The contribution of single and double cones to spectral sensitivity in budgerigars during changing light conditions

Abstract

Bird colour vision is mediated by single cones, while double cones and rods mediate luminance vision in bright and dim light, respectively. In daylight conditions, birds use colour vision to discriminate large objects such as fruit and plumage patches, and luminance vision to detect fine spatial detail and motion. However, decreasing light intensity favours achromatic mechanisms and eventually, in dim light, luminance vision outperforms colour vision in all visual tasks. We have used behavioural tests in budgerigars (Melopsittacus undulatus) to investigate how single cones, double cones and rods contribute to spectral sensitivity for large (3.4°) static monochromatic stimuli at light intensities ranging from 0.08 to 63.5 cd/m². We found no influences of rods at any intensity level. Single cones dominate the spectral sensitivity function at intensities above 1.1 cd/m², as predicted by a receptor noise-limited colour discrimination model. Below 1.1 cd/m², spectral sensitivity is lower than expected at all wavelengths except 575 nm, which corresponds to double cone function. We suggest that luminance vision mediated by double cones restores visual sensitivity when single cone sensitivity quickly decreases at light intensities close to the absolute threshold of colour vision.

Bird colour vision: behavioural thresholds reveal receptor noise

“…1B). The unrewarded colours were chosen such that the colour difference to the rewarded colour ranged from well below 1 to above 3 just-noticeable differences (JNDs; see supplementary material Table S1), according to the RNL model, using a Weber fraction of 0.1 (Eqn 1 in the Materials and methods) for the LWS mechanism that is commonly adopted for birds Lind et al, 2014). Colour differences larger than 1 JND should be discriminable if correct parameters are assumed and the model correctly describes behaviour (Vorobyev et al, 2001;Lind et al, 2014).…”

“…Maier, 1992;Prescott and Wathes, 1999;Goldsmith and Butler, 2003;Goldsmith and Butler, 2005;Lind et al, 2014). These studies suggest that bird colour vision is tetrachromatic, mediated by the single cones, which are compared in opponent processing and well described as being limited by receptor noise .…”

“…Over a wide range of relatively high light intensities, we expect that Weber's law holds, so that sensitivity changes proportionally to light intensity and a constant Weber fraction (ω) describes the signal-to-noise ratio that sets discrimination thresholds (Donner et al, 1990;Osorio et al, 2004;Lind et al, 2014). Under these conditions, the main source of noise is probably transducer noise (Lillywhite and Laughlin, 1979;Howard and Snyder, 1983) originating at the later stages of signal transduction in the photoreceptors, possibly by fluctuations in cGMP levels (Angueyra and Rieke, 2013).…”

“…Under these conditions, the main source of noise is probably transducer noise (Lillywhite and Laughlin, 1979;Howard and Snyder, 1983) originating at the later stages of signal transduction in the photoreceptors, possibly by fluctuations in cGMP levels (Angueyra and Rieke, 2013). There are no electrophysiological measurements of noise in bird photoreceptors, but rough estimates of noise have been deduced from the results of behavioural experiments on spectral sensitivity (Maier, 1992;Lind et al, 2014).…”

The evolution of opsins and color vision: connecting genotype to a complex phenotype

“…This however, has never been demonstrated. There are rare and valuable behavioral studies aimed at understanding bird color vision (Osorio et al, 1999;Ham and Osorio, 2007;Lind and Kelber, 2009;Lind et al, 2014;Olsson et al, 2015) among others. Some have contributed some initial insight into opponency in birds (Wright, 1972;Goldsmith and Butler, 2005).…”