Understanding insect colour constancy.

Colour constancy is the ability to recognize the colour of objects despite spectral changes in the natural illumination. As such, this phenomenon is important for most organisms with good colour vision, and it has been intensively studied in humans and primates. Colour constancy is also documented for several species of insects, which is not surprising given the ecological importance of colour vision. But how do insects, with their small brains, solve the complex problem of colour vision and colour constancy? In an interspecies approach, this review reports on behavioural studies on colour constancy in bees, butterflies, moths and humans, corresponding computational models and possible neurophysiological correlates. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

Exploring the Determinants of Color Perception Using #Thedress and Its Variants: The Role of Spatio-Chromatic Context, Chromatic Illumination, and Material-Light Interaction.

The colors that people see depend not only on the surface properties of objects but also on how these properties interact with light as well as on how light reflected from objects interacts with an individual's visual system. Because individual visual systems vary, the same visual stimulus may elicit different perceptions from different individuals. #thedress phenomenon drove home this point: different individuals viewed the same image and reported it to be widely different colors: blue and black versus white and gold. This phenomenon inspired a collection of demonstrations presented at the Vision Sciences Society 2015 Meeting which showed how spatial and temporal manipulations of light spectra affect people's perceptions of material colors and illustrated the variability in individual color perception. The demonstrations also explored the effects of temporal alterations in metameric lights, including Maxwell's Spot, an entoptic phenomenon. Crucially, the demonstrations established that #thedress phenomenon occurs not only for images of the dress but also for the real dress under real light sources of different spectral composition and spatial configurations.

Safety and Vision Outcomes of Subretinal Gene Therapy Targeting Cone Photoreceptors in Achromatopsia: A Nonrandomized Controlled Trial.

Importance: Achromatopsia linked to variations in the CNGA3 gene is associated with day blindness, poor visual acuity, photophobia, and involuntary eye movements owing to lack of cone photoreceptor function. No treatment is currently available. Objective: To assess safety and vision outcomes of supplemental gene therapy with adeno-associated virus (AAV) encoding CNGA3 (AAV8.CNGA3) in patients with CNGA3-linked achromatopsia. Design, Setting, and Participants: This open-label, exploratory nonrandomized controlled trial tested safety and vision outcomes of gene therapy vector AAV8.CNGA3 administered by subretinal injection at a single center. Nine patients (3 per dose group) with a clinical diagnosis of achromatopsia and confirmed biallelic disease-linked variants in CNGA3 were enrolled between November 5, 2015, and September 22, 2016. Data analysis was performed from June 6, 2017, to March 12, 2018. Intervention: Patients received a single unilateral injection of 1.0 × 1010, 5.0 × 1010, or 1.0 × 1011 total vector genomes of AAV8.CNGA3 and were followed up for a period of 12 months (November 11, 2015, to October 10, 2017). Main Outcomes and Measures: Safety as the primary end point was assessed by clinical examination of ocular inflammation. Systemic safety was assessed by vital signs, routine clinical chemistry testing, and full and differential blood cell counts. Secondary outcomes were change in visual function from baseline in terms of spatial and temporal resolution and chromatic, luminance, and contrast sensitivity throughout a period of 12 months after treatment. Results: Nine patients (mean [SD] age, 39.6 [11.9] years; age range, 24-59 years; 8 [89%] male) were included in the study. Baseline visual acuity letter score (approximate Snellen equivalent) ranged from 34 (20/200) to 49 (20/100), whereas baseline contrast sensitivity log scores ranged from 0.1 to 0.9. All 9 patients underwent surgery and subretinal injection of AAV8.CNGA3 without complications. No substantial safety problems were observed during the 12-month follow-up period. Despite the congenital deprivation of cone photoreceptor-mediated vision in achromatopsia, all 9 treated eyes demonstrated some level of improvement in secondary end points regarding cone function, including mean change in visual acuity of 2.9 letters (95% CI, 1.65-4.13; P = .006, 2-sided t test paired samples). Contrast sensitivity improved by a mean of 0.33 log (95% CI, 0.14-0.51 log; P = .003, 2-sided t test paired samples). Conclusions and Relevance: Subretinal gene therapy with AAV8.CNGA3 was not associated with substantial safety problems and was associated with cone photoreceptor activation in adult patients, as reflected by visual acuity and contrast sensitivity gains. Trial Registration: ClinicalTrials.gov Identifier: NCT02610582.

Development of Methodology and Study Protocol: Safety and Efficacy of a Single Subretinal Injection of rAAV.hCNGA3 in Patients with CNGA3-Linked Achromatopsia Investigated in an Exploratory Dose-Escalation Trial.

Achromatopsia is an autosomal recessively inherited congenital defect characterized by a lack of cone photoreceptor function, leading to severely impaired vision. In this clinical study, achromatopsia patients were treated with a single subretinal injection of rAAV.hCNGA3 to restore cone function. The focus of this trial was on the safety of the treatment. After surgery, patients were monitored in eight extensive visits during the first year, followed by a 4-year follow-up period with annual visits. For essential complementation of the standard ophthalmological and systemic examinations, disease-specific methods were developed to assess the safety, efficacy, and patient-reported outcomes in this trial.

The Clinical Phenotype of CNGA3-Related Achromatopsia: Pretreatment Characterization in Preparation of a Gene Replacement Therapy Trial.

Purpose: The purpose of this study was to clinically characterize patients with CNGA3-linked achromatopsia (CNGA3-ACHM) in preparation of a gene therapy trial. Methods: Thirty-six patients (age 7-56 years) with complete (cACHM) or incomplete (iACHM) CNGA3-ACHM were examined, including detailed psychophysical tests, extended electrophysiology, and assessment of morphology by fundus autofluorescence and spectral-domain optical coherence tomography (SD-OCT). Results: Mean best-corrected visual acuity was 0.78 ± 0.14 logMAR. Color vision tests were consistent with a rod-dominated function in every cACHM patient. Microperimetry indicated an overall lowered retinal sensitivity within 20° of visual field. In electroretinography (ERG), photopic responses were nondetectable in cACHM patients, but residual cone responses were observed in the iACHM patients. Scotopic responses were altered referring to anomalies of photoreceptor and postreceptor signaling, whereas in voltage versus intensity functions, Vmax was significantly below normal values (P < 0.05). In contrast, slope (n) and semisaturation intensity (K) were found to be within normal limits. Spectral-domain OCT examination showed no specific changes in 14.7%, disruption of the ellipsoid zone (EZ) at the fovea in 38.2%, absent EZ in 17.7%, a hyporeflective zone in 20.5%, and outer retinal atrophy in 8.9% of all cases and foveal hypoplasia in 29 patients (85%). No correlation of retinal morphology with visual function or with a specific genotype was found. The severity of morphologic and functional changes lacked a robust association with age. Conclusions: Our extended investigations prove that even among such a genetically homogenous group of patients, no specific correlations regarding function and morphology severity and age can be observed. Therefore, the therapeutic window seems to be wider than previously indicated.

Object Recognition in Flight: How Do Bees Distinguish between 3D Shapes?

Honeybees (Apis mellifera) discriminate multiple object features such as colour, pattern and 2D shape, but it remains unknown whether and how bees recover three-dimensional shape. Here we show that bees can recognize objects by their three-dimensional form, whereby they employ an active strategy to uncover the depth profiles. We trained individual, free flying honeybees to collect sugar water from small three-dimensional objects made of styrofoam (sphere, cylinder, cuboids) or folded paper (convex, concave, planar) and found that bees can easily discriminate between these stimuli. We also tested possible strategies employed by the bees to uncover the depth profiles. For the card stimuli, we excluded overall shape and pictorial features (shading, texture gradients) as cues for discrimination. Lacking sufficient stereo vision, bees are known to use speed gradients in optic flow to detect edges; could the bees apply this strategy also to recover the fine details of a surface depth profile? Analysing the bees' flight tracks in front of the stimuli revealed specific combinations of flight maneuvers (lateral translations in combination with yaw rotations), which are particularly suitable to extract depth cues from motion parallax. We modelled the generated optic flow and found characteristic patterns of angular displacement corresponding to the depth profiles of our stimuli: optic flow patterns from pure translations successfully recovered depth relations from the magnitude of angular displacements, additional rotation provided robust depth information based on the direction of the displacements; thus, the bees flight maneuvers may reflect an optimized visuo-motor strategy to extract depth structure from motion signals. The robustness and simplicity of this strategy offers an efficient solution for 3D-object-recognition without stereo vision, and could be employed by other flying insects, or mobile robots.

Spatial and temporal aspects of chromatic adaptation and their functional significance for colour constancy.

Illumination in natural scenes changes at multiple temporal and spatial scales: slow changes in global illumination occur in the course of a day, and we encounter fast and localised illumination changes when visually exploring the non-uniform light field of three-dimensional scenes; in addition, very long-term chromatic variations may come from the environment, like for example seasonal changes. In this context, I consider the temporal and spatial properties of chromatic adaptation and discuss their functional significance for colour constancy in three-dimensional scenes. A process of fast spatial tuning in chromatic adaptation is proposed as a possible sensory mechanism for linking colour constancy to the spatial structure of a scene. The observed middlewavelength selectivity of this process is particularly suitable for adaptation to the mean chromaticity and the compensation of interreflections in natural scenes. Two types of sensory colour constancy are distinguished, based on the functional differences of their temporal and spatial scales: a slow type, operating at a global scale for the compensation of the ambient illumination; and a fast colour constancy, which is locally restricted and well suited to compensate region-specific variations in the light field of three dimensional scenes.

Colour constancy in insects.

Colour constancy is the perceptual phenomenon that the colour of an object appears largely unchanged, even if the spectral composition of the illuminating light changes. Colour constancy has been found in all insect species so far tested. Especially the pollinating insects offer a remarkable opportunity to study the ecological significance of colour constancy since they spend much of their adult lives identifying and choosing between colour targets (flowers) under continuously changing ambient lighting conditions. In bees, whose colour vision is best studied among the insects, the compensation provided by colour constancy is only partial and its efficiency depends on the area of colour space. There is no evidence for complete 'discounting' of the illuminant in bees, and the spectral composition of the light can itself be used as adaptive information. In patchy illumination, bees adjust their spatial foraging to minimise transitions between variously illuminated zones. Modelling allows the quantification of the adaptive benefits of various colour constancy mechanisms in the economy of nature. We also discuss the neural mechanisms and cognitive operations that might underpin colour constancy in insects.

Computational model of the effect of light scattering from cataracts in the human eye.

A model of the human eye has been developed, including scattering from cataracts inside the nucleus of the lens. The cataracts are modeled as spherical particles with refractive index different from that of the surrounding lens medium. Scattering from the retina is also included in the simulations. Variations of scattering particle diameter, number of particles, and wavelength of the illuminating light are investigated. It is shown that particle size is the most important parameter affecting the scattered light, and that the scattering from the retina can mask the effect of the scattering particles, for some range of the parameters.

Effects on colour discrimination during long term exposure to high altitudes on Mt Everest.

AIM: To investigate changes in colour discrimination as a result of chronic hypoxic exposure induced by extreme altitudes (above 8000 m) during an expedition to Mt Everest. METHODS: Colour discrimination thresholds for tritan, protan and deutan axes were measured extensively in two male participants (four eyes) during an expedition to Mt Everest, using a quantitative, computer controlled psychophysical colour vision test (modified version of the Cambridge Colour Test). The tests were carried out over a period of 54 days at altitudes of 1300 m, 3450 m, 4410 m, 5060 m, 5300 m, 6450 m, 7200 m and 8000 m. RESULTS: Colour vision tests 1 week before and 6 months after the expedition indicated normal colour discrimination in both participants. With increasing altitude, colour discrimination thresholds were found to rise, predominantly for the tritan (blue) axes in both observers. Deutan (green) thresholds were minimally elevated at high altitude, whereas protan (red) was altered in one observer. Tritan colour discrimination thresholds decreased as a function of time spent at a given altitude and normalised upon return to low altitude. CONCLUSIONS: Chronic hypoxia induced by high altitude exposure transiently affects colour discrimination, in particular tritan axis discrimination. Decreased tritan discrimination is partly reversible upon physiological adaptation to high altitude and completely normalised upon return to low altitude.

Colour in action: evidence for a redundancy signal effect when driving motor responses by combined colour and spatial cues

Several studies have demonstrated that either chromatic or spatial information can guide motor behaviour, but so far interactions between these two visual features are little understood. Here we addressed this issue by measuring reaction times (RT) for pointing hand movements which were instructed by either cues for spatial (thought to be predominantly processed in the dorsal visual stream), colour (thought to be predominantly processed in the ventral visual stream) or redundant (combination of colour and spatial information) conditions. While faster responses were found for spatial than for colour cues, most importantly, the shortest RTs were measured for the combined cues (redundancy signal effect, RSE). The data are inconsistent with the predictions of the race model which assumes parallel and independent input from the two streams to the motor system. Instead, the data are better explained by the coactivation model, which proposes combined sensory information from the different stimuli and detection process from the sum of the signals. Here, the redundancy signal effect results from the combination of colour and spatial information. The results provide behavioural evidence for an integration of colour and spatial cues when guiding hand movements.

Colour in action: evidence for a redundancy signal effect when driving motor responses by combined colour and spatial cues

Several studies have demonstrated that either chromatic or spatial information can guide motor behaviour, but so far interactions between these two visual features are little understood. Here we addressed this issue by measuring reaction times (RT) for pointing hand movements which were instructed by either cues for spatial (thought to be predominantly processed in the dorsal visual stream), colour (thought to be predominantly processed in the ventral visual stream) or redundant (combination of colour and spatial information) conditions. While faster responses were found for spatial than for colour cues, most importantly, the shortest RTs were measured for the combined cues (redundancy signal effect, RSE). The data are inconsistent with the predictions of the race model which assumes parallel and independent input from the two streams to the motor system. Instead, the data are better explained by the coactivation model, which proposes combined sensory information from the different stimuli and detection process from the sum of the signals. Here, the redundancy signal effect results from the combination of colour and spatial information. The results provide behavioural evidence for an integration of colour and spatial cues when guiding hand movements.(AU)

Color constancy improves, when an object moves: high-level motion influences color perception.

Color constancy refers to our remarkable ability to perceive the color of objects nearly constant despite considerable changes in the spectral content of the illumination. As such it is most important for object recognition. Visual motion can make object recognition harder because it limits the viewing time and increases the likelihood that an object encounters illumination changes. However, color constancy, as human color perception in general, has long been thought to be "motion blind." Here I show that, on the contrary, human color constancy is influenced by motion and improves when a color surface moves. Psychophysical experiments revealed that color constancy is influenced specifically by slow object motion and depends on the saliency of the moving figure. These surprising findings cannot be explained by low-level co-processing of color and motion signals. Instead they demonstrate a previously unknown influence of attention-driven, high-level motion processes on cortical color computation. Since motion is a frequent aspect of natural visual scenes, the synergistic integration of color and motion signals is an important mechanism for improving color identification. The new findings speak against a strict segregation of color and motion processing in the human visual cortex and suggest a network for encoding object color, which includes specialized ventral as well as dorsal visual areas.

The influence of depth segmentation on colour constancy.

In real scenes, surfaces in different depth planes often differ in the luminance and chromatic content of their illumination. Scene segmentation is therefore an important issue when considering the compensation of illumination changes in our visual perception (lightness and colour constancy). Chromatic adaptation is an important sensory component of colour constancy and has been shown to be linked to the two-dimensional spatial structure of a scene (Werner, 2003 Vision Research 43 1611 - 1623). Here, the question is posed whether this cooperation also extends to the organisation of a scene in depth. The influence of depth on colour constancy was tested by introducing stereo disparity, whereby the test patch and background were perceived in either the same or one of five different depth planes (1.9-57 min of arc). There were no additional cues to depth such as shadows or specular highlights. For consistent illumination changes, colour constancy was reduced when the test patch and background were separated in depth, indicating a reduction of contextual influences. An interaction was found between the influences of stereo depth and spatial frequency on colour constancy. In the case of an inconsistent illumination change, colour constancy was reduced if the test patch and background were in the same depth plane (2-D condition), but not if they were separated in depth (3-D condition). Furthermore, colour constancy was slightly better in the 3-D inconsistent condition than in the 2-D inconsistent condition. It is concluded that depth segmentation supports colour constancy in scenes with inconsistent illumination changes. Processes of depth segmentation are implemented at an early sensory stage of colour constancy, and they define visual regions within which the effects of illuminant changes are discounted for separately. The results support recent models that posit such implementation of scene segmentation in colour constancy.

Scotopic threshold responses to infrared irradiation in cats.

Infrared (IR) irradiation is frequently used in ophthalmological diagnosis and treatment. It has been used to selectively stimulate photodiode-based retinal prostheses to prove their function. Data concerning the natural IR-sensitivity of the retina are contradictory. In our experiments in dark-adapted cats an IR-laser (826 nm) and IR emitting diodes (875 nm) elicited clear scotopic threshold responses. Comparison of the two lasers (IR and a visible laser at 670 nm) using Lambs template and our experimental data revealed very similar differences in retinal sensitivity (4.28 and 3.94+/-0.29 log units, respectively). The fact that the cat retina is sensitive to IR-irradiation under certain conditions has important implications in interpreting the results from retinal prostheses and rewards further attention in its use in many ophthalmological applications.

The spatial tuning of chromatic adaptation.

A key question in colour research is how the colour and spatial analysis of an image interact. Traditionally, colour and form analysis have been regarded as parallel and separate processes, and documented effects of image complexity on chromatic adaptation have been attributed to a temporal integration process during eye movements. Evidence is presented here for a spatial mechanism, which tunes chromatic adaptation to the luminance structure (spatial frequency and orientation) of an image. This in turn suggests a close cooperation between colour and form analysis during chromatic adaptation. The results are discussed in relation to the "segregated pathway hypothesis" and the role of spatial aspects for the computation of colour constancy and adaptation to natural scenes.