Color coding in the primate visual pathway: a historical view

Gaining the system: limits to compensating color deficiencies through post-receptoral gain changes

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.

Using equiluminance settings to estimate the cardinal chromatic directions for individuals

Color information is processed by the retina and lateral geniculate along principal dimensions known as the cardinal directions of color space. Normal differences in spectral sensitivity can impact the stimulus directions that isolate these axes for individual observers and can arise from variation in lens and macular pigment density, photopigment opsins, photoreceptor optical density, and relative cone numbers. Some of these factors that influence the chromatic cardinal axes also impact luminance sensitivity. We modeled and empirically tested how well tilts on the individual's equiluminant plane are correlated with rotations in the directions of their cardinal chromatic axes. Our results show that, especially for the SvsLM axis, the chromatic axes can be partially predicted by luminance settings, providing a potential procedure for efficiently characterizing the cardinal chromatic axes for observers.

Color Vision: Decoding Color Space

A new study has used magnetoencephalography to track cortical responses to color as they emerge in time. Similarities and differences within these neural responses parallel characteristics of the perceptual experience of color.

Plasticity in perception: insights from color vision deficiencies

Inherited color vision deficiencies typically result from a loss or alteration of the visual photopigments absorbing light and thus impact the very first step of seeing. There is growing interest in how subsequent steps in the visual pathway might be calibrated to compensate for the altered receptor signals, with the possibility that color coding and color percepts might be less severely impacted than the receptor differences predict. These compensatory adjustments provide important insights into general questions about sensory plasticity and the sensory and cognitive processes underlying how we experience color.

Comparison of two methods of hue scaling

Hue-scaling functions are designed to characterize color appearance by assessing the relative strength of the red versus green and blue versus yellow opponent sensations comprising different hues. However, these judgments can be non-intuitive and may pose difficulties for measurement and analysis. We explored an alternative scaling method based on positioning a dial to represent the relative similarity or distance of each hue from the labeled positions for the opponent categories. The hue-scaling and hue-similarity rating methods were compared for 28 observers. Settings on both tasks were comparable though the similarity ratings showed less inter-observer variability and weaker categorical bias, suggesting that these categorical biases may reflect properties of the task rather than the percepts. Alternatively, properties that are concordant for the two paradigms provide evidence for characteristics that do reflect color appearance. Individual differences on both tasks suggest that color appearance depends on multiple, narrowly tuned color processes, which are inconsistent with conventional color-opponent theory.

Predicting color matches from luminance matches

Color vision and spectral sensitivity vary among individuals with normal color vision; thus, for many applications, it is important to measure and correct for an observer's sensitivity. Full correction would require measuring color and luminance matches and is rarely implemented. However, luminance matches (equiluminance settings) are routinely measured and simple to conduct. We modeled how well an observer's color matches could be approximated by measuring only luminance sensitivity, since both depend on a common set of factors. We show that lens and macular pigment density and $L/M$L/M cone ratios alter equiluminance settings in different ways and can therefore be estimated from the settings. In turn, the density variations can account for a large proportion of the normal variation in color matching. Thus, luminance matches may provide a simple method to at least partially predict an observer's color matches without requiring more complex tasks or equipment.

The Verriest Lecture: Adventures in blue and yellow

Conventional models of color vision assume that blue and yellow (along with red and green) are the fundamental building blocks of color appearance, yet how these hues are represented in the brain and whether and why they might be special are questions that remain shrouded in mystery. Many studies have explored the visual encoding of color categories, from the statistics of the environment to neural processing to perceptual experience. Blue and yellow are tied to salient features of the natural color world, and these features have likely shaped several important aspects of color vision. However, it remains less certain that these dimensions are encoded as primary or "unique" in the visual representation of color. There are also striking differences between blue and yellow percepts that may reflect high-level inferences about the world, specifically about the colors of light and surfaces. Moreover, while the stimuli labeled as blue or yellow or other basic categories show a remarkable degree of constancy within the observer, they all vary independently of one another across observers. This pattern of variation again suggests that blue and yellow and red and green are not a primary or unitary dimension of color appearance, and instead suggests a representation in which different hues reflect qualitatively different categories rather than quantitative differences within an underlying low-dimensional "color space."

A Neuronal Network Model of the Primate Visual System: Color Mechanisms in the Retina, LGN and V1

Color plays a key role in human vision but the neural machinery that underlies the transformation from stimulus to perception is not well understood. Here, we implemented a two-dimensional network model of the first stages in the primate parvocellular pathway (retina, lateral geniculate nucleus and layer 4C[Formula: see text] in V1) consisting of conductance-based point neurons. Model parameters were tuned based on physiological and anatomical data from the primate foveal and parafoveal vision, the most relevant visual field areas for color vision. We exhaustively benchmarked the model against well-established chromatic and achromatic visual stimuli, showing spatial and temporal responses of the model to disk- and ring-shaped light flashes, spatially uniform squares and sine-wave gratings of varying spatial frequency. The spatiotemporal patterns of parvocellular cells and cortical cells are consistent with their classification into chromatically single-opponent and double-opponent groups, and nonopponent cells selective for luminance stimuli. The model was implemented in the widely used neural simulation tool NEST and released as open source software. The aim of our modeling is to provide a biologically realistic framework within which a broad range of neuronal interactions can be examined at several different levels, with a focus on understanding how color information is processed.

Chromatic clocks: Color opponency in non-image-forming visual function

During dusk and dawn, the ambient illumination undergoes drastic changes in irradiance (or intensity) and spectrum (or color). While the former is a well-studied factor in synchronizing behavior and physiology to the earth's 24-h rotation, color sensitivity in the regulation of circadian rhythms has not been systematically studied. Drawing on the concept of color opponency, a well-known property of image-forming vision in many vertebrates (including humans), we consider how the spectral shifts during twilight are encoded by a color-opponent sensory system for non-image-forming (NIF) visual functions, including phase shifting and melatonin suppression. We review electrophysiological evidence for color sensitivity in the pineal/parietal organs of fish, amphibians and reptiles, color coding in neurons in the circadian pacemaker in mice as well as sporadic evidence for color sensitivity in NIF visual functions in birds and mammals. Together, these studies suggest that color opponency may be an important modulator of light-driven physiological and behavioral responses.

Colour in the eye of the beholder: receptor sensitivities and neural circuits underlying colour opponency and colour perception

Colour vision-the ability to discriminate spectral differences irrespective of variations in intensity-has two basic requirements: (1) photoreceptors with different spectral sensitivities, and (2) neural comparison of signals from these photoreceptors. Major progress has been made understanding the evolution of the basic stages of colour vision-opsin pigments, screening pigments, and the first neurons coding chromatic opponency, and similarities between mammals and insects point to general mechanisms. However, much work is still needed to unravel full colour pathways in various animals. While primates may have brain regions entirely dedicated to colour coding, animals with small brains, such as insects, likely combine colour information directly in parallel multisensory pathways controlling various behaviours.

A simple principled approach for modeling and understanding uniform color metrics

An important goal in characterizing human color vision is to order color percepts in a way that captures their similarities and differences. This has resulted in the continuing evolution of "uniform color spaces," in which the distances within the space represent the perceptual differences between the stimuli. While these metrics are now very successful in predicting how color percepts are scaled, they do so in largely empirical, ad hoc ways, with limited reference to actual mechanisms of color vision. In this article our aim is to instead begin with general and plausible assumptions about color coding, and then develop a model of color appearance that explicitly incorporates them. We show that many of the features of empirically defined color order systems (those of Munsell, Pantone, NCS, and others) as well as many of the basic phenomena of color perception, emerge naturally from fairly simple principles of color information encoding in the visual system and how it can be optimized for the spectral characteristics of the environment.

Advances in understanding the molecular basis of the first steps in color vision

Serving as one of our primary environmental inputs, vision is the most sophisticated sensory system in humans. Here, we present recent findings derived from energetics, genetics and physiology that provide a more advanced understanding of color perception in mammals. Energetics of cis-trans isomerization of 11-cis-retinal accounts for color perception in the narrow region of the electromagnetic spectrum and how human eyes can absorb light in the near infrared (IR) range. Structural homology models of visual pigments reveal complex interactions of the protein moieties with the light sensitive chromophore 11-cis-retinal and that certain color blinding mutations impair secondary structural elements of these G protein-coupled receptors (GPCRs). Finally, we identify unsolved critical aspects of color tuning that require future investigation.

The discovery of spectral opponency in visual systems and its impact on understanding the neurobiology of color vision

The two principal theories of color vision that emerged in the nineteenth century offered alternative ideas about the nature of the biological mechanisms that underlie the percepts of color. One, the Young-Helmholtz theory, proposed that the visual system contained three component mechanisms whose individual activations were linked to the perception of three principal hues; the other, the Hering theory, assumed there were three underlying mechanisms, each comprising a linked opponency that supported contrasting and mutually exclusive color percepts. These competing conceptions remained effectively untested until the middle of the twentieth century when single-unit electrophysiology emerged as a tool allowing a direct examination of links between spectral stimulation of the eye and responses of individual cells in visual systems. This approach revealed that the visual systems of animals known to have color vision contain cells that respond in a spectrally-opponent manner, firing to some wavelengths of stimulation and inhibiting to others. The discovery of spectral opponency, and the research it stimulated, changed irrevocably our understanding of the biology of color vision.