The colour vision of the pigeon

Abstract

The pigeon's colour vision was examined, using behavioural and physiological techniques Avian colour vision has aroused interest because of the suggestion that chromatic discrimination in birds is mediated by a single cone pigment, combined with several types of retinal oil-droplets which act as differential colour filters. Using an operant conditioning method, difference thresholds were measured throughout the spectrum (400 - 680 nm) to generate a wavelength discrimination function, which yields information about the type of visual system an animal possesses. Earlier work had suggested that birds are trichromatic, but the finding of three clearly defined regions of optimum discrimination at 595, 530 and 460 nm indicates instead that the pigeon's colour vision is at least tetrachromatic. The pigeon's saturation discrimination abilities were also studied using a similar technique Saturation increased towards the spectral extremes while a point of least saturation occurred at 597 nm. Additional subsidiary saturation minima were found at 443, 496, 536 and 662 nm. These results largely corroborated those of the wave length discrimination experiment but indicated that the pigeon's visual system may be more complex than a tetrachromatic one Preliminary to an extension of the wavelength discrimination study, the pigeon's spectral sensitivity was measured electroretinographically The resulting spectral sensitivity curve peaked at 560 - 580 nm, in agreement with previously reported data. Furthermore, spectral sensitivity extended well into the ultraviolet region (<400 nm), where sensitivity was quite high In a second study of wavelength discrimination, results of the first experiment showing three threshold minima were confirmed and, additionally, pigeons maintained good discrimination between wavelengths within the ultraviolet range. Experimental findings were discussed in terms of the physiological mechanisms underlying visual performance, in particular, the present results, together with other evidence, suggest that the retinal oil-droplets are not basic to avian colour vision The functional significance of the pigeon's colour vision was also considered