Abstract
Humans group components of visual patterns according to their colour, and perceive colours separately from shape. This property of human visual perception is the basis behind the Ishihara test for colour deficiency, where an observer is asked to detect a pattern made up of dots of similar colour with variable lightness against a background of dots made from different colour(s) and lightness. To find out if fish use colour for object segregation in a similar manner to humans, we used stimuli inspired by the Ishihara test. Triggerfish (Rhinecanthus aculeatus) were trained to detect a cross constructed from similarly coloured dots against various backgrounds. Fish detected this cross even when it was camouflaged using either achromatic or chromatic noise, but fish relied more on chromatic cues for shape segregation. It remains unknown whether fish may switch to rely primarily on achromatic cues in scenarios where target objects have higher achromatic contrast and lower chromatic contrast. Fish were also able to generalize between stimuli of different colours, suggesting that colour and shape are processed by fish independently.
Highlights
The survival of visually adept animals depends on their ability to detect and identify prey, predators and conspecifics that are often concealed by shadows and/or camouflaged by disruptive patterns [1,2,3]
We have shown that R. aculeatus groups dots to segregate shape and generalizes shape irrespective of colour and lightness
We conclude that R. aculeatus relied more heavily on chromatic cues for object segregation
Summary
The survival of visually adept animals depends on their ability to detect and identify prey, predators and conspecifics that are often concealed by shadows and/or camouflaged by disruptive patterns [1,2,3]. Colour vision enhances the ability of animals to detect objects and it has been suggested that colour vision originally evolved as an adaptation for object detection in conditions of changing and patchy illumination [4,5]. Humans group the components of visual patterns according to their colour, and perceive colours largely separate from shape [6,7]. Both these features of our perception can be explained as a byproduct of certain aspects of processing of visual information in the human retina and brain [8]. The two information streams are combined during the final decision
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