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 (ΔS) 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 ΔS 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 ΔS from the average distractor colour. We found the relationship between ΔS and conspicuousness followed a sigmoidal function, with high ΔS colours perceived as equally conspicuous. We found that the relationship between ΔS 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 ΔS.

中文翻译：

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显着性是否与颜色距离成线性关系？使用珊瑚鱼进行的测试

为了有效，动物颜色信号必须引起注意——因此需要引人注目。为了理解信号函数，在各种环境条件下评估它们对相关观众的显眼性是有用的，包括当视觉场景被不同颜色的物体弄乱时。一种广泛使用的色差 (ΔS) 度量基于受体噪声限制 (RNL) 模型，该模型最初被提议用于确定两种相似颜色何时出现彼此不同，称为区分阈值（或仅显着差异）。对超过此阈值的颜色之间的感知距离的估计（称为“超阈值”色差）通常假设颜色的显着性与颜色距离成线性比例，并且该比例与色彩空间中的方向无关。目前，几乎没有行为证据支持这些假设。本研究使用 Ishihara 式测试对珊瑚礁鱼 Rhinecanthus aculeatus 评估 ΔS 与超阈值颜色的显着性之间的关系。作为我们对显眼性的衡量标准，我们测试了当遇到两个彩色目标时，鱼是否更喜欢啄食与平均干扰颜色相比具有更大 ΔS 的目标。我们发现 ΔS 和显眼性之间的关系遵循 sigmoidal 函数，高 ΔS 颜色被认为同样显眼。我们发现 ΔS 和显眼性之间的关系在颜色空间中（即对于不同的色调）有所不同。Sigmoidal 可检测性曲线几乎不受背景颜色变化的影响，或者当使用不包含受体噪声的模型计算颜色距离时。这些结果表明，即使在复杂的观看环境中，RNL 模型也可以为小超阈值距离颜色的感知距离提供准确的估计，但在感知距离超过 10 ΔS 时必须谨慎使用。