PHOTORECEPTION

any of the biological responses of organisms to stimulation by light. Most organisms, including humans, respond to visible light; some react to wavelengths of light not seen by humans; and still others can react to properties of light not detectable by humans, such as polarization (vibration of light waves in a definite pattern). This article is concerned with the sensory processes by which animals detect information carried by light. Light energy is necessary for life on Earth. Green plants require light for photosynthesis, the process by which water and carbon dioxide are transformed into carbohydrates; plants also show adaptive responses (e.g., germination and flowering) to annual changes in daily light periods. Animals depend on plants for food and thus are indirectly dependent upon photosynthesis. In some animals, response to variations in day length is of great importance in the regulation of annual reproductive cycles. (For additional information about the above responses to light, see photosynthesis and stereotyped response.) Light, the name given to the mediator of the sensation of sight in higher animals, including man, and the equivalent of this sensation in lower animals, is the part of the electromagnetic spectrum that is visible to animals; it includes the range of wavelengths from about 300 nanometres (1 nm = 10-6 millimetre) in the near ultraviolet to about 700 nanometres in the deep red (300 nanometres is beyond violet and does not evoke sensation in the human eye). The entire cell of a unicellular animal such as Amoeba may be sensitive to light so that the cell moves toward or away from it. Some unicellular animals (e.g., Euglena) have developed a light-sensitive receptor, or eyespot-a region with a lower threshold for light stimulation than occurs in the rest of the cell. Some multicellular animals have photoreceptive cells, or eyespots, scattered in various parts of or throughout the body; those in the outer covering of the earthworm (Lumbricus) serve in directional orientation, which involves comparison of light intensities at different directions. Most animals have localized photoreceptors of varying complexity-e.g., the ocellus of certain mollusks and arthropods; the compound eyes of arthropods; and the camera eyes of cephalopods and vertebrates. Evidence indicates that the eyes of certain insects can make use of the information carried by near ultraviolet wavelengths of light as well as that carried by visible wavelengths; both carry information related to the sensation of colour. The eyes of many invertebrates, such as certain arthropods and mollusks, have evolved in such a way that they can detect polarized light; i.e., it evokes a sensation and provides information used for navigation. Visual sensation in higher organisms is primarily a complex response to the intensity and the spatial and temporal distribution of light on the photosensitive retina (the innermost layer of nervous tissue within the eye). Different eyes, different specialized parts of the same eye, and even the same parts of the same eye vary in their responses to illumination. Both the properties of light and those of the eye are thus important determinants of visual sensation. The great differences in the light-analyzing capacities of animals are reflected in the great diversity of gross structural organization involved in photoreception. The fundamental mechanism of photoreception-photochemical activation of a light-receptive pigment and the primary excitation-initiating process-seems to be similar among most animals, however. This section deals with the optical properties of eyes, including the basic arrangements for image formation and light detection and the morphology of photoreceptors; the photochemistry of light detection; and the physiological functioning of the receptor cells that initiate nervous activity. These initial processes of photoreception provide information for the neural centres of the retina and higher nervous centres. The neural events involving the higher centres lead to visual perception. any of the biological responses of organisms to stimulation by light. In plants the primary photoreceptive response is photosynthesis, the conversion of carbon dioxide and water to the essential nutritive elements of all life, using the energy of the Sun. A secondary consequence of photoreception in plants and in many animals is the indirect determination of growth and reproductive patterns by annual cyclic fluctuations in the availability of sunlight. Most frequently, however, the term photoreception is used in reference to the mechanism by which animals receive sensory data transmitted by light of different qualities and wavelengths, only some of which are perceptible by humans. The apparatus for the reception of light may involve an entire organism, as in many single-celled animals, or may range in specialization and complexity from the simple eyespots, or light-receptors, distributed over the surface of some animals, to the intricate structure of the mammalian eye. The operation of most eyes relies on the chemical response of a light-sensitive pigment that begins a chain of nerve responses. The camera-like apparatus of vertebrate eyes uses a lens and an external layer of transparent membrane, the cornea, to focus and project an inverted image onto the retina, a lining within the eye structure. The size of the lens and its distance from the photosensitive retina are fixed in a ratio that provides the optimal balance between light-gathering and image-focussing capacities for each organism, according to its habits and its environment. In nocturnal animals such as the opossum, for instance, a wide lens relatively near the retina allows for a high light-gathering ability, but results in poor resolution. The amount of light entering the eye is also determined by the diameter of the pupil, the central channel through which light enters, which is controlled by the musculature of the surrounding iris, the coloured portion of the eye. In humans, because the pupil retains its circular shape, its range of expansion and contraction is limited, while in some nocturnal species such as cats the pupil contracts into a slit that can block nearly all light. Image reception in the vertebrate retina is controlled by two types of photoreceptor cells: the longer rod cells are responsive to light under relatively dark conditions (scotopic reception), and cone cells are responsible for colour perception and for vision under brighter conditions (phototopic reception). The two types are found together on the retinas of animals that are active both day and night. Animals that function nocturnally have retinas composed mostly of rods. Diurnal vertebrates focus light on the fovea, a retinal depression containing only cones. The photoreception of both rods and cones is controlled by an outer sheath of disks known as lamellae. The optic structure of cephalopods, such as the octopus, is essentially the same as in vertebrates, though the retina is differently organized. The most rudimentary optical apparatus is the eyespot, found in many unicellular animals and some primitive worms. Its basic structure in multicellular animals consists of an internal pocket coated with photosensitive pigment, known as a rhabdomere. A more sophisticated form of eye is the ocellus, found in mollusks and arthropods, which, like the eye of the vertebrates, inverts the image it focusses on a photosensitive membrane. Certain worms and arthropods make use of compound eyes, which consist of a number of visual structures known as ommatidia that are densely set together. These take a rectangular or octagonal shape, and contain components analogous to those of the more complex, camera-like vertebrate eye. Each one is capable of receiving light independently, and they are sometimes separated by a thin film of pigment, as in the appositional eye of the honeybee, or by cilia that divide the rhabdomeres of different cells. Photoreceptor cells absorb light through a layer of pigment and convert it into a stimulus directed toward the nervous system. In many photoreceptors small, hairlike projections, cilia, help in the internal and intercellular transport of essential substances. This is the case with mitochondria, minute structures that produce the adenosine triphosphate needed for photoreception. The function of the cilia is performed in vertebrate rod and cone cells by the surrounding lamellae. In rod cells old lamellae are replaced by new ones throughout the life of the cell, while in cone cells disks are renewed through the replacement of exhausted tissue materials. Photoreceptor cells in vertebrates are connected by means of a neural projection known as a foot piece, to the retina, which is itself an extension of the brain. The photoreceptors of invertebrates are linked to nerves or nerve ganglia within the eye through thin fibres called axons. The pigment responsible for nerve excitation in both vertebrates and invertebrates consists of the chemical compound chromophore, which absorbs light, and a protein complex known as opsin. Though the chromophore contained in all visual pigments is virtually identical, variations in the range of wavelength reception by different pigments are the result of differences in the structures of animal proteins. In the conversion of light reception to nervous responses by photoreceptor cells, stimulation by light results in changes in the electrochemical equilibrium of cell membranes, which in turn produce nervous stimuli. Additional reading M.H. Pirenne, Vision and the Eye, 2nd ed. (1967), optics and physiology of vision (vertebrate and invertebrate eyes) for the beginner and nonspecialist; Handbook of Sensory Physiology; vol. 7, pt. 1, H.J.A. Dartnall (ed.), The Photochemistry of Vision; vol. 7, pt. 2, M.G.F. Fourtes (ed.), Physiology of Photoreceptor Organs (1971-72), authoritative treatises by leading scientists; C.G. Bernhard (ed.), The Functional Organization of the Compound Eye (1966), on the optics, morphology, photochemistry, and physiology of the compound eye and other primitive eyes; Gordon L. Walls, The Vertebrate Eye and Its Adaptive Radiation (1942, reprinted 1963), a classic and encyclopaedic store of knowledge of the vertebrate retina and eye; Tsuneo Tomita, "Electrical Activity of Vertebrate Photoreceptors," Q. Rev. Biophys., 3:179-222 (1970), a summary of vertebrate photoreceptor physiology; T.H. Goldsmith and G.D. Bernard, "The Visual System of Insects," in Morris Rockstein (ed.), The Physiology of Insecta (1972), on the visual system of insects, optics, visual pigments, and physiology; three articles on photoreceptor structure and function: M.F. Moody, "Photoreceptor Organelles in Animals," Biol. Rev., 39:43-86 (1964); and Richard M. Eakin, "Lines of Evolution of Photoreceptors," in Daniel Mazia and Albert Tyler (eds.), General Physiology of Cell Specialization (1963), and "Structure of Invertebrate Photoreceptors," in H.J.A. Dartnall (ed.), Photochemistry of Vision (1972); Bradley R. Straatsma et al. (eds.), The Retina (1969), on the morphology, function, and clinical characteristics of the vertebrate retina; Hugh Davson (ed.), The Eye, vol. 2, The Visual Process (1962), a textbook on photoreception and function of the visual system; three works containing research on photoreception and retinal function: Journal of the Optical Society of America, vol. 53, no. 1 (1963); the Cold Spring Harbor Symposia on Quantitative Biology, vol. 30, Sensory Receptors (1965); and the Proceedings of the International School of Physics, "Enrico Fermi," course 43, ed. by W. Reichardt, Processing of Optical Data by Organisms and by Machines (1969); two reviews of visual pigment chemistry: H.J.A. Dartnall, The Visual Pigments (1957); and George Wald, "Molecular Basis of Visual Excitation," Science, 162:230-239 (1968); T.H. Bullock and G.A. Horridge, Structure and Function in the Nervous Systems of Invertebrates, vol. 2 (1966), contains a broad survey of invertebrate sensory receptors; H.K. Hartline, "Visual Receptors and Retinal Interaction," Science, 164:270-278 (1969); Floyd Ratliff, Mach Bands: Quantitative Studies on Neural Networks in the Retina (1965), physiological effects on vision of neural activity in the retina. Physiology of vision is also explored in Jonathan Stone, Parallel Processing in the Visual System (1983); Leo M. Hurvich, Color Vision (1981); Eberhart Zrenner, Neurophysiological Aspects of Color Vision in Primates (1983); Gerald H. Jacobs, Comparative Color Vision (1981). William H. Miller