Vision (comparative physiology)

Often taken for granted, vision is one of the most interesting and complex sensory systems. Over the course of their evolution, each species independently fine-tuned their visual systems to adapt to their unique environments and needs. An excellent visual system system can be the factor dictating a species’ survival. Vision is essential for many animal behaviors, such as foraging for food, prey avoidance, and mate choice. Considering the diversity of Earth's species, it is evident that there must be many different types of vision; for example, a fish in its unique underwater environment would have a vastly different visual world than an insect in the rainforest.

When animals forage for food, vision is used along with many other senses, especially smell and hearing, to make a good food choice. Many animals scan a visual field before deciding to forage; thus, the visual system must be acute, enabling the animal to understand what is in its field of view, often in a very short time. The brain must be able to determine objects' shape, form, and color. Many animals utilize color vision when foraging. Cues about the suitability of a food choice are often indicated by color, for example, the difference between a poisonous berry and an innocuous one. Eye placement (the exact positioning of eyes on the head) is crucial; appropriate positioning of the eyes from each other, as in humans, makes binocular vision possible. Binocular vision gives the viewer a sense of depth in the field of view, which is critical for many animals when catching prey. Species with laterally spaced eyes have less binocular vision, and some of their visual field has to be viewed monocularly.

Vision is essential in most interspecies communication. Many species use body markings and displays as mating signals. Birds provide a striking example of visual communication in animals, where specific body markings, a dance, or an offering of food attract mates. Some birds attract mates by building elaborate nests; the bird with the most elegant nest attracts a mate and ensures reproduction and the passing of its genes to the next generation.

The Physiology of Vision

In simple terms, the visual system takes a signal in the form of light and translates it into a chemical change and later a nervous impulse in the brain; this nervous impulse is what the animal perceives as sight. The two main characteristics of eyes, whether complex or simple, are light-sensitive receptors (photoreceptors) and a mechanism to control light. In simple eyes, the light-sensitive receptors make up a layer known as the retina. The nature of a photoreceptor is dictated by opsin, the photosensitive proteinaceous pigment within the membranes in the photoreceptor. The photoreceptors are variable in size, shape, and content. Vertebrates have two types—rods and cones. Rods are especially sensitive to light, making up the majority of the retina of nocturnal species, requiring as sensitive a system as possible. The photopigment in rods is called rhodopsin. Cones are less sensitive to light, but different types are sensitive to light of various wavelengths (or colors) and can give animals color vision. There are many types of cones, and hence, many types of color vision; possessing these cone types and their specific positioning within the retina is an evolutionary adaptation in animals who benefit from color vision.

There are two main types of eye design in the animal kingdom: simple eyes and compound eyes. Simple eyes have a single layer of photoreceptors, which, in the least complicated case, form a cup of photosensitive material. The human eye, with its complex light-focusing apparatus, is still a simple eye. Compound eyes, which are present in most insects, have many separate optical units called ommatidia. Each ommatidium has a rhabdom, containing a group of up to nine tubular rhabdomeres with ciliary or microvillar (finger-shaped) photoreceptors. The orientation of groups of photosensitive cilia is structured, often with pairs of rhabdomeres organized at right angles to each other. This is especially key in the analysis of polarized light.

Many visual systems have mechanisms to control light. Restricting the amount of light entering the eye is useful; the opening through which light enters is referred to as an aperture, as in a camera. Many animals have a contractible iris, which constricts and dilates to control light entry through the pupil. For example, in dim light conditions, the iris can dilate and let in as much light as possible. Some eyes have lenses that enable light to be focused on the retina, allowing for better resolution of objects in the visual field. Many can change the shape of the lens to bring an image to focus on the retina, a process called accommodation. Other animals use a cornea to bend light onto the retina, although the cornea is rigid and cannot change shape. On the other hand, many species with much simpler eyes do not possess any kind of light control apparatus.

Upon reception of light by a photoreceptor, a biochemical cascade of events occurs within the photoreceptor itself, which amplifies the original signal received. The result of this cascade is a nervous signal which proceeds through many neural layers to the brain. Throughout most of the retina in the simple eyes of vertebrates, several photoreceptors connect to one neuron (convergence), but there is often an area of the retina where one photoreceptor connects to one neuron. This area is called the fovea and is the part of the retina that has the best acuity. The area within the retina, which comprises the fovea, is variable. Fish, dogs, cats, and rabbits, among other animals, possess what is known as a “visual streak," which allows excellent, clear vision along a horizontal slice of the visual field at far distances. The streak is a concentrated area of ganglion cells, which, in humans and many mammals, is most dense in the round pupil. Given their habitat and natural predators, this is an ideal adaptation for animals like fish and rabbits.

Information is passed through the nervous system in layers of neurons. The retina is an extension of the brain and contains many nerve cells. The exact arrangement and mechanism of action of neural cell types and the precise pathway to and within the brain differ significantly between species. Phototransduction in vertebrates is different from that of invertebrates, from the arrangement of the retina to the biochemical cascade and the types of neurons involved. Most invertebrates are sensitive to polarized light and see a large range of colors. They adapt well to dark conditions because of an intercellular messenger molecule called Cyclic adenosine monophosphate (cAMP), among other controllers, rather than calcium feedback that occurs in vertebrates.

Color Vision

Reflected natural light has a unique property that may be exploited by the visual system of an animal, namely its wavelength or color. Color vision can be vital for a species regarding mate choice and foraging. Photoreceptors can be sensitive to different wavelengths of light or colors; in vertebrates, the color-sensitive receptors are called cones because of their shape. The maximum sensitivity of a photoreceptor is dictated by the nature of the photo-sensitive protein (opsin) within the receptor. Opsins can be classified according to the approximate wavelength of light that stimulates them maximally. Opsins have been studied that are sensitive to light from the ultraviolet region of the visible spectrum all the way to the far red region.

To have the possibility of color vision, an animal must possess at least two photoreceptors with differing sensitivities. The brain must then be able to compare the outputs of both these receptors and discriminate color. Often, more than two types of photopigment type are present, as in the fish retina, which results in very complex color vision, including sensitivity to ultraviolet light. Color vision has been shown to exist in many animals within the animal kingdom. Positioning and distributing the different types of photoreceptors within the retina are also key to the ability to discriminate color. In vertebrates, this aspect of the retinal structure is called the cone mosaic, and its nature is often closely related to some behavioral aspect of the animal in question. Color vision also requires a central processing system that can decode the various light signals and turn them into a brain output that is useful to the animal.

Researchers at the University of Sussex and George Mason University successfully developed a camera system that allows users to experience how other animals see colors. Using four color channels, the camera records images in the same way various eyes in the animal kingdom would experience them. In the hopes that other scientists will build on this technology and further the science, the software they developed is available open-source.

Principal Terms

Accommodation: changing the shape of the lens to keep objects at different distances focused on the retina

Fovea: area, often a pit in the retina, of maximal acuity, where each photoreceptor has its own nerve cell, as opposed to many receptors converging on one nerve cell

Opsin: a membrane-bound protein or pigment which absorbs light

Photon: a unit used to describe light intensity

Photoreceptor: cell containing membranes that house light-sensitive pigments

Retina: the light-sensitive film at the back of the eye

Bibliography

Barber, A. L., et al. "Functional Performance of the Visual System in Dogs and Humans: A Comparative Perspective." Comparative Cognition & Behavior Reviews, vol. 15, 2020, pp. 1–16. doi.org/10.3819/CCBR.2020.150002.

Baylor, D. “How Photons Start Vision.” Proceedings of the National Academy of Science, vol. 93, Jan. 1996, pp. 560-565. doi:10.1073/pnas.93.2.560.

Buschbeck, Elke, and Michael Bok. Distributed Vision: From Simple Sensors to Sophisticated Combination Eyes. New York City, Springer, 2023.

Jacobs, G. H. “The Distribution and Nature of Color Vision Among the Mammals.” Biological Reviews 68, 1993, pp. 413-471.

Land, Michael F. Eyes to See: The Astonishing Variety of Vision in Nature. Oxford University Press, 2018.

Lodish, H., et al. Molecular Cell Biology. 9th ed., New York City, W. H. Freeman, 2021.

Marshall, J. “Visual Function: How Spiders Find the Right Rock to Crawl Under.” Current Biology 9, no. 24, Dec. 1999, pp. 918-921.

Solomon, E., et al. Biology. 3rd ed., Fort Worth, Saunders College Publishing, 1993.

Thorsberg, Christian. "See the World Through the Eyes of Animals With These Stunning New Videos." SmithsonianMagazine, 25 Jan. 2024, www.smithsonianmag.com/smart-news/see-the-world-through-the-eyes-of-animals-with-these-stunning-new-videos-180983647. Accessed 15 Sept. 2024.