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

Area of Science:

• Animal behavior
• Visual ecology
• Sensory neuroscience

Background:

• Animal color signals must be conspicuous to attract attention for signaling functions.
• Evaluating signal conspicuousness requires understanding visual perception in complex environments.
• The receptor noise limited (RNL) model quantifies color difference (ΔS) near the discrimination threshold.

Purpose of the Study:

• To investigate the relationship between suprathreshold color differences (ΔS) and conspicuousness in a coral reef fish.
• To determine if conspicuousness scales linearly with ΔS and is independent of color direction.
• To assess the influence of background complexity on color conspicuousness.

Main Methods:

• An Ishihara-style test was used with the coral reef fish *Rhinecanthus aculeatus*.
• Conspicuousness was measured by the fish's pecking preference between two targets with varying ΔS from a distractor.
• Color distance was calculated using the RNL model and a non-RNL model.

Main Results:

• The relationship between ΔS and conspicuousness followed a sigmoidal function, not a linear one.
• High ΔS colors were perceived as equally conspicuous, indicating a saturation point.
• The relationship varied across different hues, showing direction-dependent conspicuousness.
• Background color variation had minimal impact on the sigmoidal detectability curve.

Conclusions:

• The RNL model accurately estimates perceptual distance for small suprathreshold differences in complex environments.
• Linear scaling of conspicuousness with ΔS is not universally applicable, especially for larger color differences.
• Caution is advised when using RNL-derived perceptual distances exceeding 10 ΔS for predicting conspicuousness.