Absence of Cone Function in Congenital Achromatopsia.

Most individuals with congenital achromatopsia (ACHM) carry mutations that affect the retinal phototransduction pathway of cone photoreceptors, fundamental to both high acuity vision and colour perception. As the central fovea is occupied solely by cones, achromats have an absence of retinal input to the visual cortex and a small central area of blindness. Additionally, those with complete ACHM have no colour perception, and colour processing regions of the ventral cortex also lack typical chromatic signals from the cones. This study examined the cortical morphology (grey matter volume, cortical thickness, and cortical surface area) of multiple visual cortical regions in ACHM (n = 15) compared to normally sighted controls (n = 42) to determine the cortical changes that are associated with the retinal characteristics of ACHM. Surface-based morphometry was applied to T1-weighted MRI in atlas-defined early, ventral and dorsal visual regions of interest. Reduced grey matter volume in V1, V2, V3, and V4 was found in ACHM compared to controls, driven by a reduction in cortical surface area as there was no significant reduction in cortical thickness. Cortical surface area (but not thickness) was reduced in a wide range of areas (V1, V2, V3, TO1, V4, and LO1). Reduction in early visual areas with large foveal representations (V1, V2, and V3) suggests that the lack of foveal input to the visual cortex was a major driving factor in morphological changes in ACHM. However, the significant reduction in ventral area V4 coupled with the lack of difference in dorsal areas V3a and V3b suggest that deprivation of chromatic signals to visual cortex in ACHM may also contribute to changes in cortical morphology. This research shows that the congenital lack of cone input to the visual cortex can lead to widespread structural changes across multiple visual areas.

Artificial nighttime lighting impacts visual ecology links between flowers, pollinators and predators

The nighttime environment is being altered rapidly over large areas worldwide through introduction of artificial lighting, from streetlights and other sources. This is predicted to impact the visual ecology of many organisms, affecting both their intra- and interspecific interactions. Here, we show the effects of different artificial light sources on multiple aspects of hawkmoth visual ecology, including their perception of floral signals for pollination, the potential for intraspecific sexual signalling, and the effectiveness of their visual defences against avian predators. Light sources fall into three broad categories: some that prevent use of chromatic signals for these behaviours, others that more closely mimic natural lighting conditions, and, finally, types whose effects vary with light intensity and signal colour. We find that Phosphor Converted (PC) amber LED lighting – often suggested to be less harmful to nocturnal insects – falls into this third disruptive group, with unpredictable consequences for insect visual ecology depending on distance from the light source and the colour of the objects viewed. The diversity of impacts of artificial lighting on hawkmoth visual ecology alone argues for a nuanced approach to outdoor lighting in environmentally sensitive areas, employing intensities and spectra designed to limit those effects of most significant concern.

How chromatic cues can guide human eye growth to achieve good focus.

The postnatal growing eye uses visual cues to actively control its own axial elongation to achieve and maintain sharp focus, a process termed emmetropization. The primary visual cue may be the difference in image sharpness as sensed by the arrays of short- and long-wavelength sensitive cone photoreceptors caused by longitudinal chromatic aberration: Shorter wavelengths focus in front of longer wavelengths. However, the sparse distribution of short-wavelength sensitive cones across the retina suggests that they do not have sufficient spatial sampling resolution for this task. Here, we show that the spacing of the short-wavelength sensitive cones in humans is sufficient for them, in conjunction with the longer wavelength cones, to use chromatic signals to detect defocus and guide emmetropization. We hypothesize that the retinal spacing of the short-wavelength sensitive cones in many mammalian species is an evolutionarily ancient adaption that allows the efficient use of chromatic cues in emmetropization.