Stability of the color-opponent signals under changes of illuminant in natural scenes

Abstract

Illumination varies greatly both across parts of a natural scene and as a function of time, whereas the spectral reflectance function of surfaces remains more stable and is of much greater relevance when searching for specific targets. This study investigates the functional properties of postreceptoral opponent-channel responses, in particular regarding their stability against spatial and temporal variation in illumination. We studied images of natural scenes obtained in UK and Uganda with digital cameras calibrated to produce estimated L-, M-, and S-cone responses of trichromatic primates (human) and birds (starling). For both primates and birds we calculated luminance and red-green opponent (RG) responses. We also calculated a primate blue-yellow-opponent (BY) response. The BY response varies with changes in illumination, both across time and across the image, rendering this factor less invariant. The RG response is much more stable than the BY response across such changes in illumination for primates, less so for birds. These differences between species are due to the greater separation of bird L and M cones in wavelength and the narrower bandwidth of the cone action spectra. This greater separation also produces a larger chromatic signal for a given change in spectral reflectance. Thus bird vision seems to suffer a greater degree of spatiotemporal "clutter" than primate vision, but also enhances differences between targets and background. Therefore, there may be a trade-off between the degree of chromatic clutter in a visual system versus the degree of chromatic difference between a target and its background. Primate and bird visual systems have found different solutions to this trade-off.

Escape Distance in Ground-Nesting Birds Differs with Individual Level of Camouflage

“…because pattern is widely thought to be encoded by achromatic vision (Stoddard and Stevens 2010). Ferret luminance was taken as the L cone sensitivity (L-cones outnumber S-cones 14:1 (Calderone and Jacobs 2003)), human luminance was taken as (L+M)/2 (Lovell et al 2005), and peafowl luminance was taken as double cone sensitivity (Hart 2002). Luminance distribution differences (Luminance diff ) were calculated by comparing absolute differences in counts of the numbers of pixels in each target (plover egg or adult nightjar plumage) to its background at 32 linear levels of luminance (lum) from 0% to 100%:…”

Color contrast and stability as key elements for effective warning signals

“…Our prediction was that warning signals are often red, orange, or yellow, because these colors have higher contrasts than colors such as blue or white against green natural backgrounds, maximizing their conspicuousness (Endler, 1992;Endler and Mappes, 2004;Stevens and Ruxton, 2012). Illuminant spectra are known to vary with time of day and atmospheric conditions (Lovell et al, 2005;Nieves et al, 2012). Short wavelengths are expected to be less stable than longer wavelengths as atmospheric particles and cloud cover alter the contribution of Rayleigh-scattered sunlight.…”

“…Studies have demonstrated that color opponency evolved as a mechanism to detect important components of a visual scene (Lovell et al, 2005). Such mechanisms can maximize the perception of color contrast, where achromatic information is unreliable due to spatial and temporal variation: for example ripe fruit against leafy backgrounds (Mollon, 1989;Maximov, 2000;Lovell et al, 2005).…”

“…Moreover, the evolution of trichromatic vision in primates is thought to be convergent to that of birds for a fruit/leaf based diet (Mollon, 1989;Maximov, 2000;Osorio et al, 2004). As such, it seems likely that birds and humans share some analogous opponent channels, and past work has modeled how these channels may encode colors to birds and primates (Lovell et al, 2005). Additional opponent systems have also been described for tetrachromatic turtles that share similar visual systems to birds (Ammermüller et al, 1998;Ventura et al, 2001).…”

“…Lovell et al (2005) compared the visual perception of humans (or old world primates with similar vision) with starlings, using ripe and unripe fruits photographed over a day. They found that red fruit is especially contrasting when processed by the RG opponent system.…”