Pattern discrimination in the near-ultraviolet by pigeons

Four pigeons were tested, using an operant conditioning method with successive stimulus presentation, on a pattern discrimination task to determine whether or not they could distinguish patterns that were illuminated by near-ultraviolet (UV) light. Tests at red, green, and UV wavelengths showed that they could significantly discriminate not only the chromatic stimuli that were visible to human observers but also the UV-illuminated patterns that humans could not see. In control sessions, the birds’ performance deteriorated to chance levels when these UV wavelengths were blocked by additional filters. Their discrimination of UV patterns precludes that the pigeon’s known sensitivity to UV is mediated by a preretinal fluorescence, as is probably the case in some other animals, since such a fluorescence does not permit a focused retinal image. Possible retinal mechanisms mediating their UV detection are discussed.