Studying animal color vision: an example

The defining conditions for formal descriptions of color vision, as well as many of the techniques used in testing, were developed from studies of human color vision. To provide a reminder of some of the basic features of color vision, and to illustrate how they can be evaluated in studies of color vision in nonhuman subjects, we consider first an example of such an investigation—a study of color vision in the domestic dog.

Human subjects can simply be asked to say whether lights, surfaces, or objects appear the same or different, or they can be directed to provide ordered descriptions of their perceptions. Establishing a dialog with a nonhuman subject typically requires either a training regimen, the goal of which is to establish a linkage between a visual stimulus and a response or, alternatively, the examination of some naturally occurring behavior in circumstances where the visual stimuli can be specified. With a compliant subject like the dog, effective communication can be easily initiated using operant conditioning procedures (Neitz et al., 1989).

Figure 62.1 summarizes results that characterize several features of dog visual performance. A basic feature is spectral sensitivity (Fig. 62.1A), in which sensitivity to lights of different wavelength content is established. In the dog, this was assessed by progressively decreasing the intensity of a monochromatic light until its presence could no longer be discriminated from a spectrally broadband light to which it had been added. This is not strictly a test of color vision, but the character of the function yields inferences about the nature of the visual system that can, in turn, be linked to color vision. Here the appearance of a sharp decline in sensitivity at a point in the spectrum that separates two regions of higher sensitivity indicates the presence of two different kinds of photopigments in the eye, and it implies that signals initiated by these two have been combined in a spectrally opponent manner. The latter is a hallmark of the neural organization for color vision in a wide variety of species, so its characteristic signature in the dog's spectral sensitivity function is strongly suggestive of the presence of color vision (Chittka et al., 1992; Jacobs, 1981).

Figure 62.1..  

A compilation of measurements relevant to understanding dog color vision. A, spectral sensitivity. B, Color discrimination test. C, Cone pigment absorption spectra. D, Wavelength discrimination. The curves in A and C are normalized to have peak values of 1.0. The details of each of these curves are discussed in the text. (Data taken from Neitz et al., 1989.)

Color vision means that an animal can independently process wavelength and intensity information, and it is typically established by asking an animal to discriminate between lights that consistently differ only in their spectral energy distributions. A key in color vision tests of this sort is to make it impossible for animals to use any additional perceptual cues. The results of such a color vision test in which three dogs were required to discriminate between various monochromatic lights and spectrally broadband lights are shown in Figure 62.1B. For most such combinations discrimination performance was nearly perfect, so dogs must have color vision. Note, however, that they failed the test when they were asked to discriminate a narrow band of wavelengths centered at about 480 nm. Such failure means that their color vision is of a particular type. About 1% of all humans experience a similar failure and, like the dog, they are defined as having dichromatic color vision. Most people, however, can easily discriminate all of these spectral lights from the broadband light. These individuals are classified as trichromatic, and thus the normative color vision of humans and dogs is discretely different.

The failure of dogs to see a difference between a 480 nm light and one that contains all spectral wavelengths reveals a fundamental feature of color vision in all animals—that stimuli having quite different spectral energy distributions may appear the same. Such perceptual identities constitute color matches, and the analysis of such matches has proven central to understanding the nature of color vision. Another color matching experiment was conducted in which dogs were asked to discriminate various additive mixtures of 500 nm and 440 nm lights from a 480 nm light. The result was that, for most proportions, the mixture of 500 nm and 440 nm lights appeared different to dogs than 480 nm lights, but for a particular ratio of 500 nm and 440 nm there was a failure of discrimination, so that combination is said to match in color the 480 nm light. The relative amounts of the two mixture lights at this point define a color matching equation to the 480 nm light. These perceptual identities are conceptually powerful because from them inferences can be drawn about the spectral properties of the underlying cone photopigments. The nature of the color matches indicated that the two cone photopigments in the retina that support dichromatic color vision have spectral peaks at about 429 nm and 555 nm, respectively (Fig. 62.1C). Later direct measurements of dog cone pigments showed that these peak estimates were quite accurate (Jacobs et al., 1993).

The tests described established that the dog has color vision, identified its dimensionality (dichromatic), and yielded some strong indications of important features of the biology of vision in this species. But how acute is their color vision? The acuteness of color vision can be assayed in a number of different ways. Shown in Figure 62.1D are results from one such assessment, measurements of wavelength discrimination in which a determination was made of the size of the wavelength change (Δλ) required for successful discrimination at various locations in the spectrum. The results indicate that at one point, around 480 nm, dogs have quite acute color vision, with differences of 5 nm or less required for successful discrimination. Away from that point discrimination quickly worsens, with the result that dogs are quite blind to wavelength differences over much of the middle and long wavelength portions of the spectrum. Human dichromats who behave in a similar fashion are often characterized as being “red/green color blind.”

This example illustrates how a number of basic features of color vision can be assessed in nonhuman species. These particular laboratory tests, which were easily accomplished for a common mammal, may be difficult or even impossible to apply to other species. In such cases, a range of other behavioral indices can be used. For example, insects like honeybees and wasps visit flowers to harvest nectar and pollen, and this natural behavior has often been exploited in controlled tests to examine insect color vision (Menzel and Backhaus, 1991). Whatever the technique, however, the goals of all animal color vision tests are usually quite similar: to assess the presence of color vision, to determine its dimensionality and acuteness, and to yield inferences about its biological basis and functional utility.