The birds were first trained on a limited set of paintings: when the shown painting was a Picasso, the pigeon was able to obtain food by repeated pecking; when it was a Monet, pecking had no effect. After a while, the pigeons would only peck when shown Picasso paintings. They were then able to generalize, and correctly discriminate between paintings of the two painters not previously shown, and even between cubist and impressionist paintings (cubism and impressionims being the two stylistic schools Picasso and Monet belong to). When the Monet paintings were shown upside down, the pigeons were not able to properly categorize anymore; showing the cubist works upside down did not have such an effect.
In 1995, the authors won the humorous Ig Nobel Prize in psychology for this work.
Similar experiments had shown earlier that pigeons can be trained to distinguish between photos showing human beings and those that do not, and between photos showing trees and those that do not, among many other examples.
In all these cases, discrimination is quite easy for humans, even though the classes are so complex that no simple distinguishing algorithm or rule can be specified. It has therefore been argued that pigeons are able to form "concepts" or "categories" similar to humans, but that interpretation is controversial. Nevertheless, the experiments remain important and often cited examples in cognitive science.