title: Bird colour vision: behavioural thresholds reveal receptor noise.
abstract: Birds have impressive physiological adaptations for colour vision, including tetrachromacy and coloured oil droplets, yet it is not clear exactly how well birds can discriminate the reflecting object colours that they encounter in nature. With behavioural experiments, we determined colour discrimination thresholds of chickens in bright and dim light. We performed the experiments with two colour series, orange and green, covering two parts of chicken colour space. These experiments allowed us to compare behavioural results with model expectations and determine how different noise types limit colour discrimination. At intensities ranging from bright light to those corresponding to early dusk (250-10 cd m(-2)), we describe thresholds accurately by assuming a constant signal-to-noise ratio, in agreement with an invariant Weber fraction of Weber's law. Below this intensity, signal-to-noise ratio decreases and Weber's law is violated because photon-shot noise limits colour discrimination. In very dim light (below 0.05 cd m(-2) for the orange series or 0.2 cd m(-2) for the green series) colour discrimination is possibly constrained by dark noise, and the lowest intensity at which chickens can discriminate colours is 0.025 and 0.08 cd m(-2) for the orange and green series, respectively. Our results suggest that chickens use spatial pooling of cone outputs to mitigate photon-shot noise. Surprisingly, we found no difference between colour discrimination of chickens and humans tested with the same test in bright light.
authors.affiliation: Lund University, Department of Biology, Sölvegatan 35, SE-226 32 Lund, Sweden peter.olsson@biol.lu.se.
authors.forename: Peter
authors.initials: P
authors.lastname: Olsson
authors.affiliation: Lund University, Department of Biology, Sölvegatan 35, SE-226 32 Lund, Sweden Lund University, Department of Philosophy, SE-221 00 Lund, Sweden The University of Auckland, Department of Optometry and Vision Science, Auckland 1142, New Zealand.
authors.forename: Olle
authors.initials: O
authors.lastname: Lind
authors.affiliation: Lund University, Department of Biology, Sölvegatan 35, SE-226 32 Lund, Sweden.
authors.forename: Almut
authors.initials: A
authors.lastname: Kelber
mesh_terms: D000328:Adult; D000818:Animals; D001522:Behavior, Animal; D002645:Chickens / Q000502:physiology; D003118:Color Perception / Q000502:physiology*; D055253:Color Vision / Q000502:physiology; D004193:Discrimination Learning; D005260:Female; D006801:Humans; D008027:Light; D008297:Male; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology; D012684:Sensory Thresholds; D059629:Signal-To-Noise Ratio
keywords: Animal behaviour; Bird vision; Gallus gallus; Psychophysics; Visual modelling