Search Results

This chapter addresses why animals have evolved different numbers, ranges, and placements of spectral channels in their color-vision systems. It also examines the factors, such as water transmission, visual task, phylogeny, and activity patterns, that drive the evolution of such diverse modes of seeing color. Even in the absence of any color sense, trees are still visible, as most of the information in natural scenes can be gained from achromatic cues alone. Color vision, however, gives an animal more information, allowing it to make quicker and more informed decisions. The chapter attempts to disentangle man's experience of color from that of other animals to provide an objective measure of what color vision is and how evolution has molded its variety of forms.

The late Knut Nordby was a real-life counterpart of Mary: a color-blind expert in the science of color vision. This chapter describes the results of empirical research on color vision and other sense modalities. Based on these results and his own experience, he argues that Mary will be able to sense and discriminate color hues, but will not be able to name them on the basis of her knowledge. He does not take a definite stand on the epistemic and metaphysical gaps. But his reflections should help inform views on these matters.

A study of the visual system is often divided into topics which are then treated as discrete fields. Two of these are spatial vision and color vision. Such an arbitrary division leads one to overlook the very important areas of intersections between the fields. This chapter considers the intersection between color vision and spatial vision. Topics discussed include color spatial contrast sensitivity function (CSF), significance of color CSFs for vision, whether color contrast sensitivity function reflects the filter characteristic of a single channel or the envelope of the sensitivities of multiple channels, and luminance-color interactions.

This chapter adopts a psychophysical approach to studying color vision and reviews the evolution of color perception in vertebrates. Color vision in goldfish is described as an example before data from other vertebrate species are given for comparison. Comparing the color vision systems in different vertebrate taxa based on behavioral data indicates that a highly developed trichromatic or tetrachromatic color vision must be a very old invention of vertebrates, as it occurs in fishes, amphibia, reptiles, and birds. In mammals, this type of color vision is widely absent and had obviously been reinvented by Old World primates.

This chapter examines how color interacts with the other visual signals from floral size, shape, and outline in enhancing floral attraction to visitors. Visual attraction by flowers is linked to floral shape and size. For most pollinators, color and color patterns are attractive. Most of today’s key pollinating taxa have good color vision, and flowers should have been selected to interact with their visitors’ visual abilities. The chapter first considers floral pigments and floral color before discussing the problems of defining and measuring color in flowers. It then explains how animals perceive flower color and proceeds by analyzing color preferences in animals, along with the ecology and evolution of flower color and color preferences. It also explores nectar guides, how floral color change can control pollinators, and other visual cues used for advertisement. Finally, it asks why flower colors diverge, citing the role of selection.

This chapter describes the evolution of vertebrate “camera” eyes and concentrates on color vision and visual pigments. The vertebrate camera eye with a lens, a variable pupil aperture, and a photosensitive receptor layer in the retina, evolved in primitive jawless fish under relatively bright light in shallow seas. With the broad spectral range of daylight, four spectral classes of cone photoreceptor rapidly evolved, offering the benefit of tetrachromatic color vision in order to take full advantage of the visual information available in the environment. This highly successful design has been greatly modified as vertebrates evolved into all the major classes, extending their environmental range into the oceans, the deep sea, freshwater, terrestrial habitats, and the air.

Color vision is trichromatic, as a result of three kinds of cone photoreceptors with different spectral sensitivities. The output of the long-wavelength cones is subtracted from the output of the middle wavelength cones to give a red/green opponent color signal in the retina, and another subtraction in different cells gives a yellow/blue opponent color signal. These signals are conveyed to the lateral geniculate nucleus without much change, then to V1, where double opponent (opponent for color in the center of the receptive field, with the opposite sign of response in the surround) cells are found in upper layers in what are called blobs. The blobs project to the thin stripes in V2, then to clusters of color-coded cells in V4, and to inferotemporal cortex. Lack of one type of cone leads to color blindness, and lesions near V4 leads to achromatopsia.

This chapter reviews evidence supporting the hypothesis that color vision is a kind of social vision, about the emotional states and moods of others. The argument from the first of the three main sections of the chapter centers on evidence that our perception of skin is organized so that we are maximally able to discriminate spectral changes around baseline skin color, akin to the way in which we are maximally able to discriminate temperature changes of skin around baseline skin temperature. The argument from the second section concerns evidence that our primate cone sensitivities have been designed by natural selection to sense the subtle spectrum changes that occur with modulations of hemoglobin oxygenation. Finally, the third section presents evidence that it is the primates with color vision that are bare-skinned; the nontrichromatic primates are furry faced, like a typical mammal. This is just as the hypothesis would predict if color vision is about skin.

Visual coding changes or adapts its properties in response to specific properties of the prevailing stimulus. Adaptation adjusts sensitivity at multiple levels of the visual system and to multiple aspects of the stimulus. This chapter considers how adaptation alters visual sensitivity, and what this can tell us about our colour vision. First, the chapter reviews the general characteristics of two distinct forms of visual adaptation, light adaptation and constrast adaptation, and considers how they combine to influence colour perception. It also considers how these two forms of adaptation adjust our colour vision to the natural visual environment.

Colour constancy is the tendency for objects to maintain stable colour appearances, despite considerable variations in the physical and neural signals mediating colour vision. Two major types of variation arise from changes in the light illuminating the objects, and from changes in the backgrounds against which the objects are seen. This chapter focuses on the problem of achieving colour constancy when both backgrounds and illuminants may be varying. While it may be easy if only one of these can change, the challenge for colour constancy to succeed in the real world is to simultaneously handle both types of change.

This chapter relates the processes of colour vision to the characteristics of the natural environment. The reference to natural scene statistics relates the two main parts of the chapter — the first five sections concerned with colour discrimination, and the seventh section concerned with colour appearance and its transformation or constancy under changes of illumination.

In this chapter the theme of structural genomic variation is continued with a particular focus on segmental duplications. The origins and extent of segmental duplications in humans and other species is described. Evidence of the important role played by segmental duplications in gene creation and evolution are described. Models for evolution of multigene families are discussed involving gene duplication with a description of olfactory and globin supergene families, together with the immunoglobulin gene family and how remarkable diversity of immunoglobulin proteins is achieved through a number of different mechanisms. The relationship between segmental duplications and chromosomal rearrangements, genomic disorders and copy number variation is discussed before a detailed review of the role of segmental duplication, deletion and gene conversion in colour vision and Rhesus blood groups. The final part of the chapter describes insertion/deletion (indel) polymorphisms notably involving sequence level variation; the extent and importance of such diversity is reviewed.

This chapter discusses research results, mainly from the author's research team, providing evidence for the first stages of the neurobiological model, showing sequential onset of different selective channels in visual cortex. Colour processing is weak or absent at birth, but emerges in the first three months. The novel VEP method developed in the Visual Development Unit is described, showing the onset of cortical orientation selectivity, with related behavioural evidence for infants' orientation discrimination. For motion selectivity, optokinetic nystagmus (OKN) shows an early subcortical directional mechanism, with characteristic monocular asymmetries. At two to three months, VEP evidence shows a cortical directional mechanism, which also contributes to symmetrical OKN. VEP and behavioural methods show cortical responses to binocular correlation and disparity, first appearing at three-four months. This is discussed in relation to vergence control, strabismus, stereo depth perception, and how the pre-binocular cortex may be organized.

This chapter demonstrates that human surface colour perception can be modelled as algorithms that, over certain ranges of environmental conditions, manage to assign colours to objects that are in correspondence with specific, objective properties of the object's surface, called intrinsic colours. The chapter suggests that under certain circumstances, human observers do seem to estimate intrinsic surface colours accurately. Environmental constraints permit us to succeed in perceiving stable surface colours. These constraints can be thought of as a list of precise assertions concerning a visual scene. If all of the assertions on the list are true of the scene, then human colour vision, confined to a specified environment, will assign colours to surfaces in that scene that are the same as those it assigns to these surfaces in another scene that also satisfies these assertions.

This chapter describes a psychophysical approach to studying the limits of the perceptual abilities of birds. Birds live in an extremely rich visual world that in many ways is similar to ours, but which also differs from our sensory experiences in a variety of important features. One major difference is the superiority of their color vision. With regard to their spatial vision, for most birds, it is inferior to ours, except that small birds can get much closer to an object and still keep it in focus, so that they get a larger visual image than we do. Thus, even though we have better acuity, many birds can see things that are too small for us to see. Avian temporal vision and sensitivity to achromatic contrast are inferior to ours; but, because of their superior color vision, they may have better chromatic contrast sensitivity than achromatic.

Vision is a significant capability of all animals, aiding them to interact in a meaningful way with their environment. With humans, vision is intervened by intricate mechanisms in the eye as well as in the brain. Color vision is an aspect of general vision. The eyes were invented by evolution many times and in different forms. Mammals have camera-type eyes comprising of a hollow spherical body with an opening of variable size covered by a lens whose curvature can be changed by muscular action. The lens focuses light energy onto a highly sensitive spot on the retina that lines the interior wall of the eye. There, light interacts with different kinds of light-sensitive cells.

The implications of multiple realization for scientific methodology have recently been hotly debated. For example, neuroscientists have discovered distinct realizations for what appears to be a single psychological property and some philosophers have recently maintained that in such cases scientists will always abandon commitment to the single, multiply realized psychological property in favour of two, or more, uniquely realized psychological properties. This chapter explores such methodological claims by building on the dimensioned theory of realization and a companion theory of multiple realization. Using concrete cases, this chapter shows that such an ‘eliminate-and-split’ methodology is not always the case in actual practice. Furthermore, this chapter also establishes that whether scientists postulate unique or multiple realizations is not determined by the neuroscience alone, but only in concert with the psychological theory under examination. Thus, in a sense this chapter articulates, in the splitting or non-splitting of properties, psychology enjoys a kind of autonomy from neuroscience.

The deutan-type colour vision deficiencies, deuteranopia, and deuteranomaly are the most common types of colour blindness. A known cause of deutan colour vision defects is the loss of genes encoding the middle wavelength sensitive (M) photopigments. Deutan defects have also been found to be associated with a deleterious point mutation in the M photopigment genes. This chapter tests the hypothesis that M gene expression is absent in all commonly occurring deutan defects. It shows that the commonly occurring deutan defects in which individuals have normal appearing M genes, are caused by a failure to express M pigment. Moreover, it appears that the failure is likely to be complete.