Does conspicuousness scale linearly with colour distance? A test using reef fish

Abstract

To be effective, animal colour signals must attract attention—and therefore need to be conspicuous. To understand the signal function, it is useful to evaluate their conspicuousness to relevant viewers under various environmental conditions, including when visual scenes are cluttered by objects of varying colour. A widely used metric of colour difference (Δ S ) is based on the receptor noise limited (RNL) model, which was originally proposed to determine when two similar colours appear different from one another, termed the discrimination threshold (or just noticeable difference). Estimates of the perceptual distances between colours that exceed this threshold—termed ‘suprathreshold’ colour differences—often assume that a colour's conspicuousness scales linearly with colour distance, and that this scale is independent of the direction in colour space. Currently, there is little behavioural evidence to support these assumptions. This study evaluated the relationship between Δ S and conspicuousness in suprathreshold colours using an Ishihara-style test with a coral reef fish, Rhinecanthus aculeatus . As our measure of conspicuousness, we tested whether fish, when presented with two colourful targets, preferred to peck at the one with a greater Δ S ­ from the average distractor colour. We found the relationship between Δ S and conspicuousness followed­­ a sigmoidal function, with high Δ S colours perceived as equally conspicuous. We found that the relationship between Δ S and conspicuousness varied across colour space (i.e. for different hues). The sigmoidal detectability curve was little affected by colour variation in the background or when colour distance was calculated using a model that does not incorporate receptor noise. These results suggest that the RNL model may provide accurate estimates for perceptual distance for small suprathreshold distance colours, even in complex viewing environments, but must be used with caution with perceptual distances exceeding­ ­10 Δ S .

How Much Pigment Should Flowers Have? Flowers With Moderate Pigmentation Have Highest Color Contrast

“…Increases in pigmentation may be not worth the energy investment once the threshold at which pollinators readily detect the flower has been reached. Color contrast does not scale linearly with detectability by animals, but generally shows a sigmoidal relationship (Olsson et al, 2015;Garcia et al, 2017Garcia et al, , 2020Santiago et al, 2020). Above the detection threshold, which is rapidly reached in the cases considered here, further increases in color contrast may not increase the likelihood of detection by pollinators.…”

A five‐channel LED display to investigate UV perception

“…Interestingly, the pure UV stimulus had a significantly lower success rate and higher latency than the UV‐grey and the blue stimuli. This detection study is a precursor to a more comprehensive colour discrimination study using Ishihara‐style stimuli, where the targets are presented against a background with the same luminance (Cheney et al., 2019; Santiago et al., 2020). Using this approach with our RGB‐V‐UV LED display could provide valuable information on UV discrimination thresholds in a variety of UV‐sensitive animals for which only non‐UV discrimination thresholds have been assessed such as the honeybee Apis mellifera (Vorobyev et al., 2001), domestic chicks Gallus gallus (Olsson et al., 2015), the guppy Poecilia reticulata (Sibeaux et al., 2019) and a cichlid Metriaclima benetos (Escobar‐Camacho et al., 2019).…”

The visual ecology of selective predation: Are unhealthy hosts less stealthy hosts?

“…As in the case of brightness contrast, chromatic contrast is measured in units of JNDs and the discriminability threshold is 1 JND. Whether two stimuli that are >1 JND different from their background are differentially conspicuous to the viewer remains a matter of debate (Fleishman et al, 2016 ; Santiago et al, 2020 ). Recent experiments suggest that the relative conspicuousness of two targets with suprathreshold chromatic contrasts (JND > 1) does increase with the difference in their JNDs (Fleishman et al, 2016 ; Santiago et al, 2020 ).…”

“…Whether two stimuli that are >1 JND different from their background are differentially conspicuous to the viewer remains a matter of debate (Fleishman et al, 2016 ; Santiago et al, 2020 ). Recent experiments suggest that the relative conspicuousness of two targets with suprathreshold chromatic contrasts (JND > 1) does increase with the difference in their JNDs (Fleishman et al, 2016 ; Santiago et al, 2020 ). However, Santiago et al ( 2020 ) found that the ability of fish to discriminate between targets saturates as the targets’ contrast with the background increases, such that fish may not be able to discriminate between two objects that contrast greatly with their background, for example, by >20 JNDS.…”