Influence of visual background on discrimination of signal-relevant colours in zebra finches (Taeniopygia guttata)

Abstract

Colour signals of many animals are surrounded by a high-contrast achromatic background, but little is known about the possible function of this arrangement. For both humans and non-human animals, the background colour surrounding a colour stimulus affects the perception of that stimulus, an effect that can influence detection and discrimination of colour signals. Specifically, high colour contrast between the background and two given colour stimuli makes discrimination more difficult. However, it remains unclear how achromatic background contrast affects signal discrimination in non-human animals. Here, we test whether achromatic contrast between signal-relevant colours and an achromatic background affects the ability of zebra finches to discriminate between those colours. Using an odd-one-out paradigm and generalized linear mixed models, we found that higher achromatic contrast with the background, whether positive or negative, decreases the ability of zebra finches to discriminate between target and non-target stimuli. This effect is particularly strong when colour distances are small (less than 4 ΔS) and Michelson achromatic contrast with the background is high (greater than 0.5). We suggest that researchers should consider focal colour patches and their backgrounds as collectively comprising a signal, rather than focusing on solely the focal colour patch itself.

Keywords: background; chromatic; contrast; distance; frequency; model.

Figure 1.

Colourful signals with achromatic backgrounds in birds. Many species of birds have colourful signals or ornaments displayed against largely achromatic surrounding backgrounds that may affect detection and discrimination of the signal. (a) Painted finch (Emblema pictum), (b) European robin (Erithacus rubecula), (c) red-bellied woodpecker (Melanerpes carolinus), (d) red-winged blackbird (Agelaius phoeniceus), (e) zebra finch (Taeniopygia guttata) and (f) tufted titmouse (Baeolophus bicolor). All images are from Wikimedia Commons. (Online version in colour.)

Figure 2.

Effect of achromatic background contrast on brightness discrimination. Evidence from humans suggests that stimuli are easiest to discriminate when placed on a background with low brightness contrast [37,54]. We can demonstrate this effect using equally spaced achromatic stimuli on a light grey (a) and dark grey (b) background. The arrows mark the brightness of the background relative to the series of stimuli. Stimuli that are closer in brightness to each background are more easily discriminable, and this effect is most pronounced when the background has an intermediate brightness to the two stimuli. This effect is apparent with colour stimuli, as well, but because digital displays are highly variable, we opt here to show only the achromatic version of this effect.

Figure 3.

Diagram of a foraging grid. Birds were presented with a foraging grid containing eight coloured stimuli. The two target stimuli (T) were a different colour than the six non-target stimuli (NT). All trials were repeated on three different backgrounds (B): black, white and grey. To calculate background contrast for our analyses, we took the geometric mean double cone catch from the target and non-target stimuli (given as the square root of the product of the two double cone catches) and used it and the double cone catch of the background to calculate Michelson contrast. The second value used in our models was the achromatic Michelson contrast between the target and non-target stimuli. (Online version in colour.)

Figure 4.

Discrimination test results. Mean (+/− s.d.) pass frequency over 10 one-apart (a) and two-apart (b) discrimination trials per bird. Results are segmented by the background and, generally, pass frequencies for darker colours are lower on brighter backgrounds, and vice-versa. The dotted line on both graphs represents the expected pass frequency if birds chose stimuli randomly (1 in 24).

Figure 5.

Model predictions of pass frequency. Using a LOESS model [23] to predict pass frequency as a function of background contrast (the achromatic Michelson contrast between the target stimuli and background) and chromatic distance between the colour stimuli, we see that high background contrast (greater than 0.6) reduces predicted pass frequency for a given chromatic distance, particularly when the chromatic distance is small. The dark grey box represents the range of background contrast values that the Munsell colours used in this experiment would produce against the grey colour of male zebra finches. (Online version in colour.)