Visual Perception

Physiology, Psychology and Ecology

Vicki Bruce, Mark A. Georgeson, Patrick R. Green

• Publisher: Psychology Press
• Year: 2014
• ISBN:
  9781136917141
• Edition: 4
• Topic: Psychology
• Subtopic: History & Theory in Psychology
• Index: Psychology

This comprehensively updated and expanded revision of the successful second edition continues to provide detailed coverage of the ever-growing range of research topics in vision. In Part I, the treatment of visual physiology has been extensively revised with an updated account of retinal processing, a new section explaining the principles of spatial and temporal filtering which underlie discussions in later chapters, and an up-to-date account of the primate visual pathway.
Part II contains four largely new chapters which cover recent psychophysical evidence and computational model of early vision: edge detection, perceptual grouping, depth perception, and motion perception. The models discussed are extensively integrated with physiological evidence. All other chapters in Parts II, III, and IV have also been thoroughly updated.

Part I
The Physiological Basis of Visual Perception

1
Light and Eyes

All organisms, whether bacteria, oak trees, or whales, must be adapted to their environments if they are to survive and reproduce. The structure and physiology of organisms are not fixed at the start of life; to some extent, adjustments to changes in the environment can occur so as to “fine-tune” the organism’s adaptation. One way of achieving this is through the regulation of growth processes, as when plants grow so that their leaves face the strongest available light. Another way, which is much more rapid and is only available to animals, is movement of the body by contraction of muscles.

If the movement of an animal’s body is to adapt it to its environment, it must be regulated, or guided, by the environment. Thus the swimming movements of a fish’s body, tail, and fins are regulated so as to bring it into contact with food and to avoid obstacles; or the movement of a person’s throat, tongue, and lips in speaking are regulated by the speech of other people, linguistic rules and so on.

In order for its movement to be regulated by the environment, an animal must be able to detect structures and events in its surroundings. We call this ability perception, and it in turn requires that an animal be sensitive to at least one form of energy that can provide information about the environment. One source of information is provided by chemical substances diffusing through air or water. Another is mechanical energy, whether pressure on the body surface, forces on the limbs and muscles, or waves of sound pressure in air or water. Further information sources, to which some animals are sensitive but people probably are not, are electric and magnetic fields.

An animal sensitive to diffusing chemicals can detect the presence of nearby food or predators, but often cannot pinpoint their exact location, and cannot detect the layout of its inanimate surroundings. Pressure on the skin and mechanical forces on the limbs can provide information about the environment in immediate contact with an animal, while sound can provide information about more distant animals but not usually about distant inanimate structures.

Sensitivity to diffusing chemicals and to mechanical energy gives an animal considerable perceptual abilities, but leaves it unable to obtain information rapidly about either its inanimate world or about silent animals at a distance from itself. The form of energy that can provide these kinds of information is light, and consequently most animals have some ability to perceive their surroundings through vision. The only large animals able to move about rapidly without vision are bats, dolphins and other cetaceans, which use an echolocation system based on ultrasonic cries (Griffin, 1958), and some fish species living in murky water, which detect objects in their surroundings by distortions of their own electric fields (Heiligenberg, 1973).

We will begin our discussion of visual perception in animals and people by considering first the physical nature of light and then how the environment structures the light that reaches an observer.

Light and the Information it Carries

Light is one form of electromagnetic radiation; a mode of propagation of energy through space which includes radio waves, radiant heat, gamma rays, and X-rays. One way in which we can picture the nature of electromagnetic radiation is as a pattern of waves propagated through an imaginary medium with a velocity of 3 × 10⁸ m/s in a vacuum. Its wavelength ranges from hundreds of metres in the case of radio waves to 10−12 or 10⁻¹³ m in the case of cosmic rays. Only a very small part of this range is visible; for human beings, radiation with wavelengths between 400 and 700 nanometres (1 nm = 10⁻⁹ m) can be seen (Figure 1.1). Light containing only a single wavelength is called monochromatic. If monochromatic light falls on our eyes, we perceive a colour that corresponds to its wavelength, in the way shown in Figure 1.1.

For some purposes, however, the model of electromagnetic radiation as a wave is not appropriate and we must instead treat it as a stream of tiny wave-like particles called photons travelling in a straight line at the speed of light. Each photon consists of a quantum of energy (the shorter the wavelength of the light the larger the energy quantum), which is given up as it strikes another particle. We need these two conceptions of the nature of electromagnetic radiation because nothing in our experience is analogous to the actual nature of it and we must make do with two imperfect analogies at the same time.

FIGURE 1.1 The spectrum of electromagnetic radiation. Wavelengths are given in nanometres (1 nm = 10⁻⁹ m). The visible part of the spectrum is shown on the right, with the perceived colours of different wavelengths of light.

These problems are of no concern in understanding how light is propagated around the environment. For these purposes, we can think of light as made up of rays, which vary in both intensity and wavelength. Rays are emitted from light sources and, in a vacuum, travel in a straight line without attenuation. A vacuum is not a congenial environment for animals, however, and the fate of light rays travelling through natural habitats is more complex.

First, as light passes through a medium, even a transparent one such as air or water, it undergoes absorption, as photons collide with particles of matter, give up their energy and disappear. Absorption is much stronger in water than in air and even in the clearest oceans there is no detectable sunlight below about 1000 metres. Longer wavelengths are absorbed more strongly, so that available light becomes progressively bluer in deeper water.

Second, light is diffracted as it passes through a transparent or translucent medium. Its energy is not absorbed, but instead rays are scattered on striking small particles of matter. Diffraction of sunlight by the atmosphere is the reason why the daytime sky is bright; without an atmosphere, the sky would be dark, as it is on the moon. The blue colour of the sky arises because light of shorter wavelengths is scattered more, and so predominates in the light reaching us from the sky.

Third, the velocity of light is lower when it passes through a transparent medium than when it passes through a vacuum. The greater the optical density of the medium, the lower is the velocity of light. When rays of light pass from a medium of one optical density to a medium of a different density, this change of velocity causes them to be bent, or refracted (unless they strike the boundary between the two media perpendicularly). Refraction therefore occurs at boundaries such as those between air and water, or air and glass, and we will consider it in more detail when we describe the structure of eyes.

Finally, when light strikes an opaque surface, some of its energy is absorbed and some of it is reflected. A dark surface absorbs most of the light falling on it and reflects little, while a light one does the opposite. The way surfaces reflect light varies in two important ways. First, the texture of a surface determines how coherently it reflects light. A perfectly smooth surface such as a mirror reflects light uniformly, but most natural surfaces have a rougher texture, made up of a mosaic of tiny reflecting surfaces set at different angles. Light striking such a surface is therefore reflected in an incoherent way (see Figure 1.2).

Second, a surface may reflect some wavelengths more strongly than others, so that the spectral composition of the reflected light (the relative proportions of wavelengths it contains) differs from that of the incident light. A leaf, for example, absorbs more (and hence reflects less) red light than light of other wavelengths. Note that light is never monochromatic in natural circumstances, and reflection changes the relative proportions of the different wavelengths that it contains. The relationship between the spectral composition of reflected light and the perceived colour of a surface is a very complex one, to which we will return later.

FIGURE 1.2 Regular reflection of rays of light from a polished surface such as a mirror (a) and irregular reflection from a textured surface (b).

Now that we have described the nature of light and the processes governing its travel through space, we turn to ask how it carries information for animals about their environments. A useful concept in understanding this is the ambient optic array, a term coined by Gibson (1966). Imagine an environment illuminated by sunlight and therefore filled with rays of light travelling between surfaces. At any point, light will converge from all directions, and we can imagine the point surrounded by a sphere divided into tiny solid angles. The intensity and spectral composition of light will vary from one solid angle to another, and this spatial pattern of light is the optic array. Light carries information because the structure of the optic array is determined by the nature and position of the surfaces from which it has been reflected.

Figure 1.3 illustrates the relationship between environment and a cross-section through an optic array. The array is divided into many segments, containing light reflected from different surfaces, and differing in average intensity and spectral composition. The boundaries between these segments of the optic array provide information about the three-dimensional structure of objects in the world. At a finer level of detail, each segment of the array will be patterned in a way determined by the texture of the surface from which its light is reflected. Any movement in the environment will cause change in the spatial pattern of the optic array, as the boundaries of some segments move relative to others. This spatiotemporal pattern in the optic array can carry information about the direction, speed, and form of the movement involved.

We have taken as an example an optic array in daylight in an open terrestrial environment, but the same principles apply in any illuminated environment. At night, the moon and stars illuminate the world in the same way as the sun, though with light that is many orders of magnitude less intense. In water, however, there are some differences. First, refraction of light at the water surface means that the segment of the optic array specifying “sky” is of a narrower angle than on land (Figure 1.4). Second, light is absorbed and scattered much more by water than by air, so that information about distant objects is not specified in the pattern of intensities and wavelengths in the optic array. Third, in deep water, light from below is not reflected from the substrate but scattered upwards.

These examples all illustrate one important point; the spatial and temporal pattern of light converging on a point provides information about the structure of the environment and events occurring in it. The speed of light ensures that, in effect, events in the environment are represented in the optic array instantaneously. Only in deep oceans and completely dark caves is no information at all available in light, although the phenomenon of bioluminescence—the emission of light by organisms—means that even in these habitats light may carry information about the biological surroundings.

FIGURE 1.3 Section through the optic array at a point above the ground in an environment containing objects. The optic array is divided into segments through which light arrives after reflection from different surfaces. Each segment has a different fine structure (not shown) corresponding to the texture of each surface.

Up until now, we have considered a point just above the ground, or in open water, and asked what sort of information is available in the optic array converging on it. For an animal at the centre of this optic array to detect any information at all, it must first have some kind of structure sensitive to light energy, and our next topic is the evolution of such structures among animals. How do different kinds of light-sensitive structures allow light energy to influence the activity of animals’ nervous systems, and what scope do these structures have for detecting the fundamental informationcarrying features of optic arrays; spatial pattern and changes in spatial patterns?

FIGURE 1.4 Section through the optic array at a point below the water surface. Because light rays from the sky and sun are refracted at the air– water boundary, they are “compressed” into an angle (A) of 98°. Incident light in angle B has been scattered by water or reflected from underwater objects.

The Evolution of Light-Sensitive Structures

Many biological molecules absorb electromagnetic radiation in the visible part of the spectrum, changing in chemical structure as they do so. Various biochemical mechanisms have evolved that couple such changes to other processes. One such mechanism is photosynthesis, in which absorption of light by chlorophyll molecules powers the biochemical synthesis of sugars by plants. Animals, on the other hand, have concentrated on harnessing the absorption of light by light-sensitive molecules to the mechanisms that make them move.

In single-celled animals, absorption of light can modulate processes of locomotion directly through biochemical pathways. Amoeba moves by a streaming motion of the cytoplasm to form extensions of the cell called pseudopods. If a pseudopod extends into bright light, streaming stops and is diverted in a different direction, so that the animal remains in dimly lit areas. Amoeba possesses no known pigment molecules specialised for light sensitivity, and presumably light has some direct effect on the enzymes involved in making the cytoplasm stream. Thus, the animal can avoid bright light despite having no specialised light-sensitive structures.

Other protozoans do have pigment molecules with the specific function of detecting light. One example is the ciliate Stentor coeruleus, which responds to an increase in light intensity by reversing the waves of beating of its cilia that propel it through the water. Capture of light by a blue pigment causes a change in the membrane potential of the cell, which in turn affects movement of the cilia (Wood, 1976).

Some protozoans, such as the flagellate Euglena, have more elaborate light-sensitive structures, in which pigment is concentrated into an eyespot, but Stentor illustrates the basic principles of transduction of light energy that operate in more complex animals. First, when a pigment molecule absorbs light, its chemi...