ABSTRACT
How well can a bird discriminate between two red berries on a green background? The absolute threshold of colour discrimination is set by photoreceptor noise, but animals do not perform at this threshold; their performance can depend on additional factors. In humans and zebra finches, discrimination thresholds for colour stimuli depend on background colour, and thus the adaptive state of the visual system. We have tested how well chickens can discriminate shades of orange or green presented on orange or green backgrounds. Chickens discriminated slightly smaller colour differences between two stimuli presented on a similarly coloured background, compared with a background of very different colour. The slope of the psychometric function was steeper when stimulus and background colours were similar but shallower when they differed markedly, indicating that background colour affects the certainty with which the animals discriminate the colours. The effect we find for chickens is smaller than that shown for zebra finches. We modelled the response to stimuli using Bayesian and maximum likelihood estimation and implemented the psychometric function to estimate the effect size. We found that the result is independent of the psychophysical method used to evaluate the effect of experimental conditions on choice performance.
Footnotes
Competing interests
The authors declare no competing or financial interests.
Author contributions
Conceptualization: P.O., A.K.; Methodology: P.O., R.D.J.; Software: J.J.F., J.D.K.; Validation: P.O., J.J.F., J.D.K.; Formal analysis: P.O., J.J.F., J.D.K., A.K.; Investigation: P.O., R.D.J., J.J.F., J.D.K., A.K.; Data curation: P.O., J.J.F., J.D.K.; Writing - original draft: P.O., R.D.J., J.J.F., J.D.K., A.K.; Writing - review & editing: P.O., J.J.F., J.D.K., O.L., A.K.; Visualization: P.O., J.J.F., J.D.K.; Supervision: O.L., A.K.
Funding
This work was supported by Human Frontier Science Program (grant no. RGP0017/2011), the Swedish Research Council (Vetenskapsrådet; 2012–2212 to A.K. and 637-2013-388 to O.L.) and the Knut och Alice Wallenbergs Stiftelse (Ultimate Vision).
Data availability
The R code used for the statistical analysis of the behavioural data, the response data derived from the behavioural experiments, the HTML outputs of the R markdown versions of each script, supplementary figures, and spreadsheets of light measurements used in these experiments are available from GitHub (https://github.com/JohnKirwan/Olsson_colour_discrimination/) and from figshare (https://figshare.com/articles/figure/Olsson2020_bonus_material_zip/13233812).
Supplementary information
Supplementary information available online at https://jeb.biologists.org/lookup/doi/10.1242/jeb.209429.supplemental
- Received June 23, 2019.
- Accepted October 19, 2020.
- © 2020. Published by The Company of Biologists Ltd
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