Chapter 5 Summary

Transmission of a reflected light signal from sender to receiver occurs in a series of steps depicted in the ARTS diagram. (1) White light from the sun is modified by attenuation, scattering, and environmental filtering to produce the ambient light spectrum available to the sender. (2) The sender’s surface selectively reflects certain wavelengths, described by a reflected light spectrum. (3) Reflected light is then transmitted a given distance through the environment to arrive at the receiver, and may be degraded by the combined effects of attenuation, scattering, and medium absorption as depicted in a transmission spectrum. (4) The color sensitivity of the receiver’s eye determines the range of detectable wavelengths. The spectra in steps 2–4 are transfer functions which, when multiplied together with the ambient light spectrum in step 1, determine what is actually perceived by the receiver. A parallel pathway also describes the background spectrum arriving at the receiver’s eyes at the same time.
Ambient light varies in hue and overall brightness in different habitats. The color of ambient light is often green in forests, but microhabitats may be bluish, yellowish, or purplish depending on the distribution of vegetation and the time of day. Ambient light in marine environments generally becomes bluer with increasing depth because of the strong absorption of long wavelengths. In shallow coastal and fresh water, bottom characteristics and particulate matter can result in other characteristic colors. Viewing angle (up, down, horizontal) also affects available light in the three-dimensional aquatic world.
The background against which a sender’s body or color patch is viewed has a very large effect on its conspicuousness. Strategies to increase conspicuousness involve maximizing brightness, hue, pattern, or motion contrast. Additional contrast strategies include outlining, symmetrical patterns, contrasting concentric circles, and repetitive patterns. It is to the advantage of prey animals to minimize these types of contrasts.
Transmission of a signal or target depends on the distance between sender and receiver and the combined effects of global attenuation, medium absorption, and scattering, which are expressed by the beam attenuation coefficient. Objects are visible from a greater distance in environments with low attenuation, such as clear air and ocean water. Fog, dust, and other particulate matter reduce transmission distance, along with large opaque environmental objects such as trees and rocks.
Aspects of receiver sensory ability affect the final detectability of a signal. The just-detectable distance of a signal or object depends on its apparent size and the resolving power of the receiver’s eye. Large and contrasting objects are detectable from a greater distance as long as the visual angle subtended by the object is greater than the eye’s angular resolution. If this constraint has been overcome, the contrast threshold for detection depends on receiver color sensitivity. Visual color models are constructed to quantify the perceived hue and brightness contrast between a signal and its background.
Well-developed eyes are complex organs with sophisticated optical and cellular systems for collecting and concentrating light and with intricate nerve connections for spatial pattern analysis and resolution. The first eye took the form of a simple pigment cup eye lined with a retina that could detect the direction of light shining on it but that failed to form an image. Image formation became possible with the development of a focusing mechanism such as the pinhole eye, which suffered from low sensitivity. The advent of the refracting lens greatly improved sensitivity and eventually led to the camera-lens eye, which is both sensitive and capable of producing a well-resolved image. Another development involved multiple cups that were grouped and lengthened into tubes to restrict the angle of light acceptance to a narrow field. This design led to the compound eyes of insects and crustaceans.
A lens composed of transparent tissue is required to collect light over a large area and concentrate it for image formation. Transparent lenses contain specialized cells lacking organelles and other boundary structures that would cause scattering. The cells are densely and uniformly packed with proteins that give the tissue a high refractive index. The lens shape is usually doubly convex to refract light twice and focus the image on the light-sensitive retina. Strategies of accommodation (focusing ability) in camera-lens eyes depend critically on the environment. The cornea performs most of the refraction in terrestrial animals, while the soft lens can change shape for fine-tuning. Aquatic eyes possess a hard round lens that is moved forward and backward.
The energy of electromagnetic radiation in the visible range is captured by the visual pigment rhodopsin. It consists of a bent 11-cis carotenoid derivative (retinal) attached to a large protein (opsin). Upon absorption of a photon of light, the retinal is straightened to the all-trans isomer. This change causes a cascade of reactions that culminates in a nerve impulse. Rhodopsin molecules are embedded in the multiply folded membranes of long, thin photoreceptor cells. The photoreceptor cells are packed side by side in arrays to form a retina that retains the spatial arrangement of the incoming spatial stimulus patterns.
Evaluating the image falling on the retina requires a complex network of nerve connections. Most vertebrates have a duplex retina, with two parallel systems for use under high- and low-illumination situations. The low-light system uses rod receptor cells that are very long and highly sensitive to light. The nerve outputs from many neighboring rods sum together to increase the probability of photon capture and nerve response. While the rod system is highly sensitive in dark environments, its spatial resolution is poor due to the summation. The bright-light system uses cone receptors. Cone outputs are not extensively summated, so a fine-grain image is transmitted. They are connected to neighboring cones in complex additive and negative ways to enhance boundaries in the visual field and detect certain types of temporal and spatial patterns.
A good eye should have high acuity to resolve fine details and be sensitive to light under a wide range of ambient conditions. It is difficult to maximize both features because improvements in one tend to reduce the other. Resolution is maximized by increasing the lens focal length and by decreasing the diameter and length of receptors. Sensitivity is maximized by increasing the eye aperture, decreasing the lens focal length, increasing the diameter and length of receptors, and increasing photopigment density. Sensitivity and resolution therefore trade off against each other. Compromises include the rod/cone system of vertebrates and a central eye region of high resolution (area centralis or fovea) surrounded by a peripheral region of high sensitivity. Other modifications include reflecting tapeta and tubular eyes to improve sensitivity, and lenses that correct for spherical and chromatic aberration.
Field of view depends critically on the placement of eyes on the head. Lateral placement maximizes total field of view up to 360° but permits only a small region of binocular overlap. Front-facing eyes maximize binocular overlap. Temporal resolution, or flicker fusion rate, is much higher in cones than in rods, and is higher in rhabdomeric photoreceptors than in ciliary receptors.
Color perception requires at least two sets of photoreceptor cells with clearly different spectral sensitivities, and more complete chromatic differentiation requires three or four sets. Monochromats possess only one pigment type; they perceive intensity differences, but not color. Dichromats possess two visual pigments with different but partly overlapping absorbance curves. Color discrimination is based on chromatically opponent cells, ganglion cells that either add or subtract the neural output from neighboring photoreceptors of each color type. Ganglion cells receiving opposite-signed input from two cone types are responsible for hue discrimination, whereas ganglion cells receiving same-signed input encode brightness and saturation information. Dichromats can distinguish a wide range of colors but possess a neutral point in the middle of their spectral range. Trichromats have three photopigments and thus two chromatically opponent systems. They can perceive a wider range of colors and have no neutral point. Species with 4–5 receptor types frequently use certain wavelengths for special purposes.
The process of visual spatial analysis and object recognition proceeds through multiple layers of the visual organ and brain nuclei, beginning with edge detectors in the retina, evaluation of contours and outlines, and the addition of color and fine structure resolution. Recognition of objects appears to occur over a large and diffuse specialized region of the brain, rather than by a hierarchical process ending in a small number of cells. Specialized feature detectors, involving a small number of dedicated cells, seem to be limited to the detection of specific types of motion and are coupled to motor actions requiring fast responses, such as predator evasion or prey attack.
Depth perception is important for predators and fast-moving or flying animals. Such animals have evolved binocular vision, in which the visual fields of the two eyes overlap. The cost is a lower overall field of view; thus herbivores and prey species usually have eyes on the sides of their heads. These animals must use relative object size or parallax to judge distances.
Most of the properties of visual receptors are determined by the environment, lifestyle, and diet of the animal. Nocturnal animals must maximize sensitivity and therefore usually have reduced spatial and temporal resolution. Diurnal animals are adapted to high ambient light levels and can therefore maximize resolution and color vision. Predators and fast-moving animals require better depth perception and spatial and temporal resolution than prey species. These trade-offs largely determine the optics and wiring of the visual system, and therefore the opportunity to use the visual modality for communication.