Spectral sensitivity of the red and yellow oil droplet fields of the pigeon (Columba livia)

Abstract

THE function of the brightly coloured oil droplets situated within the cone receptors of the avian retina has long been the subject of debate^1–4. Among other hypotheses it has been suggested that they reduce glare and chromatic abberation¹, remove unwanted short-wavelength sensitivity², and have a role in colour vision^3–5. The transmission spectra of the droplets in the visible spectrum are typically those of short wavelength cut-off filters^5,6–8, due to the presence in high concentration of carotenoids of dietry origin^9–11. Recent evidence from microspectrophotometry suggests that the droplets filter the light reaching the visual pigment in the receptor's outer segment, resulting in the modification of the sensitivity functions of some of the individual receptors, from the broad band sensitivity of the visual pigment alone to a narrow band one^12,13. It is also known that marked differences occur in the proportions of the variously coloured droplets between species, and also between different parts of the retina of a given species^4,14–16. In the pigeon (Columba livia) the retina is divided into two distinct areas, known as the red and yellow fields, defined by their oil droplet populations. The red field is in the dorsal retina (frontal–ventral visual field), whereas the yellow field is in the ventral retina (dorsal visual field): the two fields differ in both the type and number of broad and narrow band receptors present, as well as in receptor density and the neural complexity of the retina^13,17,18. We report here clear differences in the photopic spectral sensitivity of the two fields, determined behaviourally, and suggest that these reflect a difference in the colour vision mechanisms of the two areas.