Abstract
Photoreceptor adaptation ensures appropriate visual responses during changing light conditions and contributes to colour constancy. We used behavioural tests to compare UV-sensitivity of budgerigars after adaptation to UV-rich and UV-poor backgrounds. In the latter case, we found lower UV-sensitivity than expected, which could be the result of photon-shot noise corrupting cone signal robustness or nonlinear background adaptation. We suggest that nonlinear adaptation may be necessary for allowing cones to discriminate UV-rich signals, such as bird plumage colours, against UV-poor natural backgrounds.
Highlights
Photoreceptor adaptation allows vision to cope with differences of several log units in ambient light intensity between night and day and between open and closed habitats [1]
Receptor adaptation contributes to colour constancy, the ability to maintain object colour appearance independent of the illuminating spectrum
Studies of animal colour vision commonly assume that cones adapt to background light independently, so-called von Kries adaptation [2]
Summary
Photoreceptor adaptation allows vision to cope with differences of several log units in ambient light intensity between night and day and between open and closed habitats [1]. At long wavelengths and in bright light (more than 1 cd m22), the results obey Weber’s law, i.e. sensitivity—the inverse of detection threshold—is invariantly proportional to background intensity. This is consistent with (i) von Kries adaptation that ensures optimal cone performance during changing light conditions and (ii) invariant signal-to-noise ratio (SNR). Sensitivity was lower than expected from Weber’s law at shorter wavelengths below 450 nm, possibly as a result of photon-shot noise [8] This noise originates in the stochastic nature of photon arrival at the photoreceptors and is given by the square root of the receptor photon catch, affecting vision at low light levels. UV-poor conditions [9], our study offers the first experimental data on the selective adaptation of UV cones
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